Does air pollution affect the energy output of solar panels?

beijing smog 2017
beijing smog 2017

Smog engulfs Beijing on January 4 (courtesy of Jim Sciutto, CNN).

This is a very me post, so bear with me. There’s a picture of my cat at the end in it for you if you finish.

In the past few days, I came across two newsy items regarding China that caught my attention.

The first is the time lapse video below from British expat Chas Pope, who lives in Beijing. It depicts the horrific scene of a cloud of smog descending upon the city on January 2.

Beijing has long had well-known pollution issues, but the smog besetting the capital, and dozens of other cities in Northern China in recent weeks, has been historic. While fine particulate matter (PM2.5) levels actually fell nearly 10% in Beijing last year, authorities have already issued red fog alerts for 24 cities – a first in Chinese history – so far in 2017. These relatively new warnings are invoked when pollution levels exceed certain thresholds for multiple days. These types of toxic smog events – like the one in Donora, Pennsylvania in 1948 – are acutely toxic and can kill thousands of people each day.

The second was this section from a Bloomberg article on the falling costs of solar power:

In China, the biggest solar market, will see costs falling below coal by 2030, according to New Energy Finance. The country has surpassed Germany as the nation with the most installed solar capacity as the government seeks to increase use to cut carbon emissions and boost home consumption of clean energy.

Because my brain works in strange ways that even I don’t fully comprehend, these two items fused in my mind to open up a new line of inquiry. We know that China is investing heavily in clean energy in order to curb its air pollution crisis, which kills some 4,400 Chinese every single day. This rapid scaling up of clean energy should help to mitigate some of the major causes for this pollution, namely reliance on coal-fired power plants for electricity and heating.

So we already understand the first side of this relationship – energy produced from solar panels affects air pollution levels. But what about the reverse? Can air pollution also affect the amount of solar energy? Does air pollution generated from fossil fuels reduce the energy output from solar panels?

Can air pollution affect output from solar panels?

On the surface, this question seems pretty straightforward. We know, for instance, that ground-level ozone (O3) inhibits plant growth. However, most of this effect seems to come from the O3 either clogging the stomata of the plants, limiting their ability to exchange oxygen and carbon dioxide, or from the O3 chemically reacting within the cells of the plants, damaging them and making them more susceptible to other stressors.

We also know that solar power output is dependent upon environmental factors, including cloud cover, solar irradiance, and ambient air temperatures. Accordingly, it would seem logical that air pollution could also play a factor.

Unfortunately, there is not a lot of research on this particular issue. That said, what research there is all seems to point in one direction; namely, yes, air pollution can limit solar power generation.

The impact of pollution deposition

One line of inquiry in this field of research centers on the impacts of airborne particulate settling on the solar panels themselves.

Throughout China and much of Asia, including India and Indonesia, the prime air pollutant of concern is PM2.5, which consists of various microscopic particles and drops of liquid suspended in air. Through a process known as deposition, the particles will settle out of the air, either due to rain (wet deposition) or wind (dry deposition).

When this occurs, particularly through dry deposition, the particles will cling to a surface nearby. This effect helps explain part of the reason why trees and other forms of vegetation are so effective at mitigating air pollution – leaves can trap and hold onto pollutants. But deposition can occur on nearly any surface, which is why you’ll hear stories from before the Clean Air Act about how people living in Cleveland or Pittsburgh had to change shirts at lunchtime, since they were soiled by soot and ash.

Logically, these particles can also deposit onto solar panels, if they are in the immediate vicinity. Keeping the surface of a panel clean is essential to ensuring it can capture solar radiation and convert it to electricity efficiently. Any dust from air pollution that accumulates on the panels may reduce power output through a process known as soiling.

Multiple studies have shown that soiling can reduce solar power output. In a 2001 study (paywall) in the journal Renewable Energy, Ebrahim Asl-Soleimani, Shahrokh Farhangi, and M.S. Zabihi examined the effects of panel tilt angle and ambient air pollution on solar output in Tehran.

Like many major cities with persistent pollution problems, Tehran is surrounded by mountains, which inhibits the movement of air through the city, allowing for pollution to accumulate over time. As a result, the city consistently ranks among the most polluted. Asl-Soleimani and his colleagues compared the power output of a solar panel on two days in December 1999 – one with high levels of pollution and one with clearer skies. They found that “air pollution can reduce the energy output of solar modules by more than 60%.”

John Kaldellis and Alexandra Kokala of the Technological Educational Institute of Piraeus (TEIP), a university in Athens, published a similar study in 2010. From August to September 2009, they conducted an experiment in which they exposed pairs of solar panels to ambient air pollution for different amounts of time. In addition to a control pair, they kept panels outdoors – without cleaning them or exposing them to rainfall – for 2, 4, 6, and 8 weeks.

Athens suffers from many of the same pollution issues as Tehran due to its geography, and particles readily accumulated on the panels during this period. The panels left outside for 2 weeks saw 0.1 grams per square meter (g/m2) of dust accumulation, while those left out for 8 weeks saw 1 g/m2. As the dust accumulated on the panels, over time, the output deteriorated due to falling levels of solar irradiance. At 2 weeks, panel output fell by around 2%. This value climbed to 6.5% by week 8.

Additionally, Miqdam Chaichan, Bashar Mohammed, and Hussein Kazem published an article (PDF) in April 2015 that examined the effects of pollution and dust on solar panel output in Iraq. They compared the output of a panel they cleaned by hand to one cleaned naturally by rainfall and one covered in particles from ambient pollution. Compared to the clean panel, power output fell for the naturally cleaned and polluted panels by 7.6% and 11.9%, respectively. The average efficiency rate of the solar cell on the polluted panel actually fell from 4.8% to just 1.7%, an astonishing 63.7% drop in efficiency.

According to these authors, PM10 and PM2.5 are particularly harmful to solar panels because, unlike larger dust particles, one cannot readily wash them off. As a result, the accumulation of PM can damage the surface of the panel, lowering its efficiency and shortening its lifespan.

Pollution also blocks out sunlight

But deposition is not the only way that air pollution can affect solar power production. Much as cloud cover can reduce the amount of solar radiation that reaches the photovoltaic (PV) cells, so too can heavy smog, like what residents of Beijing are enduring.

Mohammadreza Maghami and colleagues from Universiti Putra Malaysia (UPM) examined this issue in a 2014 article for PLOS OneIn the study, they compared the output from solar panels on the UPM campus before, during, and after a major pollution event throughout Southeast Asia from June 13-19, 2013.

They authors cleaned two solar panels on June 1, then tracked their average output from before (June 1-10), during (June 11-21), and after (June 22-30) the haze, taking power output samples every 30 minutes. As the chart shows, power output plummeted as the haze set in, and ultimately jumped back up after it ended.

power output 2013 haze

Power output from fixed flat PV (FF) and tracking flat PV (TF) solar panels before, during, and after the 2013 Southeast Asian haze event (courtesy of Maghami et al).

While daily average energy output was 8.5 kWh before and 10.6 kWh after the haze, it fell to just 6.5 kWh during the pollution event, a reduction of 23% and 39%, respectively. As the researchers concluded, “the effect on PV generation was strongly dependent on the haze pollution.”

Ultimately, while the literature is relatively sparse, its results are conclusive. Just as air pollution from fossil fuel combustion harms public health and the environment, it also undermines the productivity of the clean energy we are counting on to replace it. Fossil fuels suck.

 

As promised, here’s Gigi.

Gigi being sassy AF.

 

Don’t listen to NEOMG – closing Public Square to buses leads to more air pollution

public square bus protest
public square bus protest

Protestors, including Councilman Zack Reed, call for the opening of Public Square to buses on December 3 (courtesy of Cleveland Scene).

One can generally count on Advance Ohio/NEOMG/Cleveland.com/The Plain Dealer/whatever they are going by nowadays to defend vigorously the interests of the entrenched powers-that-be. This outcome particularly holds true when it comes to shiny, big ticket megaprojects.

Regardless of whether or not said megaprojects actually have merit, Cleveland’s largest media conglomerate and newspaper seems all too happy to eschew logic or internal consistency in their quest to carry the water for the region’s political and business elite.

One need look no further than their breathless coverage last week of the “transformation plan” for Quicken Loans Arena. Cleveland.com even created a helpful landing page for the proposal, complete with 13 separate stories. Erstwhile good journalists twisted themselves into knots trying to defend a plan that will cost taxpayers some $160 million over the next two decades to bring up to snuff an arena that just hosted the Republican National Convention, in the hopes of “boosting the city’s ability to attract major events, such as political conventions.” Check your logic at the city limits, folks.

With all of that in mind, it is really no surprise that NEOMG/the PD/whatever would happily defend Cleveland Mayor Frank Jackson’s inexplicable decision to close Superior Avenue through Public Square to buses. A lot of ink and words have been spilled on this issue, and I’m not here to relitigate this fight. Instead, I just want to focus on a relatively narrow issue.

Two weeks ago, just days after a contentious City Council hearing on the issue, the PD published an editorial that dutifully parroted the Jackson administration’s talking points on the subject, right down to the hyperbolic fear-mongering about terrorist acts and bus drivers mowing down children in the street.

