Atmosphere And Weather

Air Pollution Problems and Clean Air Gains since the 1960s

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Montreal makes good.

Whisper it if you will, but at least tell someone: an international environmental commitment is in serious danger of being fulfilled.

If researchers from the Japanese National Institute for Environmental Studies and the Meteorological Service of Canada are right, atmospheric levels of the ozone-depleting compound methyl bromide are steadily decreasing, and are even doing so at the rate required by the Montreal Protocol, the international agreement to phase out substances that deplete the ozone layer.

The chemistry of methyl bromide and ozone
In 1975, researchers from Harvard University suggested that methyl bromide could be breaking down ozone molecules in the upper atmosphere, a role in which CFCs (chlorofluorocarbons) had been implicated the previous year. If this breakdown reduced ozone levels, it could lead to an increase in UV-B rays reaching the earth's surface and a corresponding rise in skin ageing, cataracts and skin cancer.
Ozone (O3, as opposed to "normal" oxygen, O2) is constantly being created and destroyed in photochemical reactions about 20 to 30 km above the earth's surface, in the stratosphere. One mechanism by which ozone is destroyed occurs when molecules containing bromine are broken up, and lone bromine atoms react with ozone to break it down in to normal oxygen and bromine monoxide (BrO). The bromine monoxide can then be broken down, and this frees up the bromine atom to destroy another ozone molecule, and so on. The catalytic nature of this process allows one ozone-destroying molecule to destroy a large amount of ozone. Bromine-containing molecules behave in a similar way to chlorinated molecules such as CFCs. Whilst CFCs are entirely man-made, other compounds such as methyl chloride and methyl bromide have both natural and artificial sources. Natural sources of these compounds were thought to include the oceans and biomass burning, such as the burning of forests in clearance to make way for agriculture. However, some recent data has pointed to the oceans being a sink rather than a source of methyl bromide, that is they may absorb more methyl bromide than they emit to the atmosphere. The main artificial source of methyl bromide is its use as a fumigant, which accounts for about 80% of artificial emissions. Leaded petrol is also thought to be a source. Experiments on the use of methyl bromide as a soil fumigant revealed that as much as a third was lost fairly rapidly to the atmosphere. The total manmade output of methyl bromide is estimated to be 45.8 Gg/year (a gigagram is 1,000,000,000 g, or 1,000 tonnes), but this estimate carries a high degree of uncertainty. This, coupled with the uncertainty regarding natural emissions (somewhere between 112 and 454 Gg/year), according to the World Meteorological Organisation, means that we are contributing somewhere between 10% and 41% of the total amount, with a best guess of 22%. Other researchers have suggested best guesses of 40% in the northern hemisphere and 25% in the southern hemisphere. This is due to the fact that most sources are in the north, and it takes time for the methyl bromide to spread globally.

The international response
The 1987 Montreal Protocol on Substances that Deplete the Ozone Layer committed signatories to reduce the use of several CFCs to 50% of 1986 levels by 1999. This was followed by several other protocols London (1990), Copenhagen (1992), Vienna (1995), Montreal again (1997) and Beijing (1999), which all widened the scope of reductions and increased the number of signatory nations. These protocols now commit developed countries to a 70% reduction from 1990 levels in methyl bromide by this year (2003), and a total stop by 2005. Methyl bromide has a short lifetime (0.3-1.3 years) in the atmosphere, and so the reductions in use should quickly lead to lower atmospheric levels.

