The underappreciated role of agricultural soil nitrogen oxide emissions in ozone pollution regulation in North China

Intensive agricultural activities in the North China Plain (NCP) lead to substantial emissions of nitrogen oxides (NO_x) from soil, while the role of this source on local severe ozone pollution is unknown. Here we use a mechanistic parameterization of soil NO_x emissions combined with two atmospheric chemistry models to investigate the issue. We find that the presence of soil NO_x emissions in the NCP significantly reduces the sensitivity of ozone to anthropogenic emissions. The maximum ozone air quality improvements in July 2017, as can be achieved by controlling all domestic anthropogenic emissions of air pollutants, decrease by 30% due to the presence of soil NO_x. This effect causes an emission control penalty such that large additional emission reductions are required to achieve ozone regulation targets. As NO_x emissions from fuel combustion are being controlled, the soil emission penalty would become increasingly prominent and shall be considered in emission control strategies.