Development and characterization of an exposure generation system to investigate the health effects of particles from fresh and aged traffic emissions

Atmospheric photochemical reactions of vehicular primary emissions result in the formation of secondary organic aerosol (SOA). This is the first study that has investigated the toxicity of secondary particles based on fleet vehicular emissions. We developed methods for photochemical oxidation of traffic primary emissions to produce mixtures of primary and/or secondary particles suitable for animal exposures. The exposure generation system produced test atmospheres of primary (P), aged primary plus SOA (P + SOA), or SOA particles suitable for animal exposures. The system consists of (1) a sampling system to extract the traffic emissions from the plenum of a highway tunnel ventilation stack, (2) a photochemical chamber to simulate atmospheric aging, and (3) a nonselective diffusion denuder to remove gaseous pollutants prior to exposure. In the presence of traffic primary particles (P + SOA), a longer mean residence time resulted in a higher SOA yield. Higher baseline plenum primary particle mass concentration resulted in lower SOA yield. In the absence of primary particles (SOA), higher plenum gas concentrations resulted in higher SOA yield. Secondary aerosol was largely organic but contained some nitrate and sulfate. Formation of secondary aerosol is influenced significantly by reaction of primary gases with ·OH. The system (1) provides adequate flow and stable chamber output of P, P + SOA, and SOA for characterization and animal exposures and (2) generates reproducible exposure atmospheres of P, P + SOA, and SOA, all at consistent mass concentrations.