Exploring Deposition Observations of Oxidized Sulfur and Nitrogen as a Constraint on Emissions in the United States

Emissions of anthropogenic sulfur and nitrogen oxides form secondary pollutants that impact human health, ecosystems, and climate. Accurate estimates of emissions trends are needed to verify the effectiveness of the regulation of these species. We explore the utility of deposition measurements of SOx (=SO2 + SO42-) and TNO3 (=HNO3 + NO3-) as constraints on emissions trends of SOx and NOx in the conterminous United States (CONUS) from 1990 to 2021. The GEOS-Chem model captures observed annual SOx and TNO3 wet deposition at NADP-NTN sites in 2011 with a -15% and +15% normalized mean bias (NMB), respectively. The model overestimates the dry deposition of SOx and TNO3 estimated at CASTNET sites in 2011 (NMB >100%), however, this bias is substantially reduced when using an alternate derived dry deposition data set at the same sites, highlighting the uncertainty in dry deposition velocities. Despite this, we find that the model (driven by scaled NEI emissions) captures the relative trend in dry deposition of SOx (-93% observed and -94% simulated) and TNO3 (-66% observed and -68% simulated) from 1990 to 2021 and that these decreases closely reflect the trends in anthropogenic SO2 emissions (-93%) and anthropogenic NOx emissions (-71%), respectively. SOx and TNO3 wet deposition observations are dominated by soluble secondary products and are more influenced by natural and transboundary sources, and therefore have decreased more modestly over the same period (-78% and -52%). Natural sources of NOx are relatively constant during this time and therefore moderate the reduction in total NOx emissions (-55%).