Methyl iodide production in the open ocean

I Stemmler,E Maier-Reimer,I Hense,B Quack

doi:10.5194/bg-11-4459-2014

Abstract

Abstract. Production pathways of the prominent volatile organic halogen compound methyl iodide (CH3I) are not fully understood. Based on observations, production of CH3I via photochemical degradation of organic material or via phytoplankton production has been proposed. Additional insights could not be gained from correlations between observed biological and environmental variables or from biogeochemical modeling to identify unambiguously the source of methyl iodide. In this study, we aim to address this question of source mechanisms with a three-dimensional global ocean general circulation model including biogeochemistry (MPIOM–HAMOCC (MPIOM – Max Planck Institute Ocean Model HAMOCC – HAMburg Ocean Carbon Cycle model)) by carrying out a series of sensitivity experiments. The simulated fields are compared with a newly available global data set. Simulated distribution patterns and emissions of CH3I differ largely for the two different production pathways. The evaluation of our model results with observations shows that, on the global scale, observed surface concentrations of CH3I can be best explained by the photochemical production pathway. Our results further emphasize that correlations between CH3I and abiotic or biotic factors do not necessarily provide meaningful insights concerning the source of origin. Overall, we find a net global annual CH3I air–sea flux that ranges between 70 and 260 Gg yr−1. On the global scale, the ocean acts as a net source of methyl iodide for the atmosphere, though in some regions in boreal winter, fluxes are of the opposite direction (from the atmosphere to the ocean).

Highlights

Methyl iodide (CH3I) is an organic halogen of natural origin
Methyl iodide modeling was performed with the MPIOM (Max Planck Institute Ocean Model) (Marsland et al, 2003) ocean general circulation model coupled to the HAMOCC5.2 (Six and Maier-Reimer, 1996; Ilyina et al, 2013) marine carbon cycle model and the CH3I module presented in Stemmler et al (2013)
Approximately 400 Gg of CH3I are produced by phytoplankton within one year, but more than 94 % is lost via degradation and outgassing (Table 2)

Summary

Introduction

Methyl iodide (CH3I) is an organic halogen of natural origin. Following emission from the ocean (or land), it is photolyzed within days into reactive iodine species that affect the oxidative capacity of the atmosphere (e.g., via ozone depletion) (Chameides and Davis, 1980; Solomon et al, 1994; Rattigan et al, 1997; Vogt et al, 1999; Carpenter, 2003). The strength of the CH3I source to the atmosphere is estimated using two different methods It is either derived from extrapolating fluxes diagnosed from concentrations measured during ship cruises (e.g., Moore and Groszko, 1999; Chuck et al, 2005; Butler et al, 2007; Jones et al, 2010; Ziska et al, 2013) or by analyzing oceanic source and sink processes (e.g., Manley and De La Cuesta, 1997; Bell et al, 2002; Carpenter, 2003; Richter and Wallace, 2004; Youn et al, 2010).

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