The Photochemical Degradation of Chloral and Oxalyl Chloride

摘要

The major source of chloral, CCl_3CHO in the atmosphere is from the degradation of 1,1,1-trichloroethane, methyl chloroform, (Nelson et al., 1990, Platz et al., 1995). In comparison, direct emissions of chloral from its use as a feedstock chemical are negligible. While methyl chloroform was formerly widely used as a solvent, it is now strictly regulated as an ozone-depleting substance under the Montreal Protocol. As a consequence, estimates of annual global emissions have fallen by approximately 95% over the last 10 years (Midgeley and McCulloch, 1999). These calculations are borne out by recent observations of atmospheric background concentrations of methyl chloroform (Montzka et al, 1999). Methyl chloroform degrades mainly by reaction with the hydroxyl radical in the troposphere to give chloral (～85%) with minor contributions from hydrolysis in the oceans (～5%) and destruction in the stratosphere (～10%) (Cox, 1998). Chloral is expected to have a relatively short atmospheric lifetime due to its loss via reaction with the OH radical, UV photolysis and wet deposition (Nelson et a.l, 1990, Platz et al, 1995, Rattigan et al, 1993, Barry et al, 1994, Sidebottom and Franklin, 1996, Rattigan et al, 1998, Talukdar et al, 2001). It has generally been assumed that the major tropospheric loss process for chloral is by photolysis. This suggestion is based on the UV absorption cross sections of chloral and an assumed quantum yield for photodissociation of unity (Rattigan et al, 1993, Sidebottom and Franklin, 1996, Rattigan et al, 1998). This work is concerned with a determination of the quantum yields for photodissociation of CCl_3CHO and for formation of the resulting products under atmospheric conditions.