Understanding the Paradox of Atmospheric Ammonia Trends in South Asia

Abstract

Ammonia (NH3) is the most abundant alkaline gas in the atmosphere, mainly emitted by agricultural activities. Such emission is of significant concern as 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 (PM2.5), which has far-reaching effects on human health and ecosystems. For effective mitigation strategies to combat  environmental impacts of NH3, in this study, we investigated long-term atmospheric NH3 trends in South Asia (SA) using satellite observations from the Infrared Atmospheric Sounding Interferometer (IASI) instrument. 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 (p ≤ 0.05) rise in atmospheric NH3 over 51% of SA plains. Conversely, a downward trend (p ≤ 0.05) is noticed 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 the rapidly developing regions of the world. Local-level mitigation strategies can be implemented considering the various hotspot areas identified in this study.

Keywords: Ammonia, Air pollution, Trends, agriculture, remote sensing, South Asia