Atmospheric NH.sub.3 in urban Beijing: long-term variations and implications for secondary inorganic aerosol control

Ammonia (NH.sub.3) has major effects on the environment and climate. In situ measurements of NH.sub.3 concentrations taken between June 2009 and July 2020 at an urban site in Beijing were analyzed to study its long-term behavior, responses to meteorological conditions, and influences on the formation of secondary inorganic aerosols (SIAs). The 11-year average NH.sub.3 mixing ratio was 26.9±19.3 ppb (median 23.5 ppb). The annual average NH.sub.3 mixing ratio increased from 2009 to 2017 by 50 % and then decreased by 49 % from 2017 to 2020. Notably, the long-term trend for NH.sub.3 at the ground level did not align with the trends derived from satellite observations and emission estimates. The NH.sub.3 concentration exhibited a stronger correlation with the daily variation in water vapor (H.sub.2 O) concentration than with air temperature. Thermodynamic modeling revealed the nonlinear response of SIAs to NH.sub.3, with increased sensitivity when its concentration was reduced to 40 % of the initial level. Although reducing NH.sub.3 concentrations can improve air quality during winter, controlling acid gas concentrations has a greater effect than controlling NH.sub.3 concentrations on reducing SIA concentrations, until NH.sub.3 and acidic gas concentrations are reduced below 80 % of their current levels. Nevertheless, the increased mass proportion of ammonium salts in SIAs during the observation period indicates that future control measures for NH.sub.3 concentrations may need to be prioritized in Beijing.