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.

Multiphase Halogen Chemistry in the Tropical Atlantic Ocean

“…Gas-phase BrO was measured by longpath differential optical absorption spectroscopy (LP-DOAS) at CVAO 6,11 and by multiaxis differential optical absorption In the EUROPE scenario the model overestimated the observations of BrO by one or more ppt, while in the other scenarios BrO was generally within the range of the observations, although it tended to increase with time ( Figure 1g); the typical BrO peaks at dawn and dusk were several ppt larger than observed. 11 The model results were in broad agreement with the observations by ref 5, but in contrast with the observations by ref 10, who reported BrO below detection limit upwind of CVAO and very high BrO mixing ratios (up to 10.2 ppt) between Cape Verde and the African coast; however, it is likely that the latter were affected by local emissions not included in the model.…”

“…This indicates that Cl accounted for 5.4−11.6% of total methane loss in the Cape Verde region, in accord with previous estimates. 9 The Cape Verde region is typically under ozone destruction conditions, due to the low levels of NO x : diurnal ozone loss was observed at CVAO 6,21 with average values of 3.3 ± 2.6 ppb/ day. The BASE cases could reproduce ozone destruction during the day, although the calculated destruction rates were higher than the observations by a factor of 2−3 (depending on the scenario: 6−14 ppb/day).…”

“…Measurements of these species in the tropical region are sparse, but a number of recent studies 3−13 have reported observations of several halogen species in the tropical Atlantic Ocean. These studies suggested that halogen chemistry is responsible for a large fraction (∼30%) of ozone destruction in the Tropics 6 and up to 7% of the global methane destruction. 9 In this paper, we used the one-dimensional chemical model MISTRA to study halogen chemistry in the tropical Atlantic Ocean and the evolution of the chemical composition of air masses traveling across the ocean to the islands of Cape Verde, 900 km West of the coast of Africa.…”

Seasonal characteristics of tropical marine boundary layer air measured at the Cape Verde Atmospheric Observatory

“…Observations made in Cape Verde therefore provide rare information on links between climate, marine biology and atmospheric composition changes. The productive marine biology in the region of Cape Verde may be at least partly responsible for the high levels of halogen oxides observed at the CVAO Read et al, 2008). These measurements have provided important observational evidence of the widespread impacts of halogen chemistry in the marine boundary layer, which had remained elusive until now.…”

Constraints on inorganic gaseous iodine in the tropical upper troposphere and stratosphere inferred from balloon-borne solar occultation observations

“…Butz et al: Inorganic iodine in the tropical UT/LS observed regularly in the form of IO radicals in the lower troposphere with concentrations of several ppt (e.g. Frieß et al, 2001;Peters et al, 2005;Zingler and Platt, 2005;SaizLopez et al, 2006SaizLopez et al, , 2007bRead et al, 2008). Only recently, space-borne observations confirmed enhanced levels of IO near coastal Antarctica (Saiz-Lopez et al, 2007a;Schönhardt et al, 2008).…”