Index Geophysics

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 (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.

Li, Qinyi

Zhang, Li

Wang, Tao

Tham, Yee Jun

Ahmadov, Ravan

Xue, Likun

Zhang, Qiang

Zheng, Junyu

2018-09-15

Text

application/pdf

10.5194/acp-16-14875-2016

https://acp.copernicus.org/articles/16/14875/2016/

eISSN: 1680-7324

eng