Stratospheric chemistry particles

Heterogeneous chemistry occurring on polar stratospheric cloud particles of ice and nitric acid trihydrate has been estabUshed as a dorninant factor in the aggravated seasonal depletion of o2one observed to occur over Antarctica. Preliminary attempts have been made to parameterize this chemistry and incorporate it in models to study ozone depletion over the poles (91) as well as the potential role of sulfate particles throughout the stratosphere (92). 

K. R. Chan, Chlorine Chemistry on Polar Stratospheric Cloud Particles in the Arctic Winter, Science, 261, 1130-1134 (1993a). 

Peter, T., Microphysics and chemistry of polar stratospheric cloud particles. Ann Rev Phys Chem 48, 785, 1995. 

L. Lait, M.R. Schoeberl, J.W. Elkins,and K.R. Chan, Chlorine chemistry on polar stratospheric cloud particles in the Arctic winter. Science 261, 1130, 1993. 

A related issue associated with an eventual fleet of supersonic aircraft is the particulate Emission Index. If the El of small particles measured by Fahey et al. (1995) in the exhaust of the Concorde is characteristic of the proposed HSCT fleet, significant increases in particle number and surface area would be likely to occur in the lower stratosphere, with concomitant effects on stratospheric chemistry. 

It has become clear only recently that the atmospheric sierosol plays an important role for the climate on earth. It is common to distinguish between direct and indirect effects of the aerosols on the climate. Aerosols effect directly the radiation balance of the earth due to scattering and absorption of electromagnetic radiation (radiative forcing). On the other hand they influence the physics and chemistry of the atmosphere as condensation nuclei for cloud droplets and their chemical reactions with atmospheric trace gases. Though these indirect aerosol effects are difficult to quantify, they are at least as important as the direct radiative forcing. An especially important and complex example for the indirect influence of aerosols on the chemistry and radiation balance of the earth is the role of stratospheric aerosol particles on the polar ozone depletion, which is discussed in more detail below. 

The Knudsen cell reactor has been used successfully to measure reaction and uptake rates on solid and liquid surfaces, including ice, nitric acid trihydrate, soot and concentrated sulfuric acid [8,9,16,26,27]. Recent measurements of uptake and reactivity on soot surfaces are particularly intriguing. In these experiments, funded by NASA s Subsonic Assessment Program, we are investigating the impact of solid particles found in the exhaust of aircraft, i.e., soot, on stratospheric chemistry. 

Recently was estimated an expected impact on the global chemistry of the atmosphere of the indirect heterogeneous photocatalytic reactions under the much more abundant near ultraviolet, visible and near infrared solar light [2]. As photocatalysts may serve atmospheric aerosols, i.e. ultrasmall solid particles that sometimes are embedded into liquid droplets. Aerosols are known to contain Ti02, Fc203, ZnO and other natural oxides, as well as metal sulfides of volcanic or antropogenic origin, that may serve as semiconductor photocatalysts (see Fig.5). Aerosols are known to be concentrated mainly in the air layers near the surface of the Earth, i.e. in the troposphere, rather than stratosphere. 

Most of the research to date has focused on aerosols and PSCs containing inorganic species such as nitric and sulfuric acids. While CH4 is the only hydrocarbon that is sufficiently unreactive in the troposphere to reach the stratosphere, it is oxidized to compounds such as HCHO that can be taken up into sulfuric acid particles (Tolbert et al., 1993). The effects of such uptake and subsequent chemistry are not well established. 

The finding that the heterogeneous chemistry that occurs on polar stratospheric clouds also occurs in and on liquid solutions in the form of liquid aerosol particles and droplets in the atmosphere provided a key link in understanding the effects of volcanic eruptions on stratospheric ozone in both the polar regions and midlatitudes. As discussed herein, the liquid particles formed from volcanic emissions are typically 60-80 wt% H2S04-H20, and hence the chemistry discussed in the previous section can also occur in these particles (Hofmann and Solomon, 1989). We discuss briefly in this section the contribution of volcanic emissions to the chemistry of the stratosphere and to ozone depletion on a global scale. For a brief review of this area, see McCormick et al. (1995). 

Additional sulfates continue to form after the eruption as gaseous S02 is oxidized to sulfuric acid and sulfates. While we shall focus here on the effects of these sulfate particles on the heterogeneous chemistry of the stratosphere, there may be other important effects on the homogeneous chemistry as well. For example, model calculations by Bekki (1995) indicate that this oxidation of S02 by OH leads to reduced OH levels, which alters its associated chemistry. 

The chemistry in the midlatitude stratosphere follows that discussed throughout this chapter. As seen in the previous sections, the heterogeneous chemistry that was once thought to be unique to PSCs also occurs in and on the liquid solutions characteristic of sulfate particles distributed globally, with their relative importance being determined by the temperature, composition, and phase of the condensed phase. 

Carslaw, K. S., M. Wirth, A. Tsias, B. P. Luo, A. Dombrack, M. Leutbecher, H. Volkert, W. Renger, J. T. Bacmeister, and T. Peter, Particle Microphysics and Chemistry in Remotely Observed Mountain Polar Stratospheric Clouds, J. Geophys. Res., 103, 5785-5796 (1998b). 

Understanding the chemical and physical processes discussed throughout this book is key to the development of cost-effective and health-protective air pollution control strategies. Application of atmospheric chemistry to reducing stratospheric ozone depletion was discussed in Chapter 13. Here we focus on its key role in strategies for controlling tropospheric pollutants, including ozone, acids, particles, and hazardous air pollutants. 

The importance of heterogeneous processes in the chemistry of stratospheric ozone has been dramatically illustrated by the annual appearance of the "Ozone Hole" during the Antarctic spring [1-4]. Heterogeneous reactions on particle surfaces in the polar... 

The reason for the dehydration and denitrification of the Antarctic stratosphere is the formation of the PSCs, whose chemistry perturbs the composition in the Antarctic stratosphere. Polar stratospheric clouds can be composed of small (< 1 pm diameter) particles rich in HNO3 or at lower temperatures (<190 K) larger (10 pm) mainly ice particles. These are often split into two categories, the so-called Type 1PSC, which contains the nitric acid either in the form of liquid ternary solutions with water and sulfuric acid or as solid hydrates of nitric acid, or Type II PSCs made of ice particles. The ice crystals on these clouds provide a surface for reactions such as... 

Greshake A., Hoppe P., and Bischoff A. (1996) Mineralogy, chemistry, and oxygen isotopes of refractory inclusions from stratospheric interplanetary dust particles and micrometeorites. Meteorit. Planet. Sci. 31, 739-748. 

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