Atmospheric reactions of halothane and halogenated alkanes

It has been established that OH radical and Cl atom reactions are important removal mechanisms for water insoluble volatile organic chemicals (VOCs) in the troposphere. To determine the tropospheric fate of a haloalkane and hence the possible significance in stratospheric ozone depletion, we must first determine the rate of reaction of this species with both OH radicals and Cl atoms and thus the efficiency with which it is removed from the troposphere. To establish the ultimate atmospheric fate of one such haloalkane, halothane (CF3CHClBr), a widely used anaesthetic, tropospheric and stratospheric photooxidation reaction mechanisms were investigated for this species using two well established initiation steps. Results indicated that halothane will yield CF3CClBr radicals in the troposphere via hydroxyl radical attack: CF3CHClBr + OH* - CF3CClBr# + H20 and CF3CHCI radicals in the stratosphere via photolysis: CF3CHClBr + hv - CF3CHC1* + Br* ( X < 290 nm) under simulated atmospheric conditions. The stratospheric reaction mechanisms ultimately led to the formation of CF3COCl found to be a primary product, and CF2O and CO2 both secondary products, from the oxidation of CF3CHC1 radicals. A study of the quantum yields of formation of these species would confirm this result. Studies of the chlorine-sensitised photooxidation of halothane showed that the two major products were CF3CCl2Br and (CF3CClBr)2 , confirming that chlorine atom attack on halothane yielded CF3CClBr radicals. The effect of varying the [Cl2]/[CF3CHClBr] ratio on the ratio of product concentrations showed that at all times, both products were formed, although the relative amounts of the two products varied. Further work initiated bv this study would allow us to assess quantitativelv the oxidation of CF3CHC1 and CF3CClBr radicals. The kinetic data reported in this work is among the first dealing with the reaction of Cl atoms with a series of brominated and chlorinated alkanes. Their atmospheric lifetimes were established, with respect to reaction with Cl atoms using a relative rate smog chamber technique. The chlorine atoms were produced by the photolysis of chlorine gas We report the first measurements of rate constants for the reactions of the chlorine atom with 2-chloropropane, 1,3 dichloropropane 2-chloro 2 methylpropane, bromoethane, 1-bromopropane, 2 -bromopropane, 1-bromobutane, 1- bromopentane and 1-bromohexane excepting K(Cl+l-chloropropane) previously reported Cl atom reactions were measured at 298K and using a value of 2 0 x 1 0^ cm*3 for [Cl], the lifetimes of the haloalkanes were calculated The atmospheric lifetimes with respect to Cl atoms are long, 1.3 dichloropropane has the longest tropospheric lifetime of the compounds studied (1 5 years) and sufficiently long tropospheric lifetimes will allow transportation of these species to the stratosphere where they may release their chlorine atoms, ultimately resulting in ozone depletion