Methyl bromide (CH₃Br) is a pesticide used widely in California by growers of strawberries, tomatoes, and other produce. Although it is highly effective as a pesticide, it is also an ozone-depleting substance, or ODS. The Montreal Protocol, a United Nations treaty signed by more than 140 countries, stipulated a ban on production of this chemical in industrialized countries by the year 2001. (1) Recently, President Clinton vetoed a bill that would have postponed the ban until 2005.

Satellite images compare the ozone levels over the southern hemisphere in 1988 and 1998. Lower numbers of Dobson units represent lower levels of ozone. (Image courtesy of the Ozone Processing Team at NASA’s Goddard Space Flight Center.)

Approximately 76,000 tons of methyl bromide, a colorless gas, are produced each year.(1) Before planting, farmers inject the gas 12 to 24 inches underground, which kills off virtually all soil organisms. But some of the gas eventually escapes into the atmosphere, where it can react with ozone.

Found in the earth’s outer atmosphere, or stratosphere, ozone (O₃) is a protective layer of gas that absorbs ultraviolet (UV) radiation from the sun. Ozone forms when an oxygen molecule (O₂) absorbs solar radiation at wavelengths less than 200 nm and splits into two oxygen atoms. These atoms in turn react with oxygen molecules to form ozone, as shown in the equations:

I. O₂ 2 O
(absorbs radiation less than 200 nm and splits)

II.O + O₂ O₃
(formation of ozone)

The atmosphere contains three types of UV radiation:

UV-A, wavelength 320 to 400 nm
UV-B, wavelength 290 to 300 nm
UV-C, wavelength 200 to 290 nm.(Note: 1 nanometer (nm) equals 1 billionth of a meter.)

The shorter the wavelength, the more energy it gives off, so UV-C is the most harmful radiation. Ozone absorbs about 98% of the UV-B and UV-C radiation, which splits the molecule into an oxygen atom and an oxygen molecule. The oxygen atom then reacts with another ozone molecule to form two oxygen molecules.

III. O₃ O + O₂
(absorbs radiation from 200 to 300 nm)

IV.O + O₂ 2 O₂
(oxygen formed again)

The global exchange between ozone and oxygen amounts to about 300 million tons per day. It is reaction III that protects the earth from high levels of harmful UV radiation.(2

)

The best-known class of ODS is the chlorofluorohydrocarbons (CFCs), also known as freons. The Montreal Protocol banned production of these compounds in industrialized countries in 1996. (1) One mechanism for the depletion of ozone involves the absorption of UV radiation less than 260 nm, producing free chlorine atoms. These chlorine atoms are catalysts in the sequence of reactions shown below, which means the chlorine atom is capable of reacting again and again, reducing the ozone level further.(2) The chlorine atom produced in equation I reacts with an ozone molecule to produce an intermediate, ClO, and oxygen gas, O₂. The intermediate, ClO, in turn reacts with atomic oxygen to produce another Cl and oxygen gas, O₂. Note that the intermediate has only a transient existence.

I. CCl₂F₂ CClF₂ + Cl
(CFC absorbs radiation of less than 260 nm and forms Cl atom)

II.Cl + O₃ ClO + O₂
(Cl atom reacts with ozone producing intermediate and oxygen, break down of ozone)

III.ClO + O Cl + O₂
(ClO intermediate reacts with atomic O to produce Cl atom which can react with O₃ again)

The net result of equations II and III is the depletion of ozone: O + O₃ 2 O₂.

Methyl bromide reacts in a similar way as CFCs except bromine atoms act as the catalyst instead of chlorine atoms. It is estimated that methyl bromide is about 10 to 100 times more effective than CFCs in depleting ozone, since the intermediate BrO is more easily split.(2) One possible replacement of methyl bromide is methyl iodide, which does not act as an ozone depleting substance. Other chemicals may replace methyl bromide in special applications as in fumigation of imported fruits.

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