Compound Drivers of Rising Ozone Extremes in the North China Plain (2013–2023): Aerosol Decline, Hot Droughts, Afforestation‐Driven BVOCs, and Reduced Vegetation Uptake

Abstract Over the past decade (2013–2023), PM 2.5 levels in eastern China have dropped by ∼70% due to air pollution controls, yet ground‐level ozone (O 3 ) has risen by 1–3 ppbv/yr, especially in the North China Plain (NCP). Persistent O 3 extremes, with daily maximum 8‐hr average (MDA8) of 100–150 ppbv, have become more frequent. Using observations and chemistry‐climate model simulations, we identify key drivers of these trends. As aerosol declines reduce HO x scavenging and enhance radiation, afforestation‐induced increases in isoprene emissions enhance HO x supply during heat waves, thereby accelerating O 3 formation in NO x ‐rich environments. Aerosol declines have the greatest impact on lower‐percentile O 3 increases, while biosphere‐atmosphere interactions during compound heat waves and droughts amplify high‐O 3 extremes. On days when observed MDA8 O 3 exceeds 100 ppbv, impacts of aerosol decline are statistically insignificant (3.1 ± 4.1 ppbv) while contributions are 6.6 ± 2.0 ppbv from interactions of afforestation‐driven isoprene with urban nitrogen oxides (NOx) and 5.3 ± 2.5 ppbv from reduced stomatal O 3 uptake by drought‐stressed vegetation. Droughts limit dry deposition enhancement from afforestation by reducing stomatal uptake. Incorporating afforestation‐related changes in isoprene emissions improves agreement between simulated tropospheric HCHO and satellite observations. Simulated site‐days with MDA8 O 3 > 100 ppbv increase from 11% to 18%, aligning with observed 17%. Offsetting afforestation‐driven O 3 increases requires 40%–50% reductions in anthropogenic emissions of volatile organic compounds (VOCs). Marked NO x reductions over the decade have shifted O 3 production in NCP from VOC‐limited to a transitional regime. A 50% joint reduction in anthropogenic VOC and NO x emissions can lower O 3 by 13.2 ± 3.3 ppbv, eliminating events above 100 ppbv, whereas reductions below 20% in either are insufficient.