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BIOGENIC SOURCES FOR FORMALDEHYDE AND ACETALDEHYDE DURING SUMMER MONTHS
Citation:
Hutzell, W T. BIOGENIC SOURCES FOR FORMALDEHYDE AND ACETALDEHYDE DURING SUMMER MONTHS. Presented at 2004 Fall American Geophysical Union Meeting, San Francisco, CA, December 13-17, 2004.
Impact/Purpose:
The objective of this task is to improve EPA's ability to accurately predict the concentrations and deposition of air pollutants in the atmosphere that are known or suspected to cause cancer or other serious health effects to humans, or adverse environmental effects. It is an essential component of EPA's National Air Toxics Assessment (NATA), which seeks to identify and quantify the concentrations and sources of those hazardous air pollutants which are of greatest potential concern, in terms of contribution to population risk. It is a major contributor to NERL's Air Toxics Research Program.
"Air toxics" or "hazardous air pollutants" (HAPs) is a category that covers a large variety of chemicals, which range from relatively non reactive to extremely reactive; can exist in the gas, aqueous, and/or particle phases; display a large range of volatilities; experience varying deposition velocities, including in some cases revolatilization; and are emitted from a wide variety of sources at a large variety of different scales. In addition, concentrations of air toxics are needed by regulators for both short (days) as well as long (up to a year) time scales. These requirements challenge our current capabilities in air quality models far beyond the needs for other pollutants, such as ozone. The specific work being done under this task involves 1.) developing and testing chemical mechanisms which are appropriate for describing the chemistry of air toxics; 2.) incorporating these chemical and physical mechanisms into EPA's CMAQ modeling system and applying the model at a variety of scales; and 3.) developing the methods for using models to predict HAPs concentrations at subgrid or neighborhood scales; and 4.) using these tools to assess the magnitude and variability of concentrations to which urban populations are exposed.
Description:
Photochemical modeling estimated contributions to ambient concentrations of formaldehyde and acetaldehyde from biogenic emissions over the continental United States during January 2001 (Eos Trans. AGU, 83(47), Fall Meet. Suppl., Abstract A52B-0117). Results showed that maximum contributions occurred over states along the Pacific and southeastern Atlantic coasts. Biogenic contributions were between 10-50% and 20-90% over urban and rural areas, respectively. For formaldehyde, biogenic contributions were equally divided between direct emissions and photochemical production. Photochemistry accounted from 50% to 80% of acetaldehyde concentrations. Both compounds had highest production yields from Nonmethane Volatile Organic Compounds (NVOCs) that contain double bonds between carbon atoms. Ethene emissions were specifically identified to significantly contribute to formaldehyde concentrations. Acetaldehyde production also had large yields from NVOCs containing single bonds between carbon atoms. Isoprene emissions played a minor role compared to other biogenic NVOCs because winter temperatures and solar irradiances reduced its biogenic sources. We have conducted further modeling to assess biogenic contributions during July 2001. Results show that biogenic contributions dominate at most locations in the continental US. In the southern states, maximum contributions move westward to Louisiana and Arkansas but the states along the Pacific coast remain as locations of maximum contributions. Photochemical production controls most concentrations but up to one third of acetaldehyde can be traced to direct emissions. Isoprene emissions generally produced over 50% of formaldehyde concentrations. Acetaldehyde retained the same patterns in production yields from biogenic NVOCs as January 2001. Terrain and its vegetation type affect how these contributions distribute among specific compounds in biogenic emissions.
The research presented here was performed under the memorandum of understanding between the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Commerce's National Oceanic and Atmospheric Administration (NOAA) and under agreement number DW13921548. Although it has been reviewed by EPA and NOAA and approved for publication, it does not necessarily reflect their policies of views.