Identifying Ammonia Hotspots in China Using a National Observation Network.

The limited availability of ammonia (NH) measurements is currently a barrier to understanding the vital role of NH 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 NH 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 NH [1 to 23.9 μg m] were identified in typical agricultural regions, especially over the North China Plain (NCP). The spatial pattern of the NH surface concentration was generally similar to those of the satellite column concentrations as well as a bottom-up agriculture NH emission inventory. However, the observed NH 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 NH deposition fluxes at urban sites account for only half of the emissions in the NCP, suggesting the transport of urban NH emissions to downwind areas. This finding provides policy makers with insights into the potential mitigation of nonagricultural NH sources in developed regions.