Particle emissions from mobile sources: Discussion of ultrafine particle emissions and definition

At present there is no clear epidemiological evidence linking adverse human health effects to ultrafine particles (UFP), arguably because there is no universally agreed upon UFP definition. A commonly used definition is either particle number below 100 nm or total particle number, but without an agreed upon lower cut point. For example, a lower cut point of 3 nm compared to 10 nm could result in a substantially higher count. Another definition for UFP is total particle mass but without a commonly agreed upon aerodynamic diameter upper cut point, e.g., below 100 nm, 200 nm, 300 nm, etc. Yet another definition is lung deposited surface area weighted by lung deposition fraction, found mainly in the particle mobility diameter range from 20 to 400 nm. It is clear from these definitions that there are inconsistencies in the way UFP is used and defined in the literature. Sometimes these metrics are well correlated, sometimes not. In this paper we suggest three new metrics: UFP-N, UFP-M, and UFP-S, that we believe will add clarity. These metrics represent total number, mass, and surface area below 500 nm, respectively. For surface area and mass, the 500 nm cut point can be either aerodynamic or mobility diameter depending upon measurement methodology. For all metrics, this cut point captures nearly all of the primary particle emissions from mobile sources. Furthermore, UFP-N would include a lower cut point of 3–6 nm, and would not require an upper size cut point because there is very little particle number above 500 nm or even above 100 nm. Thus, our definition of UFP-N is consistent with the current definition of ultrafine number except for, importantly, the specification of a lower cut point. These new metrics are not necessarily intended to be used for regulatory tailpipe measurements, but rather for characterization of ambient exposures, and for linking them to acute and chronic health effects in epidemiological studies. They may also be useful for formulating new regulatory ambient standards to complement existing PM₁₀ and PM_2.5 standards. When formulating these new definitions, size and other properties of particles on or near roadways must be considered as this is where the highest exposures to submicron particles occur. Since modern vehicles under normal operating conditions have very low particle emissions, these particles will mainly come from older and malfunctioning vehicles as well as modern vehicles with failed or disabled exhaust aftertreatment systems. Consequently, particle emissions from such vehicles which are expected to dominate roadside exposures have been considered in the formulation in the suggested new metrics. Modern roadside measurements show particle size and modal structures similar to historical measurements going back as far as the early 1970s but at much lower concentrations.