Putting aside those claims for a minute [which, honestly, we shouldn’t, because hoo boy], there was one particular part that really caught my attention,

Public squares were designed in a quieter time before terrorist considerations and wheezing block-long buses were prevalent.

When I read that sentence, I think my eyes damn near rolled out of my head and onto the floor.

Forget the absurd claim that “public squares were designed in a quieter time before terrorist considerations,” which is, obviously, insanely ahistorical. One can easily date terrorism back the first century CE, and the word itself has its origins in Reign of Terror during the French Revolution, which took place two years before Moses Cleaveland even established this city.

Leave aside the assertion that GCRTA employs “block-long buses,” which is preposterous. The average block in downtown Cleveland is roughly 500-600 feet long. GCRTA’s longest buses are…not.

About those “wheezing” buses…

But that’s still not what I want to talk about. While you may want an analysis of the merits – or lack thereof – of the arguments put forward by the administration and its water carriers at 1801 Superior Avenue, I gotta be me. And, as Area Air Quality Nerd, I cannot get past the “wheezing” part of that ludicrous sentence.

Read literally, the PD’s editorial board argues that allowing GCRTA buses to use the dedicated bus lanes on Superior Avenue through Public Square would allow them to belch out diesel exhaust, fouling air quality and damaging the lungs of passersby.

Except that is prima facie absurd. Perhaps the members of the editorial board don’t quite understand how mobile emissions work, but that isn’t it. On the contrary, forcing buses to travel around, rather than through, the Square should produce more emissions, as the buses are forced to drive farther and sit in traffic as they compete for road space with other vehicles. But how much?

Fortunately, I do this sort of thing for a living, so I can estimate the additional bus emissions associated with closing Superior Avenue through Public Square to buses.

The method to my madness

Let me briefly lay out my methodology. According to GCRTA data, roughly 1,445 buses drive through/around Public Square on a daily basis. Because those buses are not able to access their dedicated lanes on Superior Avenue, they are forced to transit another 0.1 miles around East Roadway/West Roadway/Rockwell, adding some 52,754.5 miles per year. Additionally, because the buses are now in traffic, they must travel at reduced speeds and idle as they wait to get back on Superior Avenue.

Below, I lay out the additional emissions that result from closing the Superior Avenue bus lanes through Public Square. In one scenario, I assume each bus trip is delayed by 2 minutes – the lower estimate which the administration provided at the Council hearing. In a second scenario, I assume each bus trip is delayed by 4 minutes, which, while double the administration’s estimates, is still below observed delays of 6 to 10 minutes from GCRTA riders. The former scenario leads to 17,851 idling hours per year, while the latter adds up to 35,162 hours.

I utilized MOVES2014a, the most recent version of the U.S. EPA’s mobile emissions modeling software, to develop emissions factors per mile and for each additional hour of idling for the GCRTA bus fleet. I then converted total emissions into additional metric tons per year. The results are shown below.

additional emissions public square

Additional emissions from closing Superior Avenue through Public Square to buses (authors estimates using MOVES2014a).

Closing Superior generates more emissions

As you can see, the additional distance the buses must travel (0.1 miles per trip), leads to de minimis emissions. But when you add in the idling emissions, those numbers climb significantly. Carbon monoxide (CO) emissions total 0.71 and 1.13 tons, respectively, based 2- and 4-minute delays, while nitrogen oxide (NOx) emissions total 1.96 and 3.16 tons, respectively. Closing the Square also leads to an additional 535.98 and 860.96 tons of carbon dioxide equivalent (CO2e) per year for each scenario, respectively.

And those emissions have real costs

On the whole, these are not particularly eye-popping numbers. But they do carry real costs and consequences. When the City of Cleveland and GCRTA initially sought federal funding for the Healthline BRT project in 2001, they estimated the emissions savings the project would generate. As the table below shows, the additional emissions from closing the Square to buses nullifies a portion of those emissions.

costs from additional public square emissions

Costs associated with additional emissions from closing Superior Avenue through Public Square to buses (authors estimates).

The additional CO emissions only takes away slightly more than 1% of the estimated savings; that said, vehicle CO emissions have plummeted nationwide since that point due to new vehicle emissions controls, so that’s not surprising. But the additional NOemissions could wipe away almost half the estimated savings under a 4-minute delay scenario.

These extra emissions carry real social costs. I have also estimate the social costs of the additional emissions, using damage estimates from the Federal Highway Administration. Again, the numbers are not staggering, but they do amount to tens of thousands of dollars in additional social costs tied solely to the Mayor’s decision to close a 600-foot piece of road.

Don’t forget those unknown unknowns

Furthermore, I cannot calculate any additional emissions that may result from the ripple effects of this ill-conceived decision. GCRTA has already cut more bus revenue miles than any other major transit agency, and it recently enacted a two-step fare increase. Add to that a potentially catastrophic budget hit from the loss of sales tax revenues on managed care organizations, and you have a recipe for disaster.

Tacking on another $1.6 million in operating expenses and increasing delays will make the experience worse for riders, possibly driving those who can afford it off the bus and into private cars. Given that GCRTA buses release 19% fewer average emissions per passenger mile than single-occupant vehicles (320 grams vs. 396 grams of CO2e, per my estimates), this outcome would just add even more emissions.

So while I expected the PD to support the Jackson administration’s choice, they should tread more carefully when it comes to verifiably inaccurate statements. There are no block-long buses hurtling through Public Square, belching out emissions. Just the opposite, in fact.

Employers play a major role in shaping commuting behavior

miami rush hour traffic
miami rush hour traffic

Rush hour traffic along I-95 in Miami (courtesy of Wikimedia Commons).

For sustainable transportation advocates, changing people’s commuting behaviors can seem like our white whale.

While commutes account for just 19% of total personal trips in the US, they play an outsized role in our transportation system, accounting for 27.8% of total vehicle miles traveled (VMT).

Their timing is also critical. The concept of rush hour revolves around our commute patterns. In cities like Washington, DC and Los Angeles, rush hour congestion can make life hell commuters, costing them time, money, and sanity. But in cities that are not growing and have no real congestion issues normally, these rush hour periods are particularly important.

For a city like Cleveland, commuting patterns directly influence the transportation infrastructure we end up with. The influx of drivers heading to and from work each day provides justification to expand our already overbuilt road system, which has serious impacts on development patterns, travel choices, and mobile emissions. If we could smooth these demand spikes by reducing the number of single-occupant vehicles (SOVs) on the road, we could potentially upend this vicious cycle, which justifies the continued addition of freeway lane miles to the system.

Unfortunately, the evidence suggests that we’ve made little, if any ground in this area. In 1960, the Census Bureau reported that 64% of Americans drove to work; they did not differentiate between driving alone and carpooling at the time. Another 12.1% of commuters used public transit, while just under 10% walked. By 1980, thanks to OPEC oil embargo, 19.7% of Americans carpooled, and the drive alone number stayed at 64.4%, even as public transit use fell.

But by 2014, these trends had reversed; 76.3% of Americans drive alone to work, while just 5% take transit, and 3.4% walk or bike. To date, our efforts to get people to stop driving alone to work have failed spectacularly.

Commuting: What is the role of employers?

Part of the problem with these efforts is that we have focused far too much on the individual. Commute mode decisions are a two-way street (pun intended). They depend not only on the whims of the individual, but also on employers’ decisions. People don’t just decide to drive to work in a vacuum. Their universe of choices are shaped by a number of endogenous and exogenous factors, including things entirely in the control of their employers.

This thought really crossed my mind recently while I was reading an article on electric vehicles (EVs) by Christopher Mims in the Wall Street Journal. Mims noted the vital role that employers can play in normalizing EVs for their employees:

Placing charging stations at workplaces, where cars spend much of their time, will be uniquely powerful. When a workplace installs a charging station, employees are 20 times as likely to buy a vehicle with a plug, according to a survey from the U.S. Department of Energy.

In light of this fact, it’s important to consider what, precisely, employers can do to influence the commute patterns of their employees.

Earlier this year, the City of Cleveland Office of Sustainability and NOACA partnered together to launch what they called the Commuter Choice Challenge. The goal of the program is “encouraging Northeast Ohio organizations of all shapes and sizes to take action in sustainable transportation.”

While some people may scoff at the idea that we should reward organizations that provide pre-tax transit passes to their employees, there really are a number of steps employers can take to foster mode shift. Collectively, this effort to provide alternatives and enhance the efficiency of our transportation system is known as transportation demand management (TDM).

Changing jobs and the importance of signalling

One of the simplest things that an employer can do is to act as a knowledge broker and paragon for their employees. For most people, commuting is a habit – once people start driving to work everyday, it becomes very difficult to shake them out of it.

Because commuting is a habit, there are only so many potential points at which an intervention is likely to succeed. But habits become weaker when your personal circumstances change. This context tends to shift most abruptly after major life events, such as moving or changing jobs.

In a study published earlier this year, Ben Clark, Kiron Chatterjee, and Steve Melia from the University of the West of England explored how these life events affect people’s commutes. They found that, while one-fifth of all British commuters change their mode from one year to the next, car commuting is far stickier. Just 8.6% of car commuters changed away from driving, and the mean duration of their commute mode was 6.3 years, twice as long as those for public transit (3.0 years) or active transportation (3.2 years).

