Long-Term Trend of Particle Number Concentrations in the United States

Particle number concentration is an important factor in the public health and climate impacts of atmospheric aerosols. The indirect effects of aerosol particles on cloud properties, precipitation, radiation, and climate depend on the number concentrations of particles that can be activated to become cloud droplets. Anthropogenic emissions of SO₂ and NO_x in the United States (US) have been significantly reduced over the past decades. However, a logical and important question “How the emission reduction may affect the immediate precursors of particle formation and hence nucleation rates and particle number concentrations that is important for aerosol-cloud interactions?” is yet to be answered. We have carried out 16 years (2000-2015) of global size-resolved aerosol simulations using a state-of-art global chemistry transport model (GEOS-Chem) with size-resolved (sectional) advanced particle microphysics (APM). The GEOS-Chem/APM is driven by MERRA2 meteorology data and the model horizontal resolution is 2 degree x 2.5 degree (47 vertical layers). This presentation focuses on changes and trends of particle number concentrations in different regions of the US during 2000-2015 where anthropogenic emissions of SO₂ and NO_x in the US have been significantly reduced. The model simulations indicate that in middle and east US, the concentrations SO₂ and H₂SO₄ vapors drop respectively by a factor of ~ 5 and 3 while highly oxygenated molecules (HOMs, produced by oxidation of VOCs) increases by ~ 50%, from 2000 to 2015. During the same period, the aerosol optical depth (AOD) decreases by ~ 40% while the concentrations of condensation nuclei larger than 10 nm (CN10) and cloud condensation nuclei at 0.4% water super saturation ratio (CCN0.4) decrease by 15% and 30%, respectively. Comparisons of simulated aerosol properties with available observations (including AOD from AERONET and Satellites) will be presented. The reasons behind the long-term trends and implications will be discussed.