Evolution of particle number distribution near roadways. Part II: the ‘Road-to-Ambient’ process

The ‘road-to-ambient’ evolution of particle number distributions near the 405 and 710 freeways in Los Angeles, California, in both summer and winter, were analyzed and then simulated by a multi-component sectional aerosol dynamic model. Condensation/evaporation and dilution were demonstrated to be the major mechanisms in altering aerosol size distribution, while coagulation and deposition play minor roles. Seasonal effects were significant with winters generally less dynamic than summers. A large number of particles grew into the >10 nm range around 30–90 m downwind of the freeways. Beyond 90 m some shrink to <10 nm range and some continued growing to >100 nm as a result of competition between partial pressure and vapor pressure. Particle compositions probably change dramatically as components adapt to decreasing gas-phase concentration due to dilution, so number distribution evolution is also an evolution of composition. As a result, people who live within about 90 m of roadways are exposed to particle sizes and compositions that others are not.