NO2-Mapping using laser-radar techniques

May 27, 2017 | Autor: Anders Sunesson | Categoria: Cartography, Air Quality, Remote Sensing, Methodology, Control, LiDAR, Check, Laser Radar, LiDAR, Check, Laser Radar
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Atmospheric

Enrironmenr

Vol.

22. No.

3. pp. 569-573,

0004-6981:88

1988.

Printed in GreatBritain.

13.00f0.00

Pergamon Press pk.

N02-MAPPING Bo

USING LASER-RADAR

GALLE*,

TECHNIQUES

ANDERS SUNESSON and WILHELM

WENDT

Lund Institute of Technology, Department of Physics, Box 118, S-221 00 Lund. Sweden (First

received

1 April

1987 and received

for

publication 2

September

1987)

Abstract-Laser-radar mapping of NO1 concentration distributions in an urban area ciose toa moto~ay is described. The measurements were performed during inversion conditions leading to elevated concentrations. Horizontal mapping of the NO1 concentration was performed and the results were compared with measurements of wind speed, temperature and NO2 concentration at a point measuring station. Key

word

index: Laser radar, LIDAR, differential absorption LIDAR, NOz.

measurements

INTRODUCTION

The emission of nitrogen oxides, NO,, constitutes a significant environmental problem. Besides its own toxic properties NO,, mainly emitted as NO and oxidized to NO,, has also been recognized as a contributor to negative environmental effects such as forest damage and acid rain generation (Nat. Res. Count., 1977) It is difficult to reduce NO, emissionsNO, are formed during most combustion processes since nitrogen is normally present during combustion. Consequently, there is a growing interest in monitoring nitrogen oxides. A major source of NO, is the emission from vehicles. Traffic is estimated to contribute as much as half of the total NOz emission (Pitts and Metcalf, 1971). Urban areas will be heavily affected by these pollutants, especially if the weather conditions are such that high concentrations may occur. One such situation is the inversion (Stem, 1977). During an inversion, pollutants will not be removed by wind or convection since the air is stationary, but they will remain close to the sources or become trapped under the inversion layer. Very high concentrations and poor air quality may then be the result. It is of interest to monitor the build-up of high NO, concentrations over a large area during an inversion. The results can be used to evaluate iocal models of pollution dynamics. A point monitoring instrument gives good temporal resolution but it is limited to one sampling point, thus making determination of the spatial distribution very tedious. With the laser-radar (LIDAR) technique, however, range resolution is obtained in each measurement, and large areas can be probed (Measures, 1984). In this way areas with increased NO, ~n~ntration can be identified at distances up to about 3 km.

*Present address: Swedish Environmental Research Institute, P.O. Box 5207, S-402 24 Gliteborg, Sweden. i\E2213-Z

569

As mentioned above. the LIDAR (Light Detection and Ranging) technique yields range-resolved measurements of the pollutant concentration. The technique employs backscattering and absorption of laser pulses in the atmosphere. A description of the technique can be found in Measures (1984). Earlier works have demonstrated how NO, can be detected using LIDAR (Rothe et al., 1974a, b; Fredriksson and Hertz, 1984). At the institute we have access to a mobile laserradar system (Edner et al., 1987) that has been designed to make LIDAR measurements operational by computer-controlled data acquisition and automatic evaluation routines. Using this system the pollutant distribution can be monitored in more or less real time. Figure 1 illustrates the measurement situation for NO,. The Nd:YAG-pumped dye laser (repetition rate 10 Hz, pulse length S-10 ns, pulse energy 5 mJ) was tuned alternately to two neighbouring wavelengths, one that is absorbed by the pollutant (‘on’. 448.1 nm) and one that is less affected and is used as a reference (‘off’, 446.8 nm). None of these wavelengths are absorbed by other gases normally found in the atmosphere. The data in one direction consist of the summation of 150 measurement cycles, each cycle consisting of 18 pulses, I6 pulses on the ‘off’- and ‘on’wavelengths alternat~ngly followed by two recordings of the background light level, with the laser beam blocked. The time resolution of the detection was Ions. The DIAL (Differential Absorption LIDAR) curve (Fig. lc) was obtained by dividing the ‘on’-data with the ‘off’-data. The concentration was evaluated as a function of range from this curve. The range resolution in the evaluation was chosen to be ZOOm, and the total range was about 1500 m, depending on the light conditions. The skylight influence was minimized by reducing the duty cycle of the PMT according to Allen and Evans (1972). The result can look like the curve shown in Fig. Id. In these measurements the absorption cross-sections found in Woods and Jolliffe (1978) were used.

570

Bo

GALLE

et al

Distance

R

4

Cd)

Q

Wavelength

Distance

G

R

Distance

Dfat In CP(X,,

R)/P(X,,,.R)l

equation

R

R N IR’) dR’

= -2Ca(X,,l-o(Xott)l 0

Fig. 1. Pictorial description of the principles of the DIAL technique. In (a) the system is shown probing two industrial plumes. The lower part gives the intersection ranges. In (b) the on- and offresonance LIDAR curves are shown together with part of the absorption spectrum for the species being measured. The first plume gives rise to a slight absorption while thesecond plume absorbs

heavily. Apart from the absorptions, the LIDAR curves follow a l/R* dependence on distance. In (c)the DIAL curve obtained by division of the on-signal by the off-signal is shown. Part (d) shows the evaluated concentration curve.

