Agricultural ammonia emissions in China: reconciling bottom-up and top-down estimates

Abstract. Current estimates of agricultural ammonia (NH₃) emissions in China differ by more than a factor of 2, hindering our understanding of their environmental consequences. Here we apply both bottom-up statistical and top-down inversion methods to quantify NH₃ emissions from agriculture in China for the year 2008. We first assimilate satellite observations of NH₃ column concentration from the Tropospheric Emission Spectrometer (TES) using the GEOS-Chem adjoint model to optimize Chinese anthropogenic NH₃ emissions at the 1∕2°  ×  2∕3° horizontal resolution for March–October 2008. Optimized emissions show a strong summer peak, with emissions about 50 % higher in summer than spring and fall, which is underestimated in current bottom-up NH₃ emission estimates. To reconcile the latter with the top-down results, we revisit the processes of agricultural NH₃ emissions and develop an improved bottom-up inventory of Chinese NH₃ emissions from fertilizer application and livestock waste at the 1∕2°  ×  2∕3° resolution. Our bottom-up emission inventory includes more detailed information on crop-specific fertilizer application practices and better accounts for meteorological modulation of NH₃ emission factors in China. We find that annual anthropogenic NH₃ emissions are 11.7 Tg for 2008, with 5.05 Tg from fertilizer application and 5.31 Tg from livestock waste. The two sources together account for 88 % of total anthropogenic NH₃ emissions in China. Our bottom-up emission estimates also show a distinct seasonality peaking in summer, consistent with top-down results from the satellite-based inversion. Further evaluations using surface network measurements show that the model driven by our bottom-up emissions reproduces the observed spatial and seasonal variations of NH₃ gas concentrations and ammonium (NH₄⁺) wet deposition fluxes over China well, providing additional credibility to the improvements we have made to our agricultural NH₃ emission inventory.