Hydroxyl radical atmospheric lifetime

For polychlorinated biphenyls (PCBs), rate constants were highly dependent on the number of chlorine atoms, and calculated atmospheric lifetimes varied from 2 d for 3-chlorobiphenyl to 34 d for 236-25 pentachlorobiphenyl (Anderson and Hites 1996). It was estimated that loss by hydroxy-lation in the atmosphere was a primary process for the removal of PCBs from the environment. It was later shown that the products were chlorinated benzoic acids produced by initial reaction with a hydroxyl radical at the 1-position followed by transannular dioxygenation at the 2- and 5-positions followed by ring fission (Brubaker and Hites 1998). Reactions of hydroxyl radicals with polychlorinated dibenzo[l,4]dioxins and dibenzofurans also play an important role for their removal from the atmosphere (Brubaker and Hites 1997). The gas phase and the particulate phase are in equilibrium, and the results show that gas-phase reactions with hydroxyl radicals are important for the... 

Extensive research has been conducted into the atmospheric chemistry of organic chemicals because of air quality concerns. Recently, Atkinson and coworkers (1984, 1985, 1987, 1988, 1989, 1990, 1991), Altshuller (1980, 1991) and Sabljic and Glisten (1990) have reviewed the photochemistry of many organic chemicals of environmental interest for their gas phase reactions with hydroxyl radicals (OH), ozone (03) and nitrate radicals (N03) and have provided detailed information on reaction rate constants and experimental conditions, which allowed the estimation of atmospheric lifetimes. Klopffer (1991) has estimated the atmospheric lifetimes for the reaction with OH radicals to range from 1 hour to 130 years, based on these reaction rate constants and an assumed constant concentration of OH... 

Air photooxidation reaction rate constant of 6.30 x 10-12 cm3 molecule-1 s-1 with hydroxyl radicals and an estimated atmospheric lifetime of 22 h during summer daylight (Altshuller 1991). 

Air atmospheric t,/2 2.4-24 h for C4H10 and higher paraffins for the reaction with hydroxyl radical, based on the EPA Reactivity Classification of Organics (Darnall et al. 1976) photooxidation reaction rate constant of 1.02 x 10-11 cm3 molecule-1 s-1 with OH radical with an estimated lifetime x = 14 h during summer daylight (Altshuller 1991). 

Tropospheric chemistry is strongly dependent on the concentration of the hydroxyl radical (OH), which reacts very quickly with most trace gases in the atmosphere. Owing to its short boundary layer lifetime ( 1 s), atmospheric concentrations of OH are highly variable and respond rapidly to changes in concentrations of sources and sinks. Photolysis of ozone, followed by reaction of the resulting excited state oxygen atom with water vapour, is the primary source of the OH radical in the clean troposphere ... 

Reaction 2-6 is sufficiently fast to be important in the atmosphere. For a carbon monoxide concentration of 5 ppm, the average lifetime of a hydroxyl radical is about 0.01 s (see Reaction 2-6 other reactions may decrease the lifetime even further). Reaction 2-7 is a three-body recombination and is known to be fast at atmospheric pressures. The rate constant for Reaction 2-8 is not well established, although several experimental studies support its occurrence. On the basis of the most recently reported value for the rate constant of Reaction 2-8, which is an indirect determination, the average lifetime of a hydroperoxy radical is about 2 s for a nitric oxide concentration of 0.05 ppm. Reaction 2-8 is the pivotal reaction for this cycle, and it deserves more direct experimental study. 

Dichlorobenzene is degraded in the atmosphere by reaction with hydroxyl radicals, with an atmospheric lifetime (theoretically calculated) of about 1 month (Atkinson et al. 1985 Singh et al. 1981). 

CHC1F2. These HCFCs do react with atmospheric hydroxyl radicals, shortening their lifetime so that they do not reach the stratosphere. The problem with the HCFCs is that they cannot be used in older appliances that were designed for CFCs. When CFCs will no longer be found in the market, the older appliances will need to be replaced by new ones designed for HCFCs. 

Prinn, R.G. et al. (1995) Atmospheric trends and lifetime of trichloroethane and global average hydroxyl radical concentrations based on 1978 -1994 ALE/G AGE measurements. Science, 269,187. 

