methyl bromide

Methyl Bromide

The Crop Protection Coalition (CPC) suggests that the scientific foundation for the phase out of methyl bromide (CH3Br) may not be justified. The implication of the CPC is that the effects of CH3Br on stratospheric ozone is likely to continue to decrease because there was a decreasing effect noted as the science progressed through the 1990s. However, the science community completed a major new assessment of stratospheric ozone (Scientific Assessment of Ozone Depletion: 2002; available through the World Meteorological Organization in Geneva) where little difference was found relative to the findings of the 1998 assessment determination of the Ozone Depletion Potential (ODP) for methyl bromide. This finding was not surprising. CH3Br still accounts for roughly 50% of the organic bromine reaching the stratosphere. Determination of the ODP for methyl bromide primarily depends on the understanding of its total lifetime in the atmosphere, which in turn depends on the understanding of its removal processes from the atmosphere. During the 1990s, it was discovered that ocean and soil sinks for methyl bromide need to be considered in determining the lifetime of methyl bromide. Those sinks are now considered in our analyses (a recent reevaluation of the ocean sink gave results very similar to the analyses used in the 1998 assessment). While there remains some uncertainty in the soil sink, this is unlikely to further change the ODP for methyl bromide in any significant manner. With ocean and soil removal processes now included in the analyses of its ODP, it is unlikely that there will be major changes in the ODP for methyl bromide unless there are significant changes to the understanding of its removal processes. The current evaluated global lifetime for CH3Br is 0.7 years, resulting in an ODP of 0.38. If all of the uncertainties in the soil sinks were to go the maximum amount towards increasing the sink, the ODP for CH3Br could decrease a little more. However, this is extremely unlikely, as it would produce a huge inconsistency between the sources and sinks, exacerbating the difference already existing in the known sources and sinks. The bottom line is that the ODP for methyl bromide is unlikely to change further.

As chlorine in the stratosphere decreases under the effects of the Montreal Protocol, the fact that stratospheric bromine is currently not decreasing means that bromine will become more and more important in its effects on ozone.