Semi-confined terrain concurrently amplifies heat stress and ozone pollution in Northern China

In a warming climate, the co-occurrence of extreme heat and ozone pollution is increasing, amplifying risks to public health. While the role of subtropical high in driving such events over northern China is well recognized, the influence of terrain-atmosphere interactions remains poorly understood. Here, we analyze long-term trends in extreme heat and its link to pollution across northern China, alongside meteorology-chemistry modeling of a severe compound event in 2022. The mountains act as both barriers that inhibit dispersion, and elevated heat sources that drive thermally induced mountain-plain breeze circulations. These circulations promote the formation of upper-level heat and ozone reservoirs and enhance entrainment into the boundary layer, accelerating the buildup of near-surface extremes. Our findings highlight the amplifying role of complex terrain in compound hazards and the need for integrated mitigation strategies.