Morning commuters file briskly between rows of waiting trains. The few who attempt to chat with fellow travelers must yell to be heard over the roar of diesel engines. The rest hurry on to their train or through the station, as the stench of exhaust belches out from idling locomotives.
This scene is so enduring and familiar that it might describe any rail commute in the United States going back almost a century, but it’s now only a memory at Caltrain’s rail line on the San Francisco Peninsula. This summer, over a period of six weeks, Caltrain completely replaced its diesel fleet with electric trains.
On a September morning, a few weeks into the transition, camera crews captured images of one of the new electric trains at Caltrain’s San Francisco Station. Most worked with muted diligence, but one bespectacled photographer beamed as he jogged up and down the length of the train, snapping shot after shot as if he’d seen a celebrity. That enthusiast was no news photographer, but rather, Joshua Apte, associate professor in the Department of Civil and Environmental Engineering and in the School of Public Health.
“Electrification of our economy generally happens slowly, but this transition from diesel to electrical operations at Caltrain is happening, basically, in the blink of an eye. It’s a great opportunity for us to see improvement in real time,” says Apte, an aerosol scientist who studies how particles in the air behave and what that means for people’s exposures to pollutants.
For Apte, however, it’s not just about taking measurements and creating models, but it’s also about examining how these pollutants impact different communities — and what kind of policies might help move the needle toward equity in the realm of public health.
His latest research has identified surprising trends as far away as India while expanding our understanding of disparities on Bay Area streets. Whether looking at the scale of a city block or the entire globe, his work demonstrates how clean air policies continue to yield unequal results — and how informed progress toward a clean energy economy can ultimately benefit everyone.
A Caltrain worker crosses the tracks between an old diesel train, left, and a new zero-emission, electric train, right, at the San Francisco Caltrain Station. (Photo by Adam Lau/Berkeley Engineering)
Unequal emissions reduction policies
Much of Apte’s research has been aimed at understanding what specifically needs to be done to address systemic air quality disparities in the United States. Apte and his collaborators previously affixed particulate monitoring devices on Google Street View cars to develop block-by-block assessments of pollutant levels in 13 cities. Their work found that neighborhoods with a higher proportion of residents of color have elevated concentrations of pollutants in comparison to neighborhoods with primarily white residents.
These hyperlocal pollution maps allowed Apte to later demonstrate how historic “redlining” policies (discriminatory property appraisals in areas largely populated by people of color) led to the concentration of pollution sources like freeways, railroads and refineries in Black, Asian and Latinx majority neighborhoods. While segregation is no longer official policy anywhere in the United States, the racial composition of many neighborhoods has not shifted much. As a result, people of color are still exposed to more sources and types of pollution than white populations.
“It really is a racial issue, not an income issue,” says Apte.
In his latest paper, Apte and co-author Libby H. Koolik, a Ph.D. student in environmental engineering, examined how effectively the existing air quality standards for on-road mobile vehicles address disparities. Previous research aimed at reducing disparities in emissions exposure has proceeded from two different approaches. The first assumes that disparities are best addressed by reducing emissions from specific economic sectors — transportation, for instance. The second assumes that targeting emissions reductions within geographic areas already overburdened by pollution will be more effective.
The California Air Resources Board (CARB) regulates vehicle emissions more stringently than the federal Clean Air Act requires and has aggressively enforced policies aimed at reducing emissions for every on-road vehicle in the state. These regulations provided Apte and Koolik an excellent means of testing the approach that focuses on reducing emissions from specific economic sectors.
Using data collected by CARB, they modeled exposure concentrations of inhalable particles 2.5 micrometers and smaller in diameter (PM2.5) in California from the years 2000 through 2019. They found that, overall, exposure to PM2.5 emissions dropped 65% throughout the state, a finding that likely translates to more than 10,000 lives saved over the past two decades.
This overall finding is undoubtedly good news, but it does little to improve racial disparities because, though their total exposure dropped, people of color still live closer to emissions sources than white Californians. The vehicles are cleaner, but the freeways haven’t moved. This study suggests that targeting emissions from specific economic sectors will yield broad societal benefits but will not achieve the goal of ameliorating racial disparities.
“Despite the huge drop in emissions, we found that the relative inequality was constant through time. This suggests that a location-based approach would be more efficient at reducing disparities,” says Koolik.
Koolik plans to study what specific features of environmental policymaking are best suited to decrease disparities, and she’s created a tool that will help. ECHO-AIR is the modeling program Apte and Koolik used in the disparities study. It’s built and validated with California-specific data, but with substitute geographic data, it could potentially be used for any area. The tool was designed for non-scientists. Regulators, community activists, journalists and others can use ECHO-AIR to estimate the exposure, equity and health impacts associated with a variety of emissions scenarios — for instance, the effect of removing a freeway.
“If you have a laptop and some way of estimating emissions, you can use ECHO-AIR to do your own air pollution modeling,” says Koolik.
