Identifying Ammonia Hotspots in China Using a National Observation Network

The limited availability of ammonia (NH 3 ) measurements is currently a barrier to understanding the vital role of NH 3 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 3 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 3 [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 NH 3 surface concentration was generally similar to those of the satellite column concentrations as well as a bottom-up agriculture NH 3 emission inventory. However, the observed NH 3 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 3 deposition fluxes at urban sites account for only half of the emissions in the NCP, suggesting the transport of urban NH 3 emissions to downwind areas. This finding provides policy makers with insights into the potential mitigation of nonagricultural NH 3 sources in developed regions.