Targeting new hires can be a highly effective way to disrupt the stability of car commuting. The odds that a person will switch from driving to alternative modes increases 2.5-fold when people change jobs. This highlights the importance of providing new hires with comprehensive TDM options and information, not just a parking pass. Demonstrating from day one that your workplace acknowledges and supports non-SOV modes helps to normalize them for employees. Signalling is an important part of behavioral change.

Pull factors matter…

If employers wish to reduce their SOV share, they need to provide a suite of incentives to get them out of their cars. These pull factors can come in a variety of forms, from reduced health insurance premiums for people who use active transportation to subsidized transit passes.

Considerable evidence suggests that these sorts of TDM packages can go a long way. In a 2005 study, researchers from the U.S. EPA examined the impact of the Agency’s Best Workplaces for Commuters (BWC) program, which recognizes employers that encourage, educate, and incentivize their employees to try alternative commute modes.

The researchers compared the commute patterns among employees using the BWC programs to the average commuter in these same Census blocks. They then modeled the impacts of these commute patterns to see the associated reductions in gasoline use and mobile emissions. According to the authors:

The results of this survey indicate that where employers provide employees with incentives to commute by means other than driving alone, significant percentages of them take advantage of these benefits. Comprehensive benefits packages such as those enjoyed by commuters in the BWC group, with financial incentives, services (such as guaranteed ride home, carpool matching, etc.) and informational campaigns, appear to produce reductions of trips, VMT, pollutants, and fuel consumption of around 15 percent even under conservative assumptions.

Another 2012 paper from Virginia Tech professor Ralph Buehler found that providing bike parking, showers, and locker rooms increases the odds that employees will bike to work nearly 5-fold. Clearly pull factors, such as financial incentives and facilities investments, play a central role in this equation.

But push factors – especially parking – matter more

But, as with anything else, changing commute behaviors requires both push and pull factors. And the latter are particularly key, as the single most effective strategy that an employer can use to reduce SOV share is to remove parking subsidies.

In the US, some 95% of US commuters receive free parking at work. The provision of this benefit can increase the SOV rate for commutes by up to 50%.

UCLA professor Donald Shoup, the godfather of parking research, has explored the effects of curbing this parking subsidy. In a 2005 report, he outlined the benefits of implementing a parking cash out program, by which employers provide commuters with the option of receiving a cash incentive equal to their parking subsidy if they don’t drive alone to work. Such programs allow employees who really want to drive to work to continue getting discounted parking, but it also incentivizes alternatives for those who would rather try them.

Shoup’s research in California found that cash out programs can cut SOV share by 17%. A separate study from Daniel Hess, also of UCLA, concluded that by charging $6 per day for parking, Portland was able to cut its SOV share by 16%.

Getting parking right is even more important than these numbers show, however. The lure of free parking so strong that if an employer rolls out a TDM program but fails to price parking, the latter will simply crowd out the former. As Dr. Shoup put it, “Advocating ridesharing while offering free parking is like denouncing smoking while offering free cigarettes.”

A separate paper Buehler and his colleague Andrea Hamre explored this issue. Their research showed that providing free parking increases the share of commuters who drive alone, regardless of what other incentives the employer may provide. Without free parking, 75.9% of Washington, DC area commuters would drive alone. Free parking increases that share to an astounding 96.6%. Providing subsidized transit and incentives for active transportation, while also supplying free parking, only takes that SOV share down to 86.8%.

As the authors concluded, their research “suggests that benefit combinations that include free parking either overwhelm or render insignificant the positive effects of benefits for public transportation, walking, and cycling.”

Location, location, location

But even the most comprehensive TDM packages will struggle to overcome another factor that employers can control – their location.

Often times, sustainable transportation advocates focus on the negative effects of residential sprawl, but neglect workplace sprawl. Just as people in the US have spread farther and farther outward, so too have employers.

Consider Northeast Ohio. The region boasts five major employment hubs, like downtown Cleveland and University Circle. Yet, combined, these hubs only account for less than one-quarter of all jobs in the region. The rest are distributed broadly across the five counties.

This outcome poses a major challenge to TDM. Transit ceases to be viable when households and destinations are sprawled out. The same holds true for active transportation. No one is going to choose to walk 10 miles to work in an exurb without sidewalks.

The Clark, Chatterjee, and Melia study illustrates this clearly. If a worker’s commute increases from less than to more than two miles, the odds that s/he will switch from active commuting to driving increases 30-fold. The research seems to indicate that two miles is a key threshold; most people simply will not bike to work if their commute is longer than that.

According to the National Center for Transit Research, location may be the most important variable in the commute equation. No matter how strong the TDM package or how much the organization supports alternative modes, locating your office in the middle of an exurban office park locks in your employees’ commute options.

Ultimately, I think we have focused our attention too narrowly on the individual commuter for too long. Research has shown time and time again that the most effective TDM strategies target the employer first, as that is the critical leverage point.

Large institutions that claim to support sustainability need to back up their words through their actions when it comes to commute options. It’s not enough to simply post an annual sustainability report or get your buildings LEED certified if you subsidize parking and locate your office in exurbia. The transportation sector is now the largest source of carbon pollution in the US. It’s time for employers to act like it.

Do ‘ozone action days’ actually inspire people to act?

robert wyly cleveland pollution
robert wyly cleveland pollution

Industrial pollution obscures Cleveland’s cityscape in this 1960 photo from Robert Wyly (courtesy of Elvin Wyly).

“Ozone: Good up high, Bad nearby.” So goes the U.S. EPA’s catchy (?) refrain to help people distinguish between (good) atmospheric and (bad) ground-level ozone.

Fortunately, we have gotten some good news on the former in the past few days. A team of researchers has concluded that we are finally building up more good ozone; that is, the massive hole in the protective ozone layer over Antarctica is finally beginning to heal thanks to the phasing out of chlorofluorocarbons under the 1987 Montreal Protocol. It seems like the ozone layer may be on course to fully recover by the middle of the century.

Unfortunately, the news is not as great on the latter front, as we are also seeing an increase in ground-level ozone. On Tuesday, NOACA issued an ozone advisory, warning residents of Northeast Ohio that ambient levels of ground-level ozone may reach harmful levels, which U.S. EPA defines as those above 70 parts per billion (ppb).

As I’ve documented before, the number of these advisories has dropped significantly over the past decade; however, this summer’s hot, dry weather has stymied that downward trend somewhat. Tuesday marked the seventh time this year that ozone levels in the region exceeded 70 ppb, and NOACA encouraged people – particularly sensitive populations like the elderly and those with respiratory conditions – to limit their time outdoors during afternoon and evening hours in order to minimize their exposure. These types of warnings are commonplace; officials in some 230 metropolitan areas release similar advisories.

Air quality alerts as a call to action?

In a number of areas, these advisories are dubbed action days, highlighting the fact that the agencies see them as a call to arms around ozone pollution. Air quality officials are pushing citizens to not only limit their personal exposure to pollution but also to take steps to reduce the amount of ozone precursor emissions within the region. NOACA, for instance, encouraged people to carpool and take public transportation on Tuesday.

It was with all of this in mind that I read a recent post from the Tri-State Transportation Campaign (TSTC), a non-profit group in the New York City metro area that promotes sustainable transportation and alternatives to car dependency.

In a post titled “Air Quality Alert Days Should Be a Call for Better Streets, Not to Stay Indoors,” Emma Kilkelly, TSTC’s Communications Assistant, chided the Connecticut Department of Energy and Environmental Protection (DEEP) for telling residents to limit their time outdoors, rather than changing their transportation modes away from cars. Kilkelly wrote that “this type of messaging addresses only the symptoms, not the cause of air pollution.”

It’s worth examining this argument further. Do ozone alerts focus on minimizing exposure at the expense of promoting sustainable transportation? Is it actually harmful to tell people to avoid high levels of air pollution? And, if you assume these two points, would using ozone alerts to promote active transportation and public transit actually work?

Now if all of this seems like unnecessary semantics, that’s because it is. But I’m a huge nerd who did all of this research, and this is my website God damnit, so I’m going to write about it.

But air quality agencies are already doing just that

First, it’s pretty disingenuous to claim that air quality agencies are not already encouraging people to change their transportation patterns on ozone action days. The very agency that Kilkelly critiques, DEEP, does this. To be fair, she acknowledges this in her post, so the argument rings a bit hollow, prima facie.

Second, there does appear to be value in encouraging people, especially those most vulnerable to the deleterious impacts of high ozone levels, to limit their exposure. We know that short-term spikes in ground-level ozone levels can have significant impacts on public health.

In a landmark 2004 study, Michelle L. Bell and colleagues from Yale University studied the changes in mortality rates tied to daily fluctuations in ozone levels across 95 U.S. cities. They found that for every 10 ppb increase in daily ozone levels, all-cause mortality rates jump by 0.52% during the following week. That number goes up to 0.67% for a 20 ppb increase, which is quite common on ozone exceedance days. And this effect is even more pronounced for elderly Americans. Mortality rates for Americans aged 65 to 74 go up 0.7% for each 10 ppb spike in ozone.

Do air quality alerts benefit public health?

