Evaluating the Impacts of N2O5 Heterogeneous Reaction and ClNO2 Production on Reactive Nitrogen and Tropospheric Ozone in Southern China

Abstract:

Heterogeneous reaction of N₂O₅ on chloride-containing aerosols transforms N₂O₅ into nitrate and ClNO₂. The production of nitrate is the loss pathway of NO_x, while the formation and subsequent photolysis of ClNO₂ acts as temporary reservoir of NO_x and source of Cl atom, which reacts with VOCs like OH radical. The N₂O₅ uptake and ClNO₂ production, therefore, influences the reactive nitrogen and ozone chemistry. Hong Kong and Pearl River Delta (the latter is known as the world factory), which are located in Southern China, has been experiencing severe photochemical and haze pollution in recent years. But the role of the N₂O₅ heterogeneous chemistry has not been studied. Yet elevated concentrations of N₂O₅ and ClNO₂ have been observed at a high-altitude site in Hong Kong, suggesting the potential significance of the N₂O₅ and ClNO₂ chemistry in this region. In this study, Weather Research and Forecasting coupled with Chemistry (WRF-Chem) model, which is a widely used coupled meteorology-chemistry model, was further developed to incorporate the parameterization of N₂O₅ heterogeneous reaction, ClNO₂ production, and gas phase reactions of Cl with VOCs and then to evaluate the impacts on reactive nitrogen and tropospheric ozone in this region. The implementation of N₂O₅ and ClNO₂ chemistry improved model performance of air pollutants. The updated model was able to reproduce the observed temporal patterns of N₂O₅ and ClNO₂ at the mountain top site. We will present the simulations of effects of the N₂O₅ heterogeneous reactions and ClNO₂ production on the concentrations of NOx, total nitrate, and ozone in the planetary boundary layer of Southern China. Overall, our study suggests significant impacts of the N₂O₅ and ClNO₂ processes on reactive nitrogen budget and ozone chemistry and the necessity to consider them in future atmospheric chemistry modelling studies.