Nocturnal fine particulate nitrate formation by N2O5 heterogeneous chemistry in Seoul Metropolitan Area, Korea

This study investigated the potential of fine nitrate (NO3− in PM2.5) formation in Seoul Metropolitan Area (SMA) by nighttime dinitrogen pentoxide (N2O5) heterogeneous chemistry during March 16–18, 2016, relatively dry and stagnant early spring days, by intervening N2O5 uptake coefficients (reactive uptake probability, γN2O5) in modeling with WRF-CMAQ. Simulations of a base case and two sensitivity tests with default (Davis et al., 2008), zero and decupled (tenfold) γN2O5 showed that impacts of γN2O5 on NO3− and PM2.5 are sensitive to relative humidity (RH) and sulfate-nitrate-ammonium (SNA) conditions. The base case simulation generally underestimated NO3− and PM2.5 levels in comparison to observations. Even with decupled γN2O5, modeled NO3− and PM2.5 concentrations showed relatively small increases under conditions that RH is relatively low in the range of 20 to 40% and SNA levels are severely underestimated (e.g., lower by one third) in the base case simulation. Comparisons of NO3− and PM2.5 concentrations in SMA between simulations with differently specified γN2O5 indicated that N2O5 heterogeneous chemistry has potential to (1) form additional nitric acid (HNO3), (2) further react with ammonia (NH3) emitted from various sources including agricultural sources outside of SMA urban-core areas, and (3) contribute to NO3− and PM2.5 formation in SMA. Additional modeling and observational studies on heterogeneous N2O5 chemistry are needed to improve our understanding of NO3− and PM2.5 formation and better forecast PM2.5 pollution levels over SMA or other urban areas with abundant nitrogen oxides emissions and ammonia emissions such as agricultural emissions from surrounding areas.