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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D17, 4340, doi:10.1029/2001JD001151, 2002

Methyl iodide: Atmospheric budget and use as a tracer of marine convection in global models

N. Bell

Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA


L. Hsu

Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA


D. J. Jacob

Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA


M. G. Schultz

Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, USA


D. R. Blake

University of California, Irvine, California, USA


J. H. Butler

NOAA/Climate Monitoring and Diagnostics Laboratory, Boulder, Colorado, USA


D. B. King

NOAA/Climate Monitoring and Diagnostics Laboratory, Boulder, Colorado, USA


J. M. Lobert

Advanced Pollution Instrumentation, San Diego, California, USA


E. Maier-Reimer

Max Planck Institute für Meteorologie, Hamburg, Germany


Abstract

We simulate the oceanic and atmospheric distribution of methyl iodide (CH3I) with a global 3-D model driven by assimilated meteorological observations from the Goddard Earth Observing System of the NASA Data Assimilation Office and coupled to an oceanic mixed layer model. A global compilation of atmospheric and oceanic observations is used to constrain and evaluate the simulation. Seawater CH3I(aq) in the model is produced photochemically from dissolved organic carbon, and is removed by reaction with Cl− and emission to the atmosphere. The net oceanic emission to the atmosphere is 214 Gg yr−1. Small terrestrial emissions from rice paddies, wetlands, and biomass burning are also included in the model. The model captures 40% of the variance in the observed seawater CH3I(aq) concentrations. Simulated concentrations at midlatitudes in summer are too high, perhaps because of a missing biological sink of CH3I(aq). We define a marine convection index (MCI) as the ratio of upper tropospheric (8–12 km) to lower tropospheric (0–2.5 km) CH3I concentrations averaged over coherent oceanic regions. The MCI in the observations ranges from 0.11 over strongly subsiding regions (southeastern subtropical Pacific) to 0.40 over strongly upwelling regions (western equatorial Pacific). The model reproduces the observed MCI with no significant global bias (offset of only +11%) but accounts for only 15% of its spatial and seasonal variance. The MCI can be used to test marine convection in global models, complementing the use of radon-222 as a test of continental convection.

Published 13 September 2002.

Index Terms: 0312 Atmospheric Composition and Structure: Air/sea constituent fluxes (3339, 4504); 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry.

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Citation: Bell, N., L. Hsu, D. J. Jacob, M. G. Schultz, D. R. Blake, J. H. Butler, D. B. King, J. M. Lobert, and E. Maier-Reimer (2002), Methyl iodide: Atmospheric budget and use as a tracer of marine convection in global models, J. Geophys. Res., 107(D17), 4340, doi:10.1029/2001JD001151.