Methyl bromide emissions from agricultural field fumigations in California

Abstract

Abstract. 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. IntroductionMethyl bromide is the most abundant organic brominecontaining gas in the atmosphere [Schauf-fier et al., 1993]. Methyl bromide is destroyed by hydroxyl radicals in the troposphere, but some can be transported into the stratosphere where it is photooxidized releasing bromine atoms, which attack ozone catalytically [Wofsy et al., 1975]. Unlike chlorofluorocarbons, methyl bromide has both natural and man-made sources, and it is difficult to assess the fraction of atmospheric methyl bromide produced from man-made sources, and whether it significantly affects the global environment. Recent research is redefining our understanding of the sources and sinks of methyl bromide.Estimated budgets of atmospheric methyl bromide have been deduced by Yvon-Lewis and Butler [1997] There are many advantages in using methyl bromide for field fumigations. Because of methyl bromide's high vapor pressure and low boiling point, transport is rapid, and depth of penetration is extensive. A polyethylene film is typically used to cover the soil just after fumigation for several days to minimize emissions and maximize retention in the soil. A few days after fumigation, farmers prepare the field for planting making the waiting period between fumigation and planting very short. Because it is a broad spectrum fumigant, methyl bromide eliminates the major pests that occur in the soil, and thus a variety of chemicals is not needed. However, methyl bromide is very toxic to humans, and bromide from the decomposition of methyl bromide can be concentrated in plant tissue which is to...

A decline in tropospheric organic bromine

“…Atmosphere and coupled ocean‐atmosphere box‐model calculations [ Butler , 1994] were performed for CH 3 Br with time‐varying industrial emissions that were derived from production data reported to UNEP [1998], and additional constant emissions required to match the measured mixing ratios in 1995–1997. The calculation C 1 included a global lifetime of 0.7 yr, total CH 3 Br emissions of 205 Gg yr −1 in 1992, and an emission fraction ( f ) of 0.5 [ Williams et al , 1999] for industrially produced CH 3 Br applied to soils. Results from a range of calculations (represented by C 2 ) were derived with an [anthropogenic]/[global] emission fraction of 40–45% in 1996 (obtained with lifetimes of 0.8 < τ < 1.5 yr and corresponding releases from soils of 1.0 > f > 0.7).…”

Predicting pesticide volatilization from soils

“…This creates a series of panels down the field and a continuous plastic cover over the field. MeBr application rates range from 200 kg/ha to 400 kg/ha and large fractions (24% to 74%) of the applied chemical are lost to the atmosphere (Yagi et al, 1993(Yagi et al, , 1995Majewski et al, 1995;Yates et al, 1996b,c;Williams et al, 1999). Yates et al (1996a,b,c) conducted a field experiment to investigate the environmental fate and transport of MeBr after soil fumigation.…”

Measurements of ambient atmospheric C2H5Cl and other ethyl and methyl halides at coastal California sites and over the Pacific Ocean

“…Higher plants have been shown to produce atmospheric CH 3 Cl and CH 3 Br through a methyltransferase reaction [ Saini et al , 1995] and CH 3 Br through soil Br‐ conversion in live plants from the Brassicaceae family [ Gan et al , 1998]. These natural sources of CH 3 Br are significant contributors to a global budget once thought to be dominated by anthropogenic sources such as biomass burning [ Andreae et al , 1996], fumigation [ Yagi et al , 1993, 1995; Butler and Rodriguez , 1996; Yates et al , 1997; Williams et al , 1999b], and gasoline combustion [ Thomas et al , 1997; Chen et al , 1999; Baker et al , 1998].…”