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Methyl bromide emissions from agricultural field fumigations in California


Methyl bromide is the most abundant brominated hydrocarbon gas in the atmosphere and is significant as a source of stratospheric bromine radicals that destroy ozone. However, estimated sources and sinks of methyl bromide exhibit a deficit in sources of ∼70 Gg/yr (one Gg = 109 g), and the proportion of natural and anthropogenic sources is not known well. Known sources include agricultural fumigation (preplant and postplant), structural fumigation, biomass burning, gasoline additives, and oceans. The oceans, however, also act as a net sink for methyl bromide; that is, globally, consumption is greater than production. Early estimates of emissions of methyl bromide from fumigated agricultural fields from models were 30-60% of the amount applied. To test this estimate, we studied emissions from six field fumigations using chambers to measure the flux of methyl bromide, soil bromide analyses to measure degradation, and soil gas down to 90 cm or more to monitor methyl bromide with time in the soil profile. We found between 24-74% of applied methyl bromide was emitted. The average emission found in these experiments was 49 ± 19% based on chamber measurements and 52 ± 20% based on the soil bromide measurements. Factors affecting emissions included the polyethylene film covering the soil, the injection method, the injection depth, and the chemical and physical properties of the soil. The main factors controlling the emissions in our studies are discussed. Copyright 1999 by the American Geophysical Union.

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