Measurement and modelling of tropospheric reactive halogen species over the tropical Atlantic Ocean

Abstract

Although tropospheric reactive halogen chemistry is well studied in coastal and polar environments, the presence of halogens over the open ocean environment has not been widely reported. The impacts of halogens on the tropical open ocean marine boundary layer (MBL), in particular, are not well characterised. This paper describes observations of iodine monoxide (IO) and bromine oxide (BrO) over eight months in the tropical open ocean MBL, on the north-eastern side of São Vicente (Cape Verde Islands, 16.85° N, 24.87° W). The highest BrO mixing ratio observed was 5.6±1 pmol mol<sup>−1</sup>, while the maximum observed IO mixing ratio was 3.1±0.4 pmol mol<sup>−1</sup>. The average values seen between 09:00–17:00 GMT were ~2.8 pmol mol<sup>−1</sup> for BrO and ~1.5 pmol mol<sup>−1</sup> for IO; these averages showed little variability over the entire campaign from November 2006 to June 2007. A 1-dimensional chemistry and transport model is used to study the evolution of iodine species and quantify the combined impact of iodine and bromine chemistry on the oxidising capacity of the MBL. It appears that the measured fluxes of iodocarbons are insufficient to account for the observed levels of IO, and that an additional I atom source is required, possibly caused by the deposition of O<sub>3</sub> onto the ocean surface in the presence of solar radiation. Modelling results also show that the O<sub>3</sub> depletion observed at Cape Verde cannot be explained in the absence of halogen chemistry, which contributes ~45% of the observed O<sub>3</sub> depletion at the height of measurements (10 m) during summer. The model also predicts that halogens decrease the hydroperoxy radical (HO<sub>2</sub>) concentration by ~14% and increase the hydroxyl radical (OH) concentration by ~13% near the ocean surface. The oxidation of dimethyl sulphide (DMS) by BrO takes place at a comparable rate to oxidation by OH in this environment. Finally, the potential of iodine chemistry to form new particles is explored and conditions under which particle formation could be important in the remote MBL are discussed

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. Br inorg was generally overestimated by the model (Figure 1h): in the EUROPE scenario, Br inorg was overestimated by about a factor of 2, while in the other scenarios it was overestimated by a factor of 10.…”

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

“…The seasonal changes for both species were within the variability of the dataset. Detailed 1-D modelling of the sources of halogens and their impact on the boundary layer chemistry by Mahajan et al, (2010) indicates that the IO and BrO account for up to 45 % of the observed O 3 destruction at the surface around Cape Verde. In addition, we suggest that an extra iodine source from the surface is necessary and is most probably photolysis dependent , because the observed fluxes of iodocarbons account for only 10-25 % of the observed IO .…”

Impacts of bromine and iodine chemistry on tropospheric OH and HO<sub>2</sub>: comparing observations with box and global model perspectives

“…In 2007, the observatory was host to the RHaMBLe intensive field campaign, during which a number of additional measurements were made to complement the long-term measurements at the site , including LP-DOAS measurements of halogen species (Read et al, 2008;Mahajan et al, 2010a) and formaldehyde (Mahajan et al, 2010b), and LIF-FAGE measurements of OH and HO 2 . The halogen monoxide radicals BrO and IO exhibited a "top-hat" diurnal cycle (Vogt et al, 1999(Vogt et al, , 1996Read et al, 2008;Mahajan et al, 2010a) with essentially zero concentration in the hours of darkness and generally constant values of approximately 2.5 ppt BrO and 1.4 ppt IO during the day.…”

“…Observations of BrO and IO radicals within the MBL have demonstrated widespread impacts on atmospheric composition and chemistry (Alicke et al, 1999;Sander et al, 2003;Leser et al, 2003;Saiz-Lopez and Plane, 2004;SaizLopez et al, 2004SaizLopez et al, , 2006Peters et al, 2005;Whalley et al, 2007;Mahajan et al, 2010a;Commane et al, 2011;Dix et al, 2013;Gomez Martin et al, 2013;Le Breton et al, 2017), including significant effects on HO x concentrations and on the HO 2 : OH ratio in coastal and marine locations (Bloss et al, 2005a(Bloss et al, , 2010Sommariva et al, 2006;Kanaya et al, 2007;Whalley et al, 2010).…”

“…Measurements of halogen monoxides (Mahajan et al, 2010a) at the site have been shown to have significant impacts on local ozone concentrations, notably in the magnitude of the daily cycle (Read et al, 2008) and have been used to constrain box model calculations used to explore observations of OH and HO 2 made during the RHaMBLe campaign in 2007 . The model calculations showed generally good comparisons with the observed OH and HO 2 concentrations, apart from a period characterised by unusually high concentrations of HCHO.…”