PNNL

Abstract

Long-term (1980-2014) trends and source apportionments of aerosols and their direct radiative forcing (DRF) over the continental United States are quantified in this study using a global aerosol-climate model equipped with an aerosol source tagging technique. Due to decreases in US domestic aerosol and precursor emissions during 2010–2014, as compared to those in 1980–1984, the annual mean near-surface concentration of particles, consisting of sulfate, black carbon, and primary organic aerosol, decreases by about –1.2 µg m-3 in western US and –3.1 µg m-3 in eastern US. Non-US emissions have negligible influence on the change in US near-surface aerosol concentrations. Meanwhile, decreases in US emissions lead to a warming of +0.47 W m-2 in western US and +1.37 W m-2 in eastern US through changes in aerosol DRF. Increases in emissions from East Asia strengthen the DRF, offsetting the warming effect from the decreases in US emissions by 25% in western US and 7% in eastern US. The results suggest that changes in non-US emissions did not exert a large impact on US air quality improvement during recent three decades, but mitigated the warming effect induced by reductions in US aerosols. As US domestic emissions continue to decrease due to current polices, foreign emissions may become increasingly important to climate change in US in the near future.