Grantee Research Project Results
Final Report: Kinetics of Semi-volatile Organic Compounds and Hydroxyl Radical Reactions
EPA Grant Number: R825377Title: Kinetics of Semi-volatile Organic Compounds and Hydroxyl Radical Reactions
Investigators: Hites, Ronald A.
Institution: Indiana University - Bloomington
EPA Project Officer: Hahn, Intaek
Project Period: October 14, 1996 through October 13, 1999
Project Amount: $356,212
RFA: Exploratory Research - Air Engineering (1996) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Land and Waste Management , Air , Safer Chemicals
Objective:
This project focused on the atmospheric fates of polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and related compounds. One goal was to obtain a detailed accounting of the behavior of these compounds while in the atmosphere. One of the largest gaps in our knowledge was the rate constants of chemical degradation in the gas phase; therefore, our specific goal was to measure second-order rate constants for the reactions of these compounds with the hydroxyl radical (OH) in the gas phase. Another goal was to use these rate constants to develop a mass balance of emissions, degradation, and deposition. The latter goal required some measurement effort focused on air collected over the remote north Atlantic.
Summary/Accomplishments (Outputs/Outcomes):
To make the rate constant measurement, we had to get the compounds of interest into the gas phase. We did this with a heated, glass reaction chamber monitored directly by a mass spectrometer operated in the electron capture, negative ionization mode. We gain sensitivity by this highly selective and sensitive ionization technique, but there was a limit to the elevated temperature approach—the sources of OH also tended to decompose thermally as the temperature increased. We overcame this limitation by using both O3/H2O and H2O2 as the OH source. The atmospheric concentrations of dioxins in air over the remote north Atlantic were very low; thus, we had to achieve ultra-high sensitivity to make these measurements. This was achieved by the simple expedient of sampling or compositing very large volumes of air (up to 20,000 m3) and by using electron capture, negative ionization mass spectrometry to quantitate these samples.
Gas-phase reactions with the OH were expected to be an important removal pathway of PCDDs and PCDFs in the atmosphere. Our laboratory recently developed a system for measuring the rate constants of the gas-phase reactions of OH with semivolatile organic compounds using online mass spectrometry. We incorporated electron capture mass spectrometry (EC-MS) into this system to increase its sensitivity to PCDDs and PCDFs, which tend to have low vapor pressures. OH reaction rate constants were determined in helium for 1,2,3,4-tetrachlorodibenzo-p-dioxin at 373–432 K using a heated quartz reaction chamber. The photolysis of ozone (O3) in the presence of H2O and the photolysis of H2O2 (both at = 254 nm) served as OH sources. An extrapolation using the Arrhenius equation gave a 1,2,3,4-tetrachlorodibenzo-p-dioxin-OH reaction rate constant of 8.5 x 10-13 cm3s-1 at 298°K, which strongly correlated with the value predicted by a structure-activity method. The predicted OH reaction rate constants for tetra- through octachlorodibenzo-p-dioxin and dibenzofuran isomers were used in a simple model of the atmospheric removal of PCDDs and PCDFs. The results of our model indicated that atmospheric removal was a combination of gas-phase removal processes of lower chlorinated dioxins and furans, and particle-phase removal processes of higher chlorinated ones.
In addition, rate constants for the gas-phase reactions of the OH with x- and
y-hexachlorocyclohexane ( x- and yx-HCH) and hexachlorobenzene (HCB) were measured over the temperature range 346–386°K. These experiments were conducted in helium (He) diluent gas at approximately 1 atm in a 160-ml quartz chamber. OH was produced by the photolysis of O3 in the presence of H2O, and reactants in the chamber were monitored by online mass spectrometry. The rate constants measured at elevated temperatures were extrapolated by the Arrhenius equation, and OH reaction rate constants at 298°K were estimated for each compound. These rate constants at 298°K (in units of 10-13 cm3s-110-13 cm3s-1) were: x-HCH, 1.4; y-HCH, 1.9; and HCB, 0.27. Atmospheric lifetimes based on OH reactions ( OH) also were estimated (in units of days): x-HCH, 120; y-HCH, 96; and HCB, 940. These relatively high atmospheric lifetimes indicated that these compounds could be transported great distances.
Rate constants also were measured for gas-phase reactions of the OH with six polycyclic aromatic hydrocarbons (PAHs) and four other PCDDs and PCDFs. These OH reaction rate constants were determined in He at about 1 atm over various temperature ranges between 306–405°K. The experiments were carried out in a small, heated quartz reaction chamber sampled by online mass spectrometry, and OH was produced by the photolysis of O3 in the presence of H2O. Arrhenius regressions were performed with the rate constants of each compound, and the temperature dependencies were found to be slight to nonexistent. The OH reaction rate constants of PCDD/PCDF were in agreement with those predicted by a structure-reactivity method. The resulting rate constants at 298°K (in units of 10-12 cm3s-1) were: naphthalene, 23; acenaphthene, 58; fluorene, 13; phenanthrene, 27; anthracene, 190; fluoranthene, 11; dibenzo-p-dioxin, 12; 2,7-dichlorodibenzo-p-dioxin, 4.4; dibenzofuran, 3.5; and 2,8-dichlorodibenzofuran, 2.2.
