Measurement report: Exploring the NH₃ behaviours at urban and suburban Beijing: Comparison and implications

Abstract. Ammonia (NH₃) plays an important role in particulate matter formation; however, few long-term observations with a high temporal resolution have been conducted on the NH₃ concentrations in Beijing. In this study, online ammonia analyzers were used to observe continuously the atmospheric NH₃ concentrations at an urban site and a suburban site in Beijing from January 13, 2018, to January 13, 2019. The average mixing ratio of NH₃ at the urban site was 21 ± 14 ppb (range: 1.6–133 ppb) and that at the suburban site was 22 ± 15 ppb (range: 0.8–199 ppb). The NH₃ mixing ratios at the urban and suburban sites exhibited similar seasonal variations, with high values being observed in the summer and spring and low values being observed in the 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 indicated the different contributions of 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 the relative humidity (temperature). The relative humidity was the factor with the strongest influence on the NH₃ mixing ratio in different seasons at the two sites. In general, a high wind speed promoted a reduction in the NH₃ mixing ratio. Similar with other primary pollutants in Beijing, the NH₃ mixing ratios were high when winds originated from the south and low when winds originated from the north and northwest.