Model investigation of NO₃ secondary organic aerosol (SOA) source and heterogeneous organic aerosol (OA) sink in the western United States

Abstract. The relative importance of NO₃-initiated source and heterogeneous sink of organic aerosol in the western United States is investigated using the WRF/Chem regional weather and chemistry model. The model is run for the four individual months, representing the four seasons, of January, May, August, and October, to produce hourly spatial maps of surface concentrations of NO₃, organic aerosol (OA), and reactive organic gases (ROG, a sum of alkene species tracked in the lumped chemical mechanism employed). These "baseline" simulations are used in conjunction with literature data on secondary organic aerosol (SOA) mass yields, average organic aerosol composition, and reactive uptake coefficients for NO₃ on organic surfaces to predict SOA source and OA heterogeneous loss rates due to reactions initiated by NO₃. We find both source and sink rates maximized downwind of urban centers, therefore with a varying location that depends on wind direction. Both source and sink terms are maximum in summer, and SOA source dominates over OA loss by approximately three orders of magnitude, with large day-to-day variability. The NO₃ source of SOA (peak production rates of 0.4–3.0 μg kg⁻¹ h⁻¹) is found to be significantly larger than the heterogeneous sink of OA via NO₃ surface reactions (peak loss rates of 0.5–8 × 10⁻⁴ μg kg⁻¹ h⁻¹).