Our laboratory measured the gas-phase reaction rate constants of polychlorinated biphenyls (PCBs) with the OH. We concluded that OH reactions are the primary removal pathway of PCBs from the atmosphere. With the reaction system previously employed for kinetics, we investigated the products of these PCB-OH reactions. Experiments were carried out in either air or He as the diluent gas at approximately 1 atm in a 160-ml quartz chamber. Temperatures ranged between 318–363°K to enhance the vapor pressures of these less volatile compounds. OH was produced in situ by the photolysis of O3 in the presence of H2O. Reaction products from biphenyl, the three monochlorobiphenyls, and five dichlorobiphenyls were extracted from the chamber, derivatized by diazomethane, and analyzed by gas chromatographic mass spectrometry. Experiments gave benzoic acid and the appropriate chlorinated benzoic acids in significant product yields (8–17 percent).
We developed a sampling strategy that allowed us to determine fg/m3 concentrations of PCDDs and PCDFs in remote marine atmospheres. Using this sampling strategy, a total of 37 air samples were taken during two extended sampling periods at Bermuda between September 1993 and August 1997. During this time, the average total PCDD/PCDF concentrations at Bermuda decreased from 105 ± 30 fg/m3 to 35 ± 10 fg/m3, giving a half-life of about 2 years for these compounds in the remote marine atmosphere. PCDD/PCDF concentrations during both sampling periods were somewhat higher in the winter when air parcels originated from North America. A second air sampling station was established at Barbados, where 22 air samples were taken between March 1996 and August 1997; an average total PCDD/PCDF concentration at Barbados of 15 ± 7 fg/m3 was found. This value was not significantly different than the 27 ± 7 fg/m3 found at Bermuda during this time when air arrived from the east. This indicated that the remote marine background concentration for these compounds was on the order of 20 fg/m3. Using this background concentration, the dry depositional rate of PCDD/PCDF to the world's oceans was estimated to be 200 ± 80 kg/yr, and the wet depositional rate was estimated to be 900 ± 300 kg/yr. This is a total deposition rate of about 1 ton/yr to the oceans, as compared to our previous estimate of about 10 ton/yr PCDD/PCDF deposition from the atmosphere to the land.
Journal Articles on this Report : 8 Displayed | Download in RIS Format
Other project views: | All 20 publications | 8 publications in selected types | All 8 journal articles |
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Baker JI, Hites RA. Comment on "Evidence for the presence of PCDD/Fs in the environment prior to 1900 and further studies on their temporal trends." Environmental Science & Technology 1999;33(1):205. |
R825377 (1998) R825377 (Final) |
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Baker JI, Hites RA. Polychlorinated dibenzo-p-dioxins and dibenzofurans in the remote North Atlantic marine atmosphere. Environmental Science & Technology 1999;33(1):14-20. |
R825377 (1998) R825377 (Final) |
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Baker JI, Hites RA. Siskiwit Lake revisited: time trends of polychlorinated dibenzo-p-dioxin and dibenzofuran deposition at Isle Royale, Michigan. Environmental Science and Technology 2000;34(14):2887-2891. |
R825377 (Final) |
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Brubaker Jr. WW, Hites RA. Polychlorinated dibenzo-p-dioxins and dibenzofurans:gas phase hydroxyl radical reactions and related atmospheric removal. Environmental Science & Technology 1997;31(6):1805-1810. |
R825377 (1998) R825377 (Final) |
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Brubaker Jr. WW, Hites RA. Gas phase oxidation products of biphenyl and polychlorinated biphenyls. Environmental Science & Technology 1998;32(24):3913-3918. |
R825377 (1998) R825377 (Final) |
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Brubaker Jr. WW, Hites RA. OH reaction kinetics of polycyclic aromatic hydrocarbons and polychlorinated dibenzo-p-dioxins and dibenzofurans. Journal of Physical Chemistry A 1998;102(6):915-921. |
R825377 (1998) R825377 (Final) |
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Brubaker Jr. WW, Hites RA. OH reaction kinetics of gas-phase α-and γ-hexachlorocyclohexane and hexachlorobenzene. Environmental Science & Technology 1998;32(6):766-769. |
R825377 (1998) R825377 (Final) |
Exit Exit Exit |
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Willett KL, Hites RA. Chemical actinometry:using o-nitrobenzaldehyde to measure lamp intensity in photochemical experiments. Journal of Chemical Education 2000;77(7):900-902. |
R825377 (1998) R825377 (Final) |
Exit Exit |
Supplemental Keywords:
atmosphere, troposphere, dioxins, organics, environmental chemistry., RFA, Scientific Discipline, Air, Environmental Chemistry, tropospheric ozone, Engineering, Chemistry, & Physics, fate, dioxin, hydroxyl radical, semi-volatile organic compounds, fate and transport, food chain, toxicology, PCBs, VOCs, chemical kinetics, furans, photodegradationProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.