Models for predicting the ratio of particulate pollutant concentrations inside vehicles to roadways.

Under closed-window driving conditions, the in-vehicle-to-outside (I/O) concentration ratio for traffic-related particulate pollutants ranges from nearly 0 to 1 and varies up to 5-fold across a fleet of vehicles, thus strongly affecting occupant exposures. Concentrations of five particulate pollutants (particle-bound polycyclic aromatic hydrocarbons, black carbon, ultrafine particle number, and fine and coarse particulate masses) were measured simultaneously while systematically varying key influential parameters (i.e., vehicle type, ventilation, and speed). The I/O ratios for these pollutants were primarily determined by vehicle air exchange rate (AER), with AER being mostly a function of ventilation setting (recirculation or outside air), vehicle characteristics (e.g., age and interior volume), and driving speed. Small (±0.15) but measurable differences in I/O ratios between pollutants were observed, although ratios were highly correlated. This allowed us to build on previous studies of ultrafine particle number I/O ratios to develop predictive models for other particulate pollutants. These models explained over 60% of measured variation, using ventilation setting, driving speed, and easily obtained vehicle characteristics as predictors. Our results suggest that I/O ratios for different particulate pollutants need not necessarily be measured individually and that exposure to all particulate pollutants may be reduced significantly through simple ventilation choices.