Soil Emissions of Reactive Nitrogen Accelerate Summertime Surface Ozone Increases in the North China Plain

Summertime surface ozone in China has been increasing since 2013 despite the policy-driven reduction in fuel combustion emissions of nitrogen oxides (NO_x). Here we examine the role of soil reactive nitrogen (N_r, including NO_x and nitrous acid (HONO)) emissions in the 2013-2019 ozone increase over the North China Plain (NCP), using GEOS-Chem chemical transport model simulations. We update soil NO_x emissions and add soil HONO emissions in GEOS-Chem based on observation-constrained parametrization schemes. The model estimates significant daily maximum 8 h average (MDA8) ozone enhancement from soil N_r emissions of 8.0 ppbv over the NCP and 5.5 ppbv over China in June-July 2019. We identify a strong competing effect between combustion and soil N_r sources on ozone production in the NCP region. We find that soil N_r emissions accelerate the 2013-2019 June-July ozone increase over the NCP by 3.0 ppbv. The increase in soil N_r ozone contribution, however, is not primarily driven by weather-induced increases in soil N_r emissions, but by the concurrent decreases in fuel combustion NO_x emissions, which enhance ozone production efficiency from soil by pushing ozone production toward a more NO_x-sensitive regime. Our results reveal an important indirect effect from fuel combustion NO_x emission reduction on ozone trends by increasing ozone production from soil N_r emissions, highlighting the necessity to consider the interaction between anthropogenic and biogenic sources in ozone mitigation in the North China Plain.