The EPA National Emissions Inventory shows U.S. anthropogenic nitrogen oxide (NOx) emissions—an air pollutant that can cause severe health and environmental consequences—have declined from 2005-2019. However, satellite data from NASA’s OMI satellite shows a decreasing trend in NOx emissions until 2009 and then a flat trend after that. A new study supported in part by CPO’s Atmospheric Chemistry, Carbon Cycle, & Climate (AC4) program helps explain this disagreement between emission inventories and satellite data. The study is published as an early access article in Environmental Research Letters.
Much of the inconsistency is due to variations in background emissions from soil and lightning, which are both natural sources of NOx emissions. Led by researchers from the University of Iowa, a team of scientists developed improved estimates of soil NOx emissions in order to better understand how soil and lighting emissions contribute to this disagreement in anthropogenic NOx trends.
Their work highlights the importance of soil and lighting NOx in explaining the observed emissions trend slowdown captured by satellites starting in 2009. Soil emissions of NOx are particularly relevant in the summer, mainly due to increases in fertilizer application, soil moisture, and soil temperature.
Atmospheric chemistry and anthropogenic emissions both also play a role in the slowdown, but the researchers emphasize that the role of soil, especially across the U.S. Midwest, should not be overlooked. Moreover, as anthropogenic emissions continue to decline and natural emissions from soil, fire, and lightning increase in proportion, it will be difficult to study trends of anthropogenic NOx emissions in rural areas using only satellite data without improved methods of incorporating and estimating natural emission sources.