Modeling ammonia uptake by secondary organic aerosols in the North China Plain

Being the most important alkaline gas in the atmosphere, ammonia (NH₃) can react with acidic species to form ammonium salts, which have significant impacts on air quality and human health. Laboratory studies confirm that NH₃ can heterogeneously react with carbonyl groups in secondary organic aerosol (SOA) to form nitrogen-containing organic compounds (NOCs), consuming gaseous ammonia and potentially influencing ammonia levels and aerosol composition. In order to study the possible impact of this reaction, we incorporated a first-order loss rate representing the NH₃-SOA uptake reaction into the WRF-Chem air quality model and conducted simulations over the North China Plain (NCP) during November 2017. With an uptake coefficient γ of 10^-5, the modeled average NOCs concentration was 1.60 μg m^-3, closely matching the observed average of 1.52 μg m^-3. However, given the presence of other significant sources contributing to NOCs, we consider γ = 10^-5 to represent the upper limit for the uptake coefficient of this specific NH₃–SOA reaction. Sensitivity tests indicate only minor changes in NH₃ concentrations, with an average decrease of 0.69% (0.04 μg m⁻³). The average percentage changes for NO₃^-, NH₄⁺, and SO₄^2- were -0.08%, -0.06%, and -0.01%, respectively, while SOA and PM_2.5 exhibited negligible variations of -0.03% and +0.03%. These results suggest that, although the NH₃-SOA heterogeneous uptake can contribute to NOCs formation, its overall effect on atmospheric NH₃ and particulate matter is limited, and it does not constitute a significant factor in regional air quality modeling in NCP.