On-road concentrations of traffic-related pollutants are typically much higher than concentrations measured at ambient monitoring stations. This results in in-vehicle microenvironments contributing disproportionately to the total exposure with exposures frequently being as high as on-road concentrations. However, under conditions of low air exchange rate, pollutants with significant in-vehicle losses, such as particles, can have in-vehicle concentrations that are significantly lower than those outside the vehicle. We tested a large sample of vehicles selected to be representative of the California fleet for air exchange rate (AER) at various speeds and found that AER is a predictable function of vehicle age or mileage, speed, and ventilation setting choice (outside air, recirculation, or open windows). We demonstrated that AER is the dominant factor in determining the inside-to-outside ratio for pollutants like ultrafine particles. Models were developed that explain over 79% of the variability in AER and ultrafine particle indoor/outdoor ratios across the California fleet and across the expected range of normal driving conditions. To better determine on-road concentrations, we also conducted extensive on-road measurements using a mobile platform hybrid vehicle with real-time instrumentation. Models were developed and validated to estimate on-road traffic-related pollutant concentrations (variance explained was 37% to 73% depending on the air pollutant and modeling method). Models developed in this study can be combined with subject information about their vehicle, ventilation choices, and commute route to estimate in-vehicle exposures in future studies.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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