the meteorology and vertical distribution of pollutants in air pollution episodes in philadelphia

16
Atmospheric Environment, Pergamon Press 1968. Vol. 2, pp. 559-574. Printed in Great Britain. THE METEOROLOGY AND VERTICAL DISTRIBUTION OF POLLUTANTS IN AIR POLLUTION EPISODES IN PHILADELPHIA* FRANCIS K. DAVISand HERMAN NEWSTEIN Drexel Institute of Technology and WFIL-TV, Philadelphia, Pa., U.S.A. (First received 8 January 1968 and in revised form 17 May 1968) Abstract-An analysis is presented here of the general weather patterns, the vertical tempera- ture structure and the wind conditions associated with two periods of high air pollution con- centrations in Philadelphia. Measurements of the vertical distribution of pollutants and their variations are presented for the second period. 1. RELATION OF METEOROLOGICAL CONDITIONS TO HIGH POLLUTION LEVELS IN PHILADELPHIA RELATING meteorological parameters to air pollution concentrations in a given city is not a simple task. Although most large cities across the nation make air pollution measurements, there are still no fixed standards for types of measuring instruments, for location or height of exposure of instruments, or for the atmospheric conditions under which such measurements are made. Wind speed and direction measurements are often available consistently only at U.S. Weather Bureau Stations with the instruments exposed at various heights and at airports located at some distance from the urban areas they represent. Furthermore, in most cities no vertical temperature measurements are available, and atmospheric stability must be inferred by methods such as those employed by TURNER (1961) in his Nashville study. Observations by DEMARRAIS (1961) and DAVIDSON (1967) show that stability conditions over an urban complex are quite different from those over open areas. The study presently being conducted by DAVIDSON (1967) and his group in New York is an attempt to provide meteorological and air pollution measurements in greater quantity and in more appropriate forms to more clearly define the relation- ships between the two. Meanwhile, attempts have been made to analyse available observations to gain a greater understanding of the air pollution problem in a par- ticular city. DAVIS (1960) analysed 10 years of weather data and 3 years of air pollution data for Philadelphia in an effort to relate general weather patterns to periods of high air pollution levels. Some consistent weather characteristics were evident to the extent that DAVIS (1962) was able to demonstrate some success in forecasting high air pollu- tion levels in Philadelphia on the basis of forecasting weather conditions. + This work supported in part by U.S. Weather Bureau Contract No. Cwb 11133 and by Public Health Service Research Grant No. APOO33501Al from the Division of Air Pollution. 559

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Atmospheric Environment, Pergamon Press 1968. Vol. 2, pp. 559-574. Printed in Great Britain.

THE METEOROLOGY AND VERTICAL DISTRIBUTION OF POLLUTANTS IN AIR POLLUTION EPISODES IN

PHILADELPHIA*

FRANCIS K. DAVIS and HERMAN NEWSTEIN

Drexel Institute of Technology and WFIL-TV, Philadelphia, Pa., U.S.A.

(First received 8 January 1968 and in revised form 17 May 1968)

Abstract-An analysis is presented here of the general weather patterns, the vertical tempera- ture structure and the wind conditions associated with two periods of high air pollution con- centrations in Philadelphia.

Measurements of the vertical distribution of pollutants and their variations are presented for the second period.

1. RELATION OF METEOROLOGICAL CONDITIONS TO HIGH POLLUTION LEVELS IN PHILADELPHIA

RELATING meteorological parameters to air pollution concentrations in a given city is not a simple task. Although most large cities across the nation make air pollution measurements, there are still no fixed standards for types of measuring instruments, for location or height of exposure of instruments, or for the atmospheric conditions under which such measurements are made.

Wind speed and direction measurements are often available consistently only at U.S. Weather Bureau Stations with the instruments exposed at various heights and at airports located at some distance from the urban areas they represent. Furthermore, in most cities no vertical temperature measurements are available, and atmospheric stability must be inferred by methods such as those employed by TURNER (1961) in his Nashville study. Observations by DEMARRAIS (1961) and DAVIDSON (1967) show that stability conditions over an urban complex are quite different from those over open areas.

