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Environmental fate & pathways

Phototransformation in air

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Acetonitrile is degraded in the atmosphere by reaction with photochemically-produced hydroxyl radicals; the half-life of this reaction in air has been estimated to range from 20-620 days.  An overall half-life for acetonitrile in the troposphere of about 42 days has been estimated considering both the OH radical and ozone reaction rate constants (USEPA, 1987). However, based on average tropospheric conditions reported by Klopffer (1988), this number would be about one year (321 days) according to the 2000 EU Risk Assessment for acetonitrile.  Acetonitrile absorbs light only in the far UV regions, and therefore is not expected to be susceptible to direct photolysis by sunlight.

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The maximum of absorption for acetonitrile in the UV range is lower than 160 nm, therefore direct photolysis of acetonitrile in the atmosphere is not expected to be an important fate process (Silverstein and Bassler, 1967; Howard, 1991), (Arijs, 1983; HSDB, 2009).

Acetonitrile is reactive to oxidising materials and compounds in the atmosphere, being the reaction with hydroxyl radicals quoted as one of the main mechanisms for its removal from the environment (Lobert et al., 1990). The rate constant for the reaction of acetonitrile with hydroxyl radicals has been determined by several authors to range from 1.9 - 4.94 x 10-14cm3. molec-1. sec-1for the temperature range 20-27ºC. In a moderately polluted atmosphere, with a mean concentration of 107hydroxyl radicals/cm3, the calculated half-life was about 20 days (Atkinson, 1985; Guesten et al., 1981; Güsten et al., 1984; Harris et al. 1981; Wallington et al., 1988, Lyman et al., 1982). The Arrhenius activation energy, as determined by Harris et al (1981), was 1500 cal. mole-1.  Howard, 1993 presented a much longer half life for this reaction of 535 days based on data in (Atkinson, 1985), and HSDB (2009) estimated the atmospheric half-life of acetonitrile to be 620 days based on a hydroxyl radical reaction rate constant of 2.63 x 10E-14 cm3/molecule-sec at 25 deg C (Atkinson, 1994).

In conditions of an average tropospheric OH radical concentration, [OH] = 5 x 105cm-3, Klöpffer et al. (1988) obtained a half-life of >160 days, or about 10 times longer than that reported above. Poschl (2001) specified the photochemical lifetime of acetonitrile to be approximately 500 days in the lower troposphere and relatively longer in the upper troposphere (HSDB, 2009).

The reaction of acetonitrile with ozone is slow, with a reported reaction rate constant of 1.5 x 10-19cm3. molec-1. sec-1(Harris et al., 1981), which will yield a half-life of 54 days, considering an average ozone abundance of 1 x 1012molec/cm3(Munshi et al., 1989). Using a reaction rate of 0.15 x 10-18 cm3/mol.sec and an atmospheric ozone concentration of 7 x 10 E11 mols/cm3, at 25 degree C or at room temperature, the half-life has been estimated to be 76.4 days (Atkinson and Carter, 1984; Envirofate, 1994). A much longer half life for acetonitrile in the presence of ozone of about 860 days has been reported based on data in (Atkinson and Carter, 1984; Atkinson, 1985) by Howard, 1993).

A reaction rate constant between acetonitrile and single oxygen of 2.4 x 10-16cm3. molec-1. sec-1 has been reported (Graedel, 1978), which yields an estimated atmospheric half-life of >5000 years. Acetonitrile reaction with chlorine radicals is not thought to be significant in relation to hydroxyl radical reaction (Arijs et al., 1983).

An overall half-life for acetonitrile in the troposphere of about 42 days has been estimated, considering both the OH radical and ozone reaction rate constants (USEPA, 1987), based on rate constants quoted by Harris et al (1981). Nevertheless, as indicated above, that value should be about 10 times higher for average tropospheric conditions as reported by Klopffer (1988), which would give a half live of about one year (321 days) according to the 2000 EU Risk Assessment of acetonitrile.

Due to the nonreactivity of acetonitrile in the atmosphere, transport of acetonitrile from troposphere to stratosphere is expected to occur, and acetonitrile has been detected in the stratosphere (Arijs et al, 1983)