What Has Been Observed Recently About The Levels Of Ozone Depleting Compounds In The Atmosphere?


Shuttle Data Show Long-Term Increase In Concentrations Of Stratospheric Halogens

Observations of hydrogen chloride (HCl) and hydrogen fluoride (HF) in the upper stratosphere and lower mesosphere (altitude greater than 50 km) were carried out by the Atmospheric Trace Spectroscopy (ATMOS) sensor on the ATLAS-1 mission in March-April, 1992, and compared to measurements by the ATMOS instrument on the Spacelab-3 flight in April-May, 1985. At these altitudes, nearly all of the chlorine and fluorine atoms in the stratosphere are found in the form of HCl and HF, respectively, making these molecules good indicators of the total burdens of halogens in the stratosphere. Mixing ratios of 3.44 and 1.23 parts per billion by volume (ppbv) were obtained, corresponding to a 37% and 62% increase since the Spacelab-3 flight. The derived trend in HCl is in good agreement with the model-predicted increase in chlorine loading of 0.13 ppbv/year. This consistency lends further support to previous observations that the Mt. Pinatubo eruption did not deposit significant chlorine into the stratosphere. The fluorine increase can only be interpreted as being due to anthropogenic sources, such as CFCs, HCFCs, and halons.
Reference: Increase in Levels of Stratospheric Chlorine Loading Between 1985 and 1992, Gunson, M. R., et al., Geophysical Research Letters, Vol. 21, pp. 2223-2226, 1994.

The Rate Of Increase Of CFCs In The Atmosphere Is Slowing

Global atmospheric measurements continue to confirm decreases in the growth rates of the ozone depleting chemicals CFC-11, CFC-12, and several halon compounds. The rates of increase in total organic chlorine and bromine in the troposphere have slowed significantly over the past few years. These measurements indicate that actions taken in response to the Montreal Protocol and its amendments are having the desired effect. Total stratospheric chlorine/bromine loading is expected to peak in the late 1990s, followed by a slow recovery of the ozone layer over the next few decades.
Reference: Decrease in the Growth Rates of Atmospheric Chlorofluorocarbons 11 and 12, Elkins, J. W., et al. Nature, Vol. 364, pp. 780-783, 1993.

First Global Distribution Of Stratospheric Hydrogen Fluoride Determined

Hydrogen fluoride (HF) is the end breakdown product of fluorine which reaches the stratosphere from industrial gases such as chlorofluorocarbons (CFCs) and hydrogen-containing CFCs (HCFCs). There are no natural sources of stratospheric hydrogen fluoride, so its presence in the stratosphere is a clear manifestation of the ability of CFCs and related molecules to reach the stratosphere. The Halogen Occultation Experiment (HALOE) instrument on UARS provided the first-ever global distribution of hydrogen fluoride in the stratosphere. Previous measurements of this quantity were exceedingly limited in spatial and temporal coverage, as well as vertical resolution (aircraft and ground measurements obtain only total column). The dynamical signatures in the observed HF fields show that in much of the atmosphere it serves as a conserved tracer. Amounts observed are significantly higher than those observed from space by the Atmospheric Trace Spectroscopy (ATMOS) instrument in 1985, corresponding to an exponential growth rate of 4.9-6.6% /year.
Reference: Observations of Stratospheric Hydrogen Fluoride by the Halogen Occultation Experiment (HALOE), Luo, M., R. Cicerone, J. M. Russell III, and T. Y. W. Huang, Journal of Geophysical Research, Vol. 99, pp. 16,691-16,705, 1994.

Biomass Burning Found To Be A Significant Source Of Methyl Bromide, An Efficient Depletor Of Stratospheric Ozone

Human initiated biomass burning is estimated to be a significant source of the emissions of methyl bromide globally. Because a bromine atom is about 40 times as efficient as a chlorine atom in destroying stratospheric ozone, understanding its emission sources is critical. Other major anthropogenic sources of methyl bromide include soil fumigation and exhaust from automobiles using leaded gasoline. Further research is needed to major uncertainties in quantifying methyl bromide sinks. Recent amendments to the Montreal Protocol call for a January 1, 1995 freeze in methyl bromide production at 1991 levels.
Reference: Emission of Methyl Bromide from Biomass Burning, Mano, S. and M. O. Andreae, Science Vol. 263, pp. 1255-1257, 1994.

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Last updated 04/10/96