Source Apportionments of Aerosols and Their Direct Radiative Forcing and Long-Term Trends Over Continental United States

Due to U.S. air pollution regulations, aerosol and precursor emissions have decreased during recent decades, while changes in emissions in other regions of the world also influence U.S. aerosol trends through long-range transport. We examine here the relative roles of these domestic and foreign emission changes on aerosol concentrations and direct radiative forcing at the top of the atmosphere over the continental United States. Long-term (1980-2014) trends and aerosol source apportionment are quantified in this study using a global aerosol-climate model equipped with an explicit aerosol source tagging technique. Due to U.S. emission control policies, the annual mean near-surface concentration of particles, consisting of sulfate, black carbon, and primary organic aerosol, decreases by about -1.1 (+/- 0.1)/-1.4 (+/- 0.1) mu g/m(3) in western United States and -3.3 (+/- 0.2)/-2.9 (+/- 0.2) mu g/m(3) in eastern United States during 2010-2014, as compared to those in 1980-1984. Meanwhile, decreases in U.S. emissions lead to a warming of +0.48 (+/- 0.03)/+0.46 (+/- 0.03)W/m(2) in western United States and +1.41 (+/- 0.07)/+1.32 (+/- 0.09)W/m(2) in eastern United States through changes in aerosol direct radiative forcing. Increases in emissions from East Asia generally have a modest impact on U.S. air quality but mitigated the warming effect induced by reductions in U.S. emissions by 25% in western United States and 7% in eastern United States. As U.S. domestic aerosol and precursor emissions continue to decrease, foreign emissions may become increasingly important to radiative forcing over the United States.