Low-level summertime isoprene observed at a forested mountaintop site in southern China: implications for strong regional atmospheric oxidative capacity
2018
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 × 10 6 molecules cm −3 ) and nighttime NO 3 radicals ( 6.0 ± 0.5 × 10 8 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 × 10 6 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.
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