Regional transport and urban emissions are important ammonia contributors in Beijing, China.

Measuring ammonia (NH3) is important for understanding the role of NH3 in secondary aerosol formation and the atmospheric deposition of reactive N. In this study, NH3 was measured in an urban area, a background region, and a tunnel in Beijing. The average NH3 concentrations between September 2017 and August 2018 were 24.8 ± 14.8 ppb and 11.6 ± 10.3 ppb in the urban area and background region, respectively. Higher NH3 concentrations at both the urban and background sites, relative to some earlier measurements indicated a likely increase in the NH3 concentrations in these regions. The urban NH3 level in Beijing was much higher than that typically observed at urban and industrial sites in other domestic and foreign cities, suggesting that the Beijing urban area was affected by greater NH3 emissions than other regions. Based on the relationship among NH3, wind direction, and wind speed, the urban area was affected by both local emissions and air transported from North China Plain (NCP). Potential source contribution function analyses suggested that regional transport from the NCP could greatly affect local concentrations of NH3 in both urban and background areas in spring and autumn; however, in addition to the NCP, urban emissions could also affect NH3 levels in the background region in summer and winter. The average NH3 concentration at the Fenshuiling Tunnel was 8.5 ± 7.7 ppb from December 2017 to February 2018. The NH3:CO emission ratio measured in the tunnel test was 0.022 ± 0.038 ppb/ppb, which was lower than values in the USA and South Korea. The contribution of traffic to NH3 in Beijing did not agree well with the available emission inventories, suggesting that vehicular emissions were underestimated and further evaluation is necessary.