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

The spatiotemporal variations of fine particulate matter (PM2.5) chemical composition have occurred in the U.S. and increasing evidence indicated differential toxicity of PM2.5 chemical composition. Thus, the comprehensive explanation of PM2.5-related adverse impacts in the U.S. necessitated a detailed analysis of spatiotemporal trends of PM2.5 chemical composition. This research aims to analyze the changes of concentrations of PM2.5 and its chemical composition in spatial and temporal scales in the conterminous U.S. The PM2.5 mass concentration and chemical speciation data were downloaded from U.S. EPA Air Quality System (AQS) (2006–2020) to investigate the spatiotemporal changes of PM2.5 and its chemical components. The results indicated that national annual average PM2.5 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 PM2.5 reductions (i.e., ammonium (NH4+), nitrate (NO3−), and sulfate (SO42−)) 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 PM2.5 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 SO42−, NO3−, and NH4+ almost levelled off in recent years, further air quality improvements may require targeting carbonaceous species. The heavily polluted days occurred less 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). This analysis revealed the spatial and temporal trends of various PM2.5 chemical composition in the conterminous U.S. and provided insights into source contributions, atmospheric chemical conditions, and future emissions control strategies.