Relationships between atmospheric organic compounds and air-mass exposure to marine biology S. R. Arnold, A,G D. V. Spracklen, A S. Gebhardt, B T. Custer, B J. Williams, B I. Peeken C,D,E and S. Alvain F A Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK. B Max Planck Institute for Chemistry, Joh.-Joachim-Becher-Weg 27, D-55128 Mainz, Germany. C Ifm GEOMAR, Du ¨ sternbrooker Weg 20, D-24105 Kiel, Germany. D Center for Marine Environmental Sciences (MARUM), Leobener Strasse, D-28359 Bremen, Germany. E Alfred-Wegener-Institute for Polar- and Marine Research, Biological Oceanography, Am Handelshafen 12, D-27570 Bremerhaven, Germany. F Centre National de la Recherche Scientifique (CNRS), Laboratoire d’Oce ´anologie et de Ge ´osciences (LOG), Unite ´ Mixte de Recherche (UMR) 8187, 32 Avenue Foch, F-62930 Wimereux, France. G Corresponding author. Email: s.arnold@leeds.ac.uk Environmental context. The exchange of gases between the atmosphere and oceans impacts Earth’s climate. Over the remote oceans, marine emissions of organic species may have significant impacts on cloud properties and the atmosphere’s oxidative capacity. Quantifying these emissions and their dependence on ocean biology over the global oceans is a major challenge. Here we present a new method which relates atmospheric abundance of several organic chemicals over the South Atlantic Ocean to the exposure of air to ocean biology over several days before its sampling. Abstract. We have used a Lagrangian transport model and satellite observations of oceanic chlorophyll-a concentra- tions and phytoplankton community structure, to investigate relationships between air mass biological exposure and atmospheric concentrations of organic compounds over the remote South Atlantic Ocean in January and February 2007. Accounting for spatial and temporal exposure of air masses to chlorophyll from biologically active ocean regions upwind of the observation location produces significant correlations with atmospheric organohalogens, despite insignificant or smaller correlations using commonly applied in-situ chlorophyll. Strongest correlations (r ¼ 0.42–0.53) are obtained with chlorophyll exposure over a 2-day transport history for CHBr 3 , CH 2 Br 2 , CH 3 I, and dimethylsulfide, and are strengthened further with exposure to specific phytoplankton types. Incorporating daylight and wind-speed terms into the chlorophyll exposure results in reduced correlations. The method demonstrates that conclusions drawn regarding oceanic trace-gas sources from in-situ chlorophyll or satellite chlorophyll averages over arbitrary areas may prove erroneous without accounting for the transport history of air sampled. Introduction Ocean biology impacts the Earth’s climate through its effects on atmospheric composition both by taking up atmospheric CO 2 and by releasing various organic compounds back into the atmo- sphere. The production of dimethylsulfide (DMS) by oceanic phytoplankton, leading to the formation of sulphate aerosol and perturbation to cloud condensation nuclei (CCN) abundance in the marine atmosphere has been studied extensively in this context (e. g. refs [1– 4]). More recently, an observed link between oceanic chlorophyll content and cloud droplet number over the remote Southern Ocean [5] and observations of enhanced organic mass in remote marine aerosol during periods of enhanced ocean biology, [6–8] has led to the postulation that a natural source of organic carbon from the oceans may also exert control on marine CCN concentrations. The magnitude of this source, [9,10] and contributions from primary emission and secondary production from ocean-emitted volatile organic compounds (VOCs) such as isoprene [11–13] and monoterpenes [14] remain highly uncertain. Ocean biology also provides a source of halogens to the marine atmosphere through the emission of halogenated organic com- pounds from phytoplankton and macro algae. [15,16] These natural sources of halogens can impact the oxidising capacity of the remote troposphere. [17,18] The rapid transport of short-lived halogenated organics in deep convection to the tropopause region can also provide a halogen source to the stratosphere, contributing to stratospheric ozone depletion (e.g. Solomon et al. [19] ). Our understanding of natural processes in the Earth’s climate system underpins our estimates of impacts from anthropogenic changes. An improved quantification of the dependence of atmospheric composition on ocean biological activity is therefore important to our understanding of the Earth’s climate system and its response to anthropogenic influence. The attribution of an ocean-biology dependent source of a trace constituent is commonly based on observed correlation CSIRO PUBLISHING S. R. Arnold et al., Environ. Chem. 2010, 7, 232–241. doi:10.1071/EN09144 www.publish.csiro.au/journals/env Ó CSIRO 2010 1448-2517/10/030232 232 Research Paper