Soil Emissions of Reactive Nitrogen Accelerate Summertime Surface Ozone Increases in the North China Plain
Wanshan Tan, Haolin Wang, Jiayin Su, Ruize Sun, Cheng He, Xiao Lu, Jintai Lin, Chaoyang Xue, Haichao Wang, Yiming Liu, Lei Liu, Lin Zhang, Dianming Wu, Yujing Mu, Shaojia Fan
Abstract
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.