Kinetics of Semi-volatile Organic Compounds and Hydroxyl Radical ReactionsEPA Grant Number: R825377
Title: Kinetics of Semi-volatile Organic Compounds and Hydroxyl Radical Reactions
Investigators: Hites, Ronald A.
Institution: Indiana University - Bloomington
EPA Project Officer: Shapiro, Paul
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 , Engineering and Environmental Chemistry
The polychlorinated dibenzo-p-dioxins and dibenzofurans include some of the most toxic compounds produced by man. These compounds are primarily formed by anthropogenic combustion throughout the world and injected into the atmosphere. Once there, they are transported for considerable distances before they deposit on vegetation, soil, and water. Those dioxins and furans that deposit on vegetation may enter the Nation's food supply. To determine people's risk of exposure to atmospheric dioxins and furans, many facts are necessary. These include a knowledge of the sources and toxicology of these compounds, an understanding of the routes by which these compounds can enter people, and a detailed accounting of the behavior of these compounds while they are in the atmosphere. In the latter case, one of the largest gaps in our knowledge is rate constants of photodegradation. In fact, the U.S. Environmental Protection Agency's dioxin reassessment specifically says, "research is needed which provides photodegradation rate constants for these compounds in the air." This is the point of this project.
Prof. Hites' laboratory has previously studied the reactions of particle-associated dioxins and furans; they are stable under these conditions. It is now necessary to study the reactions of vapor phase dioxins and furans. Thus, this project will measure temperature dependent second-order rate constants for the reaction of dioxins with the hydroxyl (OH) radical. For these measurements, Prof. Hites' laboratory will use a heated, glass reaction chamber monitored directly by a mass spectrometer. It has been demonstrated that this system works well for toluene, biphenyl, and selected polychlorinated biphenyls (PCBs). Modifications to the system include electron capture negative ionization mass spectrometry, a tool which is capable of exquisite sensitivities.
In addition to the simple (but technically challenging) measurements of these important rate constants, this project's research plan has two other features. First, the measured second order dioxin rate constants will be converted to atmospheric residence times. Using this approach, the atmospheric residence times of each compound in the study will be determined, information which will be very useful in assessing the relative risk of these compounds. For example, imagine that compound x has an atmospheric residence time of 3 days but that compound y has a residence time of 60 days. Assuming similar toxicities of these two compounds, it is clear that compound y poses the greater risk. Second, the relative rates of degradation by reactions with OH will be estimated for all of the dioxin and dibenzofuran homologues with four to eight chlorines, and these relative rates will be used to compare this degradation process to other simple deposition processes.