OH reactivity in a South East Asian tropical rainforest during the oxidant and particle photochemical processes (OP3) project
OH (hydroxyl radical) reactivity, the inverse of the chemical lifetime\nof the hydroxyl radical, was measured for 12 days in April 2008 within a\ntropical rainforest on Borneo as part of the OP3 (Oxidant and Particle\nPhotochemical Processes) project. The maximum observed value was 83.8\n+/- 26.0 s(-1) with the campaign averaged noontime maximum being 29.1\n+/- 8.5 s(-1). The maximum OH reactivity calculated using the diurnally\naveraged concentrations of observed sinks was similar to 18 s(-1),\nsignificantly less than the observations, consistent with other studies\nin similar environments. OH reactivity was dominated by reaction with\nisoprene (similar to 30%). Numerical simulations of isoprene oxidation\nusing the Master Chemical Mechanism (v3.2) in a highly simplified\nphysical and chemical environment show that the steady state OH\nreactivity is a linear function of the OH reactivity due to isoprene\nalone, with a maximum multiplier, to account for the OH reactivity of\nthe isoprene oxidation products, being equal to the number of isoprene\nOH attackable bonds (10). Thus the emission of isoprene constitutes a\nsignificantly larger emission of reactivity than is offered by the\nprimary reaction with isoprene alone, with significant scope for the\nsecondary oxidation products of isoprene to constitute the observed\nmissing OH reactivity. A physically and chemically more sophisticated\nsimulation (including physical loss, photolysis, and other oxidants)\nshowed that the calculated OH reactivity is reduced by the removal of\nthe OH attackable bonds by other oxidants and photolysis, and by\nphysical loss (mixing and deposition). The calculated OH reactivity is\nincreased by peroxide cycling, and by the OH concentration itself.\nNotable in these calculations is that the accumulated OH reactivity from\nisoprene, defined as the total OH reactivity of an emitted isoprene\nmolecule and all of its oxidation products, is significantly larger than\nthe reactivity due to isoprene itself and critically depends on the\nchemical and physical lifetimes of intermediate species. When\nconstrained to the observed diurnally averaged concentrations of primary\nVOCs (volatile organic compounds), O-3, NOx and other parameters, the\nmodel underestimated the observed diurnal mean OH reactivity by 30%.\nHowever, it was found that (1) the short lifetimes of isoprene and OH,\ncompared to those of the isoprene oxidation products, lead to a large\nvariability in their concentrations and so significant variation in the\ncalculated OH reactivity; (2) uncertainties in the OH chemistry in these\nhigh isoprene environments can lead to an underestimate of the OH\nreactivity; (3) the physical loss of species that react with OH plays a\nsignificant role in the calculated OH reactivity; and (4) a missing\nprimary source of reactive carbon would have to be emitted at a rate\nequivalent to 50% that of isoprene to account for the missing OH sink.\nAlthough the presence of unmeasured primary emitted VOCs contributing to\nthe measured OH reactivity is likely, evidence that these primary\nspecies account for a significant fraction of the unmeasured reactivity\nis not found. Thus the development of techniques for the measurement of\nsecondary multifunctional carbon compounds is needed to close the OH\nreactivity budget.