Halocarbon emissions and sources in the equatorial Atlantic Cold Tongue

Abstract. Halocarbons from oceanic sources contribute to halogens in the troposphere, and can be transported into the stratosphere where they take part in ozone depletion. This paper presents distribution and sources in the equatorial Atlantic from June and July 2011 of the four compounds bromoform (CHBr₃), dibromomethane (CH₂Br₂), methyl iodide (CH₃I) and diiodomethane (CH₂I₂). Enhanced biological production during the Atlantic Cold Tongue (ACT) season, indicated by phytoplankton pigment concentrations, led to elevated concentrations of CHBr₃ of up to 44.7 and up to 9.2 pmol L⁻¹ for CH₂Br₂ in surface water, which is comparable to other tropical upwelling systems. While both compounds correlated very well with each other in the surface water, CH₂Br₂ was often more elevated in greater depth than CHBr₃, which showed maxima in the vicinity of the deep chlorophyll maximum. The deeper maximum of CH₂Br₂ indicates an additional source in comparison to CHBr₃ or a slower degradation of CH₂Br₂. Concentrations of CH₃I of up to 12.8 pmol L⁻¹ in the surface water were measured. In contrary to expectations of a predominantly photochemical source in the tropical ocean, its distribution was mostly in agreement with biological parameters, indicating a biological source. CH₂I₂ was very low in the near surface water with maximum concentrations of only 3.7 pmol L⁻¹. CH₂I₂ showed distinct maxima in deeper waters similar to CH₂Br₂. For the first time, diapycnal fluxes of the four halocarbons from the upper thermocline into and out of the mixed layer were determined. These fluxes were low in comparison to the halocarbon sea-to-air fluxes. This indicates that despite the observed maximum concentrations at depth, production in the surface mixed layer is the main oceanic source for all four compounds and one of the main driving factors of their emissions into the atmosphere in the ACT-region. The calculated production rates of the compounds in the mixed layer are 34 ± 65 pmol m⁻³ h⁻¹ for CHBr₃, 10 ± 12 pmol m⁻³ h⁻¹ for CH₂Br₂, 21 ± 24 pmol m⁻³ h⁻¹ for CH₃I and 384 ± 318 pmol m⁻³ h⁻¹ for CH₂I₂ determined from 13 depth profiles.