Spatiotemporal trends in PM2.5 chemical composition in the conterminous U.S. during 2006–2020.

The spatiotemporal variations of fine particulate matter (PM 2.5) chemical composition have changed over time in the U.S. and increasing evidence indicated differential toxicity of PM 2.5 chemical composition. Thus, comprehensive explanation of PM 2.5 -related adverse health impacts in the U.S. necessitated a detailed analysis of spatiotemporal trends of PM 2.5 chemical composition. This research aims to analyze the changes in concentrations of PM 2.5 and its chemical composition in spatial and temporal scales in the conterminous U.S. The PM 2.5 mass concentration and chemical speciation data were downloaded from U.S. EPA Air Quality System (AQS) (2006–2020) to investigate the spatiotemporal changes of PM 2.5 and its chemical components. The results indicated that national annual average PM 2.5 concentration was significantly reduced from 11.38 ± 2.94 μg m−3 in 2006 to 8.20 ± 2.76 μg m−3 in 2020 with an average reduction of 0.21 μg m−3yr−1, mainly attributed to inorganic PM 2.5 reductions (i.e., ammonium (NH 4 +), nitrate (NO 3 −), and sulfate (SO 4 2−)) and the average reductions were 0.09 μg m−3yr−1, 0.02 μg m−3yr−1, and 0.06 μg m−3yr−1, respectively. The largest air quality improvements occurred in areas with the worst baseline air quality. Moreover, observed spikes in PM 2.5 in California in 2020 were due to higher concentrations of organic matter (OM) and elemental carbon (EC) caused by 2020 wildfires. Furthermore, while levels of SO 4 2−, NO 3 −, and NH 4 + almost levelled off in recent years, further air quality improvements may require targeting carbonaceous species. The heavily polluted days occurred less frequently in recent years and primary organic carbon (OC) accounted for a larger portion of OC in winter than in summer because of the relatively reduced formation rate of secondary organic aerosol (SOA). Our analysis revealed the spatial and temporal trends of various PM 2.5 chemical composition in the conterminous U.S. and provided insights into source contributions, atmospheric chemical conditions, and development of future emissions control strategies. [Display omitted] • Novel spatiotemporal trends analyses focusing on both PM 2.5 and chemical components. • Annual average PM 2.5 concentration was significantly reduced in 2006–2020 in U.S. • Larger air quality improvements occurred in areas with worse baseline air quality. • Spikes in 2020 annual average PM 2.5 in California were likely due to wildfires. • Further air quality improvements may require targeting carbonaceous species. [ABSTRACT FROM AUTHOR]