Particle Number Concentrations and Their Controlling Parameters over the United States

Particle number concentrations are an important aspect of adverse health impacts, aerosol-cloud interactions and the subsequent climate effects. We examine the dependence of aerosol number concentrations on meteorology and atmospheric chemical species using their long-term (30 years, 1989-2018) simulations in GEOS-Chem-APM: a global 3D atmospheric chemistry model driven by GEOS meteorology of the NASA GMAO with an incorporated size resolved Advanced Particle Microphysics (APM) model. Our motivation is examining the causality of these parameters and atmospheric composition towards improved understanding of processes determining cloud condensation nuclei (CCN) number concentrations. Previous studies have examined the robustness of the GEOS-Chem-APM model with measurements from surface networks, aircraft campaigns, and satellite observations. The dependence of CN10 (condensation nuclei > 10nm) and CCN0.4 (CCN at 0.4% supersaturation) on inorganic and organic atmospheric species and meteorological variables are quantified from the surface to upper troposphere over a number of locations in the US. CCN0.4 shows strongest correlation with the secondary fraction of PM_2.5 (r = 0.75±0.11) while CN10 is anti-correlated with RH (r = -0.42±0.06) at the surface, well-correlated with solar radiation (r = 0.32±0.08) in the lower troposphere, and NO_x and SO₂ (r = 0.45±0.07) in the middle-upper troposphere at the hourly temporal scale. The dependence with other meteorological factors and chemical species and the subsequent development of a predictive model for particle number concentrations from commonly available parameters will be discussed.