Identifying Ammonia Hotspots in China Using a National Observation Network

The limited availability of ammonia (NH₃) measurements is currently a barrier to understanding the vital role of NH₃ in secondary aerosol formation during haze pollution events and prevents a full assessment of the atmospheric deposition of reactive nitrogen. The observational gaps motivated us to design this study to investigate the spatial distributions and seasonal variations in atmospheric NH₃ on a national scale in China. On the basis of a 1-year observational campaign at 53 sites with uniform protocols, we confirm that abundant concentrations of NH₃ [1 to 23.9 μg m^–3] were identified in typical agricultural regions, especially over the North China Plain (NCP). The spatial pattern of the NH₃ surface concentration was generally similar to those of the satellite column concentrations as well as a bottom-up agriculture NH₃ emission inventory. However, the observed NH₃ concentrations at urban and desert sites were comparable with those from agricultural sites and 2–3 times those of mountainous/forest/grassland/waterbody sites. We also found that NH₃ deposition fluxes at urban sites account for only half of the emissions in the NCP, suggesting the transport of urban NH₃ emissions to downwind areas. This finding provides policy makers with insights into the potential mitigation of nonagricultural NH₃ sources in developed regions