Atmospheric methyl bromide: Trends and global mass balance

Abstract

Atmospheric methyl bromide is of considerable environmental importance as the largest reservoir of gaseous bromine in the atmosphere. Bromine gases can catalytically destroy stratospheric ozone. Since agricultural activities, automobiles, biomass burning, and other human activities produce CH3Br , it is of interest to know its global mass balance and particularly the specific sources and sinks. In this paper we provide a decadal time series of global CH3Br concentrations in the Earth's atmosphere. The data show that average concentrations are about 10 pptv and during the last 4 years may be increasing at 0.3 +_ 0.1 pptv/yr (3%/yr +_ 1%/yr). We estimate that the atmospheric lifetime of CH3Br that is due to reaction with OH, is about 2 years, resulting in a calculated global emission rate of about 100 GgJyr. Ocean supersaturations of 140-180% are observed, and atmospheric concentrations over the open oceans are higher than at comparably located coastal sites. The ocean source is estimated to be about 35 GgJyr. The remaining emissions must come from other natural sources and anthropogenic activities. Our results are based on some 2200 samples obtained over more than a decade. Mass balance calculations explain most aspects of the present data but other implications are not easily reconciled, leaving open the possibility of undiscovered sources and sinks. 1. INTRODUCTION Methyl bromide is probably the most abundant reservoir of gaseous bromine in the Earth's atmosphere. While it is generally believed to come from the oceans, there are identified man-made sources such as automobiles, agricultural fumigants, and biomass burning that have been increas!ng in recent decades. Bromine gases, particularly methyl bromide (CH3Br) because of its relatively high concentrations, may be of considerable importance in the global environment since such gases can destroy the ozone layer and contribute to forming the Antarctic ozone hole. According to recent calculations bromine gases, in their capacity to destroy stratospheric ozone, may be up to 100 times more effective than chlorine from the well known chlorofiuorocarbons [Wofsy et al., 1975; Yung et al., 1980; McElroy et al., 1986; Solomon, 1990; World Meteorology Organization, 1989]. Methyl bromide is considered of sufficient importance for ozone depletion that plans to reduce production are being considered by the parties to the Montreal Protocol [Global Enviromnental Change Report, 1992]. In spite of its importance, there are very few published data on CH3Br , and little is known about its trends and global mass balance. We have three goals for this paper. (1) We present global concentration data that include a time series at six sites, latitudinal distributions in air and in the waters of open oceans and the variation of CH3Br with altitude in the troposphere.(2) These data are taken into account to construct a global mass balance from which we estimate the total global emissions and discuss the apportionment of these emissions between natural and man-made sources. (3) Finally, ...

Fate of Methyl Bromide in Fumigated Soils

“…Moisture was adjusted to 0.03 bar, 0.3 bar, 1 bar, and 3 bar, respectively. MeBr was applied as a liquid at a concentration of 2,733 μg MeBr/g soil (594 g/m 3 ), which represented a typical structural fumigation rate. This rate was used instead of a typical field fumigation rate because of the difficulty of applying small quantities of this highly volatile compound.…”

“…While the quantity of MeBr released into the atmosphere from natural sources is estimated to be 75%, anthropogenic emissions (primarily through fumigation, chemical manufacturing, and car exhausts) also contribute significantly to the global MeBr budget (2,3). Large quantities of field-applied MeBr (> 80%) have been shown to volatilize into the atmosphere (4).…”

Atmospheric methyl halides and dimethyl sulfide from cattle

“…Equation (1) is more accurate for gases with Tr greater than the time constant for exchange between the Northern and Southern Hemispheres, which is about 0.8 year [Prather et al, 1987]. The most reliable quantity in (1) is the atmospheric burden: 145 Gg of CH3Br (1 Gg = 10 9 g) and 5000 Gg of CH3C1 (these data are from Butler [1994], Khalil et al [1993], and Singh et al [1983] and are probably accurate to within Gg yr -x [Yvon-Lewis and Butler, 1997]. Alternatively, current sink estimates may be too large; for relevant data and discussion of individual sources and sinks, see Yvon and Butler [1996] and Butler and Rodriguez [1996].…”

Large carbon isotope fractionation associated with oxidation of methyl halides by methylotrophic bacteria

“…Both compounds have natural and human-influenced sources and a predominant sink by reaction with OH in the troposphere (2)(3)(4). MeBr also has a bacterial soil sink (5) that represents about 20% of the estimated total removal from the troposphere, and it is likely that a soil sink of similar magnitude exists for MeCl (6).…”