United States Environmental Protection Agency Office of Research and Development National Exposure Research Laboratory Research Abstract Government Performance Results Act (GPRA) Goal 1 Annual Performance Measure 20 Significant Research Findings: Evaluation of the Atmospheric Persistence of Air Toxic Compounds Scientific In deliberating the Clean Air Act Amendments of 1990, Congress determined "that the Problem and growth in the amount and complexity of air pollution brought about by urbanization, Policy Issues industrial development, and the increasing use of motor vehicles, has resulted in mounting dangers to the public health and welfare." In response to their concerns about public health and welfare, Congress identified a list of Hazardous Air Pollutants (HAPs, also called "air toxics") for possible regulation by the Agency. The EPA Office of Air Quality Planning Standards (OAQPS), which has responsibility for regulating the HAPs, has identified 33 compounds (or compound classes) that present the greatest threat to public health in the largest number of urban areas. These 33 HAPs are often targeted in air quality modeling analyses and exposure assessments. However, it is impossible to conduct a complete exposure assessment for these HAPs without understanding their lifetime and fate in the environment. Reliable data on the environmental reactions and fate of these 33 HAPs is vital for scientifically sound decisions about potential regulatory actions. A comprehensive search of major scientific databases was completed to identify the data that are currently available on the atmospheric lifetimes, chemical reactions and reaction products, formation rates, and the chemical and physical removal rates of these 33 HAPs. The evaluation identified gaps and uncertainties in the available information. The thirty-three air toxic compounds under study fell into four categories: volatile organic compounds (VOCs), complex compound mixtures (e.g., dioxins, coke oven emissions), metals, and special cases (mercury and hydrazine). The major loss processes examined include chemical reactions with the hydroxyl radical, ozone, and the nitrate radical; and physical processes, including photolysis, dry and wet deposition, and gas-to-particle conversion followed by deposition. Following an evaluation of the loss processes, chemical mechanisms were written describing the conversion of these toxic compounds to their reaction products under atmospheric conditions in a form suitable for incorporation into the Agency's regulatory air quality models. The results of the literature review revealed the major loss process and atmospheric lifetimes for the four targeted categories of HAPs compounds. For extremely slow reacting compounds in the troposphere, such as carbon tetrachloride, removal by dry deposition is possible in addition to removal by chemical mechanisms, and only lower limits to the lifetime could be estimated. Mercury, and in some cases the transition metals (chromium, manganese, and nickel), can have complicated particle-phase Research Approach Results and Impact ------- chemistry, making the lifetimes highly dependent on environmental conditions. Chemical mechanisms for the major VOC removal processes initiated by the hydroxyl radical are provided based either on literature information or inferred from conventional atmospheric reactions. For some of the very slowly reacting VOCs, only a list of known products is given. For formaldehyde, the reaction mechanism includes the photolysis pathway in addition to the hydroxyl radical pathway. In general, very little information is available in the literature on the properties of the reaction products of the air toxic compounds. The information provided by this literature survey and evaluation is being used to develop chemical mechanisms for inclusion in air quality models to estimate ambient concentrations. Such information is needed by national, regional, tribal, state, and local offices to estimate ambient concentrations and aggregate exposures to HAPS and to evaluate strategies for reducing risk associated with exposure to these airborne toxic compounds. This evaluation was conducted by EPA's National Exposure Research Laboratory, with contractor support from ManTech Environmental Technology, Inc. At the present time this review is a stand-alone document that can be used to evaluate mechanisms and rate constants for air toxic compounds currently incorporated in air quality models: U.S. Environmental Protection Agency, National Exposure Research Laboratory, "Literature Review and Evaluation of the Atmospheric Persistence of Air Toxic Compounds" Internal Report, September 2003. Several experimental studies are currently planned to address the most significant data gaps identified in this evaluation. Future research will include smog chamber experiments to test the loss rate of and product formation from chlorinated VOCs in mixtures representative of urban atmospheres. The influence on aerosol and ozone formation on these reactions will also be investigated. Contacts for Questions and inquiries can be directed to the principal investigator: Additional Tadeusz E. Kleindienst Information jj.S. EPA, Office of Research and Development National Exposure Research Laboratory MD D205-03 Research Triangle Park, NC, 27711 Phone: 919-541-3905 E-mail: kleindienst.tad@epa.gov Research Collaboration and Research Products Future Research Federal funding for this research was administered under EPA contract number 68-D- 00-206 to ManTech Environmental Technology, Inc. ------- |