As a result of air quality control regulations, aerosol emissions from developed countries in North America and Europe had decreased during recent decades. Meanwhile, emissions from developing countries in East Asia and South Asia had significantly increased because of rapid industrial and population growth. We examined the relative roles of domestic and foreign emission changes on long-term (1980-2014) trends of aerosol concentrations and direct radiative forcing (DRF) over China and US using a global aerosol-climate model equipped with an explicit aerosol source tagging technique. We found that the impact of China’s increasing domestic emissions on PM_2.5 (particulate matter less than 2.5 μm in diameter) concentrations over the last two decades of 20^th century was partially offset by decreasing foreign emissions. A slowdown in the foreign emission reductions together with the weakening of winds explain 25% of China’s increased post-2000 wintertime PM_2.5 trend. Meanwhile, decreases in US domestic emissions led 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 generally had a modest impact on US air quality, but mitigated the warming effect induced by reductions in US emissions by 25% in western US and 7% in eastern US. As US domestic aerosol and precursor emissions continue to decrease, foreign emissions (e.g., from Asia) may become increasingly important to regional air quality and climate in US.