Determination of Atmospheric Methyl Bromide by Cryotrapping-Gas Chromatography and Application to Soil Kinetic Studies Using a Dynamic Dilution System

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Abstract
Methyl bromide (CH3Br) is considered to be a major source of stratospheric Br, which contributes to the destruction of ozone. It is therefore necessary to understand the natural sinks of this compound and to accurately measure ambient mixing ratios. Methodology is described for the measurement of atmospheric CH3Br by cryotrapping-gas chromatography and its application to soil kinetics. A 2-propanol/dry ice cryotrap was used to preconcentrate CH3Br in standard and air samples, with subsequent detection using a gas chromatograph equipped with an O2-doped electron capture detector (GC-ECD). The GC-ECD cryotrapping method had a detection limit of 0.23 pmol of CH3Br. This is equivalent to the amount of CH3Br in a 500 mL sample of ambient air at the estimated northern hemisphere atmospheric mixing ratio of 11 parts per trillion by volume (pptv). A dynamic dilution system was developed to produce mixing ratios of CH3Br ranging between 4 and 1000 pptv. Calibrated mixing ratios of CH3Br produced with the dilution system were used to determine soil uptake kinetics employing a dynamic soil incubation method.

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