Environ. Sci. Technol. 1994, 28, 528-533
Gas-Phase Atmospheric Chemistry of Dibenzo-pdioxin and Dibenzofuran Eric S. C. Kwok,t Janet Arey,'7* and Roger Atkinson*
Statewide Air Pollution Research Center, University of California, Riverside, California 9252 1 Rate constants for the gas-phase reactions of dibenzofuran and dibenzo-p-dioxin with the OH radical and O3 have been determined at 297 f 2 K and atmospheric pressure of air. Using a relative rate method, rate constants for the OH radical reactions were 3.9 x 10-l2cm3 molecule-l s-l for dibenzofuran and 1.48 X cm3 molecule-l s-l for dibenzo-p-dioxin. Upper limits to the rate constants for the 0 3 reactions of 4 3 X and 205 330 From ref 33. * For a 12-h average daytime OH radical concentration of 1.6 X IO6 molecule ~ m (51). - ~ For a 12-h average nighttime NO3 radical concentration of 5 X los molecule ~ m (52). - ~ An NO2 concentration of 2.4 X 1OI2molecule ~ m (100 - ~ppb) has been used. For a 24-h average 0 3 concentration of 7 X 10" molecule cm3 (30 ppb) (53). e Calculated lifetime is linearly dependent on the NO2 concentration. 1.48 X 1O-I'
Products. 2-Nitrodibenzo-p-dioxin was synthesized by reacting dibenzo-p-dioxin with nitric acid as described by Saint-Ruf and Lobert (38),and its identity was confirmed by lH NMR using a GE 300-MHz NMR spectrometer. Methyl nitrite was prepared and stored as described previously (39),and 0 3 was generated as needed using a Welsbach T-408 ozone generator.
Results The dark decays of dibenzofuran and dibenzo-p-dioxin in the chambers were 17% and 1 1 2 % ,respectively, over time periods up to 175 min and were generally within the measurement uncertainties of f 5 % . The addition of 0 3 at concentrations of (7.0-11.6) X 1013molecule ~ m to- ~ dibenzofuran (or dibenzo-p-dioxin)-cyclohexane-air mixtures led to no significant enhancement of the dibenzofuran or dibenzo-p-dioxin decay rates, with 1 6 % loss of either compound being measured over reaction times of 181-190 min (no corrections were made for dark decays). Assuming a 5 6 % reactive decay of dibenzofuran and dibenzo-p-dioxin over these reaction times combined with the O3concentrations leads to the upper limits to the rate constants for reaction with O3 given in Tables 1 and 2. A series of irradiations of aromatic-cyclohexane (in excess)-air and CHSONO-NO-aromatic-referenceorganic-air mixtures were carried out at 50% or 100% of the maximum light intensity. Photolyses of aromatic-cyclohexane (in excess)-air mixtures at 50 % of the maximum light intensity led to measured disappearances of gas-phase dibenzofuran of 8% over 60 min, 7% over 291 min,
