Changes in United States deposition of nitrogen and sulfur compounds over five decades from 1970 to 2020

Evaluating long-term air quality and deposition trends can demonstrate effectiveness of control strategies and guide future air quality management planning. Observations show a reduction of atmospheric concentrations of NO_x and SO_x species and their wet and dry deposition since monitoring began in 1978 and 1987, respectively, although deposition observations are sparse. This study used a regional model (CAMx) nested within a global model (GEOS-Chem) to characterize regional changes in sulfur and nitrogen deposition due to intercontinental transport and local/regional anthropogenic emissions representing six modeling years spanning five decades (1970-2020). The meteorological year and the natural emissions were fixed at 2005 to isolate the effects of changing anthropogenic emissions. The downward trend from 1970 to 2020 of mean atmospheric SO₂ concentration is 1.5-1.8%; of total S deposition is 1.0-1.7%; and of SO₂ emissions is 1.4-1.8% per year. The rate of decline in NO_y deposition of 0.6-1.4% per year is similar to the 0.5-1.6% per year reduction in NO_x emissions, thus showing that the changes in deposition have tracked changes in emissions of SO₂ and NO_x emissions quite well. Further reducing N deposition could face some challenges due to increasing NH_x contribution from NH₃ emissions and the expectation of these to increase globally. Our study also examined the transference ratios of NO_y and SO_x in the context of the secondary NAAQS for NO_x and SO_x and found them to be sensitive to emission reductions over the study period.