Increasing tropospheric ozone levels over the past 150 years have led to a significant climate perturbation1; the prediction of future trends in tropospheric ozone will require a full understanding of both its precursor emissions and its destruction processes. A large proportion of tropospheric ozone loss occurs in the tropical marine boundary layer2,3 and is thought to be driven primarily by high ozone photolysis rates in the presence of high concentrations of water vapour. A further reduction in the tropospheric ozone burden through bromine and iodine emitted from open-ocean marine sources has been postulated by numerical models4,5,6,7, but thus far has not been verified by observations. Here we report eight months of spectroscopic measurements at the Cape Verde Observatory indicative of the ubiquitous daytime presence of bromine monoxide and iodine monoxide in the tropical marine boundary layer. A year-round data set of co-located in situ surface trace gas measurements made in conjunction with low-level aircraft observations shows that the mean daily observed ozone loss is ∼50 per cent greater than that simulated by a global chemistry model using a classical photochemistry scheme that excludes halogen chemistry. We perform box model calculations that indicate that the observed halogen concentrations induce the extra ozone loss required for the models to match observations. Our results show that halogen chemistry has a significant and extensive influence on photochemical ozone loss in the tropical Atlantic Ocean boundary layer. The omission of halogen sources and their chemistry in atmospheric models may lead to significant errors in calculations of global ozone budgets, tropospheric oxidizing capacity and methane oxidation rates, both historically and in the future.
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We thank pilots C. Joseph and D. Davies from the NERC Airborne Research and Support Facility, Oxford, for their assistance in obtaining the vertically resolved observations. We thank M. Heimann for provision of CH4 data from the Cape Verde Observatory and K. Furneaux and L. Whalley for provision of J(O1D) data. We acknowledge the UK NERC Surface Ocean Lower Atmosphere programme and the EU (Tropical Eastern North Atlantic Time Series Observatory) for funding. Finally, we thank D. Wallace, M. Heimann, J. Pimenta Lima and O. Melicio for their roles in setting up the Cape Verde Observatory, and G. McFiggans for conception of the Reactive Halogens in the Marine Boundary Layer Experiment, which contributed to this paper.
Author Contributions L.J.C, J.M.C.P, M.J.P. and A.C.L. conceived the experiment, and together with K.A.R., A.S.M., B.V.E.F., D.E.H., J.R.H., J.D.L, S.J.M, L.M., J.B.M., H.O. and A.S.-L. carried it out; L.J.C., M.J.E and K.A.R. carried out the data analysis; L.J.C., A.C.L, M.J.E and K.A.R. wrote the paper.
The file contains Supplementary Figures S1-S5 with Legends, Supplementary Methods, Supplementary Table 1 and additional references.
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A comparison between hydrogen and halogen bonding: the hypohalous acid–water dimers, HOX⋯H2O (X = F, Cl, Br)
Physical Chemistry Chemical Physics (2019)