Ambient and plume processing of atmospheric ultrafine particles.

摘要

Aerosol number distributions characterize the properties of small atmospheric particles, especially in the ultrafine size range (100 nm). Our goal is to build a photochemical model able to predict the particle number distribution and concentration, which will be necessary to establish the source-to-receptor relationship needed for developing rational emission control strategies. Atmospheric processing connects source to receptor, which can be further divided into ambient processing and plume processing. As the first step of the study, each term in the aerosol dynamic equation is analyzed to estimate its influence on the overall number distributions under typical urban conditions, where ambient processing dominates.; New particle formation has been observed in conditions relevant to both ambient and plume processing. Our study on the thermodynamic and chemical properties of fresh nuclei and organic vapors lead to a hypothesis that acid-catalyzed heterogeneous reactions between organics and acidic nuclei initiate their early growth by polymerizing the organics, helping these nm-sized nuclei cross the huge Kelvin-effect barrier and facilitating further condensation of organic vapors. This hypothesis helps explain various experimental studies in marine boundary layer and engine exhausts.; The transportation sector is one of the dominant emission sources in metropolitan areas. Plume processing of traffic-generated particles usually undergoes two distinct stages after being emitted---'tailpipe-to-road' and 'road-to-ambient'. The interaction between dilution and other aerosol processes is analyzed for both stages. Based on the analysis, a modeling structure for a plume-in-grid model for mobile sources is proposed.; Finally, the 'road-to-ambient' evolution of particle number distributions near freeways in Los Angeles, California is analyzed and simulated by a multi-component sectional aerosol dynamic model. A large number of particles grew into the >10 nm range around 30 to 90 m downwind of the freeways. Beyond 90 m some shrink to 10 nm range and some continued growing to >100 nm as 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. As a result, people who live within about 90 m of roadways are exposed to particle sizes and compositions that others are not.