Significant missing OH reactivity in the tropical marine boundary layer

It has recently been highlighted that observed OH reactivity (kOH) in the remote marine boundary layer cannot be fully explained by the measured or modelled speciated VOCs [1]. Understanding the identity and magnitude of the species that contribute to OH reactivity (and influence the concentration of OH) in the tropical marine boundary layer is particularly important however, as approximately 25 % of the total tropospheric methane removal, driven by the reaction with OH, occurs in this region [2].  Ground-based observations of kOH and comparisons with calculated or modelled kOH from individually measured VOCs and inorganics offers the opportunity to investigate diel trends and variabilities driven by changing air-masses, which can provide a valuable insight into the identity and impact of any missing kOH. We made the first observations of kOH at the Cape Verde Atmospheric Observatory in the remote tropical marine boundary layer in February 2023. The observed kOH ranged from 1.5 s^-1 to 2.5 s^-1 with the highest reactivity recorded when long-range transport of Saharan air-masses reached the observatory. The calculated kOH from the different inorganic and VOC species measured during the campaign did not capture the total kOH observed and even when the contribution from model-generated species (determined from a detailed 0D box model utilising the Master Chemical Mechanism) were considered, missing reactivity on the order of 0.2 – 0.5 s^-1 remained, consistent with the levels of missing kOH previously determined in the marine boundary layer during the ATom aircraft campaign [1]. The diel profile highlighted that missing kOH was greatest during the night and morning and was at its lowest during the afternoon. Missing kOH correlated well with the OH reactivity contribution from species such as alkenes, CO and DMS, but was anti-correlated with the carbonyl reactivity suggesting the nature of the missing reactivity could be related to an unknown or unmeasured primary emission rather than a secondary species formed during the day via OH oxidation. Through modelling studies, we will present the impact that different missing primary emissions have on modelled OH concentrations and on the production of secondary oxygenated VOCs.

[1] Thames, A.B., et al., Missing OH reactivity in the global marine boundary layer, Atmospheric Chemistry and Physics, 2020, 20, 4013-4029.

[2] Bloss, W.J., et al., The oxidative capacity of the troposphere: Coupling of field measurements of OH and a global chemistry transport model. Faraday Discussions, 2005. 130: p. 425-436.