Effects of Turbulence Characteristics on the Concentration Distributions of Traffic-related Pollutants near Roadways

In urban environments, traffic emissions are one of the most critical factors controlling the levels and distributions of air pollutants concentrations. In the daytime, roadway plumes reach up to a few hundred meters due to effective mixing with ambient air, but plumes can reach up to 1 - 2 km or further when the atmosphere is stable. The effectiveness of dilution of pollutants plumes near the roadways can be affected by turbulence intensities (both on-road turbulence produced by vehicles and ambient). This study attempted to estimate the concentration decay rates with distance from the highway as well as the peak concentration of highway plumes, thus the concentration profiles. To estimate them, we assumed that the peak concentration and decay rate are controlled by traffic density and the total turbulence intensity (of vehicle-induced, structure-induced, and ambient turbulence).

Measurements were conducted near the Gyeongbu highway, one of the busiest highways, and Seoul's city highways. To obtain concentration profiles of traffic-related pollutants, we made continuous measurements of CO, NO, NO2, O3, and PM2.5 with custom-built sensor sets at upwind and five locations on the downwind side. Sensor data were corrected with the two-step correction method based on intercomparison results between sensor sets and the reference instruments. Two 3-D ultrasonic anemometers (CSAT 3) were installed at upwind and downwind locations to estimate vehicle-induced and ambient turbulence intensities.

We calculated the concentration differences between the upwind and downwind peak concentrations normalized by traffic volume (ΔCO or ΔNO), and found that ΔCO and ΔNO decreased exponentially as the turbulence intensity increased. In addition, the reduction rates of pollutants concentrations with distance from the highways were extracted by fitting them with an exponential function. The reduction rates were compared with the turbulence parameter to be empirically formulated. In this way, we could estimate the concentration profiles of highway plumes with distance from the highway. More details in the discussion of the results are presented.