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• Beijing haze is strongly related to enhanced atmospheric oxidizing capacity. • Two regimes separated by [O 3 ] of 80 μg m−3: AOC-sufficient and -deficient regimes. • Increasing oxidation capacity considerably weakens the emission mitigation effort. Although strict mitigation measures have been implemented since 2013 in Beijing-Tianjin-Hebei (BTH), China, air pollution still frequently occurs. Observations reveal that during pollution episodes in autumn, fine particulate matter (PM 2.5) concentrations have not decreased, and particularly, ozone (O 3) concentrations have increased remarkably from 2013 to 2015 in Beijing. Additionally, a concurrence of O 3 and particulate pollution with high secondary aerosol contributions has been observed frequently, indicating high atmospheric oxidizing capacity (AOC) during particulate pollution. The WRF-Chem model simulations show elevated O 3 concentrations and high fractions of oxygenated secondary aerosols (OSA) in PM 2.5 (0.53–0.73) during the severe pollution period. During daytime there exhibits an AOC-sufficient regime with the persistently high OSA fraction and an AOC-deficient regime with varied OSA fractions, separated by the O 3 level of 80 μg m−3. Our results suggest that increasing AOC can considerably weaken the emission mitigation effort by enhancing the secondary aerosol formation [ABSTRACT FROM AUTHOR]