Hydroxyl radical lifetime

Table I. Trace gas rate constants and lifetimes for reaction with ozone, hydroxyl radical, and nitrate radical. Lifetimes are based upon [O3]=40ppb [HO ]=1.0x10 molecules cm (daytime) [NO3 ]=10ppt (nighttime).

Trace-gas Lifetimes. The time scales for tropospheric chemical reactivity depend upon the hydroxyl radical concentration [HO ] and upon the rate of the HO/trace gas reaction, which generally represents the slowest or rate-determining chemical step in the removal of an individual, insoluble, molecular species. These rates are determined by the rate constant, e,g. k2s for the fundamental reaction with HO, a quantity that in general must be determined experimentally. The average lifetime of a trace gas T removed solely by its reaction with HO,... 

The lifetime of T is an inverse function of the hydroxyl radical concentration [HO>] and the rate constant kj for its reaction with a particular trace gas... 

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). 

Oosthuizen and Greyling [93] recently investigated the possibility of using chemiluminescent methods for hydroxyl radical detection. These authors concluded that the lifetime of hydroxyl radicals (10 9 s) is too short to produce a meaningful level of CL. However, in the presence of carbonate the significant levels of luminol- and MCLA-amplified CL were observed supposedly due to Reaction (18), in which the formed much more stable radical C03- is capable of interacting with luminol or MCLA. 

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 ... 

The mobile charge carrier species may either recombine or reach the semiconductor surface, where they can be trapped by the surface adsorbates or other sites. The lifetime of electron-hole (e /h+) pairs that are generated is important in determining the reaction yield. The holes are mainly trapped by water molecules or hydroxyl ions, giving rise to very reactive hydroxyl radicals ... 

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. 

The mean diffusion distance of various nitrogen and oxygen based species within one estimated lifetime. The diameter of the dot for hydroxyl radical is still 100-fold larger than the actual diffusion distance (Hutchinson, 1957). 

This prolongs tire lifetime of the hole thus increasing the probability to oxidize surface hydroxyl groups or adsorbed water to yield a hydroxyl radical ... 

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