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Environmental fate & pathways

Phototransformation in air

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Description of key information

- Atmospheric half-life versus reaction with the hydroxyl radical (OH): 2.3 d.  This is the major sink
- Atmospheric half-life versus reaction with ozone: 45 d. This is a minor sink
- Atmospheric half-life versus reaction with the nitrate radical: 155 d. This is a negligible sink
- Direct photolysis negligible in the atmosphere
- Reaction with the chlorine atom negligible in the atmosphere, except possibly in the marine boundary layer
- Reaction with the ground-state oxygen atom negligible in the atmosphere
- Overall atmospheric half-life: 2.3 d. This is an approximate mean value. The actual half-life will depend on where and when the substance is emitted
- Main primary atmospheric degradation products from reaction with OH: formyl chloride and formaldehyde, each with yields of close to 1 mole per mole of VCM degraded
- Main primary atmospheric degradation product from reaction with ozone: formyl chloride. Other products: formaldehyde, HCl, CO, CO2, formic acid, etc.

Key value for chemical safety assessment

Half-life in air:
2 d

Additional information

Degradation pathways assessed

Experimental rate constants for reactions of VCM with OH, O3 and NO3 (potential "indirect photolysis" or "sensitised" degradation pathways in the atmosphere) have been assessed. Given the availability of such experimental data, it was not deemed useful to assess publications providing purely theoretical estimations of the rate constants.

Degradation pathways not assessed in detail

The direct photolysis of VCM in the atmosphere was considered insignificant, since this compound does nor absorb radiation at wavelengths above about 230 nm (Fujimoto et al, Ber. Bunsenges. Phys. Chem. 74: 282-286, 1970; Gürtler et al, Chemosphere 29: 1671-1682, 1994; Haag et al, Environ. Sci. Technol. 30: 414-421, 1996), while solar radiation reaching the troposphere is virtually devoid of wavelengths below 290 nm.

Futhermore, the reaction of VCM with the chlorine atom (Cl) has been considered to be a negligible atmospheric sink. Indeed, while the rate constant for this reaction is 18 times greater than that for the reaction OH + VCM, the annually and globally averaged atmospheric abundance of Cl atoms has been shown to be < E+3 molecule cm-3 (Singh et al, Geophys. Res. Lett. 23: 1529 -1532, 1996), i.e. about 3 orders of magnitude lower than that of the hydroxyl radical (Prinn et al, Geophys. Res. Lett. 32, L07809, doi:10.1029/2004GL02228, 2005), so the Cl + VCM sink will be negligible compared to OH + VCM, on a global scale. Nevertheless, Cl + VCM may be significant in certain locations, especially in the marine boundary layer, where Cl concentrations may attain 1.0 E+4 to 1.0 E+5 molecule cm-3 (Wingenter et al, J. Geophys. Res. 110, D20308, doi:10.1029/2005JD005875, 2005).

The reaction of the ground-state oxygen atom O(3P) with VCM was also considered insignificant. This conclusion was based on a rate constant at ambient temperature of 5.2 E-13 cm3 molecule-1 s-1 (Hranisavljevic et al, Combust. Sci. Tech. 101: 231-245, 1994) and an assumed mean atmospheric O(3P) concentration of 2.5 E+4 molecule cm-3 (Graedel, Chemical Compounds in the Atmosphere, Academic Press, New York, NY, USA, 1978), leading to a half-life of 1.7 years versus this reaction.

Half-life versus reaction with hydroxyl radical

For the reaction OH + VCM, there are 5 studies reviewed in detail here (Howard 1976, Perry et al, 1977, Liu et al, 1989, Yamada et al, 2001; Becker et al, 1984) that used a range of measurement techniques and yet reported rate constants in good agreement with each other. There is also a critical review by an expert panel that leads to a recommended value of 6.9 E-12 cm3 molecule-1 s-1 based on the first four of these studies. Combining this recommended value with the default global annual average atmospheric hydroxyl radical concentration of 5 E+5 molecule cm-3 stipulated in the European Chemicals Bureau Technical Guidance Document for Risk Assessment Part II (2003) leads to an estimated atmospheric half-life of 2.3 days days for VCM versus reaction with OH. The seemingly pertinent study by Cox RA, Eggleton AEJ & Sandalls FJ (Photochemical reactivity of vinyl chloride, Report AERE-R7820, Atomic Energy Research Establishment, Harwell, UK, September 1974) was not taken into account, given that the mechanistic interpretation of the study was, in the words of its authors "subject to considerable uncertainty" and that the results of the four peer-reviewed studies cited above were available.

It must be emphasised that the calculated half-life of 2.3 days is an approximate mean value. Given the large spatial and temporal variations in the atmospheric OH concentrations, the actual persistence of VCM will depend very much on where and when it is emitted to the atmosphere.

