Atmospheric half-life

The a- and [3-isomers of endosulfan undergo photolysis in laboratory tests after irradiation in polar solvents and upon exposure to sunlight on plant leaves. The a-isomer also undergoes isomerization to the P-isomer, which is relatively more stable (Dureja and Mukerjee 1982). A photolytic half-life of about 7 days was reported for endosulfan by EPA (1982c). The primary photolysis product is endosulfan diol, which is subsequently photodegraded to endosulfan a-hydroxyether. Endosulfan sulfate is stable to direct photolysis at light wavelengths of >300 nm however, the compound reacts with hydroxy radicals, with an estimated atmospheric half-life of 1.23 hours (HSDB 1999). 

The principal pathway leading to degradation of acrylonitrile in air is believed to be photooxidation, mainly by reaction with hydroxyl radicals (OH). The rate constant for acrylonitrile reaction with OH has been measured as 4.1 x 10" cm /molecule/second (Harris et al. 1981). This would correspond to an atmospheric half-life of about 5 to 50 hours. This is consistent with a value of 9 to 10 hours measured in a smog chamber (Suta 1979). 

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. 

X 10 cmVmolecule-sec and 1 h, respectively. The overall atmospheric half-life was estimated to range from 0.191 to 1.27 h (Atkinson, 1987). Similarly, measured rate constants for the gas-phase reaction of acenaphthylene with OH radicals and ozone are 1.24 x 10 cmVmolecule and 1.6 x lO" cmVmolecule, respectively (Reisen and Arey, 2002). 

Photolytic. The atmospheric half-life was estimated to range from 1.43 to 14.3 h (Atkinson, 1987). 

Photolytic. The atmospheric half-life was estimated to range from 0.321 to 3.21 h (Atkinson, 1987). Behymer and Hites (1985) determined the effect of different substrates on the rate of photooxidation of benzo[ Ar]perylene using a rotary photoreactor. The photolytic half-lives of benzo[, / ]perylene using silica gel, alumina, and fly ash were 7.0, 22, and 29 h, respectively. 

Tuazon et al. (1984a) investigated the atmospheric reactions of TV-nitrosodimethylamine and dimethylnitramine in an environmental chamber utilizing in situ long-path Fourier transform infared spectroscopy. They irradiated an ozone-rich atmosphere containing A-nitrosodimethyl-amine. Photolysis products identified include dimethylnitramine, nitromethane, formaldehyde, carbon monoxide, nitrogen dioxide, nitrogen pentoxide, and nitric acid. The rate constants for the reaction of fV-nitrosodimethylamine with OH radicals and ozone relative to methyl ether were 3.0 X 10 and <1 x 10 ° cmVmolecule-sec, respectively. The estimated atmospheric half-life of A-nitrosodimethylamine in the troposphere is approximately 5 min. 

An atmospheric half-life of 216 h was reported for the reaction of pentachlorophenol and OH radicals in January (Bunce et al, 1991). 

The estimated photooxidation half-life of TCDD in the atmosphere via OH radicals ranged from 22.3 to 223 h (Atkinson, 1987a). An atmospheric half-life of 58 min was reported for TCDD exposed to summer sunlight at 40 ° N latitude (Buser, 1988). 

The major fate mechanism of atmospheric 2-hexanone is photooxidation. This ketone is also degraded by direct photolysis (Calvert and Pitts 1966), but the reaction is estimated to be slow relative to reaction with hydroxyl radicals (Laity et al. 1973). The rate constant for the photochemically- induced transformation of 2-hexanone by hydroxyl radicals in the troposphere has been measured at 8.97x10 cm / molecule-sec (Atkinson et al. 1985). Using an average concentration of tropospheric hydroxyl radicals of 6x10 molecules/cm (Atkinson et al. 1985), the calculated atmospheric half-life of 2-hexanone is about 36 hours. However, the half-life may be shorter in polluted atmospheres with higher OH radical concentrations (MacLeod et al. 1984). Consequently, it appears that vapor-phase 2-hexanone is labile in the atmosphere. 

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. 

Isophorone has a water solubility of 12,000 ppm, a log octanol/water partition coefficient of 1.67, a Henry s Law constant of 4.55 X 10 atm m mof, a vapor pressure of 0.3 mm Hg at 20 C, a log sediment sorption coefficient of approximately 1.46, and a log bioconcentration factor (BCF) of 0.85. Isophorone is released to air and water from its manufacturing and use. Based on its water solubility, some isophorone may wash out of the atmosphere however, only limited amounts will be washed out because of the short atmospheric half-life of isophorone. Particularly during the day, when hydroxyl radical (HO) concentrations are highest, very little atmospheric transport will occur due to its fast reaction with HO. ... 

