Understanding the spatial patterns of atmospheric ammonia trends in South Asia.

Ammonia (NH) is the most abundant alkaline gas in the atmosphere, mainly emitted by agricultural activities. NH readily reacts with other atmospheric acidic pollutants, such as the oxidation products of sulfur dioxide (SO) 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 NH trends in South Asia (SA) using satellite observations from the Infrared Atmospheric Sounding Interferometer (IASI). We analyzed 15 years (2008-2022) of IASI-NH 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-NH data reveals a significant rise in atmospheric NH 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 NH trends. However, our results reveal that the chemical conversion of NH to ammonium compounds, driven by the positive trends in NOₓ and SO pollution, is driving the apparently declining trend of NH in the other regions. Our results provide important insights into the NH trends detected by satellite data and can better inform the policy design aimed at reducing NH emissions and improving air quality for developing regions of the world.