Characteristics of ambient ammonia and its effects on particulate ammonium in winter of urban Beijing, China.

To understand the characteristics of winter fine aerosol pollution in Beijing, we conducted continuous measurements of the atmospheric trace gas ammonia (NH₃), PM_2.5, and inorganic ions in PM_2.5 at an urban site in Beijing from February 13 to March 17, 2015. The hourly average concentration of NH₃ throughout the campaign was 15.4 ± 17.5 ppb. NH₃ concentrations correlated well with NH₄⁺ in PM_2.5, indicating the dominant precursor role of NH₃ on NH₄⁺ formation. The diurnal profile indicated an increase in NH₃ concentrations during the morning rush hours, which was likely due to vehicle emissions. The mean ammonium conversion ratio (NHR) was 0.26, with the highest value of 0.32 in the afternoon. Elevated NHR, nitrate oxidation ratio (NOR), and NH₄⁺ coincided with the significant increase in O₃ levels in the afternoon, indicating the large daytime formation of NH₄NO₃ via photochemical reactions. Moreover, higher NHR values occurred under higher relative humidity (RH >60%) and lower temperature (0–10 °C). NHR increased during the nighttime and correlated well with RH, indicating the dominant role of heterogeneous reactions on gas-particle partitioning. The sulfate oxidation ratio (SOR) and NOR showed positive correlations with RH, which suggests that the conversions of SO₂ to SO₄²⁻ and NO₂ to NO₃⁻ were sensitive to changes in RH. The sustained increase in SO₄²⁻ concentrations at RH >60% suggests that RH had a higher influence on SO₄²⁻ formation than on NO₃⁻ formation. As the sole precursor of NH₄⁺, NH₃ significantly enhanced daytime NH₄NO₃ formation via homogeneous gas-phase reactions and also promoted sulfate formation via both homogeneous and heterogeneous reactions. Moreover, the back trajectory results inferred a high contribution of southwestern air masses to atmospheric NH₃ and NH₄⁺ aerosol variations in Beijing. The result suggests the need for controlling the vehicle emissions to reduce the high levels of NH₃ and alleviate PM_2.5 pollution in winter in Beijing. [ABSTRACT FROM AUTHOR]