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 (NH), PM, and inorganic ions in PM at an urban site in Beijing from February 13 to March 17, 2015. The hourly average concentration of NH throughout the campaign was 15.4 ± 17.5 ppb. NH concentrations correlated well with NH in PM, indicating the dominant precursor role of NH on NH formation. The diurnal profile indicated an increase in NH 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 NH coincided with the significant increase in O levels in the afternoon, indicating the large daytime formation of NHNO 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 SO to SO and NO to NO were sensitive to changes in RH. The sustained increase in SO concentrations at RH >60% suggests that RH had a higher influence on SO formation than on NO formation. As the sole precursor of NH, NH significantly enhanced daytime NHNO 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 NH and NH aerosol variations in Beijing. The result suggests the need for controlling the vehicle emissions to reduce the high levels of NH and alleviate PM pollution in winter in Beijing.