Soil Emissions of Reactive Oxidized Nitrogen Reduce the Effectiveness of Anthropogenic Source Control in China

Abstract

Nitrogen dioxide (NO 2 ) has decreased by ∼33% across over 1200 monitoring sites in China during 2015−2023, following a series of clean air policies. However, most of these sites are located in or near cities, leading to uncertainties in NO 2 trends beyond urban regions due to limited observations. Here, we used satellite measurements to examine the differences in NO 2 trends between urban and rural China. In urban areas, NO 2 columns decreased by 4.0% per annum (a −1 ) during summer 2011− 2023, consistent with bottom-up anthropogenic emission inventory and in situ measurements. In contrast, rural NO 2 columns showed a slower than expected reduction (−2.6 to −0.0% a −1 ) during the same period. Model simulations with updates in the soil reactive oxidized nitrogen (N r ) scheme indicated that increasing soil N r emissions can be an important factor contributing to the observed slow NO 2 decrease in rural areas. This unregulated source increased summertime pollutant levels, partially offsetting the national efforts to mitigate NO 2 , ozone (O 3 ), and particulate nitrate (NO 3 − ) levels by 20.9%, 15.4%, and 4.7%, respectively, from 2011 to 2020. In the agriculture-intensive North China Plain, the increase in soil N r emissions offset 46.6% of the NO 2 reductions achieved by clean air policies. Our results highlight the increasing significance of soil emissions and the need to control them in future air-quality policies.