Heavy ozone pollution episodes in urban Beijing during the early summertime from 2014 to 2017: Implications for control strategy

Highlights: • HOP episode occurrence frequency was the highest in May or June from 2014 to 2017. • HOP episodes were mainly caused by local photochemical formation. • VOCs, NO_X, and CO concentrations increased during HOP episodes. • Vehicular exhaust contributed the most to anthropogenic VOCs. • Control strategy adjustment is needed according to the changes of precursors. Ground-level ozone (O₃) has become the principal air pollutant in Beijing during recent summers. In this context, an investigation of ambient concentrations and variation characteristics of O₃ and its precursors in May and June from 2014 to 2017 in a typical urban area of Beijing was carried out, and the formation sensitivity and different causes of heavy O₃ pollution (HOP, daily maximum 8-h O₃ (MDA8h O₃)>124 ppbv) were analyzed. The results showed that the monthly assessment values of the O₃ concentrations (the 90^th percentile MDA8h O₃ within one month) were highest in May or June from 2014 to 2017, and the values presented an overall increasing trend. During this period, the number of O₃ pollution days (MDA8h O₃ > 75 ppbv) also showed an increasing trend. During the HOP episodes, the concentrations of volatile organic compounds (VOCs), nitrogen oxides (NO_X), and carbon monoxide (CO) were higher than their respective mean values in May and June, and the meteorological conditions were more conducive to atmospheric photochemical reactions. The HOP episodes were mainly caused by local photochemical formation. From 2014 to 2017, O₃ formation during the HOP episodes shifted from VOC and NO_X mixed-limited to VOC-limited conditions, and O₃ formation was most sensitive to anthropogenic VOCs. Six categories of VOC sources were identified, among which vehicular exhaust contributed the most to anthropogenic VOCs. The VOC concentrations and OFPs of anthropogenic sources have decreased significantly in recent years, indicating that VOC control measures have been effective in Beijing. Nevertheless, NO_X concentrations did not show an evident decreasing trend in the same period. Therefore, more attention should be devoted to O₃ pollution control in May and June; control measure adjustments are needed according to the changes in O₃ precursors, and coordinated control of VOCs and NO_X should be strengthened in long-term planning.