Vehicular Exposures and Potential Mitigations Downwind

By Thomas Cahill
Sacramento, CA (October 23, 2006)- This study confirms previous finding that areas immediately downwind (east) of Watt Avenue, including Arden Middle School, are heavily impacted by Watt Avenue traffic. The mass of very fine and ultra fine particles that are strongly implicated in health impacts by both their size (Dp 0.25 um diameter lung capture efficiency) and toxic composition (PAHs, other).
Literature values of emission rates were collected, and roadway concentrations of both diesel vehicles and automobiles were calculated by two separate methods- tracer and mass balance. These values were displaced downwind via diffusion modeling from linear source to calculate values at the sampling sites. Meteorology data was generated both locally and via trajectory analysis by NOAA’s HYSPLIT program. Model results were close to measured values, giving confidence in assigning a wider variety of traffic pollutants to the downwind site than we had been able to measure previously.
In considering these data, it is important to understand that this study is an ambient exposure study, not a health study. The potential health impacts of automotive particulate matter are poorly known at this time, although they have some degree of similarity (as well as differences) from the health impacts of the better studied diesel particulate matter. Note that diesel particulate matter is responsible for 70 percent of the potential cancer impacts of all toxic air contaminants combined (California Almanac, 2006). Exhaust from diesels and automobiles, almost all of which lies in particles finer than 0.25 um, is efficiently captured deep in the lung. Health studies have shown that this efficient capture of very fine and ultrafine particles allows for their transport from the lung into the blood stream, which may result in adverse effects to the heart and brain.
A key unknown is the toxicity of smoking automobile exhaust. Data are presented from the literatures indicating that used spark emission lubricating oil has 10 to 20 times more PAHs than used diesel oil, (Fujita et al, 2005, see Appendix A) but diesel combustion has additional sources of toxicity not anticipated in automobile exhaust. Research is in progress on this matter at the ARB and elsewhere. In addition, roadways emit a variety of other pollutants, gaseous and particulate, with uncertain health impacts.
In considering these data, it is also important to understand that this study is an ambient exposure study, not a regulatory exercise.
Mitigation alternatives have been examined in four categories. Some of these would be applicable only to new development and need additional regulations, while some are available immediately:
a. Mitigation by source reduction on the roadway While strategies to reduce pollution from on-road sources are determined by state and federal mobile emission standards, changes made locally, such as adjusting the signal timing at Watt Avenue and Arden Way, as well as encouraging green transportation modes, can reduce congestion directly upwind of Arden Middle School.
b. Mitigation via improved roadway design. This is a promising but little examined option, including using vegetation on the median and within the right-of-way to foster both the natural temperature driven plume rise from the roadway waste heat and trap very fine/ultra fine particles on needles and leaves. Studies are in progress on this option by the HETF at UC Davis.
c. Mitigation from the right-of-way fence to receptors (homes, schools, etc.). The most effective mitigation is distance, with many studies giving a 160 m to 240 m distance as adequate to achieving pollution concentrations only 10% greater than upwind values. Barriers and vegetation are also included, and limited evidence indicates potential important mitigation is possible. Studies are in progress on this option by the HETF at UC Davis.
d. Mitigation in the indoor air of the receptors Indoor mitigation is both the most immediate and most effective mitigation available, supported by models and data, with the potential of effectively eliminating the impact of Watt Avenue (to a few %) in indoor air at modest cost. The HETF – UC Davis studies of 2006 showed a 75% reduction on very fine/ultra fine pollution inside the Arden Middle School teacher ready room with a standard (non HEPA) upgrade to an electrostatic filter.
American Lung Association Aerosol Transect Study- Part 2
The Sacramento/Interstate-5 Aerosol Transect Study, Part 1, delivered data of extraordinary scope and utility, and also identified ways to improve the credibility of this report in several ways, which were not possible within the time and resources available in the original study.
These include:
a. Data on the size and composition of diesel exhaust, with Dr. Zielenska and co-workers at Desert Research Institute (DRI), Nevada, the University of Minnesota, and the National Renewable Energy Lab, Golden, Colorado.
b. Recent tests of the toxicity of diesel and smoking car exhaust by Erik Fujita of DRI, Reno, Nevada.
c. Detail size information of the samplers to make simultaneous upwind measurements.
d. No concurrent truck and traffic data were available.
e. Quality assurance and inter-comparisons of mass from samplers at Yosemite National Park.
From the new data available from our joint studies with Zielenska and co-workers at DRI, this study identifies that diesel exhaust almost entirely exists in the 0.25-0.05 um size range, and that diesel exhaust mass has elemental tracers of sulfur, found in diesel fuel, as well as zinc, phosphorus, and other elements characteristic of lubricating oil.
This study confirms the previous finding that Arden Middle School is heavily impacted by Watt Avenue.
Related Resources:
Vehicular Exposures and Potential Mitigations Downwind
American Lung Association Aerosol Transect Study- Part 1
American Lung Association Aerosol Transect Study- Part 2