Accordingly, limiting the amount of time that people from higher-risk groups spend outside during the late afternoon and early evening hours can benefit their well-being. But does encouraging people to shift the time that they exercise or go outdoors – so-called avoidance behaviors – actually work?

This answer appears to be yes. Economists Alison L. Sexton Ward and Timothy K. M. Beatty published a paper (paywalled) last year studying this very question. They found that, on air quality alert days, individuals do engage in avoidance behaviors.

Overall, people limit the amount of time they spend outdoors engaged in vigorous physical activity by 18%. The number is even higher among the elderly, who reduced the amount of time by 59%. A similar study (PDF) from Australia noted that cyclists reduce the amount of time they spend biking on air quality alert days by 18.2% for commuting and 38.2% for recreation, respectively.

But can they drive mode shift?

Third, the question of efficacy remains. If air quality agencies devoted all their public outreach efforts to promoting mode shift on ozone action days, would they suceed?

Unfortunately, the evidence suggests they wouldn’t. Several agencies have tried this approach for years. San Francisco, for instance, operates the Spare the Air Program, which encourages alternative commute modes and subsidizes transit passes on exceedance days. The problem is that it doesn’t work.

Multiple studies cast doubt on the effectiveness of Spare the Air. A 2009 study from W. Bowman Carter and Matthew Neidell noted (PDF) a small increase in transit ridership on ozone action days, but it was statistically insignificant. Stephen Sexton, in turn, argues (paywall) that ozone alerts may actually push some transit users to drive to work in order to limit their exposure to higher pollution levels. His review of the program found an increase in both transit trips and VMT.

As a result, Spare the Air apparently “has the perverse effect of increasing car trips,” making it, essentially “a pay-for-pollution program.”

Studies from other cities back up these results. Calvin Tribby and colleagues, for example, looked at the impact of air quality alerts on traffic volumes in Salt Lake City from 2001 to 2011. They concluded (paywall) that “messages regarding air quality and voluntary reductions in vehicle trips are not only ineffective at reducing traffic but apparently increased average daily traffic levels.” Their work again suggests that normal transit users shift to driving in an attempt to limit their personal exposure to pollution.

All told, the weight of the available evidence contradicts Kilkelly’s central argument. While ozone action days may drive people to take certain actions, they are not necessarily the actions she (and I) would like to see. Instead of criticizing air quality officials for encouraging people to take avoidance behaviors on exceedance days, we should acknowledge the limits of what their pleas can realistically achieve.

While ozone alerts are highly unlikely to cut down on VMT, they can and do provide a clear public health benefit. Let’s focus our attention and advocacy on those actors who can actually influence the nature of our transportation system in the long-run. Criticize the underlying system that forces your friendly area air quality planner to issue these advisories in the first place, not her/him for doing so.

Does sprawl make the urban heat island effect worse?

urban heat island effect by city
urban heat island effect by city

Cleveland experiences the fourth strongest urban heat island effect in the United States. Could our sprawling development patterns be to blame? (courtesy of Debbage and Shepherd, 2015).

A few weeks ago, NASA officially announced that the record-breaking, “Godzilla” El Niño event that dominated much of our weather over the past year plus had finally come to an end.

But while the monster has returned to its hibernation deep below the surface of the Pacific Ocean, its impacts have already been and will continue to be felt across the United States. Around the same time that it made this announcement, NASA also revealed that April and May were the warmest such months on record in the US, meaning that every month since October 2015 has broken the existing record for that month. This eight-month streak of heat is, obviously, unprecedented. To date, the average temperature in 2016 is 1.9°F (1.08°C) above the average for the 20th century, making it a full 0.43°F (0.24°C) above the mark for the first five months of 2015.

You remember 2015, right? The warmest year on record? Well, not for long. NASA scientists are already more than 99% certain that 2016 will break that record, just as 2015 had claimed the mantle from 2014.

The impacts of 2016’s extreme heat

The extreme heat is having clear effects. It is contributing to wildfires consuming wide swathes of the West. Ozone levels are higher than normal across the country, as high temperatures foster the development of harmful, ground-level ozone more readily. So far, Greater Cleveland has already experienced six days when ozone levels exceed 70 parts per billion (ppb), the most at this point since 2012.

But the most acute impact of high temperatures heat-related mortality, a subject that I’ve written about considerably. Extreme heat is the deadliest type of disaster in the US, killing more people than hurricanes, floods, tornadoes, and lightning strikes combined each year. As I’ve discussed in the past, climate change is only exacerbating this issue; the World Meteorological Organization (WMO) noted that the global death toll from extreme heat rose by around 2,300% from 2000-2010, compared to the previous decade.

change in disaster deaths by decade

The change in the number of deaths, by disaster, from 1991-2000 to 2001-2010 (courtesy of WMO).

Nearly all regions have seen a spike in dangerous heat, but the risk of heat-related mortality is not distributed evenly. While an individual’s vulnerability to extreme heat is the function of a number of factors, one of these is where s/he lives. Generally speaking, those of us living in cities are at greater risk due to the so-called urban heat island (UHI) effect. I won’t go too far into the science behind the UHI effect; suffice it to say that the combination of dark surfaces, a lack of urban trees, and the production of waste heat from various sources like air conditioners increases the temperature of cities, relative to rural areas. According to the U.S. EPA, the temperature of a large city can be more than 20°F higher than surrounding rural areas under the right (or wrong?) conditions.

Last September, Forbes published an article examining the scale of the UHI in various cities throughout the US. Strikingly, it included a map (see above) stating that Cleveland has the fourth strongest UHI effect in the country. Now, if you’re one of the literally tens of people who has inexplicably read something I’ve posted on this site, you may be familiar with my general dislike of sprawl. I’ve discussed research linking it to population decline, limited social mobility, climate change, and poor air quality, among other things.

So, I wondered, could Cleveland’s strong UHI effect be the result of our development pattern? Given that sprawl affects so many important phenomena, it seems reasonable to assume it would have an effect on UHI, right? To the peer-reviewed literature! [Cue 1970s Batman transition music].

Is suburban sprawl actually linked to the urban heat island effect?

At first glance, it may seem odd to posit that suburban sprawl would play a role here; the phenomenon is called the urban heat island effect, after all. But a handful of studies strongly suggest that sprawling development patterns do, in fact, exacerbate the UHI effect.

Two of the most convincing papers come from Brian Stone, Jr., a professor at the Georgia Tech School of City and Regional Planning and an expert on urban environmental planning and climate change.

In a 2006 study (paywalled) that he coauthored with Jon Norman from the University of Wisconsin-Madison, Stone examined the link between land use patterns and the UHI effect in Atlanta. The researchers broke properties into groups based on four variables: extent of impervious surfaces, lawn and landscaping, tree canopy, and the number of bedrooms per residential structure. This categorization enabled them to study the magnitude of surface warming produced by property type.

Stone and Norman concluded that the size of residential lots – in other words, residential density – was closely tied to black body flux, a measure of surface warming. As one moves from the highest density lot type to the lowest density, the amount of surface heat released increased 6-fold. Other land use features closed tied to suburban and exurban development – namely large lawns – also exacerbate the UHI effect. A one unit increase in the area of a plot covered by lawn and landscaping increases the net black body flux by 0.51 units.

As the authors conclude,

The results of this analysis provide compelling evidence that the size and material composition of single-family residential parcels is significantly related to the magnitude of surface warming in the Atlanta study region. Specifically, smaller, higher density parcels were found to be associated with a lower net black body flux than larger, lower density parcels…

[The] results of this study support the hypothesis that lower density, dispersed patterns of urban residential development contribute more surface energy to regional heat island formation than do higher density, compact forms.

Connecting sprawl and the UHI across cities

On its own, one study does not prove the relationship. Fortunately, Stone followed up with a 2010 paper that he co-wrote with Jeremy Hess and Howard Frumkin of the University of Washington, which studied the connection between urban sprawl and the number of extreme heat events (EHEs) in 53 cities from 1956-2005.

To measure the relationship, they took the correlation between the mean annual change in the number of EHEs from 1956-2005 and the sprawl ranking for each of the cities in 2000. Whereas the most compact cities experienced 5.6 more extreme heat days in 2005 than in 1956, that number was 14.8 for the most sprawling cities. In other words,

The most sprawling cities experienced a rate of increase in EHEs that was more than double that of the most the most compact cities…These findings are consistent with the hypothesis that urban sprawl contributes to EHE frequency.

Exploring some competing research

Now, I should note that there is other research that does not jibe with Stone’s work. Last year, Neil Debbage and Marshall Shepherd of the University of Georgia took another look at urban form and the urban heat island effect. Using a different measure for UHI (the difference in average rural and urban temperatures) and a different measure for urban form (an index measuring various variables of city shape, contiguity, and land uses), Debbage and Shepherd studied the degree to which city configuration affected urban heat in the 50 largest US metro areas from 2001-2010.

