Uneven attainment pressure under China's tightened ambient air quality standard: Socioeconomic predictors and differentiated control pathways for PM2.5 and O3 in Shandong.

China's revised ambient air quality standard (GB 3095-2026) tightens limits for major pollutants, increasing attainment pressure in heavily industrialized regions. Using daily observations of fine particulate matter (PM2.5), inhalable particulate matter (PM10), sulfur dioxide (SO2), nitrogen dioxide (NO2), carbon monoxide (CO), and ozone (O3) from 16 cities in Shandong Province during 2015-2025, we developed a relative attainment pressure index (RAPI) combining concentration gaps and additional non-attainment days, and assessed socioeconomic predictors and policy pathways using random forest, Shapley additive explanations (SHAP), and monotonicity-constrained extreme gradient boosting (XGBoost). PM2.5, PM10, SO2, NO2, and CO declined by 41.0-79.6%, whereas O3 remained high at 166-191 μg/m3. Under the transition-stage limits, Heze, Dezhou, and Liaocheng had the highest relative pressure, with RAPI values of 98.7, 91.3, and 89.1, forming a western high-pressure belt. Civilian vehicle ownership and secondary-industry share were the leading model-based predictors of particulate matter and NO2. The selected socioeconomic indicators represented PM2.5 variation better than O3. Scenario analysis indicated that stronger industrial restructuring and energy-efficiency improvement reduced PM2.5, but eight cities remained above the 30 μg/m3 transition-stage limit under the deep-transition pathway in 2030. O3 was less effectively represented by the socioeconomic model; its scenario results are therefore interpreted as screening-level estimates, with eight cities remaining above 160 μg/m3 under the same pathway. The revised standard will sharpen spatial differences in attainment pressure, supporting differentiated structural measures in western industrial cities and source-specific volatile organic compound-nitrogen oxides management.