Speciation of volatile organic compound emissions for regional air quality modeling of particulate matter and ozone

Abstract

A new classification scheme for the speciation of organic compound emissions for use in air quality models is described. The scheme uses 81 organic compound classes to preserve both net gas-phase reactivity and particulate matter (PM) formation potential. Chemical structure, vapor pressure, hydroxyl radical (OH) reactivity, freezing point/boiling point, and solubility data were used to create the 81 compound classes. Volatile, semivolatile, and nonvolatile organic compounds are included. The new classification scheme has been used in conjunction with the Canadian Emissions Processing System (CEPS) to process 1990 gas-phase and particle-phase organic compound emissions data for summer and winter conditions for a domain covering much of eastern North America. A simple postprocessing model was used to analyze the speciated organic emissions in terms of both gas-phase reactivity and potential to form organic PM. Previously unresolved compound classes that may have a significant impact on ozone formation include biogenic high-reactivity esters and internal C6-8 alkene-alcohols and anthropogenic ethanol and propanol. Organic radical production associated with anthropogenic organic compound emissions may be 1 or more orders of magnitude more important than biogenic-associated production in northern United States and Canadian cities, and a factor of 3 more important in southern U.S. cities. Previously unresolved organic compound classes such as low vapour pressure PAHs, anthropogenic diacids, dialkyl phthalates, and high carbon number alkanes may have a significant impact on organic particle formation. Primary organic particles (poorly characterized in national emissions databases) dominate total organic particle concentrations, followed by secondary formation and primary gas-particle partitioning. The influence of the assumed initial aerosol water concentration on subsequent thermodynamic calculations suggests that hydrophobic and hydrophilic compounds may form external mixtures, and that separate treatment for these groups may be required in the future air quality model simulations. The post-processing model used here overestimates the organic particle formation relative to measurements, lacks the complexity of a regional air quality model, and is not intended as an alternative to latter. Results from the post-processing model do, however, provide guidance for the treatment of organic gases and particles in future air quality modeling work. Future air quality model simulations should attempt to speciate primary particulate organic compounds and include more detailed organic compound classes. Future emissions profile measurements should speciate gaseous high-molecular-mass organic compounds and primary organics emitted in particulate form (primary particle emissions are only available as a total particulate mass in currently available missions data).