Contrary to Stone, Debbage and Marshall found that both more compact and more sprawling cities experience a stronger UHI effect, provided they are highly contiguous. That is, the contiguity of urban form may matter more than its composition; designing cities so that they are made up of either cul-de-sacs or skyscrapers as far as the eye can see makes them more vulnerable to extreme heat. According to the authors,

A ten percentage point increase in the spatial contiguity of high-intensity urban development, the equivalent of shifting roughly from Orlando to Seattle, was predicted to enhance a city’s average UHI intensity by 0.4°C…[In turn] a ten percentage point increase [in low-intensity urban development] was predicted to enhance a city’s annual average UHI intensity by 0.3°C. Therefore, as suggested by the bivariate analysis, both low and high-density urban land uses appear to amplify the UHI effect if they are high contiguous.

Importantly, Debbage and Marshall note that, while compact development may not solve the UHI on its own, it does provide a litany of other benefits, from improved air quality to better public health. Accordingly, urban planners may want to promote less contiguous, higher density urban development by designing networks of smaller green spaces, expanding the urban tree canopy, and installing white and green roofs throughout cities. A 2014 study by Stone and colleagues found that implementing these types of policies can offset projected increases in heat-related mortality due to climate change by anywhere from 40-99%.

Wait, so does sprawl make the UHI effect worse?

So what can we take away from all of this?

First, yes – the evidence does suggest that sprawl exacerbates urban heat islands. Low-density, suburban-style development increases the amount of impervious surfaces, which raises lowers the surface albedo of urban areas. It also increases the amount of excess waste heat that cities produce, as larger houses require more energy. And sprawl typically leads to forest clearance for development, reducing the extent of the urban tree canopy. All told, these factors increase the amount of heat cities generate, and they prevent this additional heat from dissipating rapidly at the urban fringe.

Second, the fact that sprawling development patterns are not the only type urban form that increases the UHI effect may not be as relevant as it may seem. While dense urban areas may also promote UHIs, they also make it easier to address both the causes and effects of heat-related mortality risks. Residents of dense cities produce fewer carbon emissions per capita, mitigating climate change. And the economies of scale in these dense neighborhoods increases the efficacy of mitigating extreme heat; opening a cooling station or installing shade trees are more effective in these areas, for instance.

All told, we can add the urban heat island effect to the list of social problems that sprawl makes worse. Maybe we should rename it the (sub)urban heat island effect?

When it comes to bike lanes, if you build it, they will shift

bike to work day
bike to work day

Cleveland area commuters congregate downtown for Bike to Work Day on May 20 (courtesy of Bike Cleveland).

When it comes to mobile emissions, not all bike rides are created equal.

The cyclist who drives her bike into downtown to take part in Critical Mass or rides along the Towpath on a Saturday afternoon does not actually eliminate vehicle miles traveled (VMT) or reduce greenhouse gas (GHG) emissions to any extent. (This is why the National Bike Challenge’s methodology tends to irk me).

None of this is to say that these rides are somehow inferior or less than those taken for transportation; they’re not. Recreational riding is good for public health, enjoyable, and it increases the number and visibility of cyclists on roads. But it is somewhat disingenuous to claim they improve air quality or mitigate climate change.

How do we calculate the emissions savings from bike projects?

Now, we already know that shifting people from cars to bikes can go a long way towards promoting these ends. The problem is that we lack good tools to let us demonstrate this on the small scale. How do we prove, definitively, that investing in a particular piece of bike infrastructure gets people to change their travel mode? And how can we calculate the associated emissions reductions?

In some ways, recreational cycling may make this process more difficult. Traditional methods, like bike counts, don’t distinguish between those who are riding for recreation and those who are riding for transportation. Knowing the difference between the two and being able to isolate that segment of the latter who would have otherwise driven is essential for cycling advocates. We need to be able to quantify the demonstrable benefits of bike infrastructure in order to get funding for projects under certain programs, particularly the Congestion Mitigation and Air Quality (CMAQ) Improvement program.

In part because we struggle to get accurate data, bike infrastructure projects remain a small sliver of overall CMAQ projects. The Federal Highway Administration (FHWA) estimates, for instance, that CMAQ needs to invest $3.5 million in bike projects to reduce one ton of fine particulate matter (PM2.5), compared to just $38,000 for diesel vehicle retrofits and $76,000 for idle reduction programs. Perhaps the cost-benefit ratio for bike projects would improve if we had better data on how bike infrastructure directly affects mode choice.

New research may provide an answer

Fortunately, researchers are beginning to develop better tools to do just this. In a new study (paywall) in the journal Transportation Research Part A, researchers Seyed Amir H. Zahabi, Annie Chang, Luis F. Miranda-Moreno, and Zachary Patterson explore how the built environment and accessibility to bike infrastructure affects mode choice and GHG emissions among commuters in Montreal.

In the study, the authors broke Montreal down into a series of 500-square meter neighborhoods based on population density, employment density, cycling network density, transit accessibility, and land use mix. It defined neighborhoods using one of five typologies: downtown, urban, urban-suburb, inner suburb, and outer suburb.

Using this approach, they sought to answer two main questions. First, what are the effects of the built environment and the network connectivity of the transportation system on cycling rates during the period in question (1998-2008)? Second, how did cycling rates and the associated GHG emissions change over this period?

In order to study the first question, they estimated the effects of the neighborhood typologies on cycling rates. However, this isn’t as straightforward as it may seem. One cannot directly estimate this effect on mode choice, as people often self-select into certain types of neighborhoods that fit their preferred mode. For example, I consciously looked for apartments in certain parts of Washington, DC so that I could be within a short walk of a Red Line station. The same holds for cyclists, who may choose to live in more bike-friendly areas.

When your independent variable (in this case, neighborhood type) is not completely independent from your dependent variable (commute mode choice), we say they are endogenous. The researchers employed a statistical approach, known as a simultaneous equation model, which allows them to control for this endogeneity.

Drawing the link between bike lanes and GHG reductions

To study their second research question, they utilized a variable that allowed them to measure the distance a person lives from the nearest bike path or lane. This enabled them to consider how increasing or decreasing that distance may affect commute mode choice and GHG emissions.

It’s this second question that I want to focus on, as it gets to the heart of the issue I raised earlier. Fortunately, the authors provide some concrete evidence that investing in bike infrastructure does foster mode shift. When it comes to bike lanes, if you build it, people really do come.

Based on their results, they found that reducing the distance that a person lives from the nearest bike facility increase the odds that s/he will bike to work by 3.7%. In Montreal, the city expanded its bike network to 648 kilometers (402.6 miles) in 2014, from 603 km (374.7 miles) in 2008. The expansion directly led to a 1.7% decrease in vehicle GHG emissions within the city.

This reduction stacks up well with alternative emissions control options. As the authors conclude,

As in other studies, it is found that cycling infrastructure accessibility is positively linked to bicycle usage, playing a positive role in reducing transportation GHG emissions, by shifting the mode share of bikes. Although this effect may appear small (about 1.7%), it is as big as the estimates we have found in our previous research when converting all the transit diesel buses to hybrid technology and electrifying the commuter trains in Montreal at the same time. This is to say that the GHG benefit from adding low-cost new cycling infrastructure can be as important as other more costly strategies.

Hopefully this type of research can provide further, tangible justification for incorporating bike infrastructure in the urban toolkit to tackle climate change. We need to build real (preferably protected) bike lanes in order both to increase the number of people biking regularly and broaden the type of people biking from hardcore recreational cyclists to normal people using bikes as a transportation mode. Because, while recreational biking is great, only transportation biking can help us solve these pressing crises

Ozone levels have fallen dramatically, though you probably didn’t notice

cleveland skyline smog
cleveland skyline smog

Smog obscures the Cleveland skyline in this picture from July 20, 1973 (courtesy of the National Archives/U.S. EPA).


As someone who has spent most of his life in the city of Cleveland and bikes to work across the Lorain-Carnegie Bridge on a daily basis, I feel like I have a close, personal relationship with air pollution here.

I can tell when the steel mills and other factories in the Industrial Flats are releasing more sulfur dioxide (SO2) than normal from the distinctive odor of rotten eggs. I have entirely too much experience trying to avoid the clouds of diesel particulate matter as they belch forth from GCRTA’s older buses. I have inhaled more than my fair share of nitrogen dioxide (NO2) from passing vehicles.

The dynamics of ground-level ozone

But one common urban pollutant that I cannot and will never be able to smell or see or taste is ground-level ozone. It is completely colorless and odorless. The only way you can notice ozone is from afar, as it helps obscure your view of cities on particularly hazy days. But even then, you can’t really “see” it, as the ozone is just one component of the smog that envelops cities.

Ozone is a sneaky little bastard. It forms above us in the troposphere, travels dozens to hundreds of miles downwind, and then silently works its way into our airways. Only when you have already inhaled it can you possibly begin to notice ozone, as it irritates and inflames tissue in your nose and lungs.

Fortunately, thanks largely to regulations put in place over the past several years by the U.S. EPA, ozone levels have been falling consistently around the country. According to EPA, ozone declined by one-third nationwide, from 1980 to 2014.

But while long-term ozone concentrations certainly affect public health, environmental and public health officials typically focus more on the impacts of spikes in the pollutant over the shorter term. The short-term health effects of rising ozone levels can be significant. According to a landmark 2004 study from Michelle Bell and colleagues, when ozone increases by 10 parts per billion (ppb), mortality rates in Cleveland increase by roughly 1% during the next week. These daily spikes also lead to additional hospitalizations, missed school days, and missed workdays due to asthma and other respiratory conditions.

