Space charge density measurements downwind
from a traffic route
S. Israelsson
a,), R. Lelwala
ba
Department of Meteorology, Uppsala UniÕersity, Box 516, 75120, Uppsala, Sweden b
UniÕersity of Colombo, Colombo, Sri Lanka
Abstract
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.q1999 Elsevier Science B.V. All rights reserved.
Keywords: Space charge density; Electrical environment; Traffic route
)Corresponding author. Tel.:q46-18-542814
0169-8095r99r$ - see front matterq1999 Elsevier Science B.V. All rights reserved.
Ž .
1. Introduction
The electrical processes in the lower atmosphere are complex, and vary within a large range of space and time scales. An overview of electric phenomena in the lower
Ž .
atmosphere is given by Hoppel et al. 1986 . The basic principle that describes the vertical distribution of space charge at ground level is the electrode effect, but it strongly depends on the meteorological conditions. But other natural sources can also be included. The effects of wind and evaporation can sometimes be considerable, see
Ž . Israelsson 1994a,b .
Ž .
Reiter 1992 showed that anthropogenic processes can produce highly electric charged particles and sole measurements of the net space charge are not always so informative. Such studies have to be done under well-defined conditions both from the point of view of weather and measuring site. The use of electrostatic precipitators on an industrial stack to remove particulate matter and aerosol particles is now common.
Ž .
Jennings and Jones 1976 showed the occurrence of high electric fields from industrial
Ž .
stack plumes on long distances up to 8 km downwind from the chimney stack. Ž .
Markson et al. 1979 showed that by using electrical measurements to detect point source plumes it was capable to detect plumes in situ at long distances up to 90 km from the source by using an aircraft.
Steam or smoke from locomotives always give rise to a positive space charge, which
Ž . Ž .
was first noted by Kelvin 1860 . Whitlock and Chalmers 1956 found that the space charge could still be identified when the steam and smoke were no longer visible at a
Ž . Ž .
distance of over 1 km from the source. Lundquist 1971 and Muhleisen 1953, 1958
¨
observed high positive electric fields near the traffic due to exhaust from gasoline engines producing positive space charges.Deliberate attempts to create space charges have been made by Vonnegut and Moore Ž1958 , Vonnegut et al. 1961, 1962 and they have shown that space charges artificially. Ž . produced can be detected at a considerable distance away from the source.
Ž Within thunderstorm research Moore together with, among else, Vonnegut see
.
Moore et al., 1986 showed that by releasing negative charges emitted from a 2-km-long wire by corona discharge electrification processes in clouds above the wire can be initiated.
Measurements of particles near traffic routes have increased during the last decades
Ž .
which is of special importance from the view of environment research Chock, 1977 , and also with the increasing use of traffic routes and high power lines it becomes also of interest to measure and evaluate the effects of these on the electrical environment.
Space charge measurements downwind from a monopolar 500 kV HVDC test line Ž .
have been presented by Carter and Johnson 1988 . They found that the small ion density decays rapidly with distance downwind from the HVDC line in corona. As distance downwind increases, aerosols become the predominant carrier of the space charge and near the HVDC line the small air ions are the predominant charge carrier. They also found that for large distances the wind turbulence appears to play an increasingly important role in the space charge density measured.
viz. near-neutral atmospheric stability conditions and in an undisturbed area. The
Ž X X .
measurements were made at Marsta Observatory 59855 N, 17835 E Gr, 20 m asl and in
the surroundings of the observatory, that consists of a wide flat area with very few Ž .
buildings and no factories, see Israelsson et al. 1973 .
The study is part of an investigation of the connections between electrical, radioactive and meteorological parameters in the atmospheric surface layer. The paper can be used in the study of disturbances from the traffic route on atmospheric electrical measure-ments and how far away charged particles can be diffused.
2. Experimental methods
The distribution of the space charges downwind from a traffic route ‘E4’ situated 700 Ž .
m from the Marsta Observatory Fig. 1 is measured in the winter with snow-covered ground surface in order to avoid the effects of particles blowing up from the ground-surface.
The observatory is located in very flat farming country area 7 km north of Uppsala in southern Sweden. No industrial establishments by which condensation nuclei might be produced were in the surroundings of the observation place and no burning took place during the observations.
Ž
The average space charge density was recorded by Faraday cage method Knudsen et .
al., 1974; Israelsson, 1994b and the Obolensky filtration method used by Knudsen et al. ŽKnudsen et al., 1989a,b . The instruments have been constructed which, apart from.
Ž .
minor modifications, are a copy of the Anderson apparatus see Andersson, 1966 . The meteorological parameters were measured at Marsta Observatory in 2 masts, 10 and
32 m height. The temperature was measured with ventilated platinum thermometers and the wind-speed with anemometers. The wind direction was determined with vanes on 10 m height.
