Space charge density measurements downwind from a traffic route

The purpose of this study is to investigate the dispersion of charges generated by the gasoline engines of vehicles running on a highway and its effect on the electrical environment of the Marsta Observatory 700 m away from the highway. One quantity that helps to describe the electrical environment near a traffic route is space charge density, a quantity that consists of charged air ions and charged aerosols. This paper describes measurements of the horizontal distributions of charged particles downwind from a traffic route under near-neutral atmospheric stability conditions. The surrounding is flat with no trees and very few obstacles. The intensity of the traffic is also determined. The measurements are carried out during snow-covered surface. Wind-direction, wind-speed and temperature are measured in two masts. From the different horizontal profiles we see that the change of concentrations is greatest near the traffic route and most pronounced for the lowest wind-speeds. On distances above 2 km downwind the effect of the traffic route is limited but it can be observed. From comparison between the present study and previous investigations we observe that close to the ground surface there is due to the eddy diffusion influences on the dispersion of charge particles with time and distance. In the low levels the electrode effect can be included in the charging processes, which also diminishes the tracking distances of the plume. On higher levels above the electrode layer the electrode effect can be neglected and the eddy diffusion is less dominant which explained why space charge plume can be tracked for larger distances.