Effects of Emission Reduction during the Last Three Decades on Particle Properties and Direct and Indirect Aerosol Radiative Forcing over the United States

Tropospheric aerosols impact climate both directly by scattering and absorbing solar radiation, and indirectly by acting as cloud condensation nuclei (CCN) thus changing cloud properties and influencing precipitation. Anthropogenic emissions of SO₂ and NO_x in the United States (US) have been significantly reduced over the past several decades. An important question to be answered is how the emission reduction may affect key aerosol properties and their associated radiative forcing. We carried out 30 years (1989 to 2018) of global size-resolved aerosol simulations using a state-of-the-art global chemistry transport model (GEOS-Chem) with size-resolved (sectional) advanced particle microphysics (APM) (2º×2.5º horizontal resolution, 47 vertical layers), driven by MERRA2 meteorology fields. The present analysis focuses on changes and trends of key particle properties in different regions of the US during 1989-2018 where anthropogenic emissions of SO₂ and NO_x have been significantly reduced. Comparisons of simulated aerosol properties with available observations (including mass and number concentrations from surface stations, AOD from AERONET and satellites, and cloud droplet number concentrations (CDNC) from MODIS products) show good agreements. The model simulations indicate that the average CCN, CDNC, PM2.5, and AOD in the lower troposphere over the continental US decreased by 48%, 37%, 32%, and 51%, respectively, from 1989 to 2018. This took place with most of the decrease occurring after around 2000 and the most significant decrease occurring in the middle and eastern US. Interestingly, we found that the average aerosol volume median diameter over the US has slightly increased in the past several decades. The CDNC over the US is dominated (up to ~90%) by secondary particles nucleated and grown from precursor gases. We also carried out model simulations with anthropogenic emissions turned off (i.e., corresponding to pre-industry emissions). A comparison of model outputs with and without anthropogenic emissions shows that both direct and first indirect radiative forcing of anthropogenic aerosols has weakened in the last several decades but the reduction in the direct forcing is more significant. The spatiotemporal variations of these changes and the underlying processes will also be discussed.