Atmosphere with trace species

The simplest example of radical recycling is given by the atmospheric oxidation of carbon monoxide (there are no other important CO reactions in the air) ks 34 = 2.2 10  

H rapidly eombines with O2 to HO2 (reaction 5.1). Reaction (5.34) is the only known OH splitting reaetion. The mechanism goes via an additive intermediate  

Because reaction (5.34) is faster than OH + O3 (5.22), ozone decay is enhanced in the presence of carbon monoxide, an almost abundant gas (Chapter 2.8.2.6). This statement is only virtually in contradiction with the experience that biomass burning leads to O3 formation because CO (and organics as we will see later) in the presenee of NO results in net ozone formation. In competition with (5.23), HO2 

This concentration is the threshold between the ozone-depleting chemical regimes (in the very remote air) and ozone-producing regimes (in NO c air). Other important reactions of the HO2 radical (with the exception of H2O2 formation via reaction 5.24) are unknown. It also follows that H2O2 formation is favored (and vice versa) in low NO c and low O3 environments, which are relatively restrictive conditions. 

As mentioned at the beginning of Chapter 5.3.2.1, OH can be lost by several reactions. The first type is the abstraction of H and forming reactive intermediates without recycling radicals (but possible further radical consumption in subsequent reactions to the final oxidation product)  

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In order to estimate the extent of ozone depletion caused by a given release of CFCs, computer models of the atmosphere are employed. These models incorporate information on atmospheric motions and on the rates of over a hundred chemical and photochemical reactions. The results of measurements of the various trace species in the atmosphere are then used to test the models. Because of the complexity of atmospheric transport, the calculations were carried out initially with one-dimensional models, averaging the motions and the concentrations of chemical species over latitude and longitude, leaving only their dependency on altitude and time. More recently, two-dimensional models have been developed, in which the averaging is over longitude only. 

Many gases are mixtures of two or more species. The atmosphere, with its mixture of nitrogen, oxygen, and various trace gases, is an obvious example. Another ex-ample is the gas used by deep-sea divers, which contains a mixture of helium and oxygen. The ideal gas model provides guidance as to how we describe mixtures of gases. 

In order to calculate the steady-state concentration of ozone in the stratosphere, we need to balance the rate of production of odd oxygen with its rate of destruction. Chapman originally thought that the destruction was due to the reaction O + 03 —> 2O2, but we now know that this pathway is a minor sink compared to the catalytic destruction of 03 by the trace species OH, NO, and Cl. The former two of these are natural constituents of the atmosphere, formed primarily in the photodissociation of water or nitric oxide, respectively. The Cl atoms are produced as the result of manmade chlorofluorocarbons, which are photodissociated by sunlight in the stratosphere to produce free chlorine atoms. It was Rowland and Molina who proposed in 1974 that the reactions Cl + 03 —> CIO + O2 followed by CIO + O —> Cl + O2 could act to reduce the concentration of stratospheric ozone.10 The net result of ah of these catalytic reactions is 2O3 — 3O2. 

More information is needed about the surface emission and deposition of trace atmospheric species. These fluxes can often be best measured by the eddy correlation technique with fast chemical sensors in conjunction with micrometeorological instrumentation. As analytical techniques for trace species progress, fast and sensitive sensors are becoming available for field research. Consideration must be given to matching the chemical sensors to the eddy correlation technique. 

Measurements either from the ground or from satellites have been a major contribution to this effort, and satellite instruments such as LIMS (Limb Infrared Monitor of the Stratosphere) on the Nimbus 7 satellite (I) in 1979 and ATMOS (Atmospheric Trace Molecular Spectroscopy instrument), a Fourier transform infrared spectrometer aboard Spacelab 3 (2) in 1987, have produced valuable data sets that still challenge our models. But these remote techniques are not always adequate for resolving photochemistry on the small scale, particularly in the lower stratosphere. In some cases, the altitude resolution provided by remote techniques has been insufficient to provide unambiguous concentrations of trace gas species at specific altitudes. Insufficient altitude resolution is a handicap particularly for those trace species with large gradients in either altitude or latitude. Often only the most abundant species can be measured. Many of the reactive trace gases, the key species in most chemical transformations, have small abundances that are difficult to detect accurately from remote platforms. 

