Measurement of the Vertical and Horizontal Profile of Aerosol Concentration in Urban Air with the Integrating Nephelometer Norman C. Ahlquist and Robert J. Charlson Department of Civil Engineering, University of Washington, Seattle, Wash.
The integrating nephelometer developed by the authors has been successfully adapted for rapid mobile monitoring of the atmospheric extinction coefficient due t o scatter. The instrument has been used t o measure horizontal profiles of a city a s well as t o make vertical soundings. The instrument (particularly the adaptation for a 1-second time constant) is described in detail. Horizontal profile data obtained with the instrument mounted on a n automobile show that it may be possible t o consider a city as an entity rather than a set of unconnected monitoring sites. Vertical sounding data are also presented showing considerable structure that should be of use to the meteorologist.
T
modified integrating nephelometer which can be used for mobile or airborne monitoring of atmospheric aerosol. The integrating nephelometer, which has been discussed earlier (Ahlquist and Charlson, 1967; Charlson, Horvath, et a/., 19671, measures the extinction coefficient due to scatter of light. The scattering coefficient (meter-’) thus derived can be related to visibility (more properly, meteorological range) and and to the mass of aerosol per volume of air ( p g . per cu. meter). The utility of this technique for evaluation of horizontal profiling and vertical sounding of aerosol concentration will be demonstrated by the results of preliminary measurements. 111strum en I
Figure 1 is a functional diagram of the integrating nephelometer as designed for mobile use. The instrument consists of three physical units: the optical assembly the control unit and recorder, and the power supply unit. These are not separated in Figure 1. The optical assembly is contained in a 130-cni. (52-inch) length of 7.5-cm. (3-inch) diameter aluminum irrigation pipe. The detector is an Amperex XPlOlO 10-stage multiplier phototube, which “looks” down the center of the pipe through holes in the disks that form the collimator and the light trap. The cone of observation is defined by the first and fourth disks and does not intersect the edges of any other holes. ‘The fifth disk and the disks in the light trap serve to cast 5had0\4s on any ~
he spatial distribution of atmospheric pollutants in the vicinity of urban complexes is difficult to evaluate for several reasons. Because of inconveniently large horizontal and vertical scales, it is necessary to employ either a large number of static instruments and towers or vehicles with mobile instruments. One exception to this, the laser radar (Barrett, 1967), may facilitate vertical profiling of aerosol, but it is still experimental. For horizontal profiles, the laser radar is not currently hell adapted owing to a limited range of 10 to 20 km. The purpose of this paper is to describe a specially
Linear
H.V. Power Supply
Lag
Figure 1. Functional diagram for the mobile nephelometer The optical unit is not drawn to scale
Volume 2, Uumber 5 , Ma! 1968 363
87-
A
6-
surfaces seen b> the phototube. This is necessary to reduce the background t o a reasonable level. All inside surfaces are painted with 3-M velvet black paint. The light source illuminates the sample air in the center section of the pipe. Any light scatterer in the cone of observation will be seen by the phototube. The source consists of a xenon flashlamp and a 5-cm. (2-inch) diameter opal glass diffuser t o provide a cosine emission characteristic. That this geometry integrates over scattering angle has been illustrated elsewhere (Middleton, 1963). Three features make this device particularly useful for mobile work. First, it is built t o 2/3 scale and 1/2 diameter of earlier units. The weight of 5 kg. (11 pounds) is light enough to permit hand portability. More important, this size results in a sample volume of ca. 1 liter, allowing rapid change of the air sample. Second, the unit contains provisions for in-operation calibration. Third, the electronics operate on a two-beam principle to eliminate noise from the flashlamp, giving a reasonable signal-to-noise ratio with a 1/2 second averaging time. The primary standard for nephelometer calibration is the Rayleigh scattering of particle-free air, C02, and Freon-12. These are reproducible and cover the usual sensitivity range used in the Puget Sound area. However, they are not very convenient for rapid field calibration, so a secondary standard is provided in the instrument. A butterfly valve operated by a rotary solenoid shuts off the sample air stream. The continuous purge air flow fills the instrument with particle-free air with a sea-level Rayleigh scattering coefficient of 2.8 X 10-j meter-' at ca. 4600 A. The upscale calibration is provided by a white surface which is illuminated by light from the flashlamp through a flexible light pipe. A solenoid-actuated shutter uncovers a 1-mm. hole in the end of the light trap through which the multiplier phototube views this white surface. The signal thus introduced into the phototube is about equal t o Freon-12; this is usually set to be about half-scale on the lowest range of the instrument which is 0 to 10 x 10-4 meter-'. These two points-i.e., clean air and a half-scale signal-provide a check of calibration in the field in less than one minute. The electronics package is built in two modules: the control unit and recorder and the power supply unit. The instrument operates on 11 to 16 volts d.c. and the power unit contains square-wave converters and regulators to supply - 1500 to -1800 volts for the multiplier phototube, +450 volts for the flashlamp energ> storage, and positive and negative voltages for the signal amplifiers. The phototube supply is well regulated (0.01 and on log response is controlled by the analog signal to the recorder to obtain the nonlinear response when desired. The energy storage for the flashlamp employs resonant charging to obtain 800 volts across a 2-pf. capacitor. This charging method has high efficiency and results in a current requirement of only 0.9 ampere for the whole instrument. The control module contains the signal amplifiers, detector circuits, and all power switching controls. As mentioned previously. the circuit is a two-beam design. Most of the noise and almost all of the drift in sensitivity in earlier units (Charlson, Horvath, et al., 1967) may be traced to the flashlamp, and the
x),
364 Environmental Science and Technology
54-
3-
I
2-
.?
