Tropospheric reactions

NMHC. A large number of hydrocarbons are present in petroleum deposits, and their release during refining or use of fuels and solvents, or during the combustion of fuels, results in the presence of more than a hundred different hydrocarbons in polluted air (43,44). These unnatural hydrocarbons join the natural terpenes such as isoprene and the pinenes in their reactions with tropospheric hydroxyl radical. In saturated hydrocarbons (containing all single carbon-carbon bonds) abstraction of a hydrogen (e,g, R4) is the sole tropospheric reaction, but in unsaturated hydrocarbons HO-addition to a carbon-carbon double bond is usually the dominant reaction pathway. 

The peroxynitrate CH3C0CH200N02 formed from the N02 reaction thermally decomposes, with a rate constant of 3 s-1 at 700 Torr and 295 K. Sehested et al. (1998b) suggest that its lifetime with respect to thermal decomposition is sufficiently small even at the lower temperatures of the upper troposphere that this species cannot participate in long-range transport of NOx, as is the case for PAN, and that in the upper troposphere, reaction of CH3C0CH200N02 with OH and photolysis will be major fates. 

FIGURE 13.22 Summary of major tropospheric reactions of HCFC-f25. 

Tuazon, E. C., and R. Atkinson, Tropospheric Reaction Products and Mechanisms of the Hydrochlorofluorocarbon-141b, Hydro-chlorofluorocarbon-142b, Hydrochlorofluorocarbon-225ca, and Hydrochlorofluorocarbon-225cb, Enriron. Sci. Technol., 28, 2306-2313 (1994). 

Barbara J. Finlayson-Pitts is Professor of Chemistry at the University of California, Irvine. Her research program focuses on laboratory studies of the kinetics and mechanisms of reactions in the atmosphere, especially those involving gases with liquids or solids of relevance in the troposphere. Reactions of sea salt particles to produce photochemically active halogen compounds and the subsequent fates of halogen atoms in the troposphere are particular areas of interest, as are reactions of oxides of nitrogen at aqueous and solid interfaces. Her research is currently supported by the National Science Foundation, the Department of Energy, the California Air Resources Board, the Dreyfus Foundation, and NATO. She has authored or coauthored more than 80 publications in this area, as well as a previous book, Atmospheric Chemistry Fundamentals and Experimental Techniques. 

Indirect Photolysis in the Atmosphere (Troposphere)—Reactions with Hydroxyl Radical (HO )... 

In my filosofie licentiat thesis of 1968 at Stockholm University I analyzed this proposal and concluded that the rate constants that had been chosen for reactions R5 and R6 could not explain the vertical distribution of ozone in the stratosphere above 25 km. Furthermore, the above choice of rate constants would also lead to unrealistically rapid loss of ozone (on a timescale of only a few days) in the troposphere (9). Anticipating a possible role of HO in tropospheric chemistry, I also briefly mentioned the potential importance of a tropospheric reaction between HO with CH4. In fact, it was soon found that reactions R5 and R6 proceed about 25 and 10 times slower, respectively, than the values given above. 

In the troposphere reaction (2a) is approximately 10 times slower than (2b). However, it is of great importance because excited atomic oxygen (0 D) reaction with water vapor is the major source of hydroxyl radical (OH), the main oxidant in the troposphere. 

For the majority of gas-phase organic chemicals present in the troposphere, reaction with the OH radical is the dominant loss process (Atkinson, 1995). The tropospheric lifetime of a chemical is the most important factor in determining the relative importance of transport, to both remote regions of the globe and to the stratosphere, and in determining the possible buildup in its atmospheric concentration. Knowledge of the OH radical reaction rate constant for a gas-phase organic compound leads to an upper limit to its tropospheric lifetime. 

The oxidation scheme for halomethanes not containing a hydrogen atom is similar to that for those which do, except that it is not initiated by tropospheric reaction with hydroxyl radicals, since the fully halogenated methanes are unreactive. Consequently, substantial amounts of CFCs and halons are transported intact up into the stratosphere, where they absorb UV radiation of short wavelength and undergo photodissociation (equation 36) to a halogen atom and a trihalomethyl radical. The halogen atom Y may enter into catalytic cycles for ozone destruction, as discussed in the introduction. 

Although many advances have been made in understanding the tropospheric reactions of anthropogenic aromatic compounds, additional work is clearly needed. Specific areas of foci for future closely coordinated computational and laboratory-based studies are in the areas of ... 

Carbon monoxide is oxidized in the troposphere ((133) and (134)). With a high concentration of nitric oxide in the troposphere, reactions (135) and (136) take place. This sequence is a formation of ozone catalyzed by nitric oxide. If the nitric oxide concentration is too low, the perhydryl radicals decompose ozone to form hydroxyl radicals (136). Ozone and peroxyacylnitrates PAN are the major toxins of smog. Peroxyacylnitrates are formed from aldehydes in a reaction catalyzed by nitric oxide. 

