Inertial deposition of nanoparticle chain aggregates: Theory and comparison with impactor data for ultrafine atmospheric aerosols

Nanoparticle chain aggregates (NCAs) are often sized and collected using instruments that rely on inertial transport mechanisms. The instruments size segregate aggregates according to the diameter of a sphere with the same aerodynamic behavior in a mechanical force field. A new method of interpreting the aerodynamic diameter of NCAs is described. The method can be used to calculate aggregate surface area or volume. This is useful since inertial instruments are normally calibrated for spheres, and the calibrations cannot be directly used to calculate aggregate properties. A linear relationship between aggregate aerodynamic diameter and primary particle diameter based on published Monte-Carlo drag calculations is derived. The relationship shows that the aggregate aerodynamic diameter is independent of the number of primary particles that compose an aggregate, hence the aggregate mass. The analysis applies to aggregates with low fractal dimension and uniform primary particle diameter. This is often a reasonable approximation for the morphology of nanoparticles generated in high temperature gases. An analogy is the use of the sphere as an approximation for compact particles. The analysis is applied to the collection of NCAs by a low-pressure impactor. Our results indicate the low-pressure impactor collects aggregates with a known surface area per unit volume on each stage. Combustion processes often produce particles with aggregate structure. For diesel exhaust aggregates, the surface area per unit volume calculated by our method was about twice that of spheres with diameter equal to the aerodynamic diameter. Measurements of aggregates collected near a major freeway and at Los Angeles International Airport (LAX) were made for two aerodynamic cutoff diameter diameters (da,50), 50 and 75 nm. (Aerodynamic cutoff diameter refers to the diameter of particles collected with 50% efficiency on a low-pressure impactor stage.) Near-freeway aggregates were probably primarily a mixture of diesel and internal combustion engine emissions. Aggregates collected at LAX were most likely present as a result of aircraft emissions. In both measurements, the aggregate aerodynamic diameters calculated from the primary particle diameter were fairly close to the stage cutoff diameter. The number of primary particles per aggregate varied one order of magnitude for particles depositing on the same stage. The average aggregate surface area per unit volume was 2.41 x 106 cm-1 and 2.59 x 106 cm-1 (50 nm da,50) and 1.81 x 106 cm-1 and 1.68 x 106 cm-1 (75 nm da,50) for near-freeway and LAX measurements, respectively. These preliminary measurements are consistent with values calculated from theory