An Investigation of Turbulence and Diffusion within Vehicle Wakes and On-Road Measurements using an Instrumented Mobile Car and a Stationary Roadside Monitoring System

Moving motor vehicles emit pollutants that negatively impact human health. Stationary roadside measurements alone are not sufficient to quantify the pollutant–flow interactions that occur behind moving vehicles. The instrumented mobile car however is well–suited for on–road measurements, but has been underutilized for this purpose since limited studies have investigated its accuracy at high vehicle speeds. Thus, this work details two on–road measurement campaigns using an instrumented car, with three main objectives: (1) study the vehicle momentum wake and vehicle–induced turbulence (VIT), (2) investigate the accuracy of the mobile system for measuring atmospheric means, variances and covariances, and (3) quantify the emission of aerosols and CO2 by on–road vehicles and their subsequent diffusion. Measurements behind on–road vehicles demonstrate that VIT decays with increasing distance following a power law relationship. Comparison of measurements with prior on–road studies suggests a height dependence of VIT in vehicle wakes, and an extended parameterization is outlined that describes the total on–road turbulent kinetic energy (TKE) enhancement due to a composition of vehicles, including a vertical dependence on the magnitude of TKE. Next, a wavelet–based approach to remove the effects of sporadic passing traffic is developed and applied to a measurement period during which a heavy–duty truck passes in the opposite highway lane; removing the times with traffic in this measurement period gives a 10% reduction in the TKE. When sampling uncertainties are considered, the vertical momentum flux measured on the car is found to be not different from roadside measurements in the 95% confidence interval. The first on–road and in–traffic measurements of the vertical turbulent particle number flux and the vertical turbulent CO2 flux are presented and the results suggest this technique could be further developed to measure individual vehicle emission rates while driving. The lateral width of the wake generated by each passing vehicle is estimated using the stationary roadside measurements, and is determined to be a factor of 5 times greater for heavy–duty trucks relative to sport utility vehicles and passenger cars at a distance of 150 m behind the vehicle.