Atkinson, R., J.N. Pitts Jr. and S.M. Asohmann Tropospheric reactions of dimethyl suUide with NO3 and OH... 

Usually, in a chamber reactor (see the bottom row in Table 4) the complex chemical systems existing in the atmosphere gas phase are approximated. Especially advantageous for studying gas reactions are the two characteristic features of a chamber reactor a relatively large volume and rather insignificant wall effects. Examples of effectively studied tropospheric reactions in chamber reactors are photo-dissociation and oxidation of a selection of organic compounds, the latter reactions with such oxidants as ozone, OH and NO3. In many... 

Aldehydes are emitted by combustion processes and also are formed in the atmosphere from the photochemical degradation of other organic compounds. Aldehydes undergo photolysis, reaction with OH radicals, and reaction with N03 radicals in the troposphere. Reaction with N03 radicals is of relatively minor importance as a loss process for these compounds, but can be a minor contributor to the H02 (from formaldehyde) and peroxyacetyl nitrate (PAN) formation during nighttime hours (Stockwell and Calvert, 1983 Cantrell et al., 1985). Thus, the major loss processes involve photolysis and reaction with OH radicals. 

PROBABLE FATE photolysis could be important, photooxidation half-life in water 54.1-541 days, direct photolysis in the stratosphere may occur, but is insignificant in the troposphere, reaction with photochemically produced hydroxyl radicals yields a half-life of 1.45 yrs oxidation atmospheric photooxidation by hydroxyl radicals to COBT2 is relatively rapid hydrolysis too slow to be important, first-order hydrolytic half-life 687 yrs volatilization volatilization has been demonstrated, could be an important transport process, volatilization from moist soil surfaces expected to occur sorption no information is available biological processes slight potential for bioaccumulation/metabolization is known to occur in some organisms other reactionsAnteractions possibly produced by halogen reaction... 

Whereas the troposphere contains abundant water vapor, little H2O makes it to the stratosphere the low temperatures at the tropopause lead to an effective freezing out of water before it can be transported up (a cold trap at the tropopause). Mixing ratios of HiO in the stratosphere do not exceed about 5 to 6 ppm. In fact, about half of this water vapor in the stratosphere actually results from the oxidation of methane that has leaked into the stratosphere from the troposphere. Reaction 4.17 is the principal source of OH radicals in the troposphere in the lower stratosphere both reactions 4.17 and 4.18 are important sources of OH. 

Ruppert, L., Barnes, I, and Becker, K. H. (1995) Tropospheric reactions of isoprene and oxidation products kinetic and mechanistic studies, in Tropospheric Oxidation Mechanisms, edited by K. H. Becker. European Commission, Report EUR 16171 EN, Luxembourg, pp. 91-102. 

For most organic compounds that enter the troposphere, reactions with HO govern their disappearance (Carter and Atkinson, 1985). For example, methane, which is by far the most abundant tropospheric hydrocarbon, is virtually inert to atmospheric reactions except for its reaction with OH (4.38), which initiates a series of free-radical reactions that lead to its conversion to oxidized forms ... 

The studies undertaken within this project aimed to combine laboratory studies of the tropospheric reactions of NO3 and RO2 with relevant field measurement studies. The objective of the latter was the investigation of the validity of the current understanding of the tropospheric role of NO3 and RO2. The primary target species for the field studies was therefore RO2. 

Tropospheric reactions of isoprene and oxidation products Kinetic and mechanistic studies,... 

Davison B, Hewitt CN (1994) Elucidation of the tropospheric reactions of biogenic sulfm species from a field measurement campaign in NW Scotland. Chemosphere 28 543... 

Unfortunately, research studies that address environmentally relevant atmospheric fate processes of pesticides are relatively few in comparison to studies that measure transformations on land surfaces and in water. This scarcity of fate information is related to the difficulty in attaining relevant tropospheric photochemical and oxidative information under both environment and controlled laboratory conditions. Only a limited number of studies exist that have measured airborne pesticide reactivity under actual sunlight conditions (d, 7,8), These studies enq)loyed photochemically stable tracer confounds of similar volatility and atmospheric mobility to con5)ensate for physical dilution. The examined airborne sunlight-exposed pesticides in these limited studies had to react quickly to provide environmentally measurable reaction rate constants. The field examination of tropospheric reaction rates for the vast majority of agricultural pesticides is impractical since reaction rates for many of these compounds are probably too slow to yield reliable rate constant information. 

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