The morphology of ultrafine particles on and near major freeways

The morphology of ultrafine particles (UFPs; diameter < 100 nm) collected on and near major Los Angeles freeways in April 2006 is reported. Samples were size selected with a differential mobility analyzer, collected by a nanometer aerosol sampler, and analyzed using a transmission electron microscope. Typical observed morphologies included aggregates, spheres, irregularly shaped particles, and particles with multiple inclusions. For freeway aerosols with 50 nm electrical mobility diameter, most (>90%) electron-opaque particles were surrounded by an electron-transparent material. This suggests that much of these particles were heterogeneously internally mixed. The fraction of UFPs in a given morphology class collected on and at increasing downwind distance from the I-405 freeway was compared. The fraction of aggregates measured 90 m downwind of I-405 was significantly less than the fraction measured on the freeway (p-value < 0.01). Because aggregates are a primary aerosol (directly emitted), this may indicate that secondary aerosol (formed in the atmosphere) becomes more prevalent with increasing distance from the freeway. The fraction of particles with multiple inclusions measured 90 m downwind of I-405 was significantly greater than the fraction measured on the freeway (p-value < 0.01). The increase in the number of particles with multiple inclusions with increasing distance from the freeway suggests that dilution does not prevent particles from colliding and merging which may alter the particle size distribution.