Impacts of heterogeneous uptake of dinitrogen pentoxide and chlorine activation on ozone and reactive nitrogen partitioning: improvement and application of the WRF-Chem model in southern China
Abstract. The uptake of dinitrogen pentoxide (N2O5) on aerosol surfaces and the subsequent production of nitryl chloride (ClNO2) can have a significant impact on the oxidising capability and thus on secondary pollutants such as ozone. The range of such an impact, however, has not been well quantified in different geographical regions. In this study, we applied the Weather Research and Forecasting coupled with Chemistry (WRF-Chem) model to investigate the impact of the N2O5 uptake processes in the Hong Kong–Pearl River Delta (HK–PRD) region, where the highest ever reported N2O5 and ClNO2 concentrations were observed in our recent field study. We first incorporated into the WRF-Chem an aerosol thermodynamics model (ISORROPIA II), recent parameterisations for N2O5 heterogeneous uptake and ClNO2 production and gas-phase chlorine chemistry. The revised model was then used to simulate the spatiotemporal distribution of N2O5 and ClNO2 over the HK–PRD region and the impact of N2O5 uptake and Cl activation on ozone and reactive nitrogen in the planetary boundary layer (PBL). The updated model can generally capture the temporal variation of N2O5 and ClNO2 observed at a mountaintop site in Hong Kong, but it overestimates N2O5 uptake and ClNO2 production. The model results suggest that under average conditions, elevated levels of ClNO2 (> 0.25 ppb within the PBL) are present in the south-western PRD, with the highest values (> 1.00 ppb) predicted near the ground surface (0–200 m above ground level; a.g.l.). In contrast, during the night when very high levels of ClNO2 and N2O5 were measured in well-processed plumes from the PRD, ClNO2 is mostly concentrated within the residual layer ( ∼ 300 m a.g.l.). The addition of N2O5 heterogeneous uptake and Cl activation reduces the NO and NO2 levels by as much as 1.93 ppb ( ∼ 7.4 %) and 4.73 ppb ( ∼ 16.2 %), respectively, and it increases the total nitrate and ozone concentrations by up to 13.45 µg m−3 ( ∼ 57.4 %) and 7.23 ppb ( ∼ 16.3 %), respectively, in the PBL. Sensitivity tests show that the simulated chloride and ClNO2 concentrations are highly sensitive to chlorine emission. Our study suggests the need to measure the vertical profiles of N2O5 ∕ ClNO2 under various meteorological conditions, to consider the chemistry of N2O5 ∕ ClNO2 in the chemical transport model and to develop an updated chlorine emission inventory over China.