title: Extensive halogen-mediated ozone destruction over the tropical Atlantic Ocean.
abstract: Increasing tropospheric ozone levels over the past 150 years have led to a significant climate perturbation; 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 layer 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 models, 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 approximately 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.
authors.affiliation: Department of Chemistry, University of York, Heslington, York YO10 5DD, UK.
authors.forename: Katie A
authors.initials: KA
authors.lastname: Read
authors.forename: Anoop S
authors.initials: AS
authors.lastname: Mahajan
authors.forename: Lucy J
authors.initials: LJ
authors.lastname: Carpenter
authors.forename: Mathew J
authors.initials: MJ
authors.lastname: Evans
authors.forename: Bruno V E
authors.initials: BV
authors.lastname: Faria
authors.forename: Dwayne E
authors.initials: DE
authors.lastname: Heard
authors.forename: James R
authors.initials: JR
authors.lastname: Hopkins
authors.forename: James D
authors.initials: JD
authors.lastname: Lee
authors.forename: Sarah J
authors.initials: SJ
authors.lastname: Moller
authors.forename: Alastair C
authors.initials: AC
authors.lastname: Lewis
authors.forename: Luis
authors.initials: L
authors.lastname: Mendes
authors.forename: James B
authors.initials: JB
authors.lastname: McQuaid
authors.forename: Hilke
authors.initials: H
authors.lastname: Oetjen
authors.forename: Alfonso
authors.initials: A
authors.lastname: Saiz-Lopez
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Pilling
authors.forename: John M C
authors.initials: JM
authors.lastname: Plane
mesh_terms: D000354:Africa, Western; D001267:Atlantic Ocean; D001272:Atmosphere / Q000737:chemistry*; D056890:Eukaryota / Q000378:metabolism; D005843:Geography; D006219:Halogens / Q000737:chemistry*; D008386:Marine Biology; D008697:Methane / Q000737:chemistry; D010126:Ozone / Q000032:analysis / Q000737:chemistry* / Q000528:radiation effects; D012621:Seasons; D012623:Seawater / Q000737:chemistry* / Q000382:microbiology; D013696:Temperature; D014329:Tropical Climate
chemical_list: D006219:Halogens; D010126:Ozone; D008697:Methane