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. To investigate the atmospheric oxidative capacity (AOC) in forested highmountain areas adjacent to the photochemistry-active Pearl River Delta (PRD)region in southern China, one-month online observations of isoprene and itsoxidation products methyl vinyl ketone (MVK) and methacrolein (MACR) wereconducted at a national background station in Nanling Mountains in summer2016. The results showed that the observed daytime isoprene levels ( 377±46  pptv) were significantly lower in comparison with other forest siteswithin and outside China, although the sampling site was surrounded withsubtropical evergreen broad-leaved trees which are strong isoprene emitters.Also, high daytime (MVK  +  MACR)  ∕  isoprene ratios ( 1.9±0.5 )were observed. Based on the observations, we hypothesized that the lowerisoprene levels in the study forest might be attributable to a strong AOC inrelation to the elevated regional complex air pollution. In further dataanalyses, high site-level concentrations of daytime OH ( 7.3 ± 0.5 × 10 6  molecules cm −3) and nighttime NO3 radicals ( 6.0 ± 0.5 × 10 8  molecules cm −3) were estimated by using aphotochemical box model incorporating the master chemical mechanism(PBM-MCM), and high regional mixing ratios of OH ( 19.7 ± 2.3 × 10 6  molecules cm −3) during 09:00–15:00 LT were also obtained byapplying a parameterization method with measured aromatic hydrocarbons. Andbesides, high initial mixing ratios ( 1213±108  pptv) and shortatmospheric reaction time (0.27 h) of isoprene during the day were derivedby a sequential reaction approach. All these indicate that isoprene wasrapidly and highly oxidized in this forest, which supports our hypothesis.The study suggests that the complex air pollution in the PRD may havesignificantly elevated the background AOC of the adjacent forests, andprobably affects the regional air quality and ecological environment in thelong term. The feedback of forest ecosystems to the increasing AOC insouthern China warrants further studies.

atmospheric chemistry and physics

Low-level summertime isoprene observed at a forested mountaintop site in southern China: implications for strong regional atmospheric oxidative capacity