Halocarbon dynamics from Las Palmas to Guayaquil in winter 2021/2022 – Results of SO287-CONNECT

Short-lived bromo-, chloro- and iodocarbons from marine and anthropogenic sources contribute to the atmospheric halogen budgets and to ozone depletion in the troposphere and stratosphere. Their spatial variations are poorly known, given the sparse observations of marine and atmospheric concentrations. The distribution and air-sea fluxes of halocarbons need to be quantified in order to clarify the oceanic contributions to future tropospheric and stratospheric ozone chemistry.

Here we present the first marine and atmospheric halocarbon dataset from the research cruise SO287-CONNECT (Pan-Atlantic connectivity of marine biogeochemical and ecological processes and the impact of anthropogenic pressures). The transit of RV SONNE from Las Palmas, Spain (departure: 11.12.2021) to Guayaquil, Ecuador (arrival: 11.01.2022) was conducted to decipher the coupling of biogeochemical and ecological processes and their influence on atmospheric chemistry along the transport pathway of water from the upwelling zones off Africa into the Sargasso Sea and further to the Caribbean and the equatorial Pacific. A comprehensive work program, which combined continuous underway and station work, marine and atmospheric measurements and sampling with incubation experiments was conducted.

The distribution of short-lived halocarbons, e.g. bromoform (CHBr₃), dibromomethane (CH₂Br₂), methyl iodide (CH₃I), and trichloromethane (CHCl₃) was highly dynamic in both ocean and atmosphere. We calculate the air-sea exchange of the compounds and relate physical and biological parameters to our observations. Among these are transport processes (e.g. long-range transport, eddies) and we show the varying composition of air and water masses and the potential sources of the compounds. For the first time, we estimate the contribution of the floating macroalgae Sargassum to halocarbon cycling around the North Atlantic gyre and in the Caribbean. The evaluation of the comprehensive data set collected during SO287-CONNECT improves our knowledge on the general role of the great Atlantic Sargassum belt and anthropogenic pollution in elemental biogeochemical cycles, as well as on trace gas exchange across the ocean-atmosphere interface.