Seasonal variations of O3 formation mechanism and atmospheric photochemical reactivity during severe high O3 pollution episodes in the Pearl River Delta region

In this study, the seasonal differences of O3 formation and photochemical reactivity were investigated during the high O3 episodes observed in autumn of 2014 and winter of 2017 at a downwind rural site of Pearl River Delta region (Heshan site). The mixing ratios of O3 and NO2 were highest in the Period I of autumn, followed by the Period II of autumn and the winter period. Though the total concentrations of VOCs were comparable in these periods, alkenes made greater fraction of VOCs during Period I of autumn, while those of OVOCs and aromatics were higher in winter period, resulting in the largest contributions of alkenes (ROH(alkenes)) and OVOCs (ROVOCs) to the photochemical reactivity in autumn and winter, respectively. The ozonolysis of alkenes (especially for 1-pentene and isoprene) dominated HOx production during Period I of autumn, while in winter period, photolysis of OVOCs increased before noon, and made the greatest contribution to HOx production in late afternoon. As for the HOx destruction, reaction of OH + NO2 dominated in both seasons, with higher rates in Period I of autumn because of higher levels of radical and NO2. Furthermore, the variations in O3 precursors and radical budget resulted in different O3 formation and destruction rates, with the highest contribution from RO2 and NO to O3 formation, as well as photolysis and ozonolysis of alkenes for O3 destruction in Period I of autumn. The net O3 production from photochemistry and contributions of physical processes indicated that local photochemistry dominated the O3 accumulation at noon and in the early afternoon, while dilution in O3 levels caused by physical process increased before noon and in the late afternoon. O3 formation sensitivity could be changed during the photochemical consumption of precursors, which was more NOx-limited in the late afternoon. Overall, our findings highlighted that control measures of photochemical pollution should consider the seasonal variations in the abundance of O3 and its precursors, and O3 formation sensitivity regime.