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

## TL;DR

The paper examines how China's air quality will change under different climate scenarios and finds that only the most aggressive climate mitigation scenario meets new WHO air quality guidelines.

## Contribution

A novel framework combining CMIP6 simulations with health-based air quality indices to assess compliance with WHO guidelines under different climate scenarios.

## Key findings

- Under the most aggressive climate scenario, China's PM2.5 levels meet WHO guidelines by 2020, while O3 remains below thresholds.
- Only 58% of cities in the Yangtze River Delta meet health-based air quality targets by 2050 under the best scenario.
- The Health-based Air Quality Index (HAQI) reveals higher health burdens than traditional AQI metrics.

## Abstract

Ambient PM2.5 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 PM2.5 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 PM2.5 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, PM2.5 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.