| Semi-volatile organic compounds (SOCs) represent the most toxic pollutants produced by man. They are dispersed globally through the atmosphere, where gas-phase reactions with the hydroxyl radical (OH) are expected to be an important removal pathway. Our laboratory recently developed a reaction system that measures gas-phase OH reaction rate constants for SOCs. The system has now been modified to extend the range of SOCs that can be studied and to allow the identification of OH reaction products. The reaction system consists of a small, heated quartz chamber sampled by on-line mass spectrometry. Electron capture mass spectrometry (EC-MS) was incorporated into the reaction system to increase its sensitivity to polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), which tend to have lower vapor pressures.; OH reaction rate constants between 345-432 K were determined in helium at about 1 atm for five PCDD/F isomers. OH radicals were produced by the photolysis of O{dollar}sb3{dollar} in the presence of H{dollar}sb2{dollar}O and, on a limited basis, the photolysis of H{dollar}rmsb2Osb2.{dollar} Extrapolations using the Arrhenius equation gave the following OH reaction rate constants at 298 K (in units of 10{dollar}sp{lcub}-12{rcub}{dollar} cm{dollar}sp3{dollar} s{dollar}sp{lcub}-1{rcub}{dollar}): dibenzo-p-dioxin, 12; dibenzofuran, 3.5; 2,7-dichlorodibenzo-p-dioxin, 4.4; 2,8-dichlorodibenzofuran, 2.2; and 1,2,3,4-tetrachlorodibenzo-p-dioxin, 0.85. These values agree with those predicted by a structure-activity method. The predicted OH reaction rate constants for tetra- through octachlorinated dioxin and furan isomers were then applied to a simple model of the atmospheric removal of PCDD/Fs. The results of our model indicate that atmospheric removal is a combination of the gas-phase removal processes of the lower chlorinated dioxins and furans and the particle-phase removal processes of the higher chlorinated ones.; Rate constants were also measured for the gas-phase OH reactions of six polycyclic aromatic hydrocarbons (PAHs) and the organochlorine pesticides {dollar}alpha{dollar}- and {dollar}gamma{dollar}-hexachlorocyclo-hexane ({dollar}alpha{dollar}- and {dollar}gamma{dollar}-HCH) and hexachlorobenzene (HCB). These OH reaction rate constants were determined over various temperature ranges between 306-405 K. The extrapolated rate constants at 298 K were (in units of 10{dollar}sp{lcub}-12{rcub}{dollar} cm{dollar}sp3{dollar} s{dollar}sp{lcub}-1{rcub}{dollar}): naphthalene, 23; acenaphthene, 58; fluorene, 13; phenanthrene, 27; anthracene, 190; fluoranthene, 11; {dollar}alpha{dollar}-HCH, 0.14; {dollar}gamma{dollar}-HCH, 0.19; and HCB, 0.027. These rate constants indicate that PAHs are more reactive toward OH in the atmosphere, relative to polychlorinated biphenyls (PCBs) and PCDD/Fs, while {dollar}alpha{dollar}- and {dollar}gamma{dollar}-HCH and HCB are more inert.; With the reaction system previously employed for kinetics, gas-phase PCB-OH reaction products were investigated. Experiments were carried out in either air or He at approximately 1 atm, and temperatures ranged between 318-363 K. Biphenyl, the three monochlorobiphenyls, and five dichlorobiphenyls were reacted, and their reaction products were extracted from the chamber, derivatized by diazomethane, and analyzed by gas chromatographic mass spectrometry (GC/MS). Experiments gave benzoic acid and the appropriate chlorinated benzoic acids in significant yields (8-17%). A mechanism is suggested. |
