Simulation of particle formation and number concentration over the Eastern United States with the WRF-Chem + APM model

Abstract. Aerosol nucleation events, widely observed at various locations around the globe, are a significant source of cloud condensation nuclei (CCN) which determines aerosol indirect radiative forcing. In this study, a size-resolved, computationally efficient, advanced particle microphysics (APM) model, which has been previously incorporated into a global chemistry transport model (GEOS-Chem), is integrated into the Weather Research and Forecast model coupled with Chemistry (WRF-Chem) to study new particle formation and its contribution to particle number concentration and CCN abundance over the Eastern United States. Size- and composition-resolved aerosol properties from GEOS-Chem + APM simulations are used to initialize and provide boundary conditions for the WRF-Chem + APM model. The modeling results have been evaluated with the relevant measurements obtained during the INTEX-A field campaign in the summer of 2004. Model simulation captures the high concentrations of SO₂ and CN10 at surface layer and source regions but underpredicts the values in the upper troposphere. The particle formation and number concentrations simulated by WRF-Chem + APM are generally consistent with those based on GEOS-Chem + APM over the Eastern United States, but the WRF-Chem + APM simulation has a much higher spatial resolution and can reveal urban and even plume scale processes. Our simulations show that high values of nucleation rates are largely confined to the regions of high SO₂ emissions and that aerosol nucleation dominates the spatial and temporal distributions of condensation nuclei lager than 10 nm (CN10). Similarly, high concentrations of CCN at supersaturation of 0.4% (CCN0.4) are generally confined to SO₂ source regions, with the highest monthly (July) mean CCN0.4 value exceeding 1600 # cm⁻³ in the lower troposphere over Indiana and Ohio. Nucleation and subsequent growth of secondary particles are important sources of CCN0.4, accounting for more than 80% in most parts of the Eastern United States.