Highway measurements of vehicle-induced turbulence (VIT) and enhanced mixing in the surface layer

Motor vehicles generate vehicle-induced turbulence (VIT) while in motion, which affects the mixing and transport of emitted trace gases, heat and momentum. During July, 2016 a 40Hz sonic anemometer (ATI) and a co-located 20Hz open path CO2/H2O gas analyzer (LICOR) were assembled and driven on highways in the Greater Toronto Area. An ultra-high sensitivity aerosol spectrometer was employed to measure the total number concentration of particles between 60 and 1000nm at the position of the gas analyzer and sonic anemometer. A video camera with GPS capabilities was also utilized to assess the surrounding features while chasing a specific vehicle type. The following distance of each chased vehicle is calculated as a function of pixel location, with the back edge of each followed vehicle being identified as the initial contrast between the vehicle's shadow and the roadway. Combining the sonic anemometer data with the GPS data allows turbulence statistics to be developed and correlations with vehicle type and following distance to be examined. Vehicle-induced turbulence is assessed using turbulent kinetic energy (TKE), a measure of turbulent mixing, which maximizes near the followed vehicle and decreases with increasing following distance. Vehicle exhaust plumes are identified using the measured CO2 concentration. The CO2 concentration data in conjunction with TKE and turbulent fluxes are used to explore the potential impacts of vehicle-induced turbulence on atmospheric mixing within the surface layer.