Online characterization of a large but overlooked human excreta source of ammonia in China's urban atmosphere

In urban China, human excreta are mostly stored in septic tanks beneath various buildings, and the generated NH3 are emitted to the atmosphere through ceiling ducts on rooftops. Here we performed highly time-resolved measurements of NH3 concentrations and auxiliary parameters in the ceiling duct of a typical building complex during different seasons with varying temperature and population, allowing an in-depth investigation of the driving forces in terms of governing NH3 emissions. Extremely high levels of NH3 concentration (1013 +/- 793 mu g m(-3)) were observed throughout the campaign. Seasonally, the NH3 concentration during summer vacation (1377 +/- 1072 mu g m(-3)) was significantly higher than during school time in fall (796 +/- 432 mu g m(-3)) and winter (661 +/- 267 mu g m(-3)). Moreover, the diurnal variation of NH3 during summertime was highly correlated with temperature (R-2 = 0.95, p < 0.01), while it was not the case for school time with dense population (R-2 = 0.47, p < 0.01, R-2 = 0.57, p < 0.01 in fall, winter, respectively). The highest temperature was 35.9 degrees C, with an emission intensity peak of 4.1 mg s(-1) while the lowest temperature was 1.3 degrees C with an emission intensity of 0.8 mg s(-1). The nitrogen stable isotopic composition of NH3 (delta N-15-NH3) may be a useful tool to trace NH3 sources. Here we report the delta N-15-NH3 measured from ceiling duct collected directly, which constrains human excreta source delta N-15-NH3 values (mean =(max)( min))parts per thousand to - 35.6 parts per thousand(-31.9 parts per thousand)(-39.8 parts per thousand). These results support that temperature is the key factor in controlling NH3 emissions from human excreta and demonstrate the importance of using human excreta emitted delta N-15-NH3 to quantify excreta NH3 contribution in urban atmospheres. Our findings highlight opportunities to limit NH3 emissions from human excreta that will bring co-benefits to the air quality and human health in urban China.