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

Summertime surface ozone in China has been increasingsince 2013despite the policy-driven reduction in fuel combustion emissions ofnitrogen oxides (NO x ). Here we examinethe role of soil reactive nitrogen (N-r, including NO x and nitrous acid (HONO)) emissions in the2013-2019 ozone increase over the North China Plain (NCP),using GEOS-Chem chemical transport model simulations. We update soilNO( x ) emissions and add soil HONO emissionsin GEOS-Chem based on observation-constrained parametrization schemes.The model estimates significant daily maximum 8 h average (MDA8) ozoneenhancement from soil N-r emissions of 8.0 ppbv over theNCP and 5.5 ppbv over China in June-July 2019. We identifya strong competing effect between combustion and soil N-r sources on ozone production in the NCP region. We find that soilN(r) emissions accelerate the 2013-2019 June-Julyozone increase over the NCP by 3.0 ppbv. The increase in soil N-r ozone contribution, however, is not primarily driven by weather-inducedincreases in soil N-r emissions, but by the concurrent decreasesin fuel combustion NO x emissions, whichenhance ozone production efficiency from soil by pushing ozone productiontoward a more NO x -sensitive regime. Ourresults reveal an important indirect effect from fuel combustion NO x emission reduction on ozone trends by increasingozone production from soil N-r emissions, highlighting thenecessity to consider the interaction between anthropogenic and biogenicsources in ozone mitigation in the North China Plain. This study reveals an unexpected contribution from soilreactive nitrogen emissions to increasing ozone trends in North Chinawith reduction in anthropogenic emissions. This effect complicatesan ozone mitigation strategy.