For these reasons, U.S. EPA requires local officials to monitor ozone and advise the public when they project that ambient levels are expected to exceed 70 ppb. Unfortunately, the Cleveland area has already experienced three days this year on which concentrations exceed 70 ppb. Two of these occurred last week, given that air temperatures increased significantly as high pressure moved into the region.

Yet, as NASA pointed out recently, reductions in emissions of ozone precursors – namely nitric oxides (NOx) and volatile organic compounds (VOCs) – have gone a long way towards limiting the number of exceedance days over the past few years. Without these emissions reductions, Cleveland would have experienced roughly 4-5 more exceedance days in 2011 than we actually did, as the map below shows.

ozone exceedance days avoided 2011

Ozone exceedance days avoided in 2011 as a result of emissions reductions over the past decade (courtesy of NASA).

The benefits of this reduction are tangible, in both blood and treasure. But, at a more basic level, it provides greater peace of mind for all of us. Parents no longer have to worry as much about keeping their children indoors to protect them from pollution. Those of us with asthma don’t have to think about altering our behavior to spend less time outside.

Despite the hype, ozone levels are declining

Given the recent media coverage about worsening air quality worldwide, the fact that ozone levels continue to decline throughout most of the U.S. may come as something of a surprise. I mean, the American Lung Association just gave Cleveland an F for air quality a month ago. But, when you actually get beyond the sensationalized headlines and dig into the data, you’ll find that our air is cleaner than it is has ever been, and it is far cleaner than it was even a decade ago.

Now, none of this should be taken to mean that we can get complacent or that air quality is no longer a pressing challenge; nothing could be further from the truth. I would venture that there are relatively few people more concerned about or aware of air quality issues in this region than I, but I am also among the first to acknowledge the progress we have made and continue to make. But don’t take my word for it. Let’s actually look at the data.

Perhaps the easiest way to chart changes in ozone, over time, would be to look at the average daily ozone levels for the region. In order to do so, I collected data on daily ambient ozone concentrations for Northeast Ohio from 2005-2015 from EPA’s Air Quality System (AQS). This is charted below.

mean annual o3 level 2005-2015

Mean daily ozone levels in Northeast Ohio from 2005-2015.

While there appears to be a fairly small – but steady – decline since 2005, this is not necessarily the most valuable metric to use. First, because ozone  is a secondary pollutant, it is highly dependent upon weather conditions to form. This means that ozone levels can vary dramatically from one day to another, based upon ambient temperatures or whether or not it is raining. Secondly, there is relatively little reliable science on the health impacts of ozone at levels below 50 ppb.

The number of ozone exceedance days has fallen considerably

A more accurate way to account for changes in ozone levels is to examine the number of exceedance days per year. But, because EPA continues to update the National Ambient Air Quality Standard (NAAQS) to reflect changes in science, this does not give us a true apples-to-apples comparison. It wouldn’t be accurate, for example, to claim that air quality did not improve from 2000 to 2015 if a city had 10 exceedance days in each year, given that the NAAQS was 85 ppb during the former year and 75 ppb during the latter.

Because there are several ozone monitors operating in the region, I took the highest daily ozone value from among these monitors and used that as the regional value for a given day. To get a true comparison, I counted day as an exceedance if at least one monitor within the 8-county region registered a value of 71 ppb or more, given that the current NAAQS is 70 ppb.

annual o3 exceedance days 2005-2015

Number of ozone exceedance days per year in Northeast Ohio from 2005-2015, using a 70 ppb cutoff.

As you can see, there has been a nearly precipitous decline in the number of exceedance days over the past decade. While there is some interannual variation, based upon weather (e.g. 2012), the overall trend is undeniable. While the region averaged 43.3 exceedance days per year from 2005-2007, that number fell to just 7 per year from 2013-2015.

Another way to frame changes in ozone levels is to consider the average ozone concentration within the region on a given exceedance day. It may be more harmful for public health to have 10 exceedances with an average concentration of 80 ppb than to have 15 exceedances that average 71 ppb. Fortunately, this metric has also declined significantly since 2005. While the data are fairly noisy, they also demonstrate a strong overlap with the number of exceedance days per year. In other words, during years when we have more exceedances, ozone levels on those days tend to be higher.

mean annual o3 exceedance level 2005-2015

Mean annual ozone exceedance level per year from 2000-2015.

Clearly, by basically any measure, ozone levels have fallen considerably in the region over the past several years, which has directly enhanced public health and well being. In a 2013 study, EPA scientists Neal Fann and David Risely estimated the nationwide public health benefits due to decreases in ozone concentrations from 2000 to 2007. During this period, a 3.5 ppb decrease in national ozone levels prevented between 880 and 4,100 premature deaths. Northeast Ohio, in particular, benefited from this trend; Cuyahoga County avoided more than 30 premature deaths per year during this period, more than all but a handful of counties in the country.

But climate change threatens this trend

But, as I’ve noted before, climate change threatens to stymie this progress. Rising temperatures and changes in precipitation and wind patterns may create conditions more favorable to ozone formation in the future.  Based on a recent EPA report, ozone levels may spike by 1 to 5 ppb, depending on much surface temperatures increase. To account for this effect, I identified those days from 2005 to 2015 on which ozone concentrations peaked between 66 and 70 ppb. As the chart below illustrates, the number of exceedance days would have increased markedly during this period, if the temperature increases associated with climate change had already taken effect. On average, there would have been an additional 13.9 exceedance days per year, ranging from a low of 4 in 2009 to a high of 29 in 2006.

o3 exceedance days with & without climate change

The number of ozone exceedance days in Northeast Ohio from 2005-2015 before and after accounting for the impacts of climate change.

The system works, if you let it

Ultimately, these trends point to a clear conclusion – the air pollution control system in this country works. Donald Trump may want to ban the EPA, but – and this is shocking, I know – I’m going to go ahead and call bullshit on his claim that “we’ll be fine with the environment” afterwards. The clear improvement in air quality that we have seen in this country would not have been possible without the passage of the 1970 Clean Air Act Amendments or the creation of the EPA, which has enforced them. We are all the beneficiaries of the system that has been in place over the past four-plus decades.

But this progress is not a given. As we’ve seen, climate change – itself a product of air pollution – threatens to harm air quality in the long-term. If we get complacent or, worse yet, try to roll back these gains, we will all suffer. Ozone is a fickle and complicated bastard that can strike where and when you are not expecting it. Let’s not give it that chance.

Why we should account for air quality when planning bike lanes

critical mass
A rendering of the proposed Cleveland Midway, a network of protected cycle tracks that would run across the city (courtesy of Bike Cleveland).

A rendering of the proposed Cleveland Midway, a network of protected cycle tracks that would run across the city (courtesy of Bike Cleveland).

In a lot of ways, cyclists get a raw deal. We ride a 25-pound machine on the same roads as people driving 2,000-pound steel boxes at high rates of speed. We struggle to carve out a small piece of the road, even as we get buzzed by passing cars or get screamed at by furious drivers who could kill us at a moment’s notice. There’s no such thing as a fair fight between a bike and a car. If I get into a head on collision with a careless driver, I lose.

Transportation people define cyclists (along with pedestrians, children, the elderly, and the disabled) as “vulnerable road users.” We are the ones most at risk of getting injured, or worse, in a collision.

For the most part, cycling and transportation safety activists have worked to try and bridge the yawning gap in safety between drivers and vulnerable users. So we push to implement road diets, to install bike lanes, to lower speed limits, to educate drivers and cyclists alike about road etiquette. And we do all of this, rightly so, in the name of safety.

The positives – and negatives – of cycling

Part of the impetus behind the push for improving bike infrastructure is the myriad benefits associated with active transportation, which I laid out in detail in my last post.

We all know the advantages of expanding cycling. It reduces wear and tear on roads. It improves safety for all road users. It helps promote vibrant neighborhoods and may increase retail sales. It can fight obesity and enhance public health. And it reduces local air pollution and helps tackle climate change.

But there’s two sides to every coin. We know that individual cyclists take a real risk each time they venture onto the road, even as the rise in cycling enhances safety for all. Could this same dilemma be true for air pollution and public health? The evidence seems to say yes.

Cyclists and exposure to air pollution

On the one hand, cyclists help to improve both local and regional air quality, full stop. Bikes are emissions free and every mile spent cycling rather than driving keeps roughly one pound of carbon dioxide (CO2) out of the atmosphere. The more people who move out of cars and onto bikes, the more we can mitigate transportation-related air pollution (TRAP) and reduce everyone’s exposure to its harmful effects.

it's a trap

Admiral Ackbar hates air pollution.

Yet, on the other hand, not every road user is exposed equally to TRAP. The specific characteristics of a vehicle can dramatically affect the levels of pollution that people riding in or on it can experience. We know, for example, that pollutants can concentrate inside of school buses, ensuring that children on board may be exposed to much higher levels of particulate matter and air toxics than they would otherwise. The same is true for heavy-duty truck drivers.

When it comes to drivers, however, that 2,000-pound steel box puts you at a significant advantage. Unlike cyclists, who have no air exchange buffer, drivers can roll up their windows and turn on recirculated air, lessening their personal exposure to TRAP, even as they produce it.

