Decadal changes in atmospheric ammonia and dry deposition across China inferred from space-ground measurements and model simulations

Abstract

Ammonia (NH3), a key alkaline gas in the atmosphere, significantly influences ecosystem nitrogen cycling and the formation of fine particulate matter (PM2.5). However, limited ground-based monitoring hinders understanding of NH3's spatial and temporal dynamics and its dry deposition across China, which is ranked as one of the largest global NH3 emission hotspots. This study integrated 2013-2023 satellite-derived NH3 column concentrations from the Cross-track Infrared Sounder (CrIS) with adjustments from approximately five years ground in-situ ground observations to derive spatial-temporal variation in ground-level NH3 concentrations across China. We also used the GEOS-Chem transport model and a random forest algorithm by using emission inventories and reanalysis meteorological fields to simulate NH3 dry deposition velocity and fluxes, and explore the mechanisms driving observed trends. The CrIS observations results show that column-averaged (averages from ground to ∼ 1 km) NH3 concentrations were the highest in the North China Plain (> 10 ppb), with notable annual and seasonal increasing trends. NH3 concentrations in 2023 were 13.8 %-30.6 % higher than in 2013. CrIS retrievals aligned well with in-situ data, though were generally about twice as high. After applying the regression equation between ground in-situ observations and CrIS column-averaged NH3 concentrations, we derive the spatial-temporal ground-level (1-1.5 m) NH3 concentrations and dry deposition fluxes from 2013 to 2023. The NH3 dry deposition fluxes exhibited a clear east-west gradient, with maxima in the North China Plain, and another hotpot region is also observed in the Sichuan Basin, southwestern China. Increases in ground-level NH3 concentrations and deposition were most pronounced in urban, cropland, and forest regions, with urban areas experiencing the fastest growth and grasslands the highest total deposition. The national mean ground-level NH3 concentration and dry deposition flux were 4.98 ppb and 0.51 g NH3 m-2 yr-1, respectively. Anthropogenic emissions explained 77.4 % of the variability in ground-level NH3 concentration trend, and meteorological factors accounted for the remainder. Besides, 72.6 %-81.2 % of the NH3 dry deposition trend was governed by NH3 concentration changes. This study identifies the underlying cause of increasing ammonia pollution, which can be used to better inform nitrogen management strategies in China.