Methyl halide gases are important sources of atmospheric inorganic halogen compounds, which in turn are central reactants in many stratospheric and tropospheric chemical processes. By observing emissions of methyl chloride, methyl bromide, and methyl iodide from flooded California rice fields, we estimate the impact of rice agriculture on the atmospheric budgets of these gases. Factors influencing methyl halide emissions are stage of rice growth, soil organic content, halide concentrations, and field-water management. Extrapolating our data implies that about 1 percent of atmospheric methyl bromide and 5 percent of methyl iodide arise from rice fields worldwide. Unplanted flooded fields emit as much methyl chloride as planted, flooded rice fields.

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[1] Spatial and temporal distributions of atmospheric CH 3 Cl, CH 3 Br, C 2 H 5 Cl, and C 2 H 5 Br were determined by gas chromatography/mass spectrometry measurements at coastal California and inland sites from 1996 through 1999. The median concentrations observed at a clean coastal site were 585, 13.1, 3.3, and 0.3 ppt (parts per trillion) for CH 3 Cl, CH 3 Br, C 2 H 5 Cl, and C 2 H 5 Br, respectively. To our knowledge, these represent the first quantitative measurements of atmospheric C 2 H 5 Cl. High variation was seen in CH 3 Br and C 2 H 5 Cl measurements from the other two California sites, a rural inland site and a coastal site with urban influence. When compared to the California stationary sites, the median Northern Hemispheric results from measurements taken along Pacific Ocean transects between California and New Zealand from 1996 through 1999 were similar for CH 3 Cl (594 ppt) and C 2 H 5 Br (0.3 ppt) but lower for CH 3 Br (10.4 ppt) and C 2 H 5 Cl (2.6 ppt). Median concentrations of Southern Hemispheric CH 3 Br (9.4 ppt), C 2 H 5 Cl (1.6 ppt), and C 2 H 5 Br (less than 0.2 ppt) were lower than the Northern Hemispheric results. No significant correlation was found between the atmospheric concentrations of the methyl halides with the ethyl halides.