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A.2.1.1 Studies on source, distribution, and reactions of methyl bromide and its alternatives in the atmosphere

[Contact Person]

Yokouchi Yoko
Senior Research Scientist
National Institute for Environmental Studies
Environment Agency
Onogawa 16-2. Tsukuba Ibaraki, 305 Japan
Tel: +81-298-50-2549 Fax: +81-298-50-2574
E-mail:yokouchi@nies.go.jp

Total Budget for FY1996-1998

45,924,000Yen (FY1998; 15,111,000 Yen)

Abstract

(1) Global distribution of methyl bromide obtained from cruise- and monitoring-studies showed a small decrease of atmospheric methyl bromide (CH₃Br) from mid- to high- latitude as well as a gradient between NH and SH with the ratio of 1.2-1.3 and occasional high concentration in the tropics. These data as well as modeled distribution of man-made CH₃Br, were used to determine the distribution of natural CH₃Br. The modeled distribution was highly dependent on the assumed lifetime of atmospheric CH₃Br. An almost equivalent distribution of natural CH₃Br in both hemispheres with tropical maximum around 6pptv was obtained for the CH₃Br lifetime of 1.3yr which gave man-made contribution of 4 pptv in NH and 2 pptv in SH, consistent with the previous trend study from Antarctic firn. (2) Photooxidation processes of chloropicrin, which is one of substitutes for methyl bromide, and their impact on photochemical ozone were investigated by using a 6-m³ photochemical chamber. Phosgene was obtained as one of the main products in the photolysis of chloropicrin with a yield of unity. Photochemical ozone formation was enhanced when chloropicrin was added to the propene/NO/air/photoirradiation system after the ozone concentration maximum had been reached. This implies that the ozone concentration in suburban and/or rural areas can increase if the amount of chloropicrin as a soil fumigant increases. Atmospheric loss processes of methyl isothiocyanate and propargyl bromide were also investigated.

[Key Words]

methyl bromide, distribution, sources, chloropicrin, photochemical ozone