Impacts of the heterogeneous ammonia uptake on air quality in the North China Plain

Abstract. Previous studies have revealed that the heterogeneous uptake of ammonia (NH3) on secondary organic aerosols (SOA) profoundly influences the NH3 budget, nitrate aerosol chemistry, and the regional PM2.5 burden, also constituting an important source of nitrogen-containing organic compounds (NOCs). However, current regional modeling efforts are hindered by large uncertainties in uptake coefficients (γ) and a persistent lack of field-based observational constraints. In this study, we incorporate a first-order reactive NH3 uptake mechanism into the WRF-Chem model to evaluate its impacts on air quality and NOCs over the North China Plain (NCP) during November 2017, with a wide range of the uptake coefficient from 10-5 to 10-3. When simulations are constrained with available observations in the NCP, we conclude that the uptake coefficient of 10-5 is characterized as a conservative upper-bound estimate for this heterogeneous process. Sensitivity simulations with the uptake coefficient of 10-5 reveal that the uptake reaction decreases NH3 concentrations by 0.62 % and inorganic aerosol and PM2.5 concentrations by less than 0.06 %, but contributes 1.2 μg m-3 of NOCs in the NCP. This study suggests that the uptake reaction might be a vital path for NOCs formation, but its impact on air pollutants concentrations is insignificant.