The changes of particle mass, size, surface area, and number concentration over the Mid Atlantic States due to anthropogenic emission cuts from 2000 to 2017

In the last decades, the Clean Air Act has led to remarkable air pollutant emission cuts and air quality improvements over the United States. As reported by EPA's bottom-up emission estimations, from 2000 to 2017, US's NOx, SO₂, and NH₃ emissions are reduced 52%, 83%, and 27%, respectively. Based on satellite retrievals and ground observations, similar reduction of NO₂ and SO₂ has been found in the atmosphere. The Mid Atlantic States of the eastern US have seen remarkable improvement in air quality over the past few decades, but still suffer from high levels of fine particulate matter (PM2.5) and it is unknown how the emission reduction has affected the particle number concentration which is important for aerosol indirect forcing. In this study, we analyzed ground-based observation from IMPROVE/CSN and GEOS-Chem-APM model simulation of PM2.5 such as sulfate, nitrate, and ammonium from 2000 to 2017 and found their mass concentrations have reduced 66%, 21%, and 44%, respectively. Based on size-resolved aerosol microphysics simulations, we found precursor gases for aerosol nucleation have changed due to anthropogenic emission cuts. Sulfur acid gas concentration has reduced 80% over this region, while ammonia concentration has enhanced 140%. It led to 53% reduction of new particle formation rate, 37% reduction of aerosol number concentration, and 33% reduction of aerosol surface area. We also studied the change of cloud droplet number concentration (CDNC) over this region due to anthropogenic emission cuts and found it is reduced 27% during this period. The change of MODIS retrieved CDNC over this region from 2003 to 2016 shows similar trend over this region. Changes of aerosol mass, number, size, surface area, and CDNC at different vertical layers in different seasons will be discussed. The changes of aerosol direct radiative forcing and indirect radiative forcing due to anthropogenic emission cuts over the Mid Atlantic States will also be presented.