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 nitrogenoxides(NOx).Hereweexaminetheroleofsoilreactivenitrogen(Nr, includingNOx andnitrousacid(HONO))emissionsinthe2013−2019ozone increase over the North China Plain (NCP), using GEOS-Chem chemical transport model simulations. We update soil NOx 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)ozoneenhancementfromsoilNr emissionsof8.0ppbvover the NCP and 5.5 ppbv over China in June−July 2019. We identify a strong competingeffectbetweencombustionandsoilNr sourcesonozoneproduction in the NCP region.We find that soilNr emissions accelerate the2013−2019 June-July ozone increase over the NCP by 3.0 ppbv. The increase in soil Nr ozone contribution, however, is not primarily driven by weather-induced increases in soil Nr emissions, but by the concurrent decreasesinfuelcombustionNOx emissions,whichenhanceozoneproductionefficiencyfromsoilbypushingozoneproduction towardamoreNOx-sensitiveregime.OurresultsrevealanimportantindirecteffectfromfuelcombustionNOx emissionreduction on ozone trends by increasing ozone production from soil Nr emissions, highlighting the necessity to consider the interaction betweenanthropogenicandbiogenicsourcesinozonemitigationintheNorthChinaPlain.