470397 Real Life PM Emissions from Traffic and Human Exposure Implications
The cars with high mileage and the ones with low mileage were kindly provided by the author colleagues. Conditional codes were introduced for the cars so as to avoid specifying the car manufacturers and models, covering various individual vehicle characteristics including manufacturing year, engine capacity and fuel type. For the aims of the study, the exhaust gas suspension (EGS) method was selected. The measurement methods were patented and used according to the following plan:
1. The test car was started and ran for 1-3 minutes before all extraneous dust and soot particles that had settled there during the downtime and brought from the outside in the tailpipe were removed.
2. Then, the engine was shut off and the hose was connected to the tailpipe of the test car. After that, the hose was dipped into the plastic container, which was filled with 10 liters of deionized water.
3. After that, the car was started and worked at the neutral speed for 20 minutes. This time interval was chosen considering that the engine had been warmed up for approximately ten minutes (according to a coolant temperature sensor). Then, the warmed up engine idled for the remaining ten minutes.
Once the measurements had been completed, the distilled water container through which the exhaust gases had passed, was sealed with a lid and then sent to the laboratory. A significant advantage of this method is the lack of transformation of aerosols and solid particles. To measure particle size distribution, a laser particle size analyzer supplied with Fritch MaS software was employed. PM size distribution of the wet or dry dispersion units can be measured separately or simultaneously, with automated switching features. Optimal dispersion was accomplished in the NanoTec integral wet dispersion unit by using a combination of a robust, variable speed centrifugal pump with powerful ultrasonification.
To translate the different emission profiles identified in the study into actual exposure and uptake, taking also into account the deposition across HRT, a comprehensive methodological scheme has been developed, taking into account:
a. The amount of PM emitted per km travelled by the vehicles investigated, using the COPERT 4 methodology. PM emission factors (in mass of PM per km traveled) for the different vehicles is necessary so as to account of the overall mass of PM emitted by each individual vehicle type.
b. The PM size distribution of the emitted PM, as identified by the laser particle size analyzer.
c. The intake fraction (IF) of the emitted PM. Intake fraction is a metric of the emission-to-inhalation relationship, facilitating comparisons among sources (in our case the individual vehicles) in terms of their exposure potential. For a given emission source and pollutant, intake fraction is the cumulative mass inhaled by the exposed population divided by the cumulative emissions. Considering that IF depends on several parameters affecting the emission-to-intake process, (e.g. prevalent wind, emissions strength, population density), it is expected that it would vary with location and time. In a recent global intraurban intake fraction study, it was found that population-weighted mean intra-urban intake fraction for cities is equal to 32.
d. The deposition of PM across the HRT. To better understand the exposure implications of size-segregated particles distribution emitted by the investigated vehicles, HRT particle deposition modeling was applied to reckon the PM fraction deposited (DF) to the three parts of HRT. Major mechanisms of PM deposition across HRT include diffusion, sedimentation and impaction. Secondary mechanisms involve interception and electrostatic deposition. Different HRT regions involve different deposition mechanism as a function of PM size as follows:
- Naso-pharyngeal region (or upper respiratory tract – URT): impaction, sedimentation, electrostatic (particles > 1 μm)
- Tracheo-bronchial (TB) region: impaction, sedimentation, diffusion (particles < 1 μm)
- Pulmonary (P) region: sedimentation, diffusion (particles < 0.1 μm)
Several parameters that affect HRT deposition have been taken into account, including PM properties (concentration and size distribution), air flow parameters (lung capacity and breathing frequency) and HRT physiology (structure and morphology). HRT deposition was carried out using the Multiple Path Particle Deposition (MPPD) v. 2.1 model. Software inputs include morphological parameters of the pulmonary system – functional residual volume (FRC), tidal volume (TV), upper respiratory tract (URT) volume, as well as breathing frequency (BF) for each age group.
The results of the study indicated that the mean diameter of particulate emitted from high mileage cars ranges in hundreds of microns. Smaller particles were found in cars that were manufactured over 10 years ago and have considerable mileage. Thus, particles were detected in five of the 17 cars. The same percentage of cars (30%, 5 out of 17) is the source of PM50 class. Particles with a mean diameter of more than 100 µm were found in most samples of the cars without mileage (58%, 10 out of 17), but 42% of the cars without mileage are a source of particles of up to 20 µm. It appears that these aerosols correspond to different agglomeration stages. Data on the surface area of the exhaust gas particulate showed that entirely new cars emit extremely small particles with a large surface area (up to ca. 90,000 cm2/cm3).
The qualitative analysis of the exhaust gas particulates showed that there are limited differences in their composition between cars with high and low mileage respectively.
In terms of implications for population exposure and public health, age-dependent physiological differences are also of special importance. Taking into account the vehicle emission profile (mass of PM emitted per km travelled and PM size distribution), the respective intake fraction and the HRT deposition based on the size distribution, the amount of PM retained by all HRT regions was estimated. Among the newest vehicles, the highest amount of PM retained is associated to the emissions of the diesel vehicle VT 2012; the very low mean diameter of the emitted PM implies that its vast majority is within the respirable size spectrum. In general, among the newer vehicles, 0.0096 μg were deposited per km travelled for the diesel vehicles, while the respective amount for the gasoline cars was 0.0074 μg. Regarding older cars, gasoline cars contribution was the same to the newer ones (0.0074 μg per km traveled), while for diesel vehicles the amount (0.031 μg per km traveled) was the highest among all investigated vehicle categories. This observation is attributed to the higher amount of overall PM emitted (per km travelled) by older vehicles. Considering that newer diesel vehicles emit PM of lower aerodynamic diameter compared to older diesel vehicles, a higher fraction of the PM taken up is expected to deposit to the lower HRT.
Among the investigated vehicles, the diesel vehicle encoded VT 2012 combines all characteristics that may be potentially harmful for public health; the lowest average PM diameter (3.84 µm) that results in significant PM deposition across HRT (and especially the lower part), the highest active surface (89,871 cm2/cm3), and the highest adsorption potential of organic (e.g. PAHs) and metals.
A key finding of the study is that car mileage has virtually no effect on the size of the solid particles released into the environment, and that even cars with very low mileage (up to 1,000 km) are not "clean" in terms of emissions. On the contrary, special attention has to be paid to the lower aerodynamic diameter related to newer diesel vehicles, their higher specific surface (as well as the higher capacity of toxic substances adsorption) and how this is translated into actual increased human exposure and, consequently, health risk.
See more of this Group/Topical: Topical Conference: Environmental Aspects, Applications, and Implications of Nanomaterials and Nanotechnology