Hydroxyl atmospheric concentration

The dominant transformation process for trichloroethylene in the atmosphere is reaction with photochemically produced hydroxyl radicals (Singh et al. 1982). Using the recommended rate constant for this reaction at 25 °C (2.36x10 cm /molecule-second) and a typical atmospheric hydroxyl radical concentration (5x10 molecules/cm ) (Atkinson 1985), the half-life can be estimated to be 6.8 days. Class and Ballschmiter (1986) state it as between 3 and 7 days. It should be noted that the half-lives determined by assuming first-order kinetics represent the calculated time for loss of the first 50% of trichloroethylene the time required for the loss of the remaining 50% may be substantially longer. 

The reaction of volatile chlorinated hydrocarbons with hydroxyl radicals is temperature dependent and thus varies with the seasons, although such variation in the atmospheric concentration of trichloroethylene may be minimal because of its brief residence time (EPA 1985c). The degradation products of this reaction include phosgene, dichloroacetyl chloride, and formyl chloride (Atkinson 1985 Gay et al. 1976 Kirchner et al. 1990). Reaction of trichloroethylene with ozone in the atmosphere is too slow to be an effective agent in trichloroethylene removal (Atkinson and Carter 1984). 

Photolysis of O3 yields O2 and electronically excited 0( D), which can either be collisionally stabilized (reaction 6.18) or react with a water molecule to yield two hydroxyl radicals (reaction 6.19). Atmospheric concentrations of hydroxyl radical on a 24-h seasonal average basis are estimated at 1 X 10 molecules cm while peak daytime concentrations of 46 X 10 molecules cm have been observed. ... 

The most reliable kinetic data are for atmospheric oxidation by hydroxyl radicals. These data are usually reported as second-order rate constants applied to the concentration of the chemical and the concentration of hydroxyl radicals (usually of the order of 10s radicals per cm3). The product of the assumed hydroxyl radical concentration and the second-order rate constant is a first-order rate constant from which a half-life can be deduced. 

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 ... 

Phenol is released into the air and discharged into water from both manufacturing and use. Based on its high water solubility (see Table 3-2) and the fact that it has been detected in rainwater, some phenol may wash out of the atmosphere however, it is probable that only limited amounts wash out because of the short atmospheric half-life of phenol. During the day, when photochemically produced hydroxyl radical concentrations are highest in the atmosphere, very little atmospheric transport of phenol is likely to occur. 

SinghHB. 1977. Atmospheric halocarbons Evidence in favor of reduced average hydroxyl radical concentration in the troposphere. Geophys Res Lett 4 101-104. 

Blake, N. J., S. A. Penkett, K. C. Clemitshaw, P. Anwyl, P. Lightman, A. R. W. Marsh, and G. Butcher, Estimates of Atmospheric Hydroxyl Radical Concentrations from the Observed Decay of Many Reactive Hydrocarbons in Well-Defined Urban Plumes, J. Geophys. Res., 98, 2851-2864 (1993). 

HDI and HDI prepolymers can be released to the atmosphere during spray applications of polymer paints containing residual amounts (0.5-1.0%) of monomeric HDI (Alexandersson et al. 1987 Hulse 1984 Karol and Hauth 1982). These substances could also be released to the atmosphere from waste streams from sites of HDI or polymer production. No information is available in the Toxic Chemical Release Inventory database on the amoimt of HDI released to the atmosphere from facihties that produce or process HDI because this compound is not included under SARA, Title 111, and therefore, is not among the chemicals that facilities are required to report (EPA 1995). There is also a potential for atmospheric release of HDI from hazardous waste sites however, no information was found on detections of HDI in air at any NPL or other Superfund hazardous waste sites (1996). Beeause of the relatively rapid reaction of HDI with hydroxyl radicals in the atmosphere an possible hydrolysis (see Seetion 5.3.2.1), significant atmospheric concentrations are not expeeted to oeeur exeept near emission sourees. 

Except for occupational settings, no information was formd in the available literature on eoncentrations of HDl or HDl prepolymers in air. Because of the relatively short atmospheric half-life (approximately 2 days) from reaction with hydroxyl radicals (see Section 5.3.2.1), significant atmospheric concentrations of HDl would be expected to be found only near sources of this substance (e.g., waste streams from manufacturing or processing facilities, hazardous waste sites, occupational settings). Atmospherie eoneentrations of HDl and HDI-BT found in occupational settings are siunmarized in Section 5.5. 

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. 

Simons, J.H., Mausteller, J.W. (1952) The properties of w-butforane and its mixtures with n-butanc. J. Chem. Phys. 20, 1516-1519. Singh, H.B. (1977) Atmospheric hydrocarbons Evidence in favor of reduced average hydroxyl radical concentrations in troposphere. Geophys. Res. Lett. 4, 101-104. 

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). 

Increased fossil fuel combustion has presmnably increased atmospheric hydrogen concentrations significantly in the last century, but there has been no detectable increase since 1990. If hydrogen uptake in the soil were becoming saturated, we would expect the concentration of hydrogen in the atmosphere to have increased, even if hydroxyl radical concentrations were increasing as well. 

The two processes likely to remove dinitrocresols from the atmosphere are reactions with hydroxyl and nitrate radicals (Atkinson et al. 1992). No experimental kinetic data are available for these two reactions (Grosjean 1991). The rate constant for the gas phase reaction of dinitrocresols with OH radicals is 3.0x10 cm /molecule-second (Grosjean 1991). Using the method of Atkinson (1988), the estimated rate constant for this reaction is 2.1x10 cm /molecule-second. Based on an average ambient atmospheric concentration of OH radicals in the northern hemisphere of 5x10 radicals/cm... 