The study presently being conducted by DAVIDSON (1967) and his group in New York is an attempt to provide meteorological and air pollution measurements in greater quantity and in more appropriate forms to more clearly define the relation- ships between the two. Meanwhile, attempts have been made to analyse available observations to gain a greater understanding of the air pollution problem in a par- ticular city.

DAVIS (1960) analysed 10 years of weather data and 3 years of air pollution data for Philadelphia in an effort to relate general weather patterns to periods of high air pollution levels. Some consistent weather characteristics were evident to the extent that DAVIS (1962) was able to demonstrate some success in forecasting high air pollu- tion levels in Philadelphia on the basis of forecasting weather conditions.

+ This work supported in part by U.S. Weather Bureau Contract No. Cwb 11133 and by Public Health Service Research Grant No. APOO33501Al from the Division of Air Pollution.

559

560 FRANCIS K. DAVE and HERMAN NEWSTEIN

These studies indicate that the general weather patterns associated with high air pollution levels in Philadelphia develop as follows. A continental polar high pressure center moves from central Canada across the Great Lakes and off the east coast. This high becomes stationary, or very slow moving, at or near the coast, and the Philadel- phia area is influenced by very light winds between west and south for several days, with southwest flow heavily predominant. Wind speeds are generally less than 8 mile/hr for periods of many hours.

2. GENERAL WEATHER PATTERNS FOR THE TWO

HIGH POLLUTION PERIODS

Two recent periods of very high air pollution levels in Philadelphia occurred 21 into 26 November, 1966 and 23 into 27 January, 1967. These “episodes” actually covered the entire northeast and mid-east coastal area. The weather patterns were similar to those generally associated with high air pollution levels in Philadelphia, and might well have been forecast.

In the first case, early on 20 November a large high pressure center representing continental polar air stagnated over New York, resulting in light north to northeast winds along the Atlantic coastal area. It showed little or no movement during the next 24 hr, then drifted slowly eastward, stretching from Maine to North Carolina on 22 and 23 November. The center was placed over Virginia and the Carolinas on 24 November and over Alabama and Georgia on the 25th. Throughout the entire period winds were very light, predominantly less than 8 knots from the 22nd through the 25th.

The second case was similar to the first. A continental polar high pressure center moved over New York on 19 January, and drifted slowly southward where it stagnated over the mid-east coast on 20 January and remained almost stationary on or just off the coast of the Carolinas through the 25th. By midday on the 26th it had moved east- ward to allow a cold front to bring a chnnge of air mass to the Philadelphia region. From 23 January through most of the 26th wind speeds were almost always 8 knots or lower, and in most instances were reported less than 5 knots at the airport station of the Weather Bureau.

3. TOWER INSTRUMENTATION AND LOCATION

Continuous vertical soundings of the atmosphere are available for the first time for analysis in connection with high air pollution levels in Philadelphia. Through a United States Weather Bureau contract and with the permission of both stations and the active cooperation and participation of WFIL-TV, instruments have been installed on the 1000 ft WFIL-KYW TV tower which is located about 6$ miles north-north- west of Drexel Institute of Technology and 12 miles north of the U.S. Weather Bureau Station at the Philadelphia International Airport. This area is within the boundary of the City of Philadelphia and is of a general suburban character. There are rolling hills with the ground elevation varying from 50 ft to about 350 ft above mean sea level.

The installation consists of Bendix-Friez Aerovane wind instruments, Leeds & Northrup Thermohm resistance wire thermometers and Minneapolis Honeywell Lithium Chloride Dew Probes. Instruments are located at the ground and at levels of 40 ft, 100 ft, 200 ft, 350 ft, 570 ft and 890 ft above the ground. Anemometers are exposed 10 ft away from the tower structure and thermometers about 4 ft away.

Meteorology and Vertical Distribution of Pollutantn in Philadelphia 561

The overall system includes the meteorological sensors on the tower and an instru- ment shelter at the ground all connected by cables to recorders in a building about 80 ft from the base of the tower. In addition to the analog record thus produced continu- ously on individual strip chart recorders, an analog voltage for each of the sensed variables is automatically produced and converted to digital form by a precise and stable digital voltmeter. The digital information is then transmitted via Bell Telephone Company Data Phone system to Drexel Institute of Technology, a distance of bout 6$ miles from the tower site, where IBM cards are automatically punched for each observation.

