Impacts of meteorological conditions on nocturnal surface ozone enhancement during the summertime in Beijing.

The ozone (O 3) concentration usually exhibits a secondary maximum at night in Beijing. To investigate the characteristics and reasons behind this nocturnal O 3 increase (NOI), surface O 3 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 O 3 maxima (NSOM) ranged from 45 to 85 μg/m3. When compared with the previous daytime maximum O 3 concentration, 72.2% of the NSOM could account for more than 40.0% of the daytime O 3 maxima. The nocturnal O 3 concentration in northern suburban areas was 18.7% higher than that in urban areas before a nocturnal O 3 increase, and this contrast reduced under the effect of vertical transport of O 3 during the NOI period. The reasons for such a downward transport of O 3 involved two mechanisms: downdrafts in convective storms and boundary layer low-level jets. • 88.9% of the NOI cases occur after midnight during the summertime in Beijing. • Nocturnal O 3 contrast between urban and suburban areas decreased with vertical transport effect. • Nocturnal O 3 maxima were typically caused by convective storms and LLJs. [ABSTRACT FROM AUTHOR]