Improved Simulation of Particle Number Concentrations Over the US: Integrating a Size-Resolved Advanced Particle...

Accurate representation of aerosol size distributions and total number concentrations is critical for evaluating particle impacts on climate, clouds, and public health. An Advanced Particle Microphysics model (APM) has been integrated into the USEPA's Community Multiscale Air Quality Modeling System (CMAQ) to improve its representation of aerosol size distribution. CMAQ is a state-of-the-science air quality model simulating the emission, transport, formation, evolution, and deposition of air pollutants. Unlike the original CMAQ's simplified modal approach, the sectional APM model distinguishes primary and secondary particles, keeps track of secondary species coated on each type of primary particle (black carbon, primary organic carbon, dust, and sea salt), and uses flexible binning for different particle types. This introduces 116 new APM-related tracers increasing computational cost by 83% but enabling high-size resolution aerosol simulations. The updated CMAQ-APM incorporates a ternary H₂SO₄-H₂O-NH₃ ion-mediated nucleation scheme to investigate particle formation and growth over the United States. Predicted monthly mean nucleation rates, particles larger than 10 nm (CN10), and cloud condensation nuclei (CCN) at 0.4% supersaturation (CCN0.4) in the surface in summer and winter of 2013 range from ~0.001 to ~2 cm^-3 s^-1, ~2,000-15,000 cm^-3, and ~300-5,000 cm^-3, respectively. Modeled CN10 and CCN0.4 generally align well with observations with normalized mean bias in the range of -6.42%-192.32% and 11.44%-36.06%, respectively. The incorporation of APM into CMAQ substantially improves the capability of CMAQ in representing size-resolved particles, especially ultrafine particles and CCN activation thresholds, hence offering a robust tool for quantifying aerosol-climate interactions and exposure risks.