Underappreciated Soil NO x Emissions Amplify Background O ₃ 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 (O 3 ) concentrations across natural background regions of North China (>35°N) unexpectedly exceed those in urban areas, accompanied by an increasing trend in tropospheric NO 2 vertical column densities (VCDs). This compelling spatial anomaly confirms that natural nitrogen oxides (NO x ) sources, particularly soil NO x emissions (SNO x ), may play a pivotal role in driving O 3 formation. To test this hypothesis, numerical experiments with the Unified Inputs for WRF-Chem (UI-WRF-Chem) are conducted, in which the SNO x scheme as a function of the soil temperature and moisture is developed with the constraint of satellite-retrieved SNO x fluxes in 2018–2024. During a severe CDHE in summer 2022, extreme dry-hot forcings triggered potent SNO x in natural backgrounds. Although natural soils have substantially lower nitrogen availability than croplands, their SNO x could contribute to 43% of O 3 production. Our study highlights the urgent need to incorporate NO x emissions from natural lands in air quality management strategies as CDHEs are projected to intensify under climate change.