Tropospheric radicals, examples

Measurements of tropospheric HO and H02 concentrations have been accomplished by both direct and indirect means. Direct techniques are based on the measurement of the hydroxyl or hydroperoxyl radical using some physical property of the radicals themselves e.g. optical absorption. Indirect techniques refer to methods based on the measurement of compounds that are uniquely and/or quantitatively formed from or destroyed by HO or HO2. Examples of these techniques for both [HO ] and [HO2 ] will be given. 

In an ideal troposphere, O3 would react with NO yielding NO2 and ultimately regenerating O3 [Equations (19) and (20)], thus no over-production of ozone could occur. However, in the presence of VOCs, the resultant peroxy radicals formed can compete with O3 by also reacting with NO to form excess NO2, thus resulting in the formation of excess ozone. This is just one example of the complexity of atmospheric chemistry peroxy radicals and NO , have substantial implications for reaction with climate gases, acid rain fonnation, and other aspects of air quality. Saunders et al. and Jenkin et al. have provided a wealth of information on the tropospheric degradation of aliphatic and aromatic VOCs. Additionally, the interested reader may wish to consult References 10-17 for further discussion of these important topics. 

For example, a background concentration of 03 may be 0.04 ppm this is 4 pphm, 40 ppb, or 40,000 ppt. Thus in 10x molecules of air, only 4 are 03 alternatively, in every 10x volumes of air, only 4 volumes are due to 03. While it is most convenient to express the concentration of 03 in ppm, pphm, or ppb, other important atmospheric species can be present in much smaller concentrations. For example, the hydroxyl free radical (OFI), which, as we saw, drives the daytime chemistry of both the clean and polluted troposphere, is believed to have typical concentrations of only < 0.1 ppt. Hence either ppt or an alternate unit discussed in the next section (number per cm3) is used. 

Other spectral regions are also important because the detection and quantification of small concentrations of labile molecular, free radical, and atomic species of tropospheric interest both in laboratory studies and in ambient air are based on a variety of spectroscopic techniques that cover a wide range of the electromagnetic spectrum. For example, the relevant region for infrared spectroscopy of stable molecules is generally from 500 to 4000 cm-1 (20-2.5 /Am), whereas the detection of atoms and free radicals by resonance fluorescence employs radiation down to 121.6 nm, the Lyman a line of the H atom. 

As discussed in Chapter 6.J, acetone photochemistry is of interest because this ketone is distributed globally, has both biogenic and anthropogenic sources, and has been proposed to be a significant source of free radicals in the upper troposphere. The absorption cross sections of acetone (as well as other aldehydes and ketones) are temperature dependent at the longer wavelenths, which is important for application to the colder upper troposphere. Figure 4.29, for example, shows the absorption cross sections of acetone at 298 and 261 K, respectively (Hynes et al., 1992 see also Gierczak et al., 1998). 

Second, in bi- and termolecular reactions, tl/2 and r depend on the concentration of other reactants this is particularly important when interpreting atmospheric lifetimes. For example, as discussed earlier, reaction with the OH radical is a major fate of most organics during daylight in both the clean and polluted troposphere. However, the actual concentrations of OH at various geographical locations and under a variety of conditions are highly variable for example, its concentration varies diurnally since it is produced primarily by photochemical processes. Finally, the concentration of OH varies with altitude as well, so the lifetime will depend on where in the troposphere the reaction occurs. 

In addition to these highly useful data sets, periodically there are reviews directed to the reactions of one particular species (e.g., OH, N03, or 03) or group of compounds (e.g., R02 radicals). These are referenced in the appropriate sections of Chapter 6. For example, a review of the gas-phase tropospheric chemistry of... 

The generation of OH in 03-alkene reactions has important implications for tropospheric chemistry. Thus the 03-alkene reactions could be important free radical sources at dusk and during the night when pho-tolytic sources of OH are minimal (e.g., Paulson and Orlando, 1996 Bey et al., 1997 Paulson et al., 1998). For example, Paulson and Orlando (1996) predicted that 10-15% of the total radical production may be from 03 alkene reactions in a typical rural area in the southeastern United States. As seen in Fig. 6.6, this reaction is expected to be most important at night. 

As seen in Table 6.1, the reactions of the nitrate radical with the simple aromatic hydrocarbons are generally too slow to be important in the tropospheric decay of the organic. However, one of the products of the aromatic reactions, the cresols, reacts quite rapidly with NO,. o-Cresol, for example, reacts with N03 with a room temperature rate constant of 1.4 X 10 " cm3 molecule-1 s-1, giving a lifetime for the cresol of only 1 min at 50 ppt N03. This rapid reaction is effectively an overall hydrogen abstraction from the pheno-... 

While we have focused here on CFC replacements, similar chemistry applies to replacements for the bromine-containing halons. For example, CF2 BrH is a potential halon substitute that will react with OH in the troposphere (DeMore et al., 1997). Through the subsequent reaction with 02 and then NO, the alkoxy radical CF2BrO is formed. This decomposes via scission of the weak C-Br bond to form COF2 (Bilde et al., 1996). 

