Measurement report: Exploring NH₃ behavior in urban and suburban Beijing: comparison and implications

Abstract

Ammonia (NH3) plays an important role in particulate matter formation; hence, its atmospheric level is relevant to human health and climate change. Due to different relative distributions of NH3 sources, concentrations of atmospheric NH3 may behave differently in urban and rural areas. However, few parallel long-term observations of NH3 exist to reveal the different behaviors of NH3 concentrations at urban and rural sites in a same region. In this study, online ammonia analyzers were used to continuously observe atmospheric NH3 concentrations at an urban site and a suburban site in Beijing from 13 January 2018 to 13 January 2019. The observed mixing ratio of NH3 averaged 21±14 ppb (range of 1.6–133 ppb) at the urban site and 22±15 ppb (range of 0.8–199 ppb) at the suburban site. The NH3 mixing ratios at the urban and suburban sites exhibited similar seasonal variations, with high values in summer and spring and low values in autumn and winter. The hourly mean NH3 mixing ratios at the urban site were highly correlated (R=0.849, P<0.01) with those at the suburban site; however, the average diurnal variations in the NH3 mixing ratios at the urban and suburban sites differed significantly, which implies different contributions from NH3 sources and sinks at the urban and suburban sites. In addition to the emission sources, meteorological factors were closely related to the changes in the NH3 concentrations. For the same temperature (relative humidity) at the urban and suburban sites, the NH3 mixing ratios increased with relative humidity (temperature). Relative humidity was the factor with the strongest influence on the NH3 mixing ratio in different seasons at the two sites. The relationships between the NH3 concentrations and temperature (relative humidity) varied from season to season and showed differences between the urban and suburban sites. The reasons for the different relationships need to be investigated in future studies. Higher wind speed mainly from the northwest sector lowered the NH3 mixing ratios at both sites. Similarly to other primary pollutants in Beijing, the NH3 mixing ratios were high when impacted by air masses from the southern sector.