Sources, concentrations, and transport models of ultrafine particles near highways: a Literature Review

Population growth is forcing people to build near highways. Outdoor air quality affects the indoor environment. Highway traffic-generated ultrafine particles (UFPs) carry toxic substances, posing a serious threat to pedestrians and residents. Existing literature has shown that the primary sources of UFPs are vehicle emissions, combustion sources, and atmospheric transformation. Particle number concentration and size distribution are mainly influenced by the type of vehicle, traffic volume, wind speed and direction, atmospheric dilution and coagulation or distance from the road. The toxicological relationship between UFPs and human health needs to be further investigated. Prediction models of UFPs transport include line source dispersion, street canyon, and Gaussian models. Line source diffusion models can be used in open streets and highways. Street canyon models are mainly used in metropolitan areas. Gaussian models can be used on flat roads. The results of these three models have randomness and uncertainty and are suitable for short-term models. Numerical models can handle diffusion problems of complex street patterns. Commonly used non-numerical models include the STREET, CPBM, and OSPM models. The CPBM model had higher accuracy and practical value than the STREET model. The OSPM model considered the effect of street geometry and weather conditions in the calculation of pollution diffusion. In addition, coupling CFD and GIS technology has become a promising approach in recent years.