Multiple studies back this up. In a recent paper (paywall), Carlos Ramos, Humbert Wolterbeek, and Susana Almeida compared the exposure of cyclists and drivers to various air pollutants, using samples from Lisbon, Portugal. Though the authors found that drivers actually inhale five time as much carbon monoxide (CO) and more than twice as much CO2 as cyclists, respectively, the same was not true for other, more harmful pollutants. Cyclists were exposed to 30% higher concentrations particle pollution and ground-level ozone, on average.

As Ramos, Wolterbeek, and Almeida note, drivers tend to face higher concentrations of primary pollutants, like CO, because they remain in direct proximity to the pollutant source. Cyclists, in contrast, are able to limit their exposure to primary pollutants, but they breathe in much higher levels of secondary pollutants (ozone, PM2.5).

Exposure to pollution isn’t the whole story

It would be really consider convenient to end the discussion here and wash our hands of this whole issue. Drivers are exposed to higher levels of one type of pollution, while cyclists face higher levels of another.

But, like most things, this isn’t as simple as it can seem on the surface. The health effects of air pollution isn’t simply a product of pollution levels. Rather, it’s a function of concentration, length of exposure, extant health status (e.g. is the person elderly or asthmatic), respiratory rate, and inhalation route (nose or mouth).

When you account for these factors, the deck becomes decisively stacked against cyclists. Because cyclists spend more time on the road (due to their slower speeds) and breathe more heavily, they inhale higher levels of pollution in nearly every instance.

How cyclists can reduce their exposure to pollution

Now, there are steps that cyclists can take, at least in theory, to reduce their exposure to TRAP. Much like a cyclist can reduce his/her chances of being hit by using off-street paths or side streets, s/he can alter the amount of pollution inhaled by changing routes.

A group of scientists, headed up by Nathan Good from Colorado State University, explored this issue in a study published last fall. They selected a group of 8 commuters (4 bike, 4 car) in Fort Collins and equipped each of them with portable air monitors to document their levels of exposure along their daily commutes.

They found that, on average, cyclists were exposed to 18% more black carbon (a particular harmful component of TRAP) and 25% more PM2.5. Because cyclists spent more time commuting, the actually inhaled 92% more black carbon and 96% more PM2.5.

But Good et al. also found that cyclists could reduce these numbers by shifting to alternate, lower trafficked routes. Cyclists who used these roads less traveled actually took nearly one-quarter less black carbon.

critical mass

Cleveland Critical Mass in July 2015 as seen from my bike.

That said, there are some real issue with this study’s implications. Some people (including me) don’t have a viable, less trafficked route we can follow to work. Additionally, this approach shifts the responsibility for avoiding pollution intake from the public sector (policy makers, urban planners) to the individual cyclist. That’s a crappy way of doing things.

Including air pollution when planning bike lanes

Fortunately, additional research provides at least a partial answer.

In a 2014 study, Piers MacNaughton and colleagues looked at (paywall) how different types of bike routes affected TRAP intake among cyclists in Boston. They compared pollution levels along bike paths (those separated from vehicular traffic) and on-road bike lanes.

Unsurprisingly, the authors found that cyclists experienced significantly lower levels of air pollution while using off-road bike paths. But set that aside for now.

The important findings of this study are related to particular components of bike infrastructure. MacNaughton et al. found that two bike lane variables – vegetative cover and the number of intersections – significantly affect TRAP intake among cyclists.

Reducing the number of intersections a cyclist has to cross not only cuts his/her travel time, it also limits the number of idling vehicles s/he will face. And increasing the amount of vegetation between cars and cyclists can help slash pollution levels, as plants filter out a variety of air pollutants. According to the authors, a one unit increase in vegetative cover lowers black carbon and nitrogen dioxide levels by 3.4% and 11.6%, respectively.

As the authors conclude,

Cyclists can reduce their exposure to TRAP during their commute by using bike paths preferentially over bike lanes regardless of the potential increase of traffic along these routes. Based on these results and the relevant cyclist safety literature, urban planners should push for the development of bike paths instead of bike lanes whenever possible and should design bike paths with vegetation between the cyclists and the vehicle traffic.

Redefining the “vulnerable” in vulnerable road users

With all this in mind, the concept of “vulnerable road users” takes on a new meaning. Cyclists are not only at a greater risk of being injured or killed in a collision, we are also at a heightened risk of suffering the ill effects of TRAP.

Planners must start taking this into account. Bike infrastructure that may make sense from a safety standpoint may not hold up when we account for air pollution. And don’t get me started on vehicular cycling advocates. Cleveland’s decision to design bike lanes that buffer the curb already made no sense from a safety perspective. When you add air quality to the equation…?

Other projects seem to make more sense, in contrast. Both the Midway and the Eastside Greenway place vegetative buffers between cyclists and traffic. This feature provides a double dividend, as they would improve safety and help reduce pollution levels.

Ultimately, it’s time to broaden our horizons on bike infrastructure. Just as we shouldn’t expect indicidual cyclists to bear the risk of being run over to improve road safety, so too shouldn’t we expect cyclists to inhale poison so the rest of us can breathe cleaner air. Let’s start accounting for air pollution exposure and intake when planning bike lanes.

Increasing mode shift is a great tool for improving air quality, public health

bike ferdinand
bike ferdinand

My trusty 2012 Trek FX 7.3, Ferdinand. Yes, like Magellan.

If it’s the first week of May, that can only mean one thing! No, not May Day. No, not Star Wars Day. No, not Cinco de Mayo. No, not Mother’s Day. Look, clearly you’re not going to get this on your own.

That’s right – it’s Air Quality Awareness Week. The U.S. EPA has designated this year’s theme as “Show How You Care About The Air.” EPA and various other government entities that work on air quality, including NOACA, are encouraging people to take a few simple steps throughout the course of the week that can have a positive, tangible impact on air quality.

One of these actions is changing your commute mode. The overwhelming majority of Americans (76.4% in 2013, to be exact) drive alone to work. Here in Northeast Ohio, that number is significantly higher, with values ranging from 79.9% in Cuyahoga County to 87.9% in Lake County. If you total the five counties in the NOACA region, 772,262 of the 938,244 workers over the age of 16 – 82.3% – drive alone to work. Given that transportation accounts for a significant portion of key pollutants in the region – 50% of nitric oxides (NOx) and 15% of fine particulate matter (PM2.5) – reducing the share of single-occupancy vehicles (SOV) on the region’s roads has the potential to improve air quality.

The question becomes by how much. While active transportation undoubtedly holds the potential to cut mobile emissions, some research suggests its immediate impact is somewhat limited. As I’ve shown, increasing overall fuel economy can do more to mitigate climate change than land use planning.

Moreover, research from the Federal Highway Administration (FHWA) shows that bike and pedestrian are not the most cost-effective way to cut emissions. According to the agency’s analysis of projects funded through the Congestion Mitigation and Air Quality Improvement Program (CMAQ), active transportation lowers emissions far less, per dollar spent, than diesel vehicle retrofits, truck stop electrification, or idle reduction projects. This may help to explain why bicycle and pedestrian projects accounted for just 7% of CMAQ funding in FY2013.

Short trips and cold starts

On the aggregate, it’s likely true that, at least in the short-term, retrofitting diesel engines in heavy-duty vehicles or reducing the amount of fuel that truck drivers use overnight may be a more effective way to cut emissions. But personal vehicles account for a much larger share of mobile emissions, and a significant share of these emissions come from short trips.

According to the 2009 National Household Transportation Survey (NHTS), the median distance of a light-duty vehicle trip in the U.S. was just four miles; nearly half of all personal trips (43.4%) were less than 3.2 miles. These short trips account for an outsized share of vehicle emissions due the issue of cold starts.

A cold start occurs when both the car engine and its catalytic converter have cooled to within 10℉ of the ambient air temperature. In order for an engine to operate at peak efficiency, it needs to warm to roughly 140℉. Until it reaches this point, the vehicle will fail to fully combust gasoline, ensuring that it releases emissions at a higher rate.

One recent study (PDF) notes that cold engines can emit four times as many hydrocarbons, three times as much carbon monoxide (CO), and twice as much NOas a warm engine. All told, the authors conclude that excess emissions attributable to cold starts account for 10-30% of total mobile emissions.

The benefits of mode shift on a national scale

Given these facts, it appears that shifting travel mode for short trips could go a long way to improving air quality. Additional research backs up this hypothesis.

In a 2010 article (paywall) in the journal Transportation Research Part D, Audrey de Nazelle and her colleagues examined the benefits of shifting short vehicle trips to active transportation. While their travel data were older (they used the 1995 NHTS), they found that 62.5% of all trips less than 0.5 miles occur in cars. This share that climbs to 87.1% for 0.5 to 1-mile trips, 92.2% for 1- to 2-mile trips, and 94.3% for 2- to 3-mile trips.

