Toward healthy air quality under CMIP6 scenarios: insights into the compliance with the new WHO guidelines in China

Ambient PM_2.5 and O₃ pollution remain major threats to public health, especially under the more stringent 2021 World Health Organization Air Quality Guidelines (WHO AQGs). We develop an integrated framework that combines bias-corrected CMIP6 simulations of PM_2.5 and O₃ during 2015-2100 under three representative concentration pathways (SSP-RCPs) with the Air Quality Index (AQI) and the Health-based Air Quality Index (HAQI). Using the WHO guideline thresholds and exposure-response functions, we quantify the compliance of two air pollutants and AQI/HAQI in national, regional scales, and also 25 cities in the Yangtze River Delta (YRD), China. Under SSP-RCP1-2.6, national mean PM_2.5 concentrations declines from 17.4 to 9.0 μg/m³ and meet the WHO daily guideline shortly after 2020. While O₃ decreases from 88.3 to 62.6 μg/m³ and remains below 100 μg/m³. AQI and HAQI both improve from the WHO Target-II toward Target-III after mid-century. Under SSP-RCP2-4.5, PM_2.5 and O₃ show only moderate improvement. AQI stays in Target-II and HAQI shifts to Target-I at the beginning. Under SSP-RCP3-7.0, O₃ exceeds 100 μg/m³ after 2060s and HAQI surpasses 150 in some regions by 2090s. The YRD emerges as the long-term hotspot. In YRD, about 58% of cities achieve HAQI ≤ 50 under SSP-RCP1-2.6 after 2050s, while no city achieve this goal under the other two scenarios. Those results highlight that HAQI reveal multipollutant health burden rather than AQI and ambitious, region-specific, multipollutant control and climate-mitigation policies are essential to meet WHO health-based air quality targets.