Future projections of winter haze events in northern East Asia under different Shared Socioeconomic Pathways (SSPs)

Despite stringent environmental regulations, winter haze events remain a significant air quality issue in northern East Asia. Here, we first investigate the relationship between the Haze Weather Index (HWI), an index of winter atmospheric stagnation, and observed surface PM_2.5 concentrations. We find a strong correlation between daily HWI and PM_2.5 levels in eastern China and South Korea. In the Beijing-Tianjin-Hebei (BTH) region (winters 2014/15-2022/23), daily mean PM_2.5 concentrations increased by 32 μg m^-3 (+36 %) under moderate haze conditions (HWI > 0.5) and by 44 μg m^-3 (+51 %) under severe haze conditions (HWI > 1.5). Conversely, PM_2.5 concentrations decreased by 35 μg m^-3 (-40 %) under clean conditions (HWI < -0.5) and by 49 μg m^-3 (-55 %) under very clean conditions (HWI < -1.5). To assess the potential impact of climate change on future haze events, we analyzed an ensemble of 16 Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations under four SSP scenarios (SSP1-2.6 through SSP5-8.5) for 2020-2100. The simulations project substantially more frequent days with haze-favorable meteorological conditions, with particularly large increases in extreme events; for example, the number of severe potential haze days (HWI > 1.5) in northern East Asia could increase by up to 87 % under SSP5-8.5 by the 2090s. We identified temperature-driven atmospheric stagnation as the main driver of these increases. These findings suggest that, even with substantial reductions in anthropogenic emissions, the projected improvements in air quality could be significantly undermined by climate-driven increases in haze-favorable meteorological conditions.