Reactions in the Troposphere

Although chemical transformations in the atmosphere may seem peripheral to this discussion, these reactions should be considered since their products may subsequently enter the aquatic and terrestrial environments the persistence and the toxicity of these secondary products are therefore relevant to this discussion. 

There are a number of important reasons for discussing the reactions of organic compounds in the troposphere  

After emission, contaminants may be partitioned among the terrestrial, aqnatic, and various atmospheric phases, and those of sufficient volatility or associated with particles may be transported over long distances. This is not a passive process, however, since important transformations may take place in the troposphere during transit so that attention should also be directed to their transformation products. 

Considerable attention has been given to the persistence and fate of organic compounds in the troposphere, and this has been increasingly motivated by their possible role in the production of ozone by reactions involving NO.  

Concern has been expressed over the destruction of ozone in the stratosphere brought about by its reactions with chlorine atoms produced from chlorofluoroalkanes that are persistent in the troposphere, and that may contribute to radiatively active gases other than COj. 

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Collectively, these examples illustrate the diversity of transformations of xenobiotics that are photochemically induced in aquatic and terrestrial systems. Photochemical reactions in the troposphere are extremely important in determining the fate and persistence of not only xenobiotics but also of naturally occurring compounds. A few illustrations are given as introduction ... 

Reactions in the troposphere are mediated by reactions involving hydroxyl radicals produced photochemically during daylight, by nitrate radicals that are significant during the night (Platt et al. 1984), by ozone and, in some circumstances by 0( P). 

The kinetics of the various reactions have been explored in detail using large-volume chambers that can be used to simulate reactions in the troposphere. They have frequently used hydroxyl radicals formed by photolysis of methyl (or ethyl) nitrite, with the addition of NO to inhibit photolysis of NO2. This would result in the formation of 0( P) atoms, and subsequent reaction with Oj would produce ozone, and hence NO3 radicals from NOj. Nitrate radicals are produced by the thermal decomposition of NjOj, and in experiments with O3, a scavenger for hydroxyl radicals is added. Details of the different experimental procedures for the measurement of absolute and relative rates have been summarized, and attention drawn to the often considerable spread of values for experiments carried out at room temperature (-298 K) (Atkinson 1986). It should be emphasized that in the real troposphere, both the rates—and possibly the products—of transformation will be determined by seasonal differences both in temperature and the intensity of solar radiation. These are determined both by latitude and altitude. 

Because of the presence of these absorbing species in the upper atmosphere, only light of A > 290 nm is available for photochemical reactions in the troposphere. It is often expressed as the integrated radiation coming from all directions to a sphere and is referred to as actinic radiation, although in the strictest sense,... 

Exotic Kinetics Oscillating Reactions in the Troposphere (J. Phys. Chem. A 2001, 105, 11212-11219. "Steady State Instability and Oscillation in Simplified Models of Tropospheric Chemistry")... 

The last years attention has been also paid on the importance of heterogeneous reactions in the troposphere. On the basis of the extremely limited laboratory studies and numerous assumptions it has been calculated that scavenging of compounds such as N03, N205 and HOBr followed by reactions in clouds or/and onto aerosols can be crucial for the oxidant levels in the troposphere [24 - 27]. However, contrary to the gas phase tropospheric chemistry, it does not exist a reference scheme for heterogeneous chemistry nor any consistent compilation of recommended relevant kinetic data. Finally, although CTMs have nowadays more or less sophisticated parameterizations of heterogeneous chemistry,... 

One of the key questions in connection with studies of future ozone changes from NOx emissions in general is the non linearity in the ozone forming process with ozone formation becoming less efficient per NOx molecule emitted at high NOx levels. Ozone formation occurs via the following sequence of reactions in the troposphere, and in the lowermost part of the stratosphere ... 

There are several mechanisms whereby organic compounds released into the atmosphere may be removed (i) physical removal by precipitation ( rain-out ) (ii) chemical reaction in the troposphere (in) transport into the stratosphere (iv) chemical reaction in the stratosphere. The physical and dynamic conditions of the different atmospheric regions will usually dictate the type of mechanism that occurs2,3. 

Carbonyl halides containing a hydrogen atom are likely to undergo reaction in the troposphere with hydroxyl radicals, which dominate the day-time chemistry. Reaction 37 of carbonyl halides CHXO (where X = F or Cl) may occur by two mechanisms (i) direct hydrogen abstraction or (ii) radical addition to carbonyl. 

A few key (i.e., primary or direct) photochemical reactions are the principal drivers of overall chemical reactions in the troposphere. These reactions involve primarily O3 and NO2 photolysis. Other reactions presented in Table 1 will be discussed later. 

