Article pubs.acs.org/est
Models for Predicting the Ratio of Particulate Pollutant Concentrations Inside Vehicles to Roadways N. Hudda and S. A. Fruin*,† Keck School of Medicine, Environmental Health Division, University of Southern California, Los Angeles, California 90033, United States S Supporting Information *
ABSTRACT: 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.
1. INTRODUCTION The in-vehicle microenvironment contributes disproportionally to traffic-related pollutant exposure. For example, Dons et al.1 measured black carbon exposure for 62 subjects and found that the transport microenvironment was the highest ranking contributor to the total exposure, with 21% of exposure resulting from less than 7% of daily time spent in transport. Wu et al.2 found that time spent inside vehicles was most strongly predictive of overall particle-bound polycyclic aromatic hydrocarbon exposure in a cohort of 28 non-smoking women, explaining 48% of total variation in daily exposure and 39% of the variation in individual level exposure. At the scale of epidemiological studies, however, it is not practical to directly measure pollutant concentrations; therefore, predictive models are needed. Predictive models of onroad concentrations have been attempted with varying degrees of success,3 but exposure estimates also need to take into account differences in penetration rates and the losses that occur inside vehicles for particulate pollutants. Our previous work4 showed that inside-to-outside (I/O) ratios for ultrafine particle (UFP) number concentrations vary from nearly 0 to 1 between vehicles as well as across operating conditions for a given vehicle, resulting in widely varying personal exposures. UFP I/O ratios depended primarily upon the air exchange rate (AER), which is a function of vehicle characteristics, speed, and ventilation setting. We explained 79% of the variability in measured UFP I/O ratios using easily obtainable information, such as ventilation settings (i.e., recirculation or outside air selection and fan setting), vehicle age, and driving speed.4 © 2013 American Chemical Society
I/O ratios for other particulate pollutants, outside of UFPs, have seldom been quantified in the literature, but Ott et al.5 measured cigarette-derived fine particle mass decay rates inside vehicles and found that they correlated with AER. They also reported a fine particulate matter I/O ratio of 0.43, averaged across three vehicles and different ventilation settings. Because particle loss rates vary by particle size,6 I/O ratios do as well but potentially in a complex way because of nonlinearity in loss mechanisms and differences in particle size distributions for different pollutants. The goal of this study was to measure I/O ratios of several particulate species of different sizes [particlebound polycyclic aromatic hydrocarbons (PB-PAHs), black carbon (BC), UFP, and PM2.5 and PM10 (particulate matter ≤2.5 and ≤10 μm, respectively)], to examine the influence of driving and vehicle characteristics on these I/O ratios, and to develop predictive models for the I/O ratios of each pollutant.
2. METHODS 2.1. Instruments and I/O Ratio Calculation. Simultaneous measurements of inside and outside concentrations were conducted with paired instruments. The I/O ratio was calculated as the ratio of the average inside concentration to the average outside concentration for the duration of the run, typically 20−30 min. Instruments used were an Ecochem Received: Revised: Accepted: Published: 11048
April 10, 2013 July 31, 2013 August 19, 2013 August 19, 2013 dx.doi.org/10.1021/es401500c | Environ. Sci. Technol. 2013, 47, 11048−11055
Environmental Science & Technology
Article
“max AC” option present in many vehicles recirculates cabin air. The automatic climate-control option is difficult to generalize from vehicle to vehicle and was not tested in this study. All vehicles tested were equipped with standard, coarse air filters of low particle collection efficiency. At each ventilation setting, experiments were conducted at medium- and highventilation fan settings as well as with the fan off. Some tests were also conducted with windows open, but I/O ratios for open windows were essentially 1.0 for all but very low (