Air atmospheric lifetime of 7-8 h for reaction with hydroxyl radicals (Atkinson 1987) estimated reaction k = 0.0015 h-1 (Paterson et al. 1990) ... 

Air atmospheric lifetimes of 2.3 h in clean troposphere and 1.2 h in moderately polluted atmosphere, based on gas-phase reaction with hydroxyl radical at room temp. atmospheric lifetimes of 15.0 d in clean troposphere and 5.0 d in moderately polluted atmosphere, based on gas-phase reaction with 03 at room temp. (Atkinson et al. 1987)... 

Air photooxidation t,/2 = 284-2840 h, based on estimated rate constant for the reaction with hydroxyl radical in air (Atkinson 1987 quoted, Howard et al. 1991) atmospheric transformation lifetime t 1-5 d (Kelly et al. 1994). 

Oxidation rate constant k for gas-phase second order rate constants, koH for reaction with OH radical, kNQ3 with N03 radical and kQ3 with 03 or as indicated data at other temperatures see reference photooxidation t,/2 = 14.7-24.4 yr in water, based on measured rate data for the reaction with hydroxyl radical in aqueous solution (Dorfman Adams 1973 selected, Howard et al. 1991) k0H = (4.9 0.4) x 10 13 cm3 molecule1 s 1 with atmospheric lifetimes of 46 d in clean troposphere and 23 d in moderately polluted atmosphere kQ3 < 1.1 x 10-20 cm3 molecule1 s 1 with atmospheric lifetimes of > 4 yr in clean troposphere and > 1.3 yr in moderately polluted atmosphere at room temp, (relative rate method, Atkinson et al. 1987)... 

Hydrogen acts as a significant sink for hydroxyl radicals, and increased atmospheric concentrations of hydrogen could lead to a decrease in OH concentration. This in turn could increase the atmospheric lifetime of greenhouse gases and other pollutants, with undesirable consequences for climate change and air quality (Hauglastine and Ehhalt, 2002). 

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. 

Thus the lifetime of a constituent with a first order removal process is equal to the inverse of the first order rate constant for its removal. Taking an example from atmospheric chemistry, the major removal mechanism for many trace gases is reaction with hydroxyl radical, OH. Considering two substances with very different rate constants for this reaction, methane and nitrogen dioxide... 

In contrast to the water phase the HO radicals can have a much longer lifetime in gaseous media, i.e. up to 1 s for the OH and 60 s for the HO radical, respectively (Fabian, 1989). Despite the low concentration of OH radicals of about 10 molecules per cm in the sunlit troposphere (Ehhalt, 1999) they play an important role in controlling the removal of many organic natural and manmade compounds from the atmosphere (Eisele et al., 1997, Eisele and Bradshaw, 1993). Even in indoor environments, the formation of hydroxyl radicals is possible by ozone/alkene reactions (Atkinson et al., 1995). Steady-state indoor hydroxyl radical concentrations of about 6.7x10 ppb equivalent to 1.7x10 molecules cm were calculated at an ozone concentration of 20 ppb (Weschler and Shields, 1996). 

Reactions with hydroxyl radicals are considered one of the most efficient ways used by the atmosphere to remove natural and anthropogenic trace gases in the atmosphere [28,29]. Previous kinetics [27,30-32] and modeling studies [2, 33-35] have shown that the primary atmospheric sink of bromopropane is the reaction with OH and that it has an atmospheric lifetime of 10-16 days. Further studies have determined that the lifetime of short-lived species, such as... 

Air t,/j > 9.9 d for the gas-phase reaction with hydroxyl radical in air, based on the rate of disappearance of hydrocarbon due to reaction with hydroxyl radical (Darnall et al. 1976) estimated L, = 17.8 d in ambient atmosphere (Howard 1990) atmospheric transformation lifetime was estimated to be 1 to 5 d (Kelly et al. 1994) calculated lifetimes of 12 d and 1.0 yr for reactions with OH radical, NOj radical, respectively (Atkinson 2000). Surface water t,//aerobic) = 1 d, t,//anaerobic) = 1 d in natural waters (Capel Larson 1995)... 

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