Evolution of particle number distribution near roadways—Part I: analysis of aerosol dynamics and its implications for engine emission measurement

Studies have suggested that aerosol number concentrations may be better correlated to health effects than mass concentrations so that the high particle number concentrations in the vicinity of freeways raise concerns regarding adverse health effects on people living there. Thus, it is important to understand how particles transport and transform near roadways for regulatory purposes. Driven by different mixing forces, exhaust dilution near roadways usually experiences two distinct dilution stages after being emitted—‘tailpipe-to-road’ and ‘road-to-ambient’. The first stage dilution is induced by traffic-generated turbulence and the dilution ratio usually reaches up to about 1000:1 in around 1–3 s; the second stage dilution is mainly dependent on atmospheric turbulence, the additional dilution ratio is usually about 10:1, and the process usually lasts around 3–10 min. The aerosol dynamical processes, such as nucleation, condensation and coagulation were qualitatively investigated in the first stage. For the second stage, condensation and dilution were the major mechanisms in altering aerosol size distribution, while coagulation and deposition play minor roles. Based on the analysis, a modeling structure for a mechanistic roadway air quality model is proposed. Our study also indicates that in order to simulate the first stage, ‘in-tailpipe’ measurement of aerosol size distribution and condensable material concentrations in their original phase states is necessary. The implications for dilution tunnel design are discussed.