Underappreciated Soil NOx Emissions Amplify Background O3 Pollution During Compound Dry-Hot Extremes in North China.

Increasingly frequent compound dry-hot extremes (CDHEs) are influencing atmospheric chemistry through land-atmosphere feedback. Here we show that summertime surface ozone (O3) concentrations across natural background regions of North China (>35°N) unexpectedly exceed those in urban areas, accompanied by an increasing trend in tropospheric NO2 vertical column densities (VCDs). This compelling spatial anomaly confirms that natural nitrogen oxides (NOx) sources, particularly soil NOx emissions (SNOx), may play a pivotal role in driving O3 formation. To test this hypothesis, numerical experiments with the Unified Inputs for WRF-Chem (UI-WRF-Chem) are conducted, in which the SNOx scheme as a function of the soil temperature and moisture is developed with the constraint of satellite-retrieved SNOx fluxes in 2018-2024. During a severe CDHE in summer 2022, extreme dry-hot forcings triggered potent SNOx in natural backgrounds. Although natural soils have substantially lower nitrogen availability than croplands, their SNOx could contribute to 43% of O3 production. Our study highlights the urgent need to incorporate NOx emissions from natural lands in air quality management strategies as CDHEs are projected to intensify under climate change.