Where are we now?
A paper in Nature in 1999, by Butler and co-workers, estimated that in the early part of the twentieth century methyl bromide concentrations in the atmosphere were rising by about 0.01 pptv/year (parts per trillion by volume). It was thought that this increase was due to biomass burning, such as the burning of forests, or by some kind of global change. By the 1970s, this rate had increased to around 0.05 pptv/year.
Atmospheric samples were taken between 1996 and 2002 at the Arctic baseline observatory in Alert, Canada and in the troposphere above Sagami Bay, Japan, between 4km and 7km above the ground. This was high enough to avoid any direct pollution from the industrial area below.
At Alert, methyl bromide declined steadily, although it did show a pronounced seasonal cycle. The annual average dropped from 10.3 pptv in 1997 to 8.6 pptv by 2001. Data from other researchers during the 1990s agree with this, with figures from Alaska, the east and the northwest Pacific showing very similar values. Data from Alaska also showed seasonal fluctuations, which are thought to be caused by the seasonality of OH radicals, which can react with methyl bromide. Data from Sagami Bay also showed a downward trend, although without the seasonal cycles. The average figure in 1999 was 10.5 pptv, which fell to 9.2 pptv in 2001. Other data from the region showed that these were lower concentrations than in the middle of the 90s.
Yokouchi and colleagues have showed with this data that methyl bromide in the troposphere of the northern hemisphere has been declining by around 5% a year since the middle of the 1990s. But was this decline what would have been expected if countries were phasing out methyl bromide use on schedule? Unfortunately, because the atmospheric lifetime of methyl bromide is not precisely known, expected reductions would vary greatly depending on the lifetime assumed. With this in mind, the researchers modelled the expected decline in concentrations using a range of lifetimes, from 0.3 to 1.4 years. The results suggest that we are doing as well as or better than the Montreal Protocol requires. If we assume that the lifetime is 1 year, which is the best estimate based on data from Antarctic ice, then the Alert data fits very well indeed. Using the WMOs best guess of 0.7 years, then we are actually ahead of schedule in the phase out of methyl bromide. By 2005, the levels of methyl bromide in the northern hemisphere will have decreased by about 40%, assuming that developing countries (with more lenient responsibilities in the Protocol) do not significantly increase their usage.

By both its own standards, and especially the standards we tend to set in other international environmental agreements, the Montreal Protocol is succeeding in phasing out at least this damaging compound, methyl bromide. But should we really be that worried about the damage we have caused to the ozone layer? UV-B radiation levels are much greater the closer you get to the equator, and so, for example, Madrid (40N) receives nearly twice as much UV-B as Edinburgh (55N). By contrast, the thinning of the ozone layer thus far has increased exposure levels by about 6%. Does this mean that the Montreal Protocol has been a waste of time? Not in the long run. Given the catalytic nature of the ozone-destroying process, the situation, although hardly a crisis at the moment, could have become a lot worse had nothing been done about it so soon.
It is problematic that the nature of the natural sources and sinks of methyl bromide are not fully understood. It is possible (though unlikely) that the decline we are seeing is due to an unexpected decline in natural emissions. Additionally, if possible future climate change increases natural emissions, we may well be back where we started.
The Montreal Protocol is often held up as a shining example of the international community rising to an environmental challenge. Whilst its success, at least at a diplomatic level, cannot be denied, it is worth noting that a major contributor to this success was the relative ease with which compounds could be replaced with less damaging ones. If ozone-depleting substances such as methyl bromide were as intimately bound up in the global economy as fossil fuels, for example, it is unlikely that we would be where we are today: a good way down the road to eliminating these compounds.

Further reading
This article based on a 2002 paper by Yokouchi et al., "Recent decline in methyl bromide in the troposphere" (Atmospheric Environment 36: 4985-4989). For bromine chemistry, try Wofsy et al. (1975), "The chemistry of atmospheric bromine" (Geophysical Research Letters 2: 215-218). Background material on the Montreal Protocol was derived from short sections in Environmental Science for Environmental Management by Tim O'Riordan (Prentice Hall) and The Skeptical Environmentalist by Bjorn Lomborg (Cambridge University Press).

Brimblecombe P (1986). Air composition and chemistry. Cambridge University Press.
Butler JH, Battle M, Bender ML, Montzka SA, Clarke AD, Saltzman ES, Sucher CM, Sevringhaus JP & Elkins JW (1999). A record of atmospheric halocarbons during the twentieth century from polar firn air. Nature 399: 749-755.
Lomborg B (2001). The Skeptical Environmentalist. Cambridge University Press.
Wofsy SC, McElroy MB & Yung YL (1975). The chemistry of atmospheric bromine. Geophysical Research Letters 2: 215-218.
Yagi K, Williams J, Wang N-Y & Cicerone RJ (1995). Atmospheric methyl bromide (CH3Br) from agricultural soil fumigations. Science 267: 1979-1981.
Yokouchi Y, Toom-Sauntry D, Yazawa K, Inagaki T & Tamaru T. Recent decline of methyl bromide in the troposphere. Atmospheric Environment 36: 4985-4989.

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