Ultrafine Particles

Transport and Fate in the Environment

Particles in air are affected by a variety of forces (e.g. aerodynamics, gravity, and bouyancy) and undergo turbulent coagulation, turbulent diffusive deposition, and gravitational sedimentation (Liao and Feddes, 1991). Ultrafine Particles (UP) are an important component of atmospheric aerosols (Seinfeld and Pandis, 1998). They play an essential role in the removal of trace gases, such as sulfur dioxide and volatile organic compounds, from the atmosphere (Kane and Johnston, 2000). The UP mode of a diesel aerosol is made up of carbonaceous soot particles (black carbon) that coexist with a mixture of organic compounds (Junker et al., 2000). Some of these compounds, e.g. polycyclic aromatic hydrocarbons (PAH) grow during the combustion process on the surface of particles (Kasper and Siegmann, 1998). It is interesting to note that as particle diameters become larger particle number concentrations decrease (Whitby and Sverdrup, 1980).

Most of the concern related to UP’s are focused on the potential human health impacts. The environmental impacts of particles is wide ranging. Particles directly affect climate by (Sipin et al., 2003):

• enhancing the scattering and absorption of solar radiation, therefore, altering the amount of solar radiation reaching the Earth’s surface;
• visibility degradation due to light scattering;
• affecting cloud properties and the hydrologic cycle; and,
• they can also act as sinks for reactive species since their exposed surfaces can catalyze heterogenous reactions.
  

As particle diameter becomes larger particle concentration decreases (Whitby and Sverdrup, 1980). UFP’s that originate via direct emissions from gasoline and diesel combustion coagulate quickly through diffusion (Hinds, 1982). The rate of coagulation is dependant on the square of the particle number concentration (Junker et al., 2000). For example, a particle mass distribution of an urban aerosol has a bimodal distribution with a maximum in the accumulation range (0.1-1 or 2 µm) and another in the coarse particle range (>1 or 2 µm). Particles in the accumulation mode have a longer residence time than UFP’s because removal by diffusion is negligible. These particles grow slowly by coagulation until they exceed 2µm where sedimentation and impaction become significant (Hind, 1982). It is unlikely that UFP’s are subject to long-range transport (Pitz et al., 2001). Ultrafine particles that originate via direct emissions from gasoline and diesel combustion coagulate quickly through diffusion (Hinds, 1982). The rate of coagulation is dependant on the square of the particle number concentration (Junker et al., 2000). In general, however, UFP’s only remain in the atmosphere for time periods of days to weeks, depending on their size (Sipin et al., 2003).

To help understand the dispersion of UFP’s from highway activity, measurements were taken 30, 60, 90, 150, and 300m upwind from I-405 at the Los Angeles National Cemetery. Studies were conducted using a condensation particle counter, scanning mobility particle sizer, Dasibi CO monitor, and aethalometer, and a DataRam (Zhu et al., 2002). The data set collected during this investigation correlated to the observed traffic patterns and diurnal changes were noted. The results of this investigation indicate that the UFP number concentration measured 300 meters downwind from the freeway was indistinguishable from upwind background concentrations. The importance of this investigation relates to its use to estimate exposure to UFP’s in the vicinity of major highways.