Abstract
Abstract. 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 (377±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 + MACR) ∕ isoprene ratios (1.9±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 (7.3±0.5×106 molecules cm−3) and nighttime NO3 radicals (6.0±0.5×108 molecules cm−3) were estimated by using a photochemical box model incorporating the master chemical mechanism (PBM-MCM), and high regional mixing ratios of OH (19.7±2.3×106 molecules cm−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 (1213±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.
Highlights
Isoprene (2-methyl-1,3-butadiene) is the most abundant nonmethane volatile organic compound (NMVOC) in the atmosphere (Guenther et al, 2012)
Isoprene and its major intermediate oxidation products methyl vinyl ketone (MVK) and MACR were simultaneously observed in the summer of 2016 at a forested mountaintop site located at the Nanling Mountains in southern China
The sampling site was surrounded with subtropical evergreen broadleaved trees which are strong isoprene emitters, the average daytime isoprene level (377 ± 46 pptv) was found to be significantly lower than other forest studies, while the (MVK + MACR) / isoprene ratio (1.9 ± 0.5) and O3 (51.9 ± 1.7 ppbv) were relatively higher
Summary
Isoprene (2-methyl-1,3-butadiene) is the most abundant nonmethane volatile organic compound (NMVOC) in the atmosphere (Guenther et al, 2012). More recent works have expanded the focus from emissions (Gu et al, 2017) to impacts of isoprene on regional forest chemistry (Taraborrelli et al, 2012; Liu et al, 2016, 2018). These studies have greatly improved our understanding of the oxidation process of isoprene, revealed current uncertainties associated with isoprene emission rates and degradation schemes, and highlighted the biogenic−anthropogenic interactions in moderately polluted forests (Rohrer et al, 2014). Recent studies in pristine Amazon forests have reported the disturbance of anthropogenic influence to the oxidation of isoprene and the amplification of atmospheric oxidative capacity (AOC) (Liu et al, 2016, 2018). Studies on this kind of disturbance and amplification in certain polluted isoprene-rich environments, such as the forests surrounding megacities, remain scarce (Hofzumahaus et al, 2009)
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