Stratospheric concentrations

Quantitative understanding of the sources, sinks and atmospheric lifetime for CHa is an important future goal for several reasons. The direct increase in tropospheric CHa concentrations adds another important infrared absorbing contributor to the greenhouse effect. The calculated contribution from a CHa increase of 0.18 ppmv in a decade is a tropospheric temperature increase of 0.04 C [N.A.S., 1983], about 1/3 as large as that calculated for the observed 12 ppmv increase for CO2 over the decade from 1970-1980. As described earlier, increasing concentrations of CHa in the stratosphere have an influence on ozone-depletion by ClOx through diversion of Cl into HCl, and should in addition after oxidation increase the upper stratospheric concentrations of H2O. Methane is also a participant in tropospheric chemical reaction sequences which lead under some conditions to the formation of ozone. 

Carli B., Comparison of current models of OH stratospheric concentration with far-infrared emission measurements. Starnberger See, West Germany, 11-16 January 1984. 

Stratospheric concentrations of HF have been calculated from the photolysis rates and vertical transport coefficients of CF Cl, CFClj, COF and COCIF [1994]. The atmospheric production rate for COF was assumed to be equal to the photodecomposition rate of CF2d2 whereas its removal was assumed to be effected by reaction with 0( D) and by photolysis. The model predicts that HF is the most abundant fluorine-containing species in the stratosphere, with concentrations of COFj (and of COCIF) calculated to be appreciably less than that of HF. In addition, the model also predicts that the maximum concentration of COF 2 occurs in the stratosphere at about 30 km above sea level [1994]. 

In many respects, N2O is analogous to CO2. It has the same linear structure, the same number of electrons (isoelectronic), and a similar (low) reactivity however, CO2 is more soluble in water as a result of the acid-base reaction of CO2 and water. It is the low reactivity of N2O that results in a long tropospheric lifetime, and therefore its eventual transport to the stratosphere, where it is believed to be a primary control on the concentration of ozone in the stratosphere. Concentrations of N2O in the troposphere are increasing, and this has raised concerns that anthropogenically produced N2O could decrease stratospheric ozone concentrations (McEIroy et al., 1976 Soderlund and Svensson, 1976 Weiss, 1981). 

As in the case of nitrogen species, the value of theoretical predictions can be evaluated by comparing the measured stratospheric concentrations of halocarbons with calculated profiles. The results of stratospheric halocarbon analyses have been recently reviewed by Volz el al. (1978). Their data show that halocarbon levels decrease rather rapidly in the stratosphere with increasing altitude. The rate of this decrease is comparable to the theoretical value. 

Since its discovery the existence of the stratospheric aerosol layer has been proved by many investigators (e.g. Mossop, 1965 Friend, 1966 Kondratyev et ai, 1969). A mathematical model of the particles in the aerosol layer, constructed by Friend (1966), led to a size distribution with a maximum in the vicinity of 0.3 fim particle radius. However, according to the results of more recent measurements by Bigg (1976) the actual distribution has its maximum at smaller sizes. The observations of Kondratiev et al. (1974) show that the stratospheric concentration of aerosol particles with radii larger than 0.2 /an may be as great as 1 cm-3. However, this concentration is strongly time dependent (Bigg, 1976) as we shall discuss in Subsection 4.4.3. 

Figure 5.36. Principal chemical reactions affecting nitrogen species in the stratosphere. Concentrations (cm-3) and reaction fluxes (cm-3s-1) between different compounds calculated at 25 km altitude (24-hour average) are indicated. After Zellner (1999).

Murcray, D.G., D.D. Barker, J.N. Brooks, A.Goldman, and W.J. Williams, Seasonal and latitudinal variations of the stratospheric concentration of HNO-j. Geophys Res... 

FIGURE 5.32 Stratospheric concentration profiles of key species involved in 03 loss catalytic cycles (35°N, Sept.). 

It turns out that the HO NO, and CIO, cycles are all coupled to one another, and their interrelationships strongly govern stratospheric ozone chemistry. The NO, and CIO, cycles are coupled by reactions 4.34 and 4.35. For example, increased emissions of N2O would lead to increased stratospheric concentrations of NO and hence increased ozone depletion by the NO, catalytic cycle. Likewise, increasing CFC levels will lead to increased ozone depletion by the CIO, cycle. However, increased NO, will lead to an increased level of the CIONO2 reservoir and a mitigation of the chlorine cycle. Thus the net effect on ozone of si-... 

Figure 2 shows the evolution of the stratospheric concentration in chlorine atoms or equivalent species (for brominated compounds) since the period when the hole in the... 

Figure 2 Scenario of the variation of the stratospheric concentration in chlorine atoms or equivalents.

Studies indicate that the Montreal Protocol has been effective to date. The 2002 Scientific Assessment of Ozone Depletion shows that the rate of ozone depletion is slowing. Stratospheric concentrations of methyl chloroform are falling, indicating that emissions have been reduced. Concentrations of other ozone-depleting substances, such as CFCs, are also decreasing. It takes years for these substances to reach the stratosphere and release chlorine and bromine atoms. For this reason, stratospheric chlorine levels are still near their peak, but they are expected to decline slowly in years to come. If all parties to... 

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