3. Results
In the present article the goal is to show that, beside the electrode and other meteorological effects, there can be other man-made mechanisms that are responsible for space charge formation in the lowest layer of the atmosphere.
The different measurement-points are given in Fig. 1. Twenty-one series of measure-ments were made from December 10, 1985 to March 20, 1986. Every series consist of mean-values from five individual point-measurements.
The diffusion of particles depends on the wind and temperature conditions. We have chosen the cases with winds blowing from east within a sector ESE to NE in order to investigate the influence of wind-speed upon the horizontal distribution of charged particles from the traffic route in near-neutral stability condition. The atmospheric stability is given by the gradient Richardson number:
2
Ris
Ž
grQ. Ž
dQrd z. Ž
r d urd z.
Ž .
1where g is gravity acceleration,Q is potential temperature and dQrd zsdTrd zqG,
Ž .
where G is the dry adiabatic rate of cooling y0.98 C8r100 m . The horizontal wind
speed here is u. In our study the atmospheric stability was determined by using a
Ž .
convenient stability parameter S, Richardson bulk number , defined as:
Ss
Ž
DTq0.1.
rU2Ž .
210
where DT is a temperature difference between 0.5 and 10 m above the ground, DTq0.1sDQ is potential temperature difference between 0.5 and 10 m above the
ground. The value 0.1 refers to Gr9.5 and u is hourly mean wind speed at 10 m above
10
the ground surface. The present measurements of the space charge density were carried out for near-neutral stability values, viz.y0.01-S-q0.18C my2 s2.
Measurements of the space charge density on both sides of the traffic route are made in order to obtain the background values. These background values are then subtracted from the data in downwind conditions. Fig. 2 shows the horizontal profiles of the space charge density for the wind-speed intervals 0–2, 2–4, and )4 mrs. Wind-speeds higher than 5 mrs are excluded due to problem with wind-driven snow particles. The error bars are calculated standard deviations.
Fig. 2. The horizontal profiles of the space charge density for different intervals of wind-speeds at the height of 10 m. The background values are subtracted from the data in downwind conditions.
The space charge density decays nearly exponentially with the downwind distance from the route. On distances 2 km or more the effect of the wind velocity seems to be fairly limited. In Sundbro, 3.7 km away, however, the traffic on E4 does not seem to
Ž . Ž .
influence space charge density. Jennings and Jones 1976 and Reiter 1992 showed examples of very high electric fields measured up to more than 8 km from factories with electrostatic precipitators.
The influence of particles released from the traffic is, thus, significant up to 2 km, but it can be observed for longer distances. The atmospheric–electric measurements at Marsta Observatory consequently ought to be influenced by the traffic for all wind-speeds due to the distance 0.7 km from the E4. But in 85% of the cases the wind is not blowing from the east, the traffic route. The measurements also indicate that the traffic produces positive space charge density but the effects for distances downwind greater than 2 km seems to be limited. This can be explained by the charging of particles due to natural
Ž
small ions in the atmosphere Bricard and Pradel, 1966; Israel, 1970; Lundquist et al., .
1971 .
Theoretically diffusion of particles and gases from line-sources have been treated in Ž .
the literature, see i.e., Pasquill 1974 . Our space charge density profiles in Fig. 2 seem to follow exponentially decreasing curves, which is also postulated in many studies on
Ž .
4. Conclusions
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 a flat with no trees and very few obstacles. No industrial establishments by which condensation nuclei might be produced were in the surroundings of the observation place and no burning took place during the observations. Anthropogenic processes can produce highly electric charged particles and sole measurements of the net space charge are not always so informative. But the present study shows that, if the measurements are done under well-defined conditions, both from the point of view of weather and measuring site, it is possible to obtain informative measuring data.
From the different horizontal profile measurements we note that the change of space charge density is greatest near the traffic route and most pronounced for the lowest wind speeds. The space charge density decays exponentially with the downwind distance from
Ž . the route, which is in agreement with studies by Jennings and Jones 1976 . On distances above 2 km downwind the effect of the traffic route is limited. Other studies presented in the literature show examples on very high electric fields measured up to Ž . more than 8 km from factories with electrostatic precipitators. For higher levels 274 m it was according to the literature capable of detecting plumes in situ at long distances up to 90 km from the source by using an aircraft. 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 distances of 90 km by using aircraft.
In further work we plan to include models for line sources at ground level together with atmospheric diffusion and charging effects in the electrode layer.
Acknowledgements
The Research was supported by the Swedish Natural Science Research Council and Ž .
International Science Program ISP in Uppsala.
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