$ P,
I-
I
-
31 August 1967 1000-1030PDT Foggy Law Wind Speed
30 August 1967 1530-1615 PDT Haze
01 20
I
15
IO
I
Distance
5 N
From
0
5 S
IO
15
20
30
25 L
University H i g h B r i d g e , Km.
Figure 2. Three horizontal profiles taken with differing rneteorological conditions through the city of Seattle, Wash., on U. S. Interstate 5
circuit eliminates the effects of these fluctuations in intensity. The output from the multiplier phototube is a pulse of current about 20 microseconds long. This is fed into a n integrator with time constant of 250 microseconds. The range switch is located in this stage and varies the value of the integrating capacitor. The reference phototube (RCA type 934) is located in the sample chamber opposite the light source. A Wratten No. 47 blue filter covers the tube to match its spectral response more closely to that of the combination of the multiplier phototube and scattering by air. The output of this reference tube is similarly integrated. The outputs of these integrators drive an analog division circuit, the output of which is the ratio of signal pulse to reference pulse. Thus any fluctuations common to both phototubes-Le., from the flashlamp-are eliminated. This ratio signal is then amplified and charges a capacitor through an electronic switch. The switch is closed from 50 to 150 microseconds after the start of the flash, the delay allowing the flash to be complete and the analog divider to stabilize. The resulting d.c. voltage is coupled to the recorder by a voltage follower circuit. A d.c. bias is inserted into this stage to buck out the background signal from light reflected by the walls of the nephelometer. The control module also contains a n analog-digital converter which allows the data to be recorded on magnetic tape; however, this system has not yet been used in the field. Air flow through the nephelometer is achieved by placing it either on the roof of an automobile or in a n appropriate location on an aircraft, such as on the wing. Externally mounted
period that the relative humidity was rapidly dropping due t o increasing air temperature. Pockets of fog could be detected as distinctly different from the residuum of particulate matter by a sharp increase in light scattering correlated with a slight increase in relative humidity. Figure 2B was taken on the afternoon of a similar day after all effects of humidity had disappeared. The relative humidity during this experiment remained below about 60%. (The usual criterion for optical effects of water vapor to be insignificant is that the relative humidity should remain below 70%;. The use of a hJgrometer in conjunction with the nephelometer would provide useful information in differentiating between effects of fog and of particulate pollution.) The shape of the curve is similar to that in Figure 2 A , but is somewhat more regular and bell-shaped. Figure 2 C was taken at approximately the same time of another weekday as Figure 2B with a several-knot wind blowing from the south. The shape of the profile is similar to that of Figure ZB,but the maximum has been decreased t o about a third b> natural ventilation. To the eye, these three cases could be described as fog, haze (or smog), and normal for Figures 2.4. B . and C, respectively. One useful result shown in these figures is the apparent background value for the light-scattering coefficient of air outside the city. The accuracy of the lowest readings outside the urban area in Figure 2C is somewhat less than for the data in Figures 2A and B , owing to the above-mentioned signal-to-noise ratio. The noise at the peak of Figure 2A is about 4 % of the reading and at the minimum of Figure 2C is about 40%. Another feature is the repetition of the general shape of the profile and the scale of the region affected b\ the urban complex. Figure 3 consists of eight vertical soundings made with an aircraft (Cessna 180 on floats) on the same dab a t ditferent
glass fiber filters provide clean purge air from the same motioli. For static operation, a small axial-flow fan can be mounted on the exhaust. No adjustments t o the optical unit are necessary during a measurement so that data may be taken continuously from inside the vehicle. Calibration checks may be made whenever desired; however, the stability of both zero and gain of the instrument is such that one or two calibrations suffice per hour of operation. The accuracy of the method for measuring scattering coefficient is probably better than i.lo%, dependent on the assumed value for the Rayleigh coefficient for particle-free air. The signal-to-noise ratio at a scattering coefficient of 5 X meter-' is more than 25 with a twosecond time constant and is 20 with a li2-second time constant. A one-second time constant corresponds to about 30 meters (100 feet) at a speed of 100 km. per hour (60 m.p.h.). The main difficulty with the smaller nephelometer has been the background light-scattering signal which is about equivalent to Freon-12. The resultant increase in noise due to noise-in-signal of the multiplier phototube is undesirable at low levels of scattering coefficient. The larger instrument previously described has a smaller background more nearly equal to the Rayleigh scattering of air alone. Dutci Figure 2 shows three different but typical horizontal profiles of Seattle, Wash. These were obtained with the nephelometer attached to the roof rack of an automobile which was driven through the city from north to south on Interstate 5 , a large freeway. Although readings on the freeway were systematically somewhat higher than on less-traveled streets, no attempt was made to estimate the importance of the freeway as a source of aerosol. Figure 211 was derived on a foggy morning during the
13 September 1967, H i g h Pressure S y s t e m , L i g h t North Wind
r;
-1050
W of Edmonds
S. End Lopez Island
Tow nse nd
1,500-
1,000-
.
