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Abstract: Variables affecting children''s exposure during school bus commutes were investigated using real-time measurements of black carbon (BC), particle-bound polycyclic aromatic hydrocarbons (PB-PAH) and nitrogen dioxide (NO2) inside 3 conventional diesel school buses, a particle trap-outfitted (TO) diesel school bus and a compressed natural gas (CNG) school bus, while traveling along an urban Los Angeles Unified School District bus route. A video camera was mounted at the front of each bus to record roadway conditions ahead of the bus during each commute. The videotapes from 12 commutes, in conjunction with pollutant concentration time series, were used to determine the influence of variables such as vehicles being followed, bus type and roadway type on pollutant concentrations inside the bus. For all buses tested, the highest concentrations of BC, PB-PAH and NO2 were observed when following a diesel school bus, especially if that bus was emitting visible exhaust. This result was important because other diesel school buses were responsible for the majority of the diesel vehicle encounters, primarily due to caravanning with each other when leaving a school at the same time. Compared with following a gasoline vehicle or no target, following a smoky diesel school bus yielded BC and PB-PAH concentrations inside the cabin 8 and 11 times higher, respectively, with windows open, and ∼1.8 times higher for both pollutants with windows closed. When other diesel vehicles were not present, pollutant concentrations were highest inside the conventional diesel buses and lowest inside the CNG bus, while the TO diesel bus exhibited intermediate concentrations. Differences in pollutant concentrations between buses were most pronounced with the bus windows closed, and were attributed to a combination of higher concentrations in the exhaust and higher exhaust gas intrusion rates for the conventional diesel buses. Conventional diesel school buses can have a double exposure impact on commuting children: first, exposures to the exhaust from other nearby diesel school buses and, second, exposure to the bus''s own exhaust through “self-pollution”. [Copyright &y& Elsevier]

A potentially important, but inadequately studied, source of children''s exposure to pollutants during school bus commutes is the introduction of a bus''s own exhaust into the passenger compartment. We developed and applied a method to determine the amount of a bus''s own exhaust penetrating into the cabin in a study of six in-use school buses over a range of routes, roadway types, fuels, and emission control technologies. A tracer gas, SF6, was metered into the bus''s exhaust system using a mass flow controller whose flow rate was logged by a data acquisition system and processed with the concurrent real-time pollutant measurement data. At the same time, the SF6 concentration inside the bus was measured using an AeroVironment CTA-1000 continuous analyzer connected to a series of solenoids that switched the sample inlet between the front and rear of the bus cabin. To account for a baseline drift of the CTA-1000, SF6-free air was also drawn through a line located outside at the front of the bus. Although this third sample line generally provided a reference zero value, it also showed that under certain wind conditions (i.e., wind from the rear) when the bus was stopped and was idling, significant amounts of the bus''s own exhaust reached this location at the front of the bus. Self-pollution, the percentage of a bus''s own exhaust that can be found inside its cabin, was a function of bus type and age, and a strong function of window position (i.e., open or closed). We estimated up to 0.3% of the air inside the cabin was from the bus''s own exhaust in older buses, approximately 10 times the percentage observed for newer buses, and 25% of the black carbon concentration variance was explained by the buses’ self-pollution. Analysis of the tracer gas concentrations provided a powerful tool for identifying potentially high-exposure conditions. [Copyright &y& Elsevier]

Concentrations of polycyclic aromatic hydrocarbons (PAHs), PM2.5, and organic and elemental carbon (OC and EC) were measured in 48 h integrated samples collected in the indoor and outdoor air in Los Angeles, CA, Houston, TX, and Elizabeth, NJ from July 1999 to June 2000. The objective of the study was to evaluate the hypothesis that outdoor air pollution contributed strongly to indoor air pollution. The measured partition coefficients of PAHs, Kp,meas, in the individual samples were well correlated with the compounds’ sub-cooled liquid vapor pressure, pLo. Values of Kp,meas varied by about two orders of magnitude for any given value of vapor pressure. These variations in gas/particle partitioning of PAHs were higher than the estimated systematic and random error of Kp,meas and are related to the aerosol characteristics and sampling conditions. Stepwise multiple linear regression analysis (MLR) of the pooled data, which included pLo at 25°C, temperature, fOC and fEC as independent variables, explains 84.5% of the variability of the partition coefficients. This is higher than the explained variance when pLo is used as a single parameter (77.5%). The relative importance of each variable for prediction of PAH partition coefficient is determined by partial coefficients of determination. Vapor pressure at 25°C (RpoL2=0.84) and temperature (RT2=0.21) are the two most important predictors followed by fEC (RfEC2=0.12) and fOC (RfOC2=0.038). Both EC and OC carbon are important predictors of gas/particle partitioning of PAHs, with EC being a better predictor. Because EC is highly correlated with (and is a good tracer of) primary combustion-generated OC, this result suggests that PAHs more readily sorb on combustion-generated aerosol containing EC. Enrichment of the indoor aerosol in non-combustion OC suggests that sorption of PAHs is more important in the indoor air compared to the outdoor air. The MLR developed in this work will improve prediction of gas/particle partitioning of PAHs in indoor and outdoor air. [Copyright &y& Elsevier]