Abstract
Abstract. 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 ~ 18 s−1, significantly less than the observations, consistent with other studies in similar environments. OH reactivity was dominated by reaction with isoprene (~ 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), O3, 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.
Highlights
The hydroxyl radical (OH) is the dominant oxidant in the troposphere
The greatest discrepancies between measured and calculated OH reactivity are usually found under low NOx conditions in the presence of elevated levels of volatile organic compounds (VOCs), typical of those found in forested locations (Di Carlo et al, 2004; Sinha et al, 2008, 2010; Nölscher et al, 2012), or in highly processed continental air masses (Dolgorouky et al, 2012; Sinha et al, 2012)
Observations of OH reactivity were made for a period of 12 days during April 2008 in a tropical rainforest in Borneo
Summary
The hydroxyl radical (OH) is the dominant oxidant in the troposphere. It is highly reactive leading to low concentrations (globally averaged ∼ 1×106 molecule cm−3; Lelieveld et al, 2004) and short lifetimes (∼ 1 s; Heard and Pilling, 2003; Stone et al, 2012). The greatest discrepancies between measured and calculated OH reactivity are usually found under low NOx conditions in the presence of elevated levels of volatile organic compounds (VOCs), typical of those found in forested locations (Di Carlo et al, 2004; Sinha et al, 2008, 2010; Nölscher et al, 2012), or in highly processed continental air masses (Dolgorouky et al, 2012; Sinha et al, 2012) This failure to account for OH sinks within remote forested regions is significant, as they constitute the overwhelming source of reactive carbon entering into the troposphere. Once this conceptual system has been developed, the OP3 OH reactivity observations are placed into this context and the sources of OH reactivity in the forest are outlined and investigated
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call