| Abstrakt: |
The spatial and seasonal variability in the composition of major PM2.5 (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 PM2.5 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. Plain Language Summary: Particulate matter (PM) in the atmosphere is made of up of many types of aerosols that can differ in urban and rural settings depending on sources, atmospheric processes, and transport. Combining data from large urban and rural ground‐based aerosol monitoring programs in the United States revealed differences in the seasonal and spatial variability of urban and rural aerosols. For example, nitrate, organic carbon, and elemental carbon concentrations were higher in urban areas due to additional urban sources, while sulfate levels in urban and rural regions were similar. Some species were highly seasonal due to their formation conditions occurring in specific seasons or influence from episodic emissions like wildfire smoke. Smoke strongly influenced aerosol composition across the western United States due to high fire years during the study period. Organic carbon is now the major contributor to both urban and rural PM in the United States, in part due to the successful regulatory activity that removed sulfate mass, and the increased contributions from wildfire smoke. Examining the seasonal and spatial variability in aerosol composition is necessary to understand the role of natural and anthropogenic source impacts on PM and to develop strategies to reduce PM in the atmosphere. Key Points: Particulate organic matter is the major contributor to PM2.5 mass across the United StatesSulfate aerosols contributed an average of 20% or less to PM2.5 mass annually in both urban and rural regionsOrganic aerosols, nitrate, and fine dust exhibited strong seasonal variability due to episodic sources and seasonal formation pathways [ABSTRACT FROM AUTHOR] |
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