The Stratospheric Aerosol Layer

The stratosphere contains a natural aerosol layer at altitudes of 12-30 km. This aerosol is composed of small sulfuric acid droplets with size on the order of 0.2 pm diameter and at number concentrations of 1-lOcm-3. In the midlatitude lower stratosphere (about 16 km) the temperature is about 220 K, and the particles in equilibrium with 5 ppm H20 have compositions of 70-75 wt% H2SO4. As temperature decreases, these particles absorb water to maintain equilibrium at 195 K, they are about 40 wt% H2S04. 

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Thus, a seed layer of condensation nuclei may be formed around 30-35 km altitude. Even under conditions of a condensation-evaporation equilibrium, IN may be sufficiently efficient to maintain the stratospheric aerosol layer [39] (Fig. 9). 

If ions were, in fact, involved in stratospheric aerosol formation, a physical link between solar activity and the stratospheric aerosol layer may exist [39]. 

IwASAKA Y., Hayashida S., and Ono A., Increased backscattered light from the stratospheric aerosol layer after Mt. El Chichon eruption laser radar measurements at Nagoya (35 N, 137 E). Geophys. Res. Lett. , 10, 440-442 (1983). 

Castleman, A.W., Jr., Munkelwitz, H.R. and Manowitz, B., 1973. Contribution of volcanic sulfur compounds to the stratospheric aerosol layer. Nature, 244 345—346. 

Aneja VP, Overton JH Jr, Cupitt LT, et al. 1980. Measurements of emission rates of carbon disulfide from biogenic sources and its possible importance to the stratospheric aerosol layer. Chemical Engineering Communication 4 721-727. 

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. 

It was mentioned above that the modifications of the stratospheric aerosol layer can also induce climatic variations. For this reason the following part of this... 

Yet another problem that recently has been brought closer to a solution is the origin of the stratospheric aerosol layer discovered by Junge et al. (1961). The layer centers at altitudes near 20 km and consists mainly of particles representing a mixture of sulfuric acid and water. A number of... 

Castleman, Jr., A. W., H. R. Munkelwitz, and B. Manowitz (1974). Isotopic studies of the sulfur component of the stratospheric aerosol layer. Tellus 26, 222-234. 

On the basis of atmospheric measurements, Chin and Davis (1995) estimated the total quantity of OCS in the atmosphere to be 5.2 Tg, of which 4.63 Tg is in the troposphere and 0.57 Tg in the stratosphere. Based on the estimated global OCS source strength of 0.86 Tg yr 1, the global atmospheric lifetime of OCS is estimated to be about 6 years. We will return to the global cycle and chemistry of OCS in Chapter 5 in connection with the stratospheric aerosol layer. 

Notholt, J. et al. (2003) Enhanced upper tropical troposphere OCS Impact on the stratospheric aerosol layer, Science 300, 307-310. 

Ozone-Depleting Potential of Halocarbons 212 Effect of Aircraft Emissions on Stratospheric Ozone 215 Carbonyl Sulfide (OCS) and the Stratospheric Aerosol Layer 216... 

Figure 28.28 Correlation of (a) the particle extinction coefficient at 308 nm, (b) the back-scatter coefficient at 308 nm, (c) the extinction-to-back-scatter DIAL ratio, (d) the effective radius of the particle size distribution, (e) the particle surface-area and (f) the mass concentrations the data were recorded on 4 April 1992. The lidar signal profiles are smoothed with a height window of 600 m for the back-scatter coefficient and 2500 m in all other cases the error bars indicate the overall retrieval error. The optical depth of the stratospheric aerosol layer was 0.25. The dashed line indicates the tropopause. Adapted from Ansmann et al, J. Atmos. Sci., 1997, 54 2630, with permission ofthe American Meteorological Society...

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