Photo source: Berkeley Lab’s Environmental Energy Technologies Division
Toxicological studies have consistently found that ultrafine particles are potentially harmful. Yet only recent epidemiological studies have been able to link ultrafine particles to negative public health effects. The difficulty in finding effects in epidemiological studies may be in part because of the population exposure metrics used in previous studies. In the December 2 SOCAAR Seminar Dr. Michael J. Kleeman, a professor at UC Davis’ Department of Civil and Environmental Engineering, presented 15 years of research on the measurements and modeling of ultrafine particular matter in California.
Ultrafine particles (UFP) are emitted from various combustion sources including vehicles and can also be nucleated in the atmospheric environment. UFP that are generated from an emission source can have an atmospheric dilution effect. The particles can evaporate, condense, and quickly change in size. Such particles are difficult to characterize based on a particle number metric since they are dynamic and changing so rapidly.
Currently, most commercial instruments count particular numbers very efficiently and a lot of research have used the particle number metric when evaluating particulate matter. Such research have become very influential. In fact data from one study that found a decrease in the concentration of particulate matter with distance from the freeway, as measured by the particle number concentration, was used by legislation in the State of California to pass a law on the placement of schools with respect to their distance from the roadway. However, the surface area of particulate matter is speculated to drive health effects because that’s where all the chemical reactions take place. An alternative metric used to characterize particulate matter is by mass emissions of PM0.1, where the particles have aerodynamic diameter of less than 0.1mm.
Dr. Kleeman discussed instances of where particle number and mass emission factors have shown opposite effects in new vehicle technology adoption. When vehicles were installed with a diesel particle filter, the emission of PM2.5 had a 90% reduction. But the number of particles emitted increased because of the nucleation of semivolatile organic vapours that occurred, resulting in very small particles in the 20-40nm range that were counted as individual particles when the particle number metric was used. The study showed that the technology improves the mass efficiency but worsens the number emission efficiency.
Studies analyzing the atmospheric dilution and photochemical aging effects were also presented. Seasonal differences in how UFP changed in terms of location and time were observed. There were higher concentrations in source sites (regions where it’s generated—Central LA) in the winter time. But in the summer time, there were higher concentrations in receptor sites at the downwind reception sites where photochemical transfer occurred. Strong diurnal patterns in UFP sources were also observed. At nighttime PM0.1 is dominated by wood combustion for heating while daytime PM0.1 is dominated by wood combustion, meat cooking and other unknown sources as mixing occurs more during day.
Dr. Kleeman presented measurement and calculation studies showing that PM0.1 is a much more stable metric compared to particle number and can useful for epidemiological studies. PM0.1 could be used as a metric for surface area to measure exposure. It is also valid as a source apportionment method. Furthermore, PM0.1 have shown UFP are toxic and correlated with negative health effects. One epidemiological study associated UFP exposure with the risk of dying from heart disease. Future work will be to investigate whether health effects of PM0.1 are distinct from PM2.5. Dr. Kleeman believes that additional epidemiological studies, along with toxicological studies, are still needed to drive the development of standards and regulations to address ultrafine particles.