Global modelling of the total OH reactivity: investigations on the missing OH sink and its atmospheric implications

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. Comparison of direct measurements of the OH loss frequency, also known as total OH reactivity (kOH), with the 10 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 15 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, while tropospheric methane lifetime increased by 20 2 % when the additional OH sink was included. The UM-UKCA simulations also allowed us to establish the atmospheric implications of the newly characterised reactions of peroxy radicals (RO2) with OH. While 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 25 product information showed that this reaction cannot reconcile models with observations of atmospheric methanol, in contrast to recent suggestions.Atmos. Chem. Phys. Discuss., https://doi

The reaction of hydroxyl and methylperoxy radicals is not a major source of atmospheric methanol

“…However, a φ CH3OH of 17% (corresponding to the yield from the chamber experiments of this study, uncorrected for the trioxide interference, see Supplementary Fig. 17 ) increases the global burden of methanol by 14% from the base case scenario, which is lower than the study of Ferracci et al 20 , which found 36% increment of methanol abundances with φ CH3OH of 20% from the scenario with φ CH3OH of 0%. Under these assumptions methanol production is found to be 54.3 Tg/yr, compared to 116.7 Tg/yr (direct production of 66.1 Tg/yr and indirect production through trioxide formation of 50.6 Tg/yr) estimated for φ CH3OH = 30% by Müller et al 18 .…”

“…To reconcile modelled and measured methanol abundances, Müller et al 18 utilised a yield of 30% for reaction (2c), the upper limit of their calculated range and also the higher rate coefficient 32 , k = 2.8 × 10 −10 cm 3 molecule −1 s −1 . However, Ferracci et al 20 used k = 1.6 × 10 −10 cm 3 molecule −1 s −1 in their modelling study and found comparatively lower CH 3 OH production (60 Tg/yr) using the yield of 40%, suggesting that a far higher yield would be needed to reconcile models with measurements. The experimental data presented herein demonstrates that the branching fraction at 298 K is instead closer to the calculated value of ~7% producing only 22.4 Tg/yr methanol, which is smaller than required to rationalise atmospheric observations.…”

“…The existing error bounds on the methanol yield, therefore, leave uncertainties not merely on the magnitude but even on the direction of the impact of this reaction, which highlights the need for direct experimental quantification of the yield. Ferracci et al 20 argue that the total rate coefficient indicated by the most recent determinations 14 , 15 , lower than that used by Müller et al 18 would place even more stringent requirements on the methanol yield needed to improve model-measurement agreement; the yield of (4) would need to be in excess of 0.8 to reconcile modelled and measured methanol abundances. We report direct determinations of the methanol yield using two different experimental approaches: isotopologues of OH + CH 3 OO via multiplexed photoionization mass spectrometry (MPIMS) and a chamber study coupled to proton-transfer reaction time-of-flight mass spectrometry (PTR-TOFMS).…”

“…estimated that the uncertainty in the stationary point energies and in the ISC probability gave uncertainties of a factor of 3.5 in the branching fractions. Dramatically different tropospheric effects are encompassed by the upper and lower limits of the methanol yield, ϕ CH3OH , given by Assaf et al 19 (0–40%), Müller et al 18 (2–30%) and Ferracci et al 20 (0–40%).…”

“…Khan et al 5 included this reaction in a global model and noted the importance of this reaction with respect to background methanol if the channel (4) forming methanol were significant. Recent studies (Millet et al 23 , Khan et al 5 , and Ferracci et al 20 ) suggest that a large ϕ CH3OH would dramatically change methanol levels. Applying a yield of 0% for (4) within a global chemical transport model, Müller et al 18 find that the discrepancy between modelled and measured atmospheric methanol is significantly exacerbated, owing to the loss of CH 3 OO through reaction with OH, rather than the self-reaction, which yields CH 3 OH.…”