LAUSR

Abstract The ozone (O3) concentration usually exhibits a secondary maximum at night in Beijing. To investigate the characteristics and reasons behind this nocturnal O3 increase (NOI), surface O3 and meteorological parameters, combined with wind profile data at the Haidian and Daxing stations were utilized and analyzed in this study. We identified 35 city-wide NOI episodes during the summertime (May to September) from 2014 to 2015. A total of 88.9% of these NOI cases occurred after midnight, and more than half of the nocturnal secondary O3 maxima (NSOM) ranged from 45 to 85 μg/m3. When compared with the previous daytime maximum O3 concentration, 72.2% of the NSOM could account for more than 40.0% of the daytime O3 maxima. The nocturnal O3 concentration in northern suburban areas was 18.7% higher than that in urban areas before a nocturnal O3 increase, and this contrast reduced under the effect of vertical transport of O3 during the NOI period. The reasons for such a downward transport of O3 involved two mechanisms: downdrafts in convective storms and boundary layer low-level jets.