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

Ammonia (NH) 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 NH 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 NH from nonagricultural sources rivals agricultural NH source contributions in the Shanghai urban atmosphere. We base our conclusion on four independent approaches: (i) a full-year operation of a passive NH monitoring network at 14 locations covering urban, suburban, and rural landscapes; (ii) model-measurement comparison of hourly NH concentrations at a pair of urban and rural supersites; (iii) source-specific NH measurements from emission sources; and (iv) localized isotopic signatures of NH sources integrated in a Bayesian isotope mixing model to make isotope-based source apportionment estimates of ambient NH. Results indicate that nonagricultural sources and agricultural sources are both important contributors to NH in the urban atmosphere. These findings highlight opportunities to limit NH emissions from nonagricultural sources to help curb PM pollution in urban China.