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 PM_2.5 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 PM_2.5 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.