Atmospheric Chemistry of Gasoline-Related Emissions: Formation of Pollutants of Potential Concern

Jan 17, 2006

Gasoline powered vehicles emit a variety of volatile organic pollutants, including alkanes, alkenes, aromatic hydrocarbons and oxygenates. The pollutants that are emitted at the tailpipe and that evaporate from the fuel system are not necessarily the same as those to which the population is exposed, due to the atmospheric transformation processes that these pollutants undergo. Transformation products need to be considered in addition to primary emissions as part of conducting a comprehensive assessment of potential human health hazards associated with gasoline usage. The primary objective of this research effort was to identify secondary products associated with gasoline combustion and evaporative emissions that may pose a toxicological concern or may be present at relatively high levels in the atmosphere. A secondary objective was to assess the atmospheric lifetimes of gasoline-related pollutants, as constituents with longer lifetimes would be of greater exposure concern. This document includes a general discussion of the lifetime and fate of volatile organic compounds (VOCs) and describes removal mechanisms for gasoline-related pollutants in the atmosphere. An overview of the reactions of classes of VOCs associated with gasoline usage is provided. Some brief notes on inorganic compounds of interest and gas/particle partitioning are also included. Gasoline-related pollutants were selected based on toxicological and/or exposure concerns for a detailed review of their atmospheric chemistry, which is provided in Appendix 1. Appendix 2 describes the procedures used to estimate hydroxyl radical reaction rates and the resulting distribution of alkyl radicals for alkanes that have not been studied experimentally, and also describes the procedures used to estimate the reaction rates of alkoxy radicals under atmospheric conditions. The mass emissions ranking method that was used to prioritize the potential for exposure to gasoline-related pollutants was based on 1998 data for California Phase 2 Reformulated Gasoline and is outlined in Appendix 3.