Stratosphere chlorine

An additional area of concern with respect to stratospheric ozone is possible direct emissions of NOj into the stratosphere by high-flying supersonic aircraft. This issue has come up repeatedly over the past 20 years, as air travel and pressure from commercial airlines has increased. However, despite substantial research effort to understand stratospheric chemistry, the question is complicated by the changing levels of stratospheric chlorine, first due to a rapid accumulation of tropospheric CFCs, followed by a rapid decline in CFC emissions due to the Montreal Protocol. To quote from the from the 1994 WMO/UN Scientific assessment of ozone depletion, executive summary (WMO 1995) ... 

De Haan, D. O., I. Flpisand, and F. Stordal, Modeling Studies of the Effects of the Heterogeneous Reaction CIOOCI + HCI - Cl2 + HOOCI on Stratospheric Chlorine Activation and Ozone Depletion, J. Geophys. Res., 102, 1251-1258 (1997). 

N. Kampfer, H. A. Michelsen, M. J. Newchurch, C. P. Rinsland, R. J. Salawitch, G. P. Stiller, and G. C. Toon, The 1994 Northern Midlatitude Budget of Stratospheric Chlorine Derived from ATMOS /ATLAS-3 Observations, Geophys. Res. Lett., 23, 2357-2360 (1996). 

However, about this time, a variety of research indicated that even with full implementation of the Montreal Protocol, the atmospheric abundance of chlorine could reach as much as 6-9 ppb between the years 2050 and 2075. This delay is due to the relatively long time between emission of these compounds into the troposphere and when they reach the stratosphere and photolyze to produce an active chlorine atom. Figure 13.1, for example, compares the estimated equivalent effective stratospheric chlorine from 1960 until the year 2100 with no controls and a 3% increase per year in CFC and methylchloroform emissions to those with the controls agreed to in the Montreal Protocol. Equivalent effective stratospheric chlorine loading depends on emissions as well as removal processes, which determine what fraction of the CFCs emitted at the earth s... 

FIGURE 13.1 Estimated equivalent effective stratospheric chlorine for a continued 3% growth per year, for controls contained in the Montreal Protocol, and for those in the Copenhagen amendments (adapted from World Meteorological Organization, 1995). 

While the growth in stratospheric chlorine should clearly be slowed by the Montreal Protocol agreements, it was still substantial and expected to lead to quite large losses of ozone. This recognition, bolstered by the dramatic appearance of the Antarctic ozone hole, led to further major amendments to the Montreal Protocol. 

Figure 13.1 also shows the estimated equivalent effective stratospheric chlorine content as a function of year under the Copenhagen Amendments. Chlorine is expected to peak around the year 2000 and then decrease. As discussed by Holmes and Ellis (1996), non-compliance and allowed exemptions could lead to a much slower decline than projected in Fig. 13.1. 

It is clear from the data presented in this chapter that the effects of control strategies developed for CFCs and halons are already measurable. Although loss of stratospheric ozone with accompanying increases in ultraviolet radiation in some locations have clearly occurred, the tropospheric concentrations of CFCs are not increasing nearly as fast as in the past. Indeed, the concentrations of CFC-11 and CFC-113 appear to have peaked and have started to decline. The equivalent effective stratospheric chlorine concentrations are predicted to have peaked about 1997 and to return to levels found around 1980 at about the year 2050 (World Meteorological Organization, 1995). The significance of the 1980 level is that these levels resulted in detectable Antarctic ozone depletion. 

Only a small percentage of the chlorine released by photolysis of CFCs is present in the active forms as Cl or CIO, however. Most of it is bound up in reservoir compounds such as hydrogen chloride and chlorine nitrate, formed respectively by hydrogen abstraction (equation 10) from methane and addition (equation 11) to nitrogen dioxide. Slow transport of these reservoir species across the tropopause, followed by dissolution in tropospheric water and subsequent rain-out, provide sink processes for stratospheric chlorine. 

Theoretical and experimental studies of the interactions between water molecules and hydrogen chloride are of fundamental importance for the understanding of the production of stratospheric chlorine molecules which, in turn, take part in the catalytic ozone depletion reactions. This mainly heterogeneous atmospheric reaction begins with the adsorption of the HCl molecules on the surface of water icicles is the source of the stratospheric chlorine atoms in the polar regions380 - 382. Chlorine molecules are photolysed by solar radiation and the resultant chlorine atoms take part in the destruction of the stratospheric ozone. The study of the (H20) HC1 clusters is an important step towards understanding of the behavior of the HCl molecule on the ice surface383- 386. 

Fig. 16. The column abundance of ozone as a function of stratospheric chlorine for three assumed mixing ratios (by volume) of NO, - NO + NO2 + HONO2 taken from Prather et al.

Tabazadeh A. and Turco R. P. (1993) Stratospheric chlorine injection by volcanic eruptions HCl scavenging and implications for ozone. Science 260, 1082-1086. 

Bass, A.M., L.C. Glasgow, C. Miller, J.P. Jesson, and D.L. Filkin, Temperature dependent cross sections for formaldehyde [CH2O] The effect of formaldehyde on stratospheric chlorine chemistry. Planet Space Set 28, 675, 1980. 

Ko, M.K.W., N.D. Sze, C.J. Scott, and D.K. Weisenstein, On the relation between stratospheric chlorine/bromine loading and short-hved tropospheric source gases. J Geophys Res 102, 25,507, 1997. 

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