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

To investigate the atmospheric oxidative capacity (AOC) in forested high mountain areas adjacent to the photochemistry-active Pearl River Delta (PRD) region in southern China, one-month online observations of isoprene and its oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR) were conducted at a national background station in Nanling Mountains in summer 2016. The results showed that the observed daytime isoprene levels (inline-formula377±46 pptv) were significantly lower in comparison with other forest sites within and outside China, although the sampling site was surrounded with subtropical evergreen broad-leaved trees which are strong isoprene emitters. Also, high daytime (MVK inline-formula+ MACR) inline-formula∕ isoprene ratios (inline-formula1.9±0.5) were observed. Based on the observations, we hypothesized that the lower isoprene levels in the study forest might be attributable to a strong AOC in relation to the elevated regional complex air pollution. In further data analyses, high site-level concentrations of daytime OH (inline-formula $M5inlinescrollmathml\mathrm{normal\; 7.3}\pm \mathrm{normal\; 0.5}\times {\mathrm{normal\; 10}}^{\mathrm{normal\; 6}}$ 72pt14ptsvg-formulamathimg4ab567b4f0685721a8163971485d3ef9 acp-18-14417-2018-ie00001.svg72pt14ptacp-18-14417-2018-ie00001.png  molecules cminline-formula⁻³) and nighttime inline-formulaNO₃ radicals (inline-formula $M8inlinescrollmathml\mathrm{normal\; 6.0}\pm \mathrm{normal\; 0.5}\times {\mathrm{normal\; 10}}^{\mathrm{normal\; 8}}$ 72pt14ptsvg-formulamathimg84e4f19f6ef9fb8a310e5d91b89a673b acp-18-14417-2018-ie00002.svg72pt14ptacp-18-14417-2018-ie00002.png  molecules cminline-formula⁻³) were estimated by using a photochemical box model incorporating the master chemical mechanism (PBM-MCM), and high regional mixing ratios of OH (inline-formula $M10inlinescrollmathml\mathrm{normal\; 19.7}\pm \mathrm{normal\; 2.3}\times {\mathrm{normal\; 10}}^{\mathrm{normal\; 6}}$ 78pt14ptsvg-formulamathimgdf4d6b2943fd48a62a518aec8017dbf2 acp-18-14417-2018-ie00003.svg78pt14ptacp-18-14417-2018-ie00003.png  molecules cminline-formula⁻³) during 09:00–15:00 LT were also obtained by applying a parameterization method with measured aromatic hydrocarbons. And besides, high initial mixing ratios (inline-formula1213±108 pptv) and short atmospheric reaction time (0.27 h) of isoprene during the day were derived by a sequential reaction approach. All these indicate that isoprene was rapidly and highly oxidized in this forest, which supports our hypothesis. The study suggests that the complex air pollution in the PRD may have significantly elevated the background AOC of the adjacent forests, and probably affects the regional air quality and ecological environment in the long term. The feedback of forest ecosystems to the increasing AOC in southern China warrants further studies.