Journal article

OH reactivity in a South East Asian tropical rainforest during the oxidant and particle photochemical processes (OP3) project

Edwards P, Evans M, Furneaux K, Hopkins J, Ingham T, Jones C, Lee J, Lewis A, Moller S, Stone D, Whalley L, Heard D ...see all

Atmospheric Chemistry and Physics, vol. 13, issue 18 (2013) pp. 9497-9514

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OH (hydroxyl radical) reactivity, the inverse of the chemical lifetime
of the hydroxyl radical, was measured for 12 days in April 2008 within a
tropical rainforest on Borneo as part of the OP3 (Oxidant and Particle
Photochemical Processes) project. The maximum observed value was 83.8
+/- 26.0 s(-1) with the campaign averaged noontime maximum being 29.1
+/- 8.5 s(-1). The maximum OH reactivity calculated using the diurnally
averaged concentrations of observed sinks was similar to 18 s(-1),
significantly less than the observations, consistent with other studies
in similar environments. OH reactivity was dominated by reaction with
isoprene (similar to 30%). Numerical simulations of isoprene oxidation
using the Master Chemical Mechanism (v3.2) in a highly simplified
physical and chemical environment show that the steady state OH
reactivity is a linear function of the OH reactivity due to isoprene
alone, with a maximum multiplier, to account for the OH reactivity of
the isoprene oxidation products, being equal to the number of isoprene
OH attackable bonds (10). Thus the emission of isoprene constitutes a
significantly larger emission of reactivity than is offered by the
primary reaction with isoprene alone, with significant scope for the
secondary oxidation products of isoprene to constitute the observed
missing OH reactivity. A physically and chemically more sophisticated
simulation (including physical loss, photolysis, and other oxidants)
showed that the calculated OH reactivity is reduced by the removal of
the OH attackable bonds by other oxidants and photolysis, and by
physical loss (mixing and deposition). The calculated OH reactivity is
increased by peroxide cycling, and by the OH concentration itself.
Notable in these calculations is that the accumulated OH reactivity from
isoprene, defined as the total OH reactivity of an emitted isoprene
molecule and all of its oxidation products, is significantly larger than
the reactivity due to isoprene itself and critically depends on the
chemical and physical lifetimes of intermediate species. When
constrained to the observed diurnally averaged concentrations of primary
VOCs (volatile organic compounds), O-3, NOx and other parameters, the
model underestimated the observed diurnal mean OH reactivity by 30%.
However, it was found that (1) the short lifetimes of isoprene and OH,
compared to those of the isoprene oxidation products, lead to a large
variability in their concentrations and so significant variation in the
calculated OH reactivity; (2) uncertainties in the OH chemistry in these
high isoprene environments can lead to an underestimate of the OH
reactivity; (3) the physical loss of species that react with OH plays a
significant role in the calculated OH reactivity; and (4) a missing
primary source of reactive carbon would have to be emitted at a rate
equivalent to 50% that of isoprene to account for the missing OH sink.
Although the presence of unmeasured primary emitted VOCs contributing to
the measured OH reactivity is likely, evidence that these primary
species account for a significant fraction of the unmeasured reactivity
is not found. Thus the development of techniques for the measurement of
secondary multifunctional carbon compounds is needed to close the OH
reactivity budget.

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  • Mathew EvansUniversity of York

  • P. M. Edwards

  • K. L. Furneaux

  • J. Hopkins

  • T. Ingham

  • C. Jones

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