The morphology of ultrafine atmospheric aerosols: An analysis of inertial deposition, condensation and evaporation, chemical composition, and spatial variation.

摘要

A large fraction of the Los Angeles ultrafine atmospheric aerosol (UAA) consists of nanoparticle chain aggregates (NCAs; Xiong and Friedlander, 2001). NCAs are often sized and collected using instruments that rely on inertial transport mechanisms. A new method of interpreting the aerodynamic diameter of NCAs was described. The method can be used to calculate aggregate surface area or volume. A linear relationship between aggregate aerodynamic diameter and primary particle diameter based on published Monte-Carlo drag calculations was 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 was applied to the collection of NCAs by a low-pressure impactor. Measurements of aggregates collected near a major freeway and at Los Angeles International Airport were made for two aerodynamic cutoff diameter diameters, 50 nm and 75 nm. 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 first study of the effect of condensation and evaporation of water on atmospheric aggregate morphology was carried out. Measurements of fractal dimension indicate that the morphology of ultrafine atmospheric aggregates did not change following once cycle of condensation and evaporation at saturation ratio 1.8. It is possible that during condensation, aggregates may not become fully incorporated in water droplets, and this may prevent restructuring.; Spatial variation in the morphology of UAA near a major freeway was investigated. UAA with 50 nm electrical mobility diameter was charged and selected with a differential mobility analyzer, and collected using a nanometer aerosol sampler (NAS). The fraction of particles with multiple inclusions measured 90 m from I-405 was significantly greater than the fraction on I-405 (p 0.001). The increase in the member of particles with multiple inclusions with increasing distance from the freeway suggests that dilution does not prevent particles from colliding and merging. Thus, coagulation may play a role in altering the particle number-size distribution.