Indoor NH3 variation and its relationship with outdoor NH3 in urban Beijing.

Online measurements of indoor and outdoor ammonia (NH3 ) were conducted at a university building in Haidian District, Beijing, to investigate their variation characteristics, indoor-outdoor differences, influencing factors, and possible contribution of indoor NH3 to atmospheric NH3 . Indoor NH3 mixing ratios varied greatly among the rooms of the same building. Indoor NH3 mixing ratio peaked at 1.43 ppm in a toilet. Both indoor and outdoor NH3 mixing ratios exhibited higher values during summer and lower values during winter and correlated significantly with relative humidity and temperature. Moreover, their daily mean mixing ratios were significantly correlated with each other. But indoor and outdoor NH3 in cold months exhibited quite different diurnal variations. During the measurement period, indoor NH3 mixing ratios were substantially higher than those outdoors, by an average factor of 3.1 (1.0-6.6). This indicates that indoor NH3 could be a source of outdoor atmospheric NH3 . The contribution of indoor NH3 to atmospheric NH3 was estimated at 0.7 ± 0.5 Gg NH3 -N·a-1 , accounting for approximately 1.0 ± 0.7% of total emissions in Beijing and being comparable to industry, biomass combustion, and soil emissions, but lower than transportation emissions. The influence of COVID-19 control measures caused indoor and outdoor NH3 mixing ratios to decrease by 22.8% and 19.3%, respectively-attributable to decreased human activity and traffic flow.