Estimating global surface ammonia concentrations inferred from satellite retrievals

Ammonia (inline-formulaNH₃), as an alkaline gas in the atmosphere, can cause direct or indirect effects on the air quality, soil acidification, climate change and human health. Estimating surface inline-formulaNH₃ concentrations is critically important for modeling the dry deposition of inline-formulaNH₃ and for modeling the formation of ammonium nitrate, which have important impacts on the natural environment. However, sparse monitoring sites make it challenging and difficult to understand the global distribution of surface inline-formulaNH₃ concentrations in both time and space. We estimated the global surface inline-formulaNH₃ concentrations for the years of 2008–2016 using satellite inline-formulaNH₃ retrievals combining vertical profiles from GEOS-Chem. The accuracy assessment indicates that the satellite-based approach has achieved a high predictive power for annual surface inline-formulaNH₃ concentrations compared with the measurements of all sites in China, the US and Europe (inline-formulaR²=0.76 and RMSE inline-formula= 1.50 inline-formulaµg N minline-formula⁻³). The satellite-derived surface inline-formulaNH₃ concentrations had higher consistency with the ground-based measurements in China (inline-formulaR²=0.71 and RMSE inline-formula= 2.6 inline-formulaµg N minline-formula⁻³) than the US (inline-formulaR²=0.45 and RMSE inline-formula= 0.76 inline-formulaµg N minline-formula⁻³) and Europe (inline-formulaR²=0.45 and RMSE inline-formula= 0.86 inline-formulaµg N minline-formula⁻³) at a yearly scale. Annual surface inline-formulaNH₃ concentrations higher than 6 inline-formulaµg N minline-formula⁻³ are mainly concentrated in the North China Plain of China and northern India, followed by 2–6 inline-formulaµg N minline-formula⁻³ mainly in southern and northeastern China, India, western Europe, and the eastern United States (US). High surface inline-formulaNH₃ concentrations were found in the croplands in China, the US and Europe, and surface inline-formulaNH₃ concentrations in the croplands in China were approximately double those in the croplands in the US and Europe. The linear trend analysis shows that an increase rate of surface inline-formulaNH₃ concentrations (> 0.2 inline-formulaµg N minline-formula⁻³ yrinline-formula⁻¹) appeared in eastern China during 2008–2016, and a middle increase rate (0.1–0.2 inline-formulaµg N minline-formula⁻³ yrinline-formula⁻¹) occurred in northern Xinjiang over China. inline-formulaNH₃ increase was also found in agricultural regions in the central and eastern US with an annual increase rate of lower than 0.10 inline-formulaµg N minline-formula⁻³ yrinline-formula⁻¹. The satellite-derived surface inline-formulaNH₃ concentrations help us to determine the inline-formulaNH₃ pollution status in the areas without monitoring sites and to estimate the dry deposition of inline-formulaNH₃ in the future.