Comment on acp-2022-216

Abstract

Emissions of ammonia (NH3) to the atmosphere impact human health, climate, and ecosystems through their critical contributions to secondary aerosol formation. Estimation of NH3 emissions is associated with large uncertainties because of inadequate knowledge about agricultural sources. Here, we use satellite observations from the Infrared Atmospheric Sounding Interferometer (IASI) and simulations from the GEOS-Chem model to constrain global NH3 emissions over the period of 2008–2018. We update the prior NH3 emission fluxes with the ratio between biases in simulated NH3 concentrations and effective NH3 lifetimes against the loss of the NHx family. In contrast to about a factor of two discrepancies between top-down and bottom-up emissions found in previous studies, our method results in a global land NH3 emission of 79 (71–96) Tg a-1, ~30 % higher than the bottom-up estimates. Regionally, we find that the bottom-up inventory underestimates NH3 emissions over the South America and tropical Africa by 60–70 %, indicating under-representation of agricultural sources in these regions. We find a good agreement within 10 % between bottom-up and top-down estimates over the U.S., Europe and eastern China. Our results also show significant increases in NH3 emissions over India (13 % decade-1), tropical Africa (33 % decade-1), and South America (18 % decade-1) during our study period, consistent with the intensifying agricultural activities in these regions in the past decade. We find that inclusion of SO2 column observed by satellite is crucial for more accurate inference of NH3 emission trends over important source regions such as India and China where SO2 emissions have changed rapidly in recent years.