The authors examined the effects of shifting 35-70% of short social trips and 15-45% of commutes, respectively, from driving to active transportation. Nationwide, this mode shift would cut daily VOC emissions by 30-70 tons, CO emissions by 400-900 tons, and NOx emissions by 15-35 tons. It would also reduce vehicle miles traveled (VMT) by 0.8-1.8%, cutting greenhouse gas emissions (GHGs) by 20,000-46,000 tons per day. They compared these results to emissions reductions from existing CMAQ projects, finding that promoting widespread mode shift for short trips could lead to emissions reductions that were “orders of magnitude greater.”

How can mode shift improve air quality and public health in Cleveland?

But that study looks at the U.S. as a whole. I often hear people from people that the weather in Northeast Ohio is too harsh, making it impossible to walk or bike for 6-9 months a year. The deck is also stacked heavily towards driving in this region, as our SOV mode share attests. Are national estimates really applicable here? Surely things are different here than in Portland or Austin or San Diego.

Fortunately, a group of researchers from the University of Wisconsin-Madison already considered this issue. In a 2012 study, they analyzed the impact of replacing half of all vehicle trips less than four kilometers (2.4 miles) with biking in the 11 largest metropolitan areas in the Midwest, including Cleveland. And they assumed this mode shift would only occur during cycling season, which they defined as April-October.

The authors estimated that eliminating these short car trips would slash residential vehicle use in these cities by one-fifth. This outcome would reduce the frequency of cold starts from 59.9% to 21.9% in urban Census tracts and from 55.6% to 20.3% in suburban tracts. Across the entire study area, PM2.5 concentrations would fall by 1-2%, while NOx and VOC levels would fall by 5-12% and 10-25, respectively.

Based on their findings,

Eliminating short car trips and replacing 50% of them by bicycle would result in mortality declines of approximately 1,295 deaths per year, including 608 fewer deaths due to improved air quality and 687 fewer deaths due to increased physical activity…We estimate that the combined benefit from improved air quality and physical fitness for the region would exceed $8.7 billion/year, which is equivalent to about 2.5% of the total cost of health care for the five midwestern states in the present study.

Here in Cleveland, PM2.5 values would fall by 0.05 micrograms per cubic meter (µg/m3), preventing 53 premature deaths, 184 asthma attacks, and 1,405 lost workdays per year. The additional physical activity would save another 42 lives per year, increasing the total benefits to $664 million annually.

And these numbers don’t account for the health benefits of increased physical activity. That prevents another 687 premature deaths and provides $3.8 billion in total benefits each year. This mode shift would further reduce GHG emissions by 3.9 billion pounds.

Clearly, the air quality benefits cities can obtain by promoting mode shift for short trips are significant. While mode shift, on its own, cannot bring every city into attainment for air quality standards or halt climate change, it is an important component of a comprehensive approach to both issues. Increasing the mode share of active transportation can produce additional dividends, as it benefits public health, enhances the livability of neighborhoods, improves safety for all road users, and just generally elevates the quality of life in communities around the country.

So show you care about air quality this week and take shorter trips on foot or by bike. Even if the weather isn’t perfect, it will be well worth it.

Go hug a tree. You just might live longer.

edgewater willow tree
edgewater willow tree

The iconic willow tree at Edgewater Park (courtesy of Francis Angelone).

Once upon a time, Cleveland was the Forest City. When Moses Cleaveland arrived to survey Connecticut’s Western Reserve in 1796, the area was heavily forested. It was said that a squirrel could travel from the Atlantic Ocean to the Mississippi River without ever touching the ground.

These days, only the moniker remains. We still have Forest City Enterprises, Forest City Brewery, Forest City Portage, etc. The trees? Not so much.

According to Cuyahoga County’s Urban Tree Canopy Assessment, just 19.2% of the city remains forested. Nearly all of the trees that existed during Cleaveland’s trip to the city that (largely) bears his name are gone today. In 1946, city officials identified 150 trees that likely existed in 1796. When the city updated this inventory in 1975, just 92 remained; of these, only 15 still had the plaques that were installed in 1946.

cuyahoga county tree canopy by community

The existing tree canopy, by community, in Cuyahoga County (courtesy of Cuyahoga County Planning Commission).

Only two of Cuyahoga County’s 59 communities have less tree cover than Cleveland, and the city lags behind comparable cities, including Cincinnati (38%) and Pittsburgh (40%). According to projections, unless Cleveland reverses this trend, its tree canopy will fall to just 14% by 2040. This would represent a loss of 97 acres of urban forest annually over the next 25 years.

When you consider some of Cleveland’s pressing challenges – a 56% child poverty rate, violent crime, population loss – the number of trees within city limits may not seem like a big deal. But we cannot consider the city’s environmental challenges as distinct from its general urban challenges; they are intrinsically connected. Our tremendous urban struggles exacerbate our environmental issues, including tree cover, and these environmental issues subsequently compound these broader issues.

Cleveland’s trees are terrific

When I think about trees, my mind immediately goes to that strangely catchy 1970s commercial from the National Arbor Day Foundation:

And it’s true, trees are terrific. In fact, they’re freaking incredible. But, as the singing cardinal in that commercial indicates, sometimes we take for granted the best things ever planted.

For many Clevelanders, trees may seem like more of a hassle than they’re worth. They produce tons of leaves, fruit, and sap that coats lawns and clogs gutters. They can damage sidewalks. Their roots may get into water and sewer pipes. They may fall in a storm and damage your property or that of a neighbor.

But the costs of trees only outweigh their benefits when we fail to account properly for the latter. Fortunately, the City of Cleveland and a number of partner organizations have placed  a price tag on the myriad benefits that our trees provide in The Cleveland Tree Plan (PDF), which was released last October.

Utilizing the U.S. Forest Service’s i-Tree model, the document estimates that the city’s trees provide more than $28 million in ecosystem services each year. Cleveland’s trees intercept 1.8 billion gallons of rainwater, which helps to mitigate our ongoing challenges with flash flooding. The trees shade homes, lowering energy costs by $3.5 million each year, as well as increase property values by $4.5 million. They also play an important role in mitigating climate change, as they remove 42,000 tons of carbon dioxide per year.

In the plan’s appendices (PDF), which you have to be a massive nerd like me to read, the Tree Plan actually lays out these ecosystem services by neighborhood. As the table below shows, there is a clear overlap between the extent of a neighborhood’s tree canopy and a host of other issues, including energy costs, asthma rates, and property values. The correlation between a neighborhood’s tree canopy and its urban heat island risk, for instance, is extremely strong (0.7609) and statistically significant (p < 0.0001).

cleveland tree benefits by neighborhood

The tree canopy and related statistics in each of Cleveland’s neighborhoods (courtesy of City of Cleveland).

Trees and mortality rates

On its surface, all of this makes sense. It’s fairly obviously that trees filter out air pollution, mitigate stormwater runoff, store carbon, beautify neighborhoods, and shade homes. But trees can do so much more, including extend your lifespan.

A recent study in the journal Environmental Health Perspectives examines the relationship between “greenness,” a measure of vegetation cover (including trees) and mortality rates among a cohort of female nurses in the U.S. The researchers, led by up Dr. Peter James from the Harvard School of Public Health, utilized satellite images to measure the amount of vegetation within 250 and 1,250 meters of each woman’s residence. The 250-meter diameter represented the vegetation directly accessible from each woman’s home, while the 1,250-meter buffer accounted for vegetation within a 10- to 15-minute walk.

The authors considered four main pathways through which exposure to vegetation can affect mortality rates: physical activity, air pollution, social engagement, and mental health. They also controlled for a range of potentially confounding factors, including race/ethnicity, smoking status, socioeconomic status, region, and whether the person lived in a urban area.

According to James et al., higher levels of “greenness” significantly lowered mortality rates among the women in the study cohort.

Analyses showed a consistent relationship between higher greenness and decreased mortality that was robust to adjustment for individual- and area-level covariates. In fully adjusted models, those living in the highest quintile of cumulative average greenness in the 250m area around their home had a 12% lower rate of mortality compared to those in the lowest quintile. Results were consistent for the 1,250m radius, although the relationship was slightly attenuated.

Greater exposure to vegetation significantly reduced mortality rates from cancer, respiratory disease, and kidney disease by 13%, 35%, and 41%, respectively. Of the four pathways studied, the effects were greatest for mental health and social engagement, though “greenness” also reduced mortality related to fine particulate matter and a lack of physical activity.

Based on their research, James et al. conclude,

[T]hese findings suggest that green vegetation has a protective effect, and that policies to increase vegetation in both urban and rural areas may provide opportunities for physical activity, reduce harmful exposures, increase social engagement, and improve mental health. While the recognized benefits of planting vegetation include reducing wastewater loads, sequestering carbon, and mitigating the effects of climate change, evidence of an association between vegetation and lower mortality rates suggests a potential co-benefit to improve health, presenting planners, landscape architects, and policy-makers with an actionable tool to grow healthier places.

Clearly, city officials should work to expand urban tree canopies in order to mitigate the myriad social, environmental, and health issues that plague cities like Cleveland. Fortunately, Cleveland has taken the first step on this road with the release and adoption of its tree plan. Hopefully we can work together to expand the city’s tree canopy in order to tap into the numerous benefits that trees provide.

Maybe the next time you look out your window at your tree lawn, you will see the tree standing there in a different light. It’s time we appreciate and better care for our trees in Cleveland. They just might extend your life.