By measuring in several directions, mapping of a pollutant can be performed. This can give information on how a smoke stack plume spreads when it leaves the stack or how a pollutant is distributed over an area. It is also possible to monitor the pollutant distribution over a heavily polluted area to search for sources. Since the evaluation of data now is highly computerized, a pollutant map can be produced quickly, within S-10 min after the data collection. Both vertical and horizontal sections can be chosen for monitoring. The detection limit during these measurements corresponds to an optical depth of about 10 mgme3 m of NO,. The area chosen for the NO, surveillance is a suburban area called Miilndal, situated 10 km south of the centre of Giiteborg (population 400,ooO).A motorway runs through the area and the traffic is especially heavy during the rush hours in the mornings and in the evenings. Part of the motorway is flanked by low hills, thus creating a valley where the pollutants may become trapped during unfavourable weather conditions. During a campaign in February 1986 the system was used to monitor the NO, concentration during inversion conditions. The laser beam was scanned horizontally in 5-1.5 directions and horizontal concentration maps were produced. One scan containing 15 different

directions took about 1 h to complete since some averaging was necessary to obtain reliable data. Every scan originated from the same direction. The point measuring station referred to in the following section is a permanent station situated in the central area. Onehour averages are given, e.g. NO, concentrations, temperature and wind velocity and direction.

RESULTS

The first series of NO, concentration maps, which can be seen in Fig. 2, deals with the situation from 06:OO a.m. to 02:OO p.m. on 25 February 1986. The topography is indicated, as well as the motorway. The time refers to the time at the midpoint of the scan. Data from three directions in l-h intervals are shown. The measurements were performed at a height of approximately 20 m above ground level, and the concentrations were evaluated as running mean values over a pathlength of 200 m. High concentrations are displayed as darker areas, according to the table in the lower right corner. Concentrations varied from 0 to more than 180 pg m - 3. The concentrations can be seen to increase up to about 11: 00 a.m. At this time a wind started to blow, as can be seen from the wind data

NOz-mapping using laser-radar techniques

Fig. 2. NO* concentration maps on 25 February 1986. At the bottom the wind velocity and temperature are shown, together with the NO1 concentration measured at the point monitoring station for comparison. in the lower part of Fig. 2, that also includes the NO, concentration and the temperature at the point measuring station. The highest concentrations are found in the direction pointing over the motorway. A similar pattern can be recognized in Fig. 3 which deals with the situation over the same area from 07 : 00 a.m. until 12:00 a.m. on 28 February 1986. Here a larger area was scanned, and again the concentrations increased until about 1l:OO a.m. when the wind suddenly started to blow, according to the wind data. The abrupt start of the wind can be clearly seen from the map at 1l:OO a.m. About half the scan was performed when the wind started and the NO, was displayed

transported away, in good agreement with the point measurement data. The last map shows low concentrations in all directions. DISCUSSION

The previous section shows that the LIDAR technique is now capable of rapid probing of large areas to gain information on pollutant distributions. It has been shown that the development and break-up of an inversion can easily be observ_ed. The concentrations throughout all the measurements seem to be rather evenly distributed in the area

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Time

Fig. 3. NO2 concentration maps on 28 February 1986. Details as in Fig. 2. probed, except at the time of the highest concentrations. Here, some areas with elevated concentrations were found. It should be noted, however, that the laser beam was directed above the roof tops, 20-50 m. and that the data were evaluated using a 200-m pathaverage along the beam. This gives the evaluated data the character of urban background measurements. Higher levels and steeper gradients would probably be found at the street level. Upward measurements towards the ceiling of the inversion showed that the layer extended to a height of approximately 100 m. Clearly, mapping of the type described in this paper would be very helpful in selecting suitable locations for a point monitor for more permanent use. Interesting places and heavily exposed areas can easily be identified. The use of this technique could also be of value in the attempts to understand the atmospheric chemistry, e.g. the production of NO, from NO.

Acknowledyement-Substantial support by Leif Untus in the development of software is greatly appreciated, as well as extensive help from Erik Jansson in evaluating the data. This work was supported by the National Swedish Environmental Protection Board, the Swedish Board for Space Activities and the Gtiteborg Region Association of Local Municipalities. REFERENCES

Allen R. J. and Evans W. E. (1972) Laser-Radar (LIDAR) for mapping aerosol structure. Reo. Sci. Instr. 43, 1422-1432. Edner H., Fredriksson K., Sunesson A., Svanberg S., U&us L. and Wendt W. (1987) Mobile remote sensing system for atmospheric monitoring. Appl. Opt. 26, 43304338. Fredriksson K. and Hertz H. (1984) Evaluation of the DIAL technique for studies on NO, using a mobile LIDAR system. Appl. Opt. 23, 14031411. Measures R. M. (1984) Laser Remote Sensing. WileyInterscience, New York. National Research Council (1977) Nitrogen Oxides. National Academy of Sciences, Washington, U.S.A.

NO1-mapping using laser-radar techniques Pitts J. N., Jr and Metcalf R. L. (eds) (1971) Advances in EnuironmentaJ Science and Technology, Vol. 2. WileyInterscience, New York. Rothe K. W., Brinkmann U. and Walther H. (1974a) Atmlications of tunable dve lasers to air nollution detection: mkHsurements of atmospheric NO, ‘concentrations by differential absorption. Appl. Phys. 3, 115-l 19. Rothe K. W., Brinkmann U. and Walther H. (1974b) Remote measurement of NO, emission from a chemical factory by

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the differential absorption technique. Appl. Phys. 4, 181-182. Stern A. C. (1977) Air Pollution, Vol. 1. Academic Press, New York. Woods P. T. and Jolliffe B. W. (1978) Exoerimental and theoretical studies related to a dye laser differential LIDAR system for the determination of atmospheric SO, and NO, concentrations. Opt. Laser Techn. February 1978, 25-28.

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