Photochemical Reactivity of Perchloroethylene: A New Appraisal

Abstract

Perchloroethylene (PCE), a solvent used in dry cleaning, has been suspected of contributing significantly to photochemical ozone/oxidant (O 3 /O x ) problems in urban atmospheres. Past evidence, however, was neither complete nor consistent. To interpret more conclusively the past evidence, and further understand PCE's role in the O 3 /O x problem, a smog chamber testing program was conducted. The program's objectives were: (a) to explain the mechanism of the PCE reaction in smog chamber atmospheres, and (b) to extrapolate the smog chamber findings regarding PCE reactivity to the real atmosphere. Results showed that in smog chambers, PCE reacts and forms O 3 /O x following what appears to be a Cl instigated photooxidation mechanism rather than the OH initiated mechanism accepted in current smog chemistry. The evidence, collectively, strongly supported this conclusion even though the source of Cl atoms could not be identified with confidence. It was further concluded that in the real atmosphere neither the Cl instigated nor the OH instigated photooxidations of PCE can generate substantial concentrations of O 3 /O x . In fact, PCE contributes less to the ambient O 3 /O x problem than equal concentrations of ethane.

Chlorine-hydrocarbon photochemistry in the marine troposphere and lower stratosphere

“…However, it might be an appreciable source of CI atoms in regions where C2CI 4 is being released and in smog chamber studies involving irradiation of C2C14/N0 x mixtures (e.g. Dimitriades et al, 1983). It might also play an important role in the photooxidation of CH3I and CHBr3 and thereby help to generate I and Br atoms (Chameides and Davis, 1980;Penkett et al, 1985).…”

Halocarbenes May Deplete Atmospheric Ozone

“…Gay et al reported about 50% more HCl formed than CH 2 O and also the presence of HCOCl which decomposes to CO and HCl [10]. The rapid decrease in O 3 concentration in the PERC and TCE systems was attributed to the presence of Cl atoms [2] resulting from the photocleavage of PERC and TCE [8,9,17]. Also, the formation of the accompanying chlorinated acetyl chlorides in the TCE and PERC systems was explained by a Cl atom-initiated mechanism.…”

“…It has been noted that Tuazon et al assume without experimental confirmation that their TCE and PERC reactions proceed via the stabilisation channel [18]. Gay and co-workers also performed radical scavenging experiments in which PERC was irradiated with high concentrations (1000-10,000 ppm) of butane [17]. Comparison of these PERC disappearance rates with those of PERC/butane mixtures which just stood in the dark "did not lead to unequivocal conclusions."…”

“…is larger (it is also larger with trans-1,2-DCE and with cis-1,2-DCE it is within experimental error) than in the absence of ethane. Since the Cl atom reaction is faster than that of the OH radical, triplet oxygen or O 3 [15,17,33], if Cl was responsible for the formation of acyl chloride and ethane was scavenging Cl atoms, the Cl concentration would be less, resulting in a slower reaction as opposed to a faster reaction. The situation continues to be intriguing in that, for 1,1-DCE and TCE, an unidentified product X was formed which increased in yield by about 50% for 1,1-DCE and by a factor of five for TCE in the presence of a scavenger.…”