Methyl halide gases are important sources of atmospheric inorganic halogen radicals. We measured methyl halide emissions from three rice fields over two full growing seasons. Rice paddy emissions of methyl chloride, methyl bromide and methyl iodide are insignificant until field flooding. Rice growth stage determines methyl bromide and methyl iodide emissions while methyl chloride emissions are comparable between planted and unplanted plots. Houston, Texas, and Maxwell, California, field integrated seasonal fluxes of methyl chloride, methyl bromide and methyl iodide are consistent (values range from 2.3 to 3.9, 0.8 to 1.1, and 28.1 to 62.0 mg m-2, respectively) despite differences in multiple field parameters. We also examined field emission variability using 12 chamber placements. Methyl bromide and methyl iodide emissions within homogenous rice paddies require at least three replicates to determine field mean fluxes within 20%, and for methyl chloride emissions, over 10 replications per field are necessary.

Modification of continental land for agricultural use has increased over the last century. Atmospheric impact of this land use change has only been addressed for a few ecosystems and compounds. This paper provides, to date, the most comprehensive examination of gaseous emissions from rice paddies. We report seasonal emission ranges and integrated emission totals for 55 chemical species. This paper is the first to report emissions of isoprene, ethyl chloride, bromoform, alkyl nitrates, bromodichloromethane, hexane, and benzene from rice paddies. Emissions of alkyl nitrates, bromoform, ethyl chloride, and bromodichloromethane by terrestrial ecosystems have never before been observed. For species where emissions were observed we tentatively ascribe possible mechanisms of production; photochemical or biological production in the water column or rice plant mediated. For some compounds, during periods of maximum emissions, ambient rice paddy air concentrations may be concentrated enough to affect regional atmospheric chemistry.