Complete details of the meteorological system and characteristics of the instruments are available in a report by NEWSTEIN (1966).

The tower has also been instrumented for continuous and automatic air pollution measurements, and some spot sampling has been carried out to observe the order of magnitude of concentrations and their vertical variability. Measurements of SOz concentrations were scheduled during the January “episode”.

For SOZ measurements the Gelman sequential bubbler sampler was modified so that it could be installed on the tower and operated with a minimum of attention. The basic changes are: (1) The single box arrangement was changed to two packages: one, the power module which contains the controlling clock and blower; the other, the sampler module which contains the sequential valve and the bubblers. (2) In place of the flow meter a calibrated limiting orifice is used to control the air flow rate. (3) Heaters were installed at appropriate locations to prevent the reagents from freezing, to prevent condensation from interfering with the flow rate, and to prevent the pump from freezing. (4) The boxes are mounted in such a manner that one person can conven- iently remove the sampler module and replace it with another that has previously been prepared in the laboratory. (5) Power is secured from each of the meteorological sensor levels. Necessary fuses and safety interlocks have been provided.

These sequential bubbler samplers are mounted at the ground near the meteorolog- ical instrument shelter and on the tower at the 100 ft, 350 ft and 570 ft levels. The sampling is for concentrations of SO2 and the method of analysis is the West-Gaeke method as modtied by HOCHHEISER (1964).

Air pollution data in Philadelphia for the two air pollution periods are available from about six locations, but for purposes of relating air pollution levels to meteoro- logical conditions in this study, the sulfur dioxide concentrations obtained at the CAMP station are used as a pollution indicator. These are measured at 20th and Race Streets in downtown Philadelphia by an electrical conductivity method.

4. THE VERTICAL TEMPERATURE STRUCTURE IN THE NOVEMBER EPISODE

The vertical temperature profiles during the high air pollution period in November show several characteristics of interest and significance. First of all, the temporal changes in the lapse rates followed what might be called a “classical” pattern described in textbooks such as the one by GEIGER (1950). Heating and cooling were initiated and propagated from ground level, forming strong temperature inversions beginning in the late afternoon and persisting until after sunrise. FIGURE 1 illustrates the temperature differences between the 890 ft level and the ground level (5 ft), and it is obvious that the cycle was consistent throughout the period.

562 FRANCIS K. DAVIS and HERMAN NEWSTEIN

890’ -Ground - Adiabatic - - - 890’-40’ --__- Adiabatic - -- -

24 -

20 -

G: 16- 1 : \

l’

-12 1 1 . * 1 1 1 * . 1 00 08 16 00 08 16 00 08 16 00 08 16 00 08 16 00 08 16 00

20 21 22 23 24 25

November 1966

FIG. 1. Temperature differences between the 890 ft and ground (5 ft) levels and between the 890 ft and 40 ft levels on the Philadelphia tower.

Note that the temperature inversion did not persist through the day but that a superadiabatic lapse rate was observed for a short period each day in the middle of the day. Thus, it is not necessary for a low level temperature inversion to persist con- tinuously for several days in order for pollution levels to build up to “episode” proportions.

There appears to be a consistent dip in the curve within an hour or two of midnight, indicating a tendency toward somewhat less stable conditions. Examination of the hourly lapse rates shows this to be a result of a singular rise in ground level tempera- ture. The reason for this may be that suggested by WANTA (1962). Under stable con- ditions the wind speed decreases because momentum lost in surface friction is not readily replaced by exchange with faster moving air layers aloft. However, an increase of wind speed aloft may mix heat and momentum downward and weaken the inversion in the process. If the increase of speed aloft is of short duration, a higher degree of stability will be re-established. The wind data presented in FIG. 4 illustrate the increase of wind speed near midnight and lend evidence in support of this explanation although a more detailed analysis is necessary to be conclusive.