No ambient air monitoring for isophorone was located in the literature. The estimated atmospheric half-life of isophorone is <5 hours may account for the lack of monitoring data, since concentrations... 

CAS No. Name Henry s law constant (atm m3 mol-1) Atmospheric half-life (days) Chemical hydrolysis half-life (days)... 

Oxidation is, of course, the dominant reaction. For example, vaporized trifluralin ( a, a, < -trifluoro-2,6-dinitro-ll,ll-dipropyl-p-toluidine) was demethylated (Figure 7) (26), and its atmospheric half-life was found to be 8 minutes (27). However, the reaction occurred to a small extent even at night, and oxidation by ozone was implicated. In fact, there is evidence (28) that parathion photooxidation actually required the presence of ozone or other highly reactive oxidants. Degradation not requiring external reagents also may proceed rapidly trifluralin was cyclized to a substituted benzimidazole (11, 26), and dieldrin again formed photodieldrin (29). 

Reaction of DEHP vapor with hydroxyl radicals in the atmosphere has been predicted, with an estimated half-life of about 6 hours using the Atmospheric Oxidation Program (Meylan and Howard 1993). The atmospheric half-life, however, is expected to be longer for DEHP adsorbed to atmospheric particulates. Based on the estimated half-life alone, extensive transport of DEHP would not be expected and concentrations in Antarctic snow would not be predicted. Nonetheless, DEHP appears to be present in urban and rural atmospheres (see Section 6.4), and its transport might be mainly in the sorbed state. Data confirming this degradation pathway have not been located. Direct photolysis and photooxidation are not likely to be important (Warns 1987). 

Oxidation estimated photooxidation rate constant k = 1.43 x 10-12 cm3 molecule-1 s-1 for the vapor-phase reaction with 5 x 105 hydroxyl radicals/cm3 in air at 25°C which corresponds to an atmospheric half-life of 11 d (Atkinson 1987 quoted, Howard 1993). 

The rate constant for the gas-phase reaction of 2-nitrophenol with OH radicals is 9.0x1 O 13 cm3 -molecule/sec at 21 °C (Atkinson 1985) and 8.95x1013 cnwnolecule/sec at 27°C (Gusten et al. 1984. Assuming that a 24-hour average concentration of OH radicals in a normal atmosphere is 5x10 radicals/cm (Atkinson 1985), the atmospheric half-life of 4-nitrophenol due to this reaction is an estimated 18 days, and the reaction may not be an important fate determining process in the atmosphere. 

The atmospheric half-life is based on the reaction with the hydroxyl radical aquatic half-life via biodegradation is based on expert estimates. From EPISuite software (http //www.epa.gov/oppt/greenengineering/software.html) or ChemFate Database (http //www.syrres.com/eswc/chemfate.htm). 

GB is very volatile with a vapor pressure of 2.9 mm Hg at 2519MMacNaughton and Brewer, 1994). A vapor concentration of 22 g/m has been reported for a temperature of 25°C (DA, 1974) (although not adeqnately described in the reference, this presumably is the saturation concentration above a pure liquid). No information was found on the atmospheric half-life of GB. 

No information is available on the transport and partitioning of BCME in the environment. Due to the relatively short half-life in both air and water, it is unlikely that significant partitioning between media or transport occurs. Primary process for BCME degradation in air is believed to be reaction with photochemically generated hydroxyl radicals to yield chloromethyl formate CICHO, formaldehyde, and HCl. Atmospheric half-life due to reaction with hydroxyl radicals is estimated to be 1.36 h. Hydrolysis in the vapor phase is found to be slower with an estimated half-life of 25 h. 

MIBK has a short half-life in the atmosphere and is also biodegraded in water. It is not expected to bioaccumulate. Based on an experimental vapor pressure of 19.9 mmHg at 25°C, MIBK is expected to exist solely as a vapor in the ambient atmosphere. Vapor-phase MIBK is degraded in the atmosphere by reaction with photochemically produced hydroxyl radicals with an estimated atmospheric half-life of 27h. Methyl isobutyl ketone is expected to have high mobility in soils based upon an estimated Kqc value of 123. Volatilization from dry soil surfaces is expected based upon the vapor pressure of this compound. 

Estimate an atmospheric half-life of acrolein. The second-order rate constant for reaction with OH- is approximately 2 X ICO11 cm3/(molecule sec) and the second-order rate constant for reaction with 03 is 4 X ICR13 cm3/(molecule sec). Assume [OH-] is approximately 106 molecules per cubic centimeter and the 03 partial pressure is ICR7 atm. 

Atmospheric half-life at 25 C and 18.4 hours (calculated) Atkinson 1987 HSDB 1995... 

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