Benzene in the atmosphere exists predominantly in the vapor phase (Eisenreich et al. 1981). The most significant degradation process for benzene is its reaction with atmospheric hydroxyl radicals. The rate constant for the vapor phase reaction of benzene with photochemically produced hydroxyl radicals has been determined to be 1.3 10"12 cm3/molecule-second, which corresponds to a residence time of 8 days at an atmospheric hydroxyl radical concentration of 1.1 x 106 molecules/cm3 (Gaffney and Levine 1979 Lyman 1982). With a hydroxyl radical concentration of 1 x 108 molecules/cm3, corresponding to a polluted atmosphere, the estimated residence time is shortened to 2.1 hours (Lyman 1982). Residence times of 472 years for rural atmospheres and 152 years for urban atmospheres were calculated for the reaction of benzene with ozone (03) using a rate constant for 03 of 7 /1 O 23 cm3/molecule-second (Pate et al. 1976) and atmospheric concentrations for 03 of 9.6/1011 molecules/cm3 (rural) and 3/ 1012 molecules/cm3 (urban) (Lyman 1982). 

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). 

The primary tropospheric oxidants are OH, O3, and NO3, with "OH and O3 reactions with hydrocarbons dominating primarily during daytime hours, and NO3 reactions dominating at night. Rate constants for the reactions of many different aromatic compounds with each of the aforementioned oxidants have been determined through laboratory experiments [16]. The rate constant data as well as atmospheric lifetimes for the reactions of toluene, m-xylene, p-xylene, m-ethyl-toluene, and 1,2,4-trimethylbenzene appear in Table 14.1. Only these particular aromatic compoimds will be discussed in this review paper, since much of the computational chemistry efforts have focused on these compounds. When considering typical atmospheric concentrations of the major atmospheric oxidants, OH, O3, and NO3 of 1.5 x 10, 7 x 10, and 4.8 x 10, molecules cm , respectively [17], combined with the rate constants, it is clear that the major atmospheric loss process for these selected aromatic compounds is reaction with the hydroxyl... 

It is estimated that about 500 million tons of methane are being added to the air each year (Craig and Chou, 1982), largely by anaerobic production in rice paddies and wetlands as well as from the metabolism of ruminant domestic animals and, possibly, African termites (Rasmussen and Khalil, 1981 Zimmerman et d., 1982). This gas is slowly oxidized by reactions with Hydroxyl free radical. Its atmospheric content is around 5 gigatons, indicating that the residence time in the atmosphere is about 10 years. As Figure 12 shows, since 1965 the atmospheric concentration of methane has increased by about 3096. If this rate continues, the methane concentration will have doubled early in the 21st century. 

Atmospheric concentrations are relatively high compared with other environmental compartments because of vinylidene chloride s high vapor pressure and low water solubility. The half-life for the chemical in air has been estimated to be 16 h and 2-3 days. Atmospheric hydroxyl radicals play a major role in its degradation. The major reaction products in air are formaldehyde, phosgene, and hydroxylacetyl chloride. 

Using an estimated atmospheric hydroxyl radical concentration of 5.0x10 mol/cm (Atkinson 1985), the more recent rate constants translate to a calculated lifetime or residence time of 6 years. The estimated atmospheric lifetime of 1,1,1-trichloroethane, which incorporates all removal processes, was also estimated to be 6 years (Prinn et al. 1987 Prinn et al. 1992). This indicates that the predominant tropospheric sink of 1,1,1-trichloroethane is through its reaction with OH radicals. 

Carbon monoxide (CO) is also formed in aquatic environments from the photochemical degradation of DOM [3,4,8,22,94-105]. Strong gradients of CO have been observed in the lowest 10 metres of the atmosphere over the Atlantic Ocean [97]. The samples nearest the ocean surface were some 50 ppb higher than at the 10-metre altitude-sampling inlet. This implies that the ocean is a source of CO to the atmosphere and that this source can increase the atmospheric concentration. CO is reactive in the troposphere and thus its emissions from the ocean may influence the hydroxyl radical (OH) and ozone concentrations in the marine atmospheric boundary layer that is remote from strong continental influences. 

Little information was found in the available literature concerning the transformation of wood or coal tar creosote components in the atmosphere. Some volatile coal tar constituents may undergo oxidation by vapor phase reaction with photochemically produced hydroxyl radicals, with calculated half-lives of 2 hours to 10 days based on experimental and estimated rate constants of 1.12-103xl012 cm/molecules-second at 25 °C and using an average atmospheric hydroxyl radical concentration of 5x10s molecules/cm3 (Atkinson 1989 Meylan and Howard 1993). Rates may be slowed since some components will exist as... 

The vapor-phase reaction of ammonia with photochemically produced hydroxyl radicals is known to occur. The rate constants for this reaction have been determined to be 1.6x10 cm molecule-sec, which translates to a calculated half-life of 100 days at a hydroxyl radical concentration of 5x10 molecules/cm (Graedel 1978). This process reportedly removes 10% of atmospheric ammonia (Crutzen 1983). Since ammonia is very soluble in water, rain washout is expected to be a dominant fate process. The half-life for ammonia in the atmosphere was estimated to be a few days (Brimblecombe and Dawson 1984 ... 

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