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
The uptake of dinitrogen pentoxide (N 2O 5) on aerosol surfaces and the subsequent production of nitryl chloride (ClNO 2) 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 N 2O 5 uptake processes in the Hong Kong–Pearl River Delta (HK–PRD) region, where the highest ever reported N 2O 5 and ClNO 2 concentrations were observed in our recent field study. We first incorporated into the WRF-Chem an aerosol thermodynamics model (ISORROPIA II), recent parameterisations for N 2O 5 heterogeneous uptake and ClNO 2 production and gas-phase chlorine chemistry. The revised model was then used to simulate the spatiotemporal distribution of N 2O 5 and ClNO 2 over the HK–PRD region and the impact of N 2O 5 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 N 2O 5 and ClNO 2 observed at a mountaintop site in Hong Kong, but it overestimates N 2O 5 uptake and ClNO 2 production. The model results suggest that under average conditions, elevated levels of ClNO 2 (> 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 ClNO 2 and N 2O 5 were measured in well-processed plumes from the PRD, ClNO 2 is mostly concentrated within the residual layer ( ∼ 300 m a.g.l.). The addition of N 2O 5 heterogeneous uptake and Cl activation reduces the NO and NO 2 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 ClNO 2 concentrations are highly sensitive to chlorine emission. Our study suggests the need to measure the vertical profiles of N 2O 5 ∕ ClNO 2 under various meteorological conditions, to consider the chemistry of N 2O 5 ∕ ClNO 2 in the chemical transport model and to develop an updated chlorine emission inventory over China.