Stratospheric chemical components

The key gas-phase reactions occurring in the stratosphere are generally known. Comprehensive reviews of kinetic data have led to general consensus on the rate parameters that should be used in stratospheric models (91). Nevertheless, discrepancies are stiU apparent when the chemical components of... 

Stratospheric ozone is in a dynamic equilibrium with a balance between the chemical processes of formation and destruchon. The primary components in this balance are ultraviolet (UV) solar radiation, oxygen molecules (O2), and oxygen atoms (O) and may be represented by the following reactions ... 

Ozone A molecule made up of three atoms of oxygen. It occurs naturally in the stratosphere and provides a protective layer shielding the Earth from harmful ultraviolet radiation. In the troposphere, it is a chemical oxidant, a greenhouse gas and a major component of photochemical smog. 

Globally, the oxides of nitrogen, NO (nitric oxide), NO2 (nitrogen oxide), and N2O (nitrous oxide), are key species involved in the chemistry of the troposphere and stratosphere. NO and N2O are produced mostly by microbial soil activity, whereas biomass burning is also an important source of NO. Nitric oxide is a species involved in the photochemical production of ozone in the troposphere, is involved in the chemical produaion of nitric acid, and is an important component of acid precipitation. Nitrous oxide plays a key role in stratospheric ozone depletion and is an important greenhouse gas, with a global warming potential more than 200 times that of CO2. 

That theory not only emphasized the importance of chlorine as a component of significant chemical importance to the unperturbed stratosphere, but also introduced the concept that the surface release of an inert and otherwise benign molecule, central to many aspects of our present society, could seriously deplete the concentration of a molecule... 

In this book air chemistry is defined as a branch of atmospheric science dealing with the atmospheric part of the biogeochemical cycle of different constituents. In other words this means that we will deal mainly with the atmospheric pathways of those components that are involved in the mass flow between the atmosphere and biosphere, as well as in chemical interactions between the air and the other media of our environment (soils, oceans etc.). It follows from this definition that, on the one hand, our discussion will be restricted to the troposphere and the stratosphere4 and, on the other hand, the photochemistry of the upper layers, the subject matter of the aeronomy (e.g. Nicolet, 1964), will be omitted. This separation of the (photo) chemistry of the lower (troposphere and stratosphere) and upper atmosphere makes it possible to give a more compact treatment of our problem, including the global anthropogenic effects due to the increase of air pollution. 

To investigate their chemical composition, stratospheric particles were collected by aircraft- and balloon-borne impactors. Elements with atomic numbers of 12-30 were identified in the samples by electron microprobe and X-ray fluorescence techniques. Table 24 summarizes the results obtained (Junge, 1963). In this table the sulfur is given as sulfate since further wet chemical analyses showed that the sulfur occurred as sulfate particles. In can be seen from the data tabulated that 89 % of the mass of the components identified is sulfate. Data also indicate that the quantity of NH4 is sufficient only to neutralize about one third of the sulfate ions. Further flights carried out over a wide range of latitudes (60° S-70° N) demonstrated that this sulfate layer can be observed everywhere in the stratosphere. 

Atmospheric measurements are also challenging because they must deal with low to extremely low concentrations of trace chemical species. The major components (>99.999%) of the lowest portions of the atmosphere (the troposphere up to 10 km in altitude and the stratosphere between 10 and 50 km) are molecular nitrogen, molecular oxygen, argon, water vapor, and carbon dioxide. Chemists will recognize that all of these species are very stable, strongly bonded molecules or atoms that are essentially inert gases at normal atmospheric temperatures (190-310 K). Indeed, without solar photons to break up selected molecules, atmospheric chemistry would be very dull indeed. 

The stratospheric aerosol is composed of an aqueous sulfuric acid solution of 60-80% sulfuric acid for temperatures from — 80 to — 45°C, respectively (Shen et al. 1995). The source of the globally distributed, unperturbed background stratospheric aerosol is oxidation of carbonyl sulfide (OCS), which has its sources at the Earth s surface. OCS is chemically inert and water insoluble and has a long tropospheric lifetime. It diffuses into the stratosphere where it dissociates by solar ultraviolet radiation to eventually form sulfuric acid, the primary component of the natural stratospheric aerosol. Other surface-emitted sulfur-containing species, for example, S02, DMS, and CS2, do not persist long enough in the troposphere to be transported to the stratosphere. 

PSCs therefore provide the surface on which the heterogeneous reactions occur. If the PSC is a solid particle, then its surface area is the relevant quantity for the reaction if a supercooled liquid, then the volume of the entire particle is accessible for the reaction. In either event, the heterogeneous reaction is fast, and, furthermore, the exact chemical composition of the PSC does not appear to exert an important influence on the reaction rate. However, the abundance of PSCs, their location and persistence, is likely a function of PSC composition. As noted above, denitrification is a necessary component of the heterogeneous catalytic cycle that is, the HNO3 must be removed from the system. Denitrification is accomplished if the PSC particles are sufficiently large to fall out of the stratosphere, carrying the absorbed HNO with them. The extent to which such fallout actually occurs is still uncertain. 

As described above, sulfuric acid solutions are the main component of the stratospheric aerosol and their state is of crucial importance for the rate of heterogeneous chemical reactions on their surface or in the bulk phase. 

Ice particles found within polar stratospheric clouds (PSCs) and upper tropospheric cirrus clouds can dramatically impact the chemistry and climate of the earth s atmosphere. The formation of PSCs and the subsequent chemical reactions that occur on their surfaces are key components of the massive ozone hole observed... 

A molecule may absorb electromagnetic (em) radiation and, in the process, break down into its atomic or molecular components. Unstable atoms and molecular fragments may also combine to form more stable molecules, disposing of their excess energy in the form of em radiation. These chemical reactions are called photochemical, and the process by which a photochemical reaction occurs is called photolysis. Photochemical reactions play very important roles in many aspects of environmental chemistry. Therefore, this book concludes with a brief account of some of the basic principles of photochemistry, which we will then apply to ozone in the Earth s stratosphere and the problem of the stratospheric ozone hole. 

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