500A
-600 -500 -400
300
C 1200
.
5
1,500-
\
0
*'
,
,
1 1 I= ,
4 6 0 2 4 L i g h t Scattering Coefficient (10-4m-l)
,
6
G. 1
IO0
2
4 H. 6
0
Figure 3. Eight vertical soundings of light-scattering coefficient (abscissa). The ordinate is altitude in both feet and meters. The location in the Puget Sound basin and the approximate time (PDT) is given on each sounding
Volume 2, Number 5, Mal 1968 365
4,5001
1
19 September 196
Between Bridges, Lake Washington
,400
Il3OPDT
,200 ,000
2
800
sB
600 . .2
5 4OOp
200
-0
i
2
3 0 i 2 3 0 i 2 L i g h t Scattering Coefficient (1V4rn-’)
3
Figure 4. Four vertical soundings of light-scattering coefficient taken at one location as a function of time
locations (and times) in the Puget Sound basin. Five effects should be noted : The laminar character of the vertical profile seems to disappear later in the day as the low-level inversion is thermally dissipated. The vertical profile and the height of the haze top are quite variable over what might be considered a relatively uniform “air shed.” Although the visual evaluation of these haze layers indicated a degree of horizontal homogeneity, it was obviously a function of averaging along the entire sight path. Figure 3A shows the effect of humidity just above the water surface of Puget Sound. (The float aircraft allows operation to zero altitude over water.) A comparison of Figures 3B, C, and D shows the effect of a large industrial operation in Port Townsend, Wash. Both Figures C and D were taken upwind of the source, Figure B downwind. Figures 3€, G, and H might indicate a rough magnitude of the effect of the city itself. Where Figures E and H represent a kind of background value for air entering Seattle on the east side of Puget Sound, Figure 3G shows air that has come from the city and has about twice the light-scattering coefficient up to the haze top at ca. 480 meters (1500 feet). Figure 4 includes four vertical soundings made over Lake Washington ca. 1 km. north of the draw span of the Lacey Murrow Floating Bridge. The aircraft was operated in a spiral descent of less than 500 meters horizontal extent. The major feature of these data is a similar disappearance with increasing time of the laminar character of the profile and apparent mixing under a sharply defined haze top. Simultaneous temperature soundings revealed an essentially isothermal layer starting at about 1300 meters (4000 feet), although better temperature instrumentation than a hand-held thermometer would be necessary for reliable determination of the stability index. After touching down on the 1345 sounding, an immediate ascent
366
Environmental Science and Technology
showed that the peak at about 30 meters had disappeared. Excursions of the light-scattering coefficient of this sort of time (or space) scale have been observed earlier (Charlson, Horvath, et a/., 1967). Conclusion The integrating nephelometer has been successfully adapted for mobile monitoring of the atmospheric light-scattering coefficient. The results of preliminary measurements with both automobile and aircraft would seem to have utility for both the atmospheric scientist and the air pollution control engineer. The use of this sort of instrumentation would allow the engineer to consider the city as an entity rather than a series of unrelated stations. Acknowledgment The authors are indebted to Steven B. Smith, who supplied technical support for all phases of the development of this instrument . Literature Cited Ahlauist, N. C.. Charlson. R. J., J . Air Pollution Contro Aisoc.’17, 7, 467-9 (1967). Barrett. E. W.. Ben-Dov. Oded. J . Auul. . . Metenrol. 6, 500-515 (1967). Charlson, R. J., Horvath, H., Pueschel, R. F.. Atmospheric EnGiron. 1, 469-78 (1967). Middleton, W. E . K., “Vision through the Atmosphere,” pp. 203-206, University of Toronto Press, Toronto, 1963. Receiced for reciew January 15, 1968. Accepted March 23, 1968. This incestigation was supported bj, the U. S. Public Health Sercice grant No. AP-00336-04 and bj* the Forest Sercice, U. S. Department of Agriculture grant N o . 2 , Unicersity of Washington. First presented at the rinnual meeting of the PaciJic Northwest International Section of the Air Pollution Control Association, Salem, Ore.. Nocetiiber 1967.
