New study finds chemical composition of US air pollution changed over time

"The results from this study are especially timely given the EPA's revision of the health-based standard for PM2.5 from 12 µg/m3 to 9 µg/m3 that was just announced this month," said Saravanan Arunachalam, senior author of the study and deputy director of the UNC Institute for the Environment. "States with monitors that are exceeding the new standard for PM2.5 will be looking to understand the chemical constituents of PM2.5, and how they have changed over time, and this will be key to developing emissions reductions policies to address potential future nonattainment designations."

Using data from the U.S. EPA Air Quality System (AQS), the research team assessed trends across the 48 contiguous U.S. states. Chemicals found in PM2.5 in the U.S. include sulfate, ammonium, nitrate, organic carbon, elemental carbon and other trace elements, which come from a variety of human-created and natural sources.

The biggest air quality improvements during this 15-year period that was analyzed were observed in areas with the worst baseline air quality. The Ohio Valley and southeastern states, for example, had the biggest improvements due to regulations on emissions sources such as coal-burning power plants and industry. Sulfur dioxide emissions, a byproduct of fossil fuel combustion, decreased 91.5% from power plants during the study period. Most of the sulfur dioxide emissions in the study came from the Ohio Valley and the southeastern U.S. A spike in poor air quality along the West Coast in 2020 was likely due to widespread forest fires.

Although across the U.S. sulfate and ammonium levels have declined, the researchers suggest looking at mitigating other sources of carbon in the environment to further improve air quality, given the increasing contribution of carbon to total PM2.5. High pollution days have waned in recent years, but primary organic carbon peaked during winter months due to the reduced formation rate of secondary organic aerosols at low temperatures.

These regional differences, the researchers point out, show that regionally targeted approaches to air pollution mitigation will further improve the country's air quality and decrease healthcare expenses for air pollution-related health issues and further save human lives.

"Different chemical constituents of PM2.5 are linked to various emissions sources, thus, the development of future emissions control strategies to reduce PM2.5 concentrations should be based on the comprehensive analysis of spatiotemporal trends of PM2.5 chemical composition and deep understanding of the relationships between changes in emissions and ambient concentrations," said Bin Cheng, a co-author and research associate at the Institute.

"The results from this study will also contribute to future epidemiological studies to identify specific PM2.5 components that affect human health more than others," added Arunachalam.