More detailed pollution mapping
Many emissions policies are aimed only at reducing the carbon dioxide emissions that contribute to climate change. For example, the Inflation Reduction Act of 2022 provides tax credits to homeowners who replace natural gas appliances with all-electric models. Though it provided additional credits for low-income communities, homeowners who want to replace their gas-powered appliances still need to be able to pay the full cost upfront before receiving the rebates, and not everyone can afford to do that.
By demonstrating the additional health benefits of replacing gas-powered appliances, Apte hopes that governments will be empowered to create more substantial and inclusive emissions control policies. The Bay Area Air Quality Management District, which regulates air pollution in the counties surrounding San Francisco Bay, has mandated restrictions on the sale and installation of new gas-powered water heaters and furnaces beginning in 2027 and 2029, respectively. Apte wants to track the reduced emissions that will likely result from this policy.
The primary health concern related to natural gas-burning appliances is from nitrogen oxides (NOx) created as a byproduct. Regular exposure to NOx has been linked to asthma and increased risk of heart attacks and death. Apte has recently acquired a van that he’s planning to pack with specialized pollution sensing equipment. Once complete, Apte will take his tricked-out ride to Bay Area streets and measure levels of NOx, as well as diesel soot (black carbon), ultrafine particles, ethane, methane, volatile organic compounds and other pollutants.
Pollution and fog over northern India, captured by satellite. Heavy air pollution is common in the region between December and February (NASA image courtesy MODIS Rapid Response Team)
Visualizations created with this kind of multi-pollutant data provide insights into how pollution varies in both space and time — the weather conditions and economic activities that affect air quality — and also how the interaction of different pollutants can compound deleterious effects. Apte says the results are often surprising.
The most polluted region of the planet is South Asia, according to the 2023 IQAir World Air Quality Report. Apte estimates that people living in parts of Pakistan, Bangladesh and the Indo-Gangetic Plain in North India lose nearly two years of life expectancy due to exposure to PM2.5.
Apte has been studying pollution in South Asia for 15 years. With his network of pollution monitors installed throughout South Asia, Apte was able to show that rural areas are often more polluted than urban areas because rural populations still burn wood for fuel. Even in summer, when wood is only used for cooking and not heating, Apte found that air quality levels in rural areas are often similar to those recorded near wildfires.
Tracking rapid improvement
Back at home, Apte is working with Caltrain to track the changing emissions associated with the transition to electric operations. This summer, Apte rode a diesel-powered train from San Francisco to San Jose and back again, measuring black carbon and nitrogen dioxide (NO2) levels inside the passenger cars. Both pollutants are associated with a variety of detrimental cardiopulmonary conditions. On returning to the San Francisco station, Apte turned to a Caltrain employee to share his results.
“You were not wrong about the first car behind the locomotive being bad. It was worse-than-New Delhi-traffic-jam bad,” said Apte.
Apte recorded between 20 and 40 micrograms per cubic meter of PM2.5 black carbon in the first passenger car behind a diesel locomotive. Those numbers are similar to concentrations Apte measured while sitting in a three-wheeled rickshaw in New Delhi, which has one of the most polluted traffic environments in the world. Notably, Apte did not record similarly high levels on northbound trains because the locomotive’s smoke trailed behind the train and did not enter the passenger cars.
After the round trip to San Jose, Apte and postdoctoral researcher Samuel Cliff visited a disused switch tower southwest of the station. They climbed into a long-abandoned interior and checked results from two devices they’d installed several weeks before. Two small tubes sniffed the air outside the tower and delivered it to a spectrometer that measured NO2 and a filter device that measured black carbon. Levels of the two pollutants were relatively low because the tower was upwind of the diesel engines, so the location provided good control data.
Sam Cliff installs air quality sensors on a Caltrain platform in San Francisco. (Photo by Adam Lau/Berkeley Engineering)
Next, they made some modifications to sensors they’d installed on the train platform and then visited an office that had been temporarily converted into a third monitoring area. Levels of pollutants were, unfortunately, quite high inside the station. Cliff explained that they record spikes in the two pollutants they’re monitoring whenever the platform doors open. Exhaust from the locomotives likely gusts in through open doors and gets trapped there.
“The air in the station and on the platform is much worse than the surrounding area, but we’re seeing quite a big reduction in emissions, particularly on the weekends when they’re already running a fully electrified fleet,” said Cliff.
The transition to electric operations at Caltrain delivers huge benefits for commuters and neighbors in the form of emissions and noise reductions, but it likely won’t be noticed by traditional emissions tracking efforts. That’s because monitors used by official regulating bodies are very expensive, sparsely distributed and sample the air less frequently. They’re best suited to monitoring broad trends over long periods.
To address this limitation, Apte is bringing all of his research together. Along with co-author Chirag Manchanda, a Ph.D. student in environmental engineering, Apte has developed a method to combine data from expensive regulatory monitors, commercial-quality monitors and mobile monitors like those he installed on Google Street View cars. This high-resolution air quality mapping could provide a hyperlocal picture of emissions that can help identify unknown pollution sources and see the immediate benefit of emissions mitigation efforts.
“We want to measure how changes are happening so we can quantify the benefits,” says Apte. “People will be able to see just how much better things can be for everyone.”