Spatial and Seasonal Variability of Remote and Urban Speciated Fine Particulate Matter in the United States

The spatial and seasonal variability in the composition of major PM_2.5 (particles with aerodynamic diameters less than 2.5 μm) aerosol species in the United States were characterized using data from ground-based aerosol monitoring networks. The IMPROVE (Interagency Monitoring of Protected Visual Environments) network and the Chemical Speciation Network (CSN) operate in mostly rural/remote or urban/suburban sites, respectively. The networks have similar sampling schedules and analysis methods. Regional, monthly, and annual mean concentrations from 2019 to 2022 were calculated for ammonium sulfate (AS), ammonium nitrate (AN), particulate organic matter (POM), elemental carbon (EC), fine dust (FD), and sea salt (SS), as well as their relative contributions to reconstructed PM_2.5 mass (RCFM). Organic aerosols were the largest contributor to RCFM across the United States (>40% annually, up to 80% monthly), with significant impacts from biomass smoke on POM and EC concentrations, contributions, and seasonality. AS concentrations and contributions were similar in urban and rural regions and contributed <20% annually to RCFM, considerably less than two decades ago. In general, urban concentrations were greater for AN, POM, and EC, suggesting additional urban sources. Some species, such as POM, FD, and AN, exhibited strong seasonal variability due to episodic source impacts or seasonal formation conditions. Evaluating the urban and rural monthly variability of major aerosol species is necessary for understanding the impacts of emission sources, regional transport, and atmospheric processes governing aerosol concentrations in the atmosphere.