Half-life versus reaction with ozone

For the reaction O3 + VCM, the study by Zhang et al (1983) reviewed here reports a rate constant of 2.45 E-19 cm3 molecule-1 s-1 that, combined with an assumed typical background atmospheric ozone concentration of 30 ppb, leads to an estimated atmospheric half-life of 45 days for VCM versus reaction with O3, suggesting that this sink for VCM is minor (about 5 %) compared to the reaction with OH. Some earlier studies on the reaction O3 + VCM were not analysed in detail, since they were deemed to be less reliable than that of Zhang et al (1983) and they would have led to half-lives either comparable to that estimated here (Gay et al, Environ. Sci. Technol. 10: 58-67, 1976) or even longer (Sanhueza et al, Chem. Rev. 76: 801-826, 1976). In a later study (Ecotox. Environ. Safety 15: 298-319, 1988), Klöpffer et al also reported a somewhat lower rate constant than that of Zhang et al (1983), which would hence also lead to a longer half-life.

Half-life versus reaction with nitrate radical

For the reaction NO3 + VCM, only the study by Noremsaune et al (1997) was assessed in detail. On this basis, the atmospheric half-life of VCM versus reaction with NO3 was estimated to be 155 days, implying that this reaction is a negligible sink for VCM. Earlier studies gave rate constants that would have led to somewhat longer half-lives (see review by Atkinson, J. Phys. Chem. Ref. Data 20: 459-507, 1991).

Primary products of reaction with hydroxyl radical

The primary reaction products of the OH-initiated degradation of VCM under atmospheric conditions were deduced to be formaldehyde and formyl chloride, each with yields close to 1 mole per mole of VCM degraded. This conclusion was based on the paper by Tuazon et al (1988). Other studies on the oxidation of VCM, certain of which purported to refer to atmospheric degradation, were not deemed relevant for various reasons, as discussed below:

- Gay et al, Atmospheric oxidation of chlorinated ethylenes, Environ. Sci. Technol. 10: 58-67, 1976. When VCM was irradiated with UV lamps in the presence of NO2 and air, formic acid, formaldehyde, carbon monoxide, hydrochloric acid and ozone were observed to be the reaction products. However, the species initiating the degradation was not defined and was not necessarily the OH radical (as would be the case in the atmosphere). In particular, no steps were taken to scavenge any chlorine atoms present, which could have led to degradation via a mechanism not relevant to the atmosphere. [It should be noted however that separate experiments on the reaction O3 + VCM were also reported in this study, in addition to the photodegradation experiments].

- Kanno & Nojima, Studies on the photochemistry of aliphatic halogenated hydrocarbons II - Photochemical decomposition of vinyl chloride and ethyl chloride in the presence of nitrogen oxides in air, Chemosphere 8: 509-514, 1977. Dilute mixtures of VCM and NO in air were irradiated with xenon lamps. The production of formaldehyde and HCl was observed. This paper was not considered relevant to the atmospheric degradation of VCM for the same reasons as given for the Gay et al (1976) study, above.

- Dilling et al, Simulated atmospheric photodecomposition rates of methylene chloride, 1,1,1-trichloro-ethane, trichloroethylene, tetrachloroethylene, and other compounds, Environ. Sci. Technol. 10: 351-356, 1976. Dilute mixtures of VCM, NO and water vapour in air were irradiated with UV lamps. The data on degradation rates provided in this paper was not considered relevant to atmospheric conditions, since the nature of the initiating species and its concentration were not defined and the photolysis conditions did not closely mimic the real atmosphere. Furthermore, the nature of the degradation products was not reported, but the results would in any case not have been deemed atmospherically relevant.

- Müller & Korte, Chemosphere 6: 341-346, 1977. This study purported to examine the non-sensitised photoxidation of VCM, despite the fact that the radiation source used emitted at wavelengths longer than the long-wave absorption cutoff of about 230 nm reported in several studies. In any case, the chloroacetaldehyde reported as being the major product is indicative of a chain mechanism propagated by the chlorine atom (see Shinoda, Plastics Industry News Japan, 19: 165-169, 1973; Sanhueza & Heicklen, J. Phys. Chem. 79: 677-681, 1975), which would not be relevant to tropospheric conditions.

- Gürtler et al, Chemosphere 29: 1671-1682, 1994. Similarly to the study of Korte & Müller (1977), this investigation involved direct photolysis of VCM (at 185 nm) and led to chloroacetaldehyde and other degradation products, resulting from a chain reaction propagated by the chlorine atom, which would not be relevant to tropospheric conditions.

- Carassiti et al, Ann. Chimica 67: 499-512, 1977. This paper appears to present solely theoretical calculations of the atmospheric degradation rate and product formation, based on the prior studies by Cox et al (1974), Gay et al (1976) and Dilling et al (1976 ). The reasons for not taking into account these three studies in the present assessment have been explained above.

- Woldbaek & Klaboe, Spectrochim. Acta 34A: 481-487, 1978. This study involved irradiating VCM in the presence of air and (in some experiments) NOx. The initial concentrations of VCM were however relatively high (tens of torr) and no steps were taken to scavenge any chlorine atoms formed during the oxidation, so the species initiating the degradation was not well defined. Under these conditions, the relevance of this sudy to the atmospheric degradation of VCM is doubtful.

Products of reaction with ozone

While reaction of VCM with ozone is a relatively minor atmospheric sink, it is believed to lead (like reaction with OH) to formyl chloride as a major product, with 0.76 mol HCOCl formed per mol VCM degraded (Zhang et al, 1983). Other products include formaldehyde, CO, CO2, HCl, formic acid and vinyl chloride epoxide (Zhang et al, 1983; Vaccani et al, Chem. Phys. Lett. 50: 187-189, 1977).