Unexpected seasonal variations and high levels of ozone observed at the summit of Nanling Mountains: Impact of Asian monsoon on southern China

Five-year continuous O3 measurements were conducted for the first time at the summit of Nanling Mountains (112.88° E, 24.68° N, 1690 m a.s.l), southern China in 2013–2017. Unlike most mountains over the world with O3 maximum in spring or summer, Nanling showed O3 maximum in mid-autumn (October; 62.8 ± 0.7 ppbv, ± 95% confidence intervals) and minimum in summer (July; 46.6 ± 0.7 ppbv). This distinct seasonal cycle of O3 at Nanling was mainly caused by the special impact of Asian monsoon in southern China, which brought clean maritime background air and fresh emissions of the Pearl River Delta (PRD) region in spring/summer, and polluted northern continental air masses in autumn/winter, respectively. Further comparisons found that O3 of Nanling in four seasons were all higher than other similar mountainous background sites, making the annual mean of O3 (55.0 ± 0.2 ppbv) at Nanling the second highest that ever reported in the mountainous sites worldwide. Moreover, the diurnal variation showed the higher impact of regional transport during nighttime from the free troposphere (FT) and presented the potential air exchange between the FT and local atmosphere in the plenary boundary layer on the summit of Nanling. Both the seasonal variation and high O3 level indicated a strong impact of anthropogenic emissions from alternatively PRD/northern China on O3 pollution in this mountainous background region. This study diversifies the seasonal variations among mountains over the world, and also illustrates the complicated impact of anthropogenic emissions on the mountainous atmosphere in southern China. •The annual mean of O3 (55 ± 18 ppbv) at Nanling was the second highest in the mountainous sites worldwide.•Different from others, Nanling showed O3 maximum in mid-autumn and minimum in summer due to Asian monsoon.•Diurnal variations of O3 at Nanling showed a minimum in the morning and a maximum at early night.