Levy (155) calculated an mixing ratio of 0.7 ppm and a tropospheric residence time of 2 yr. From this agreement it appears that both the major source and the major sink for H2 are homogeneous gas-phase reactions in the troposphere. 

Niki, Brietenbrach, and Morris (184a) have estimated from limited preliminary data that the reaction of Me2S with OH proceeds at a rate similar to that observed for H2S. Thus it is possible that Me2S also has a short residence time. The products of such a reaction in the troposphere are not known, though one can reasonably speculate that oxidation products similar to those expected for HS would result. 

Schott and Davidson found K25 = 6 x 10 13 for what is an effective two-body reaction in the troposphere. 

The dominant radical chain reaction in the troposphere is the oxidation of CO by OH in the presence of 02,... 

While CO2 is the major atmospheric carbon species, it has no significant atmospheric reactions in the troposphere. The general mechanisms for keeping it in balance in the lower atmosphere have already been discussed (see Section II.F.l). C02 photochemistry in the stratosphere and mesosphere has been reported in detail [Bates and Witherspoon (12), Hays and Olivero (92), and Wofsy et al. (256)]. 

There has been recent interest in a somewhat different aspect of adsorption and reaction on metal oxides photocatalysis. The interest stems partially from that role that some transition-metal oxides can play in photochemical reactions in the atmosphere. Atmospheric aerosol particles can act as substrates to catalyze heterogeneous photochemical reactions in the troposphere. Most tropospheric aerosols are silicates, aluminosilicates and salts whose bandgaps are larger than the cutoff of solar radiation in the troposphere (about 4.3 eV) they are thus unable to participate directly in photoexcited reactions. However, transition-metal oxides that have much smaller bandgaps also occur as aerosols — the most prevalent ones are the oxides of iron and manganese — and these materials may thus undergo charge-transfer excitations (discussed above) in the pres-... 

The chemical composition of atmospheric depositions reflects the types and rates of biogeochemical reactions in the troposphere. These chemical compositions change seriously after interactions with humid acids of the soil layer, higherplant metabolites, and soil microbes. Carbon dioxide, the end product of any organic matter decay, is readily soluble in water to yield carbonic acid, the dissociation of which... 

DMS reactions in the troposphere are believed to lead to enhanced reflectivity of marine clouds [171] and thus DMS emissions may have a cooling influence on the atmosphere. One of the best demonstrations of the link between the natural atmospheric sulfur cycle and the physical climate system are the observations that link the satellite derived stratus cloud optical depth and observed DMS derived cloud condensation nuclei (CCN) concentrations at Cape Grim, Australia [175]. Statistical evidence indicates that the optical depth of the clouds is correlated with the number of CCN in the atmosphere. Thus, any UV-related changes at the surface of the ocean that result in the alteration in DMS flux to the atmosphere and the subsequent formation of CCN would also alter the atmospheric radiation budget for the affected region. 

Rather important work began at Ford Motor Company where in the late 60-ties Weinstock (1969) promoted the importance of OH reactions in the troposphere. In this laboratory Niki used a rather small photoreactor to develop the application of FTIR spectroscopy for quantitative investigation of atmospheric reactions in the laboratory (Niki et al, 1972 Wu et al, 1976 Niki et al, 1981). IR absorption spectroscopy was applied quite early to study chemical reactions of atmospheric interest (Stephens, 1958 Hanst, 1971), based mainly on mirror systems which allowed long path light absorption (White, 1942 1976 Herriott et al, 1964, 1965). However, real progress and success came with the application of modem FTIR spectrometers by Niki et al (1981) and further work in the Pitts group to quantitatively measure rate constants and products in photoreactors by long path FTIR absorption. 

Herrmann, H., H.-W. Jacobi, G. Raabe, A. Reese, Th. Umschlag and R. Zellner Free radical reactions in the tropospheric aqueous phase, in B. Larsen, B. Versino, G. Angeletti, (eds). The Oxidizing Capacity of the... 

Although complex chemical transformations — mainly photochemical — take place in the atmosphere, many chemically stable compounds may be transported intact via the atmosphere and subsequently enter the aquatic and terrestrial environments in the form of precipitation. Although the whole issue of chemical reactions in the troposphere lies outside the scope of this account, some comments are given in Chapter 4, Section 4.1.2, and reference should be made to the comprehensive account of principles given by Finlay-son-Pitts and Pitts (1986). The persistence in the troposphere of xenobiotics — even those of moderate or low volatility — is determined by the rates of transformation processes. These involve reactions with hydroxyl radicals, nitrate radicals, and ozone, or direct photolysis. Reactions with hydroxyl radicals are generally the most important. Illustrative values are given for the rates of reaction (cm3 s 1 molecule1) with hydroxyl radicals, nitrate radicals, and ozone (Atkinson 1990). 

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