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

Abstract 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 variations 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 (PBL) on the summit of Nanling. Both the seasonal and diurnal variations 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 illustrate the complicated impact of anthropogenic emissions on the mountainous atmosphere in southern China.