The fact that fine atmospheric particles are enriched in a number of toxic trace species has been known since the early 1970s. Natusch and Wallace (20, 21) observed that the fine particles emitted by a variety of high-temperature combustion sources follow similar trends of enrichment with decreasing particle size as observed in the atmosphere, and they hypothesized that volatilization and condensation of the trace species was responsible for much of the enrichment. Subsequent studies of a number of high-temperature sources and fundamental studies of fine-particle formation in high-temperature systems have substantiated their conclusions. The principal instruments used in those studies were cascade impactors, which fractionate aerosol samples according to the aerodynamic size of the particles. A variety... 

Peroxy radicals are intermediates in the atmospheric oxidation of air pollutants and in oxidation reactions at moderate temperatures. They are rapidly formed from free radicals by addition of 02. Free radicals in the atmosphere are quantitatively converted to R02 with a half-time of about 1 fis. The peroxy radicals are then removed by reaction with other trace species. The dominant pathways are reactions with NO and NOz. Only a few peroxy radicals have been detected with a mass spectrometer, and extensive research on these reactions has been done by UV absorption spectroscopy with the well-known and conveniently accessed band in the 200- to 300-nm region. Nevertheless, FPTRMS has been used for some peroxy radical kinetics investigations. These have usually made use of the mass spectrometer to observe more than one species, and have given information on product channels. The FPTRMS work has been exclusively on atmospheric reactions of chlorofluoromethanes and replacements for the chlorofluoromethanes. 

A generic scheme for the atmospheric oxidation of a C2 haloalkane is given in Fig. 6. Values in parentheses are order of magnitude lifetime estimates. Reaction with OH radicals gives a halogenated alkyl radical which reacts with O2 to give the corresponding peroxy radical (RO2). As discussed in previous sections, peroxy radicals can react with three important trace species in the atmosphere NO, NO2, and HO2 radicals. 

In summary, there are a range of trace species present in the atmosphere with a myriad of sources varying both spatially and temporally. It is the chemistry of the atmosphere that acts to transform the primary pollutants into simpler chemical species. 

Photodissociation of atmospheric molecules by solar radiation plays a fundamental role in the chemistry of the atmosphere. The photodissociation of trace species such as ozone and formaldehyde contributes to their removal from the atmosphere, but probably the most important role played by these photoprocesses is the generation of highly reactive atoms and radicals. Photodissociation of trace species and the subsequent reaction of the photoproducts with other molecules is the prime initiator and driver for the bulk of atmospheric chemistry. 

All of the trace species discussed in this paper (CChF, CCI2F2, CCIa, CH3CCI3, CHa) are increasing in concentration in the atmosphere, primarily because of increasing use by man. Further increases in these and other trace chemical species will certainly occur in the coming decade and for the foreseeable future. These increases are connected with potentially important alterations of the environment through depletion of ozone at... 

Flo. 3. Chemical cycles affecting the formation and decay of chlorine oxide trace species in the earth s atmosphere (reproduced with permission from Wayne, R. P. Chemistry of Atmospheres, 2nd ed., p. 137, Clarendon Press Oxford, 1991). 

In this section we discuss absorption spectra, quantum yields, and photodissociation coefficients for several atmospheric trace species. DeMore et al. (1985) and Baulch et al. (1980, 1982, 1984) have critically reviewed data relevant to atmospheric processes. Table 2-7 summarizes important photochemically active molecules and associated photodissociation coefficients. These were computed with the full, unattenuated solar spectrum so that they represent maximum values. But an attempt has been made to separate critical wavelength regions as a discriminator for the significance of the process in the stratosphere and troposphere, respectively. 

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