Understanding the spatial patterns of atmospheric ammonia trends in South Asia

Ammonia (NH₃) is the most abundant alkaline gas in the atmosphere, mainly emitted by agricultural activities. NH₃ readily reacts with other atmospheric acidic pollutants, such as the oxidation products of sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), to create fine particulate matter, which has far-reaching effects on human health and ecosystems. Here, we investigated long-term atmospheric NH₃ trends in South Asia (SA) using satellite observations from the Infrared Atmospheric Sounding Interferometer (IASI). We analyzed 15 years (2008-2022) of IASI-NH₃ retrievals against climate, biophysical, and chemical variables using an ensemble of multivariate statistical methods to identify the major factors driving the observed patterns in the region. Trend analysis of IASI-NH₃ data reveals a significant rise in atmospheric NH₃ over 51 % of SA plains, but a downward trend over 31 % of the region. Spatial correlation analysis reveals that biophysical factors, representing cropland expansion and agriculture intensification, have the highest positive correlation over 56 % of SA plains experiencing positive NH₃ trends. However, our results reveal that the chemical conversion of NH₃ to ammonium compounds, driven by the positive trends in NOₓ and SO₂ pollution, is driving the apparently declining trend of NH₃ in the other regions. Our results provide important insights into the NH₃ trends detected by satellite data and can better inform the policy design aimed at reducing NH₃ emissions and improving air quality for developing regions of the world.