Human Excreta as a Stable and Important Source of Atmospheric Ammonia in the Megacity of Shanghai

Although human excreta as a NH₃ source has been recognized globally, this source has never been quantitatively determined in cities, hampering efforts to fully assess the causes of urban air pollution. In the present study, the exhausts of 15 ceiling ducts from collecting septic tanks in 13 buildings with 6 function types were selected to quantify NH₃ emission rates in the megacity of Shanghai. As a comparison, the ambient NH₃ concentrations across Shanghai were also measured at 13 atmospheric monitoring sites. The concentrations of NH₃ in the ceiling ducts (2809−2661+5803 μg m^-3) outweigh those of the open air (~10 μg m^-3) by 2–3 orders of magnitude, and there is no significant difference between different seasons. δ¹⁵N values of NH₃ emitted from two ceiling ducts are also seasonally consistent, suggesting that human excreta may be a stable source of NH₃ in urban areas. The NH₃ concentration levels were variable and dependent on the different building types and the level of human activity. NH₃ emission rates of the six residential buildings (RB_NH3) were in agreement with each other. Taking occupation time into account, we confined the range of the average NH₃ emission factor for human excreta to be 2–4 times (with the best estimate of 3 times) of the averaged RB_NH3 of 66.0±58.9 g NH₃ capita^-1 yr^-1. With this emission factor, the population of ~21 million people living in the urban areas of Shanghai annually emitted approximately 1386 Mg NH₃, which corresponds to over 11.4% of the total NH₃ emissions in the Shanghai urban areas. The spatial distribution of NH₃ emissions from human excreta based on population data was calculated for the city of Shanghai at a high-resolution (100×100 m). Our results demonstrate that human excreta should be included in official ammonia emission inventories.