Vertical profiles of N2O5-related chemical species over Seoul, Korea: Modeling approach and implications for aircraft campaign

Abstract

Heterogeneous dinitrogen pentoxide (N₂O₅) is an important reactive intermediate in the atmospheric nighttime oxidization of nitrogen oxides and in the formation of nitrate (NO₃⁻) aerosols. However, vertical profiles of N₂O₅-related chemical species over Seoul Metropolitan Area (SMA) are currently lacking. In this study, the Weather Research and Forecasting-Community Multiscale Air Quality (WRF-CMAQ) model was employed and simulated vertical profiles of nighttime N₂O₅-related chemical species to facilitate aircraft-based campaigns over the SMA. The vertical structures of the NO₃ radical (NO₃•) and N₂O₅ were assessed during the meteorologically stagnant period (March 16–17, 2016), which was a typical period of nighttime N₂O₅-driven NO₃⁻ formation in the study region. The results showed that nighttime vertical structures exhibiting higher concentrations of N₂O₅-related chemical species were strongly associated with the nocturnal residual layer (RL), which was decoupled from the ground. Our model showed diminished nighttime NO₃• at the ground but active NO₃• aloft, as it was retained in the RL. We also carried out the same analysis for the stagnant period (May 16–22, 2016) during the KORUS-AQ campaign, and confirmed the similar profiles of high concentration of NO₃• near the altitude of RL. To sum up the cases from this study and KORUS-AQ campaign study, maximum nighttime NO₃• and N₂O₅ concentrations occurred at heights of 78 ± 11% of RL height and 54 ± 15% of RL height, respectively. Although further research on nighttime N₂O₅ chemistry is needed to resolve outstanding uncertainties, the vertical structures of nighttime N₂O₅-related chemical species obtained in this study offer useful reference data for future aircraft campaigns.