Two examples serve to illustrate these photochemical relationships. One member of the family of atmospheric peroxy radicals is the peroxy acetyl radical, CH3C(0)02. In at least the warm portion of the troposphere, PAN is near thermal equilibrium with the peroxy acetyl radical and NO . The equilibrium constant for this reaction has been measured in laboratory studies. Therefore, if concentrations of both PAN and N02 are measured, the concentration of these radicals can be calculated from the equilibrium constant and the ratio of the two nitrogen species as shown in Figure 2. The... 

These equations demonstrate the link that is expected between HO and R02 in the troposphere. Hydroxyl radicals are formed in processes initiated by photolysis of various precursors, for example, the ultraviolet photolysis of... 

Based on direct spectroscopic measurements of OH radical concentrations at close to ground level, peak daytime OH radical concentrations are typically (3-10) x 106 molecule cm-3 (see, for example, Brauers et al., 1996 Mather et al., 1997 Mount et al., 1997). A diur-nally, seasonally, and annually averaged global tropospheric OH radical concentration has been derived from the emissions, atmosphere concentrations, and OH radical reaction rate constant for methyl chloroform (CH3CC13), resulting in a 24-hr average OH radical concentration of 9.7 x 10s molecule cm 3 (Prinn et al., 1995). 

Rate constants have been measured for the gas-phase reactions of a large number of organic compounds with OH radicals (Atkinson, 1989,1994,1997), N03 radicals (Atkinson, 1991, 1994, 1997), and 03 (Atkinson and Carter, 1984 Atkinson, 1994, 1997). These measured rate constants can be combined with measured or estimated ambient tropospheric concentrations of OH radicals, N03 radicals, and 03 to provide tropospheric lifetimes with respect to the various loss processes (see, for example, Atkinson, 1995). 

Hydroxyl radical, OH, is the principal atmospheric oxidant for a vast array of organic and inorganic compounds in the atmosphere. In addition to being the primary oxidant of non-methane hydrocarbons (representative examples of these secondary reactions are given in Table 6), OH radical controls the rate of CO and CH4 oxidation. Furthermore, the OH reaction with ozone also limits the destruction of O3 in the troposphere, it also determines the lifetime of CH3CI, CHsBr, and a wide range of HCFC s, and it controls the rate of NO to HNO3 conversion. Concentration profiles for hydroxyl radical in the atmosphere are shown in Fig. 2. 

A primary example of the interplay between hydroxyl radical and ozone is provided by the photo-induced oxidation of methane in the troposphere. The overall sequence of stoichiometric reactions can be written as ... 

There are two sources of tropospheric ozone. First, transport from the stratosphere in meteorological events known as tropospheric folding in which a layer of stratospheric air is entrained in tropospheric air-flow and mixed into the troposphere. Second, peroxy radical reactions which oxidize NO to N02. For example, in the OH radical initiated oxidation of CO ... 

Most of the photo-initiated processes in the atmosphere are radical reactions. However, the hydroxyl radical ( OH) is of special significance for the chemistry of the atmosphere (Ehhalt, 1999). This reactive species is mainly responsible for the photooxidation of trace organic chemicals in the troposphere and hence for the oxidative cleansing of the atmosphere (Fabian, 1989). It is nature s atmospheric detergent (Comes, 1994, Ravishankara, et al. 1998). Furthermore, several years ago it was well established that the interaction of UV/VIS radiation and environmental pollution seems to be responsible for the dramatic forest decline that has been observed, for example in the higher areas of the Black Forest or the Ore Mountains in Germany (Schenck, 1985). 

There are over 70 alcohols in the atmosphere as a result of biogenic and anthropogenic emissions [67]. For example methanol and ethanol [68-70] have been used as fuels additives to reduce automobile emissions of carbon monoxide and hydrocarbons [71], in particular ethanol has been used in Brazil as a fuel for over 20 years [72]. 1-Propanol is widely used as a solvent in the manufacturing of different electronic components. The high volatility of these compounds causes their relative abundance in the troposphere and makes it relevant to determine their degradation pathways. During daytime the major loss process for alcohols is their reaction with OH radicals [68]. Accordingly, several experimental [69,70,73-84] and theoretical [85-88] kinetic studies of alcohols -F OH reactions have been performed. 

The hydroxyl radical so produced is the major oxidising species in the troposphere, and a complete picture of its chemistry holds the key to furthering progress in understanding tropospheric chemistry. The chemistry discussed in detail elsewhere, is of course very complex. To take, for example, the cycle of reactions with carbon monoxide, which may be net producers or destroyers of tropospheric ozone depending upon the concentration of oxides of nitrogen present. In the presence of NO, the cycle (16)-(20) occurs, without loss of OH or NO, whereas at low NO concentrations, the cycle (17), (18) and (21), again without loss of OH. 

It is quite ambitious for a scientist to describe a natural phenomenon in terms of a specific reaction. The situation in the atmospheric environment is however more complicated as a variety of reactions are occurring simultaneously and a certain species may take part in different reactions affecting the relative equilibria. Most data are coming from laboratory work and experimental conditions are definitely different from the ones observed in the troposphere. As an example the mechanism of oxidation of sulphur dioxide, in gas phase is usually reported occur to a large extent through free radicals. If the presence of humidity and of particulated matter is considered, specifically in the lower part of the troposphere, definitely also heterogeneous reactions play a very important role. I feel that experiments carried on in the atmosphere yield more consistent results to elucidate the chemistry of the atmospheric environment. 

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