Diagnosing Regional Contrasts in May-to-June Surface Ozone Transitions across China

Abstract Ground-level ozone (O3) is a major warm-season air-quality challenge in China, decreasing from May to June in southern China but increasing in North China (NC). Yet the distributional basis of this contrast remains unclear. Using 2015–2025 observations, reanalysis, fire data, and backward trajectories, we partitioned the May-minus-June change (ΔO3) in monthly mean maximum daily 8 h average (MDA8) O3 into contributions from high-O3-day frequency, high-O3-day intensity, and the nonhigh-O3-day component. Mean ΔO3 was +8.16 ppb in Southeast China (SEC), +5.55 ppb in Southwest China (SWC), and −6.10 ppb in NC. The frequency contribution was the largest component and closely tracked interannual ΔO3 (r = 0.92–0.98). Stronger SEC transitions were associated with a dry, strongly irradiated May followed by a wetter June, whereas stronger NC transitions coincided with warmer, sunnier, less cloudy, and drier June conditions. Western SWC showed a larger decrease than eastern SWC (11.25 versus 3.42 ppb) under comparatively dry conditions and potential exposure to biomass-burning-influenced air masses from mainland Southeast Asia; eastern SWC instead showed June moistening and reduced radiation. These results clarify how the regional contrast is statistically expressed and identify the distinct atmospheric environments associated with its interannual variability.