Spatiotemporal trends in PM_2.5 chemical composition in the conterminous U.S. during 2006-2020

The spatiotemporal variations of fine particulate matter $\left({\text{PM}}_{2.5}\right)$ chemical composition have changed over time in the U.S. and increasing evidence indicated differential toxicity of ${\text{PM}}_{2.5}$ chemical composition. Thus, comprehensive explanation of ${\text{PM}}_{2.5}$ -related adverse health impacts in the U.S. necessitated a detailed analysis of spatiotemporal trends of ${\text{PM}}_{2.5}$ chemical composition. This research aims to analyze the changes in concentrations of ${\text{PM}}_{2.5}$ and its chemical composition in spatial and temporal scales in the conterminous U.S. The ${\text{PM}}_{2.5}$ mass concentration and chemical speciation data were downloaded from U.S. EPA Air Quality System (AQS) (2006-2020) to investigate the spatiotemporal changes of ${\text{PM}}_{2.5}$ and its chemical components. The results indicated that national annual average ${\text{PM}}_{2.5}$ concentration was significantly reduced from $11.38\pm 2.94\mu \text{g}{\text{m}}^{-3}$ in 2006 to $8.20\pm 2.76\mu \text{g}{\text{m}}^{-3}$ in 2020 with an average reduction of $0.21\mu \text{g}{\text{m}}^{-3}{\text{yr}}^{-1}$ , mainly attributed to inorganic ${\text{PM}}_{2.5}$ reductions (i.e., ammonium ( ${\text{NH}}_4^{+}$ ), nitrate $\left({\text{NO}}_3^{-}\right)$ , and sulfate $\left({\text{SO}}_4^{2-}\right)$ ) and the average reductions were $0.09\mu \text{g}{\text{m}}^{-3}{\text{yr}}^{-1},0.02\mu \text{g}{\text{m}}^{-3}{\text{yr}}^{-1}$ , and $0.06\mu \text{g}{\text{m}}^{-3}{\text{yr}}^{-1}$ , respectively. The largest air quality improvements occurred in areas with the worst baseline air quality. Moreover, observed spikes in ${\text{PM}}_{2.5}$ 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 ${\text{SO}}_4^{2-},{\text{NO}}_3^{-}$ , and ${\text{NH}}_4^{+}$ 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 ${\text{PM}}_{2.5}$ chemical composition in the conterminous U.S. and provided insights into source contributions, atmospheric chemical conditions, and development of future emissions control strategies.