Chemical composition of US air pollution has changed over time

A recent study offers new insights into the battle against air pollution in the United States, focusing on the evolution of fine particulate matter (PM2.5) levels across the nation from 2006 to 2020.

This comprehensive research by the University of North Carolina at Chapel Hill reveals the impact of regulatory efforts by federal, state, and local authorities to improve air quality.

Invisible threat: The PM2.5 challenge

PM2.5, a complex mixture of tiny particles smaller than 2.5 microns, poses significant health risks, including premature death, and contributes to reduced visibility through haze formation.

The study’s findings highlight a notable reduction in the annual average concentration of PM2.5, alongside shifts in its chemical makeup, suggesting that pollution control strategies tailored to specific regional needs could further diminish air pollution and its adverse health effects.

“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.”  

Decade of progress: How America cleaned its skies

Utilizing data from the U.S. EPA Air Quality System, the research team scrutinized PM2.5 trends across 48 states, identifying a range of chemical species originating from both human activities and natural sources.

Their analysis revealed significant air quality improvements in regions with historically poor air quality, such as the Ohio Valley and southeastern states, largely due to stringent controls on emissions from coal-burning power plants and industrial sources.

The period studied saw a dramatic 91.5% reduction in sulfur dioxide emissions from power plants, particularly in the Ohio Valley and southeastern U.S., areas previously known for high sulfur dioxide levels.

Additionally, the study noted a temporary deterioration in air quality along the West Coast in 2020, attributed to extensive forest fires.

The chemical puzzle of air pollution

Despite a nationwide decrease in sulfate and ammonium levels, the researchers advocate for measures targeting carbon emissions, which increasingly contribute to PM2.5 levels.

They observed a decline in high pollution days, with a notable rise in primary organic carbon during winter, attributed to the slower formation of secondary organic aerosols in lower temperatures.

Highlighting the significance of regional differences in pollution sources and effects, the study supports the adoption of targeted air pollution mitigation strategies.

“Region-specific approaches are essential for further enhancing air quality, reducing healthcare costs related to air pollution, and saving lives,” the researchers emphasized.

Future of clean air: Removing pollution from US skies

Bin Cheng, a co-author and research associate at the Institute, stressed the importance of a detailed analysis of PM2.5’s chemical composition and its emission sources for future emissions control strategies.

“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,” explained Cheng.

In summary, this important study contributes valuable data for policy development and lays the groundwork for future epidemiological research to pinpoint the PM2.5 components most harmful to human health.

Through such comprehensive research, the journey towards cleaner air and healthier communities continues, informed by science and guided by effective policy.

The full study was published in the journal Atmospheric Environment.

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