biostudies-literatureUnknown13(2)Wang YUpper-layer ozone (O₃) intrusion (ULOI) is an important source of surface O₃, affecting gas pollutants and secondary aerosol formation. However, no robust method has been reported to identify ULOI events based on ground observations and assess their effects on surface atmospheric chemistry. We propose a novel method to identify ULOI events by ranking O₃ concentrations before dawn and evaluate their contributions to ground-level O₃ and aerosol formation across China. Our results show that ULOI events occur at a rate of 22%-74% across China, with higher frequency in eastern and southern coastal regions. ULOI enhances ground-level O₃ by 13-43 ppbv at night and 3-14 ppbv during the day. This increases atmospheric oxidation capacity (AOC) aNational science reviewnwaf593https://www.ebi.ac.uk/biostudies/studies/S-EPMC12831027biostudies-literatureUpper-layer ozone intrusion promotes wintertime secondary aerosol formation on the ground.PMC12831027Ma PJiang JPetaja TDu WQuan JKerminen VMMa WZhao XLiu YXia MZhao SXie JHua CChen XWang YZheng FWang ZLiao ZWorsnop DRBianchi FYuan QBao XHu BZhang YKulmala MDing AfalseUpper-layer ozone intrusion promotes wintertime secondary aerosol formation on the ground.Upper-layer ozone (O₃) intrusion (ULOI) is an important source of surface O₃, affecting gas pollutants and secondary aerosol formation. However, no robust method has been reported to identify ULOI events based on ground observations and assess their effects on surface atmospheric chemistry. We propose a novel method to identify ULOI events by ranking O₃ concentrations before dawn and evaluate their contributions to ground-level O₃ and aerosol formation across China. Our results show that ULOI events occur at a rate of 22%-74% across China, with higher frequency in eastern and southern coastal regions. ULOI enhances ground-level O₃ by 13-43 ppbv at night and 3-14 ppbv during the day. This increases atmospheric oxidation capacity (AOC) a2026-01-01T00:00:00Z2026 Jan2026-06-13T05:15:24.838Z2026-06-13T03:08:32.427ZS-EPMC128310274158557910.1093/nsr/nwaf593