Atmospheric NH₃ in urban Beijing: long-term variations and implications for secondary inorganic aerosol control

Abstract. Ammonia (NH₃) has major effects on the environment and climate. In-situ measurements of NH₃ concentrations taken between June 2009 and July 2020 at an urban site in Beijing were analyzed to study their long-term behaviors, responses to meteorological conditions and influences on the formation of secondary inorganic aerosols (SIAs). The total average NH₃ mixing ratio was 26.9 ± 19.3 ppb (median, 23.5 ppb). NH₃ mixing ratios initially increased and peaked in 2017 but subsequently decreased, resulting in an overall decrease of 24 % from 2009 to 2020. Notably, the long-term trend for NH₃ at the ground level did not align with the trends derived from satellite observations and emission estimates. It exhibited distinct seasonal variation but also complex diurnal patterns across multiple seasons and years. The NH₃ concentration exhibited a stronger correlation with the water vapor (H₂O) concentration than with air temperature. Thermodynamic modeling revealed the nonlinear response of SIAs to NH₃. Although reducing NH₃ concentrations can improve air quality during winter, controlling acid gas concentrations has a greater effect than controlling NH₃ concentrations on reducing SIA concentrations. The increase in the proportion (mass concentration) of ammonium salts in SIAs during the observation period indicates that measures to control NH₃ concentrations should be prioritized.