Assessing Contributions of Agricultural and Nonagricultural Emissions to Atmospheric Ammonia in a Chinese Megacity

Ammonia (NH₃) is the predominant alkaline gas in the atmosphere contributing to formation of fine particles-a leading environmental cause of increased morbidity and mortality worldwide. Prior findings suggest that NH₃ in the urban atmosphere derives from a complex mixture of agricultural (mainly livestock production and fertilizer application) and nonagricultural (e.g., urban waste, fossil fuel-related emissions) sources; however, a citywide holistic assessment is hitherto lacking. Here we show that NH₃ from nonagricultural sources rivals agricultural NH₃ source contributions in the Shanghai urban atmosphere. We base our conclusion on four independent approaches: (i) a full-year operation of a passive NH₃ monitoring network at 14 locations covering urban, suburban, and rural landscapes; (ii) model-measurement comparison of hourly NH₃ concentrations at a pair of urban and rural supersites; (iii) source-specific NH₃ measurements from emission sources; and (iv) localized isotopic signatures of NH₃ sources integrated in a Bayesian isotope mixing model to make isotope-based source apportionment estimates of ambient NH₃. Results indicate that nonagricultural sources and agricultural sources are both important contributors to NH₃ in the urban atmosphere. These findings highlight opportunities to limit NH₃ emissions from nonagricultural sources to help curb PM_2.5 pollution in urban China.