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

Abstract

Nitrogen dioxide (NO₂) 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 NO₂ trends beyond urban regions due to limited observations. Here, we used satellite measurements to examine the differences in NO₂ trends between urban and rural China. In urban areas, NO₂ 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 NO₂ 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 NO₂ decrease in rural areas. This unregulated source increased summertime pollutant levels, partially offsetting the national efforts to mitigate NO₂, ozone (O₃), and particulate nitrate (NO₃^-) 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 NO₂ 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.