Abstract
Abstract. The hydroxyl radical (OH) plays a crucial role in the chemistry of the atmosphere as it initiates the removal of most trace gases. A number of field campaigns have observed the presence of a missing OH sink in a variety of regions across the planet. A comparison of direct measurements of the OH loss frequency, also known as total OH reactivity (kOH), with the sum of individual known OH sinks (obtained via the simultaneous detection of species such as volatile organic compounds and nitrogen oxides) indicates that, in some cases, up to 80 % of kOH is unaccounted for. In this work, the UM-UKCA chemistry-climate model was used to investigate the wider implications of the missing reactivity on the oxidising capacity of the atmosphere. Simulations of the present-day atmosphere were performed and the model was evaluated against an array of field measurements to verify that the known OH sinks were reproduced well, with a resulting good agreement found for most species. Following this, an additional sink was introduced to simulate the missing OH reactivity as an emission of a hypothetical molecule, X, which undergoes rapid reaction with OH. The magnitude and spatial distribution of this sink were underpinned by observations of the missing reactivity. Model runs showed that the missing reactivity accounted for on average 6 % of the total OH loss flux at the surface and up to 50 % in regions where emissions of the additional sink were high. The lifetime of the hydroxyl radical was reduced by 3 % in the boundary layer, whilst tropospheric methane lifetime increased by 2 % when the additional OH sink was included. As no OH recycling was introduced following the initial oxidation of X, these results can be interpreted as an upper limit of the effects of the missing reactivity on the oxidising capacity of the troposphere. The UM-UKCA simulations also allowed us to establish the atmospheric implications of the newly characterised reactions of peroxy radicals (RO2) with OH. Whilst the effects of this chemistry on kOH were minor, the reaction of the simplest peroxy radical, CH3O2, with OH was found to be a major sink for CH3O2 and source of HO2 over remote regions at the surface and in the free troposphere. Inclusion of this reaction in the model increased tropospheric methane lifetime by up to 3 %, depending on its product branching. Simulations based on the latest kinetic and product information showed that this reaction cannot reconcile models with observations of atmospheric methanol, in contrast to recent suggestions.
Highlights
The removal of the vast majority of trace gases emitted into the atmosphere is initiated by reaction with the hydroxyl radical, OH
OH sources and sinks, consisting of the totality of the species that react with OH: these include volatile organic compounds (VOCs), nitrogen oxides (NOx) and many others species, both biogenic and anthropogenic
The total OH loss frequency, known as the total OH reactivity, is a useful measure of the total amount of OH sinks present in a particular environment. kOH is defined as the pseudo first-order rate constant for OH loss and is equivalent to the inverse of the OH lifetime, τOH, as shown in Eq (1): n kOH = i=1kOH+Xi [Xi ] = 1/τOH, (1)
Summary
The removal of the vast majority of trace gases emitted into the atmosphere is initiated by reaction with the hydroxyl radical, OH. A review (Yang et al, 2016) recently described these techniques in detail, whilst the various instruments developed for direct measurements of kOH have been the subject of an extensive intercomparison (Fuchs et al., 2017) These techniques, when deployed in the field along with instruments for the detection of trace species, have enabled the comparison of direct measurements of the total kOH with the sum of reactivities of the individual OH sinks. CheST with Reaction (R3) An additional OH sink, species X, is introduced in the model to account for the missing kOH The multi-channel reaction of methyl peroxy radicals (CH3O2) with OH was included in the chemistry scheme with branching ratios α = 1, γ = 0 ∗ The implications of both approaches for the oxidising capacity of the atmosphere are discussed
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