FIGURE 1 also shows the pattern for the temperature differences between the 890 ft level and the 40 ft level. It is essentially the same, but with a time lag and a smaller amplitude, as might be expected. Furthermore, the night-time dip is absent.

FIGURE 2 illustrates another interesting feature of the vertical temperature structure in that the top of the inversion layer is mostly at the 350 ft level, though the lapse rate above is often nearly isothermal during periods of inversion below. A notable ex- ception to this occurred during the night of 23-24 November when the layer above 350 ft was strongly stable. This condition developed even though winds aloft were stronger than on previous nights. As shown in FIG. 4, however, there is essentially no wind shear above 350 ft.

Meteorology and Vertical ~s~bution of Pollutants in P~elphia 563

The typical “classical” diurnal variation of the lapse rate during this high pollution period is shown in FIG. 3. This covers the period 0600 21 November through 0400 22 November. It indicates that the heating develops upward from the ground level as superadiabatic lapse rates are set up in the lowest layers, and the low level inversion is entirely broken up before the entire layer is heated. This is in contrast to some ob- servations presented by DEMARRAIS (1965) which indicate heating to a great depth immediately after sunrise without breakup of the surface inversion. The less stable conditions around midnight between the ground and the 890 ft level, as noted in FIG. 1, are represented by the 2200 temperature profile shown in FIG. 3 (b).

24

20

- 16

890’-350’ - Adiabatic - - - 35lJ’- 40’ ___.. Adiabatic ----

-12 16 00 08 16 00 08 16 00

20 21 22 23 24 2s

Mvember 1966

FIG. 2. Temperature diffemrxxa between the 890 ft and 350 ft kvcls and bctwacn the 354 ft and 40 ft kvcls on the Ph~~elp~~ tower.

5. THE VERTICAL WIND STRUCTURE IN THE NOVEMBER EPISODE

The most obvious characteristic of the wind field is persistently low wind speeds during the high pollution period. The wind speed at the 40 ft level practically never exceeded 4 knots, and at the 100 ft level speeds were almost always 8 knots or lower. This can be best seen in a time-section of the type suggested by HOLLAND (1952) and shown in FIG. 4. It shows wind speeds at various times and heights from 20 November 1966 through 25 November 1966.

A possible influence of the upward extension of the friction layer resulting from day-time heating is clearly evident. Wind speeds at the upper two levels show night- time maxima and day-time minima. Isopleths of wind speeds in FIG. 4 show low wind speeds extending upward from the 40 ft level to the top of the tower during the periods between 1000 and 1600 on the first 4 days. The wind shear has an obviously strong dependence on the temperature structure, with the exponent in a power law having larger values during stable periods at night and smaller values during the day-time periods of instability.

The tendency toward slightly higher Iow level wind speeds at times during the night, which might have been associated with enough mixing to produce the higher ground level tern~rat~~ discussed in section 4, are not clearly defined in the gross analysis

564 FRANCIS K. DAVIS and HERMAN NEWSIEIN

Meteorology and Vertical Distribution of Poliutmts in Philadelphia

r . I

566 FRANCIS K. DAVIS and HERMAN NEWSTEIN

presented in FIG. 4, but are indicated. It is probably necessary to look more closely at the wind structure below 40 ft.

6. AIR POLLUTION LEVELS IN THE NOVEMBER EPISODE

Measurements of air pollution concentrations of various pollutants at different locations in Philadelphia are available through the Philadelphia air monitoring program of the Department of Public Health. Sulfur dioxide concentrations are often used as an air pollution index, and FIG. 5 shows hourly average values of the sulfur dioxide concentrations measured at the CAMP station in Philadelphia, located at 20th and Race Streets. This is a center-city location. TABLE 1 gives peak and average values during the November episode. The average sulfur dioxide concentration in Philadel- phia is about 0.09 ppm.

TABLE 1. SULFUR DIOXIDE DAILY AVERAGES AND HOURLY PEAKS,

PHILADELPHIA CAMP STATION

Date (Nov. 1966) Average (ppm) Peak (ppm)

20 21 22 23 24 25 26

0.17 0.26 0.26 0.11 0.23 0.12

0.26 (10-l 1 p.m.) 0.30 (7-8 a.m.) 0.52 (9-10 a.m.) 0.50 (67 a.m.) 0.24 (9-10 a.m.) 0.64 (8-9 a.m.) 0.53 (mid-l a.m.)

Highest air pollution levels are not strongly correlated with greatest inversions, nor are they consistently related to periods of lowest wind speeds. In general, of course, conditions of low wind speed and temperature inversions are associated with the air pollution “episode” in November. On the other hand, TABLE 1 shows that peak pollution values at ground level were measured within a few hours after sunrise and may be largely a result of the fumigation process.

7. THE JANUARY 1967 EPISODE

FIG- 6 and 7 show that the vertical temperature structure during the January episode was similar to that during the November period with temperature inversions from late afternoon until after sunrise and superadiabatic lapse rates for a short time around noon. In January, however, the stability was generally stronger in spite of higher wind speeds and greater wind shear. The inversions in January extended up through the levels above 350 ft in a much more definite manner, and there was a less consistent interruption of the low level night-time stability. This agrees with the previously discussed condition of 23-24 November when a more strongly stable condition in the layers above 350 ft corresponded to higher wind speeds aloft.

The wind speed pattern is drawn in FIG. 8 and reveals the same features as described in connection with the November episode though wind speeds aloft are distinctly

568 F+LANCB K. DAMS and HERMAN NEWS=

higher during the January episode. It shows the pronounced minima around noon- time and the night-time maxima at the upper levels as well as wind speeds of predom- inantly 8 knots or less at the 100 ft level and 4 knots or less at the 40 ft level.

890’ Ground - Mlabatic - - - *cJ0,_403’ _-_- Adiabatic ----

J-___-- ’ 1 ’ ’ ’ ’ ’ * 1 00 - 08 16 00 OS lb 00 lx3 16 00 08 16 00 08 lb 00

23 24 25 26 27

.lanuary 19b7

FOG. 6. Temperature diRerenc&s between the 890 ft and ground (5 ft) levels and between the 890 ft and 40 ft levels on the Philadelphia tower

890’ -350’ - 3yJ’- 40’ ____

Adiabatic - - - Adiabatic -- - -

-I‘? t . . i * 1 . . I. r ,

00 08 16 00 08 16 00 08 lb 00 08 16 itO 08 lb 00 23 24 25 Zb Z.7

January 1967

FIG. 7. Temperature differences between the 890 ft and 350 ft levels and between the 350 ft and 40 ft levels on the Philadelphia tower.

The sulfur dioxide concentrations measured at the CAMP station in Philadelphia are shown in FIG. 9 and TABLE 2. They are closely comparable to those measured in the November episode, even though the wind direction is significantly different. It is

Met~~lo~ and Vertical Distribution of Po~~~~ in Pbikdelphia 569

believed that the principal SO2 sources are southwest of the CAMP station, so that measurements for the January period should have been higher if source strength were the principal factor since the wind direction was mostly from the southwest during that time.

00 04 o* 12 16 20 23 24 25 26

FIG. 8. Variation of wind speeds with height and time at the Philadelphia Tower-January 1967.

January 26

FIG. 9. Sulfur dioxide at 20th and Race Streets-1967.

c A

570 FRAXWX IL DAVIS and HERMAN NEWSTJZN

TABLE 2. SULFUR DXOXILYJE DAILY AVERAGES AND HOURLY PEAKS,

&ILAJXLPIiIA CAMP STATION

Date (Jan. 1967) Average (ppm) Peak @pm)

23 0.32 24 0.20

25 0.23 26 0.31 27 0.12

0.56 (9-10 a.m.) 0.38 (7-8 a.m.)

(P-10 a.m.) 0.50 (11 a.m.-12) 0.64 (2-3 a.m.) 0.36 (8-9 a.m.)

During the January episode some measurements of sulfur dioxide concentrations were made on the tower so that a vertical profile was obtained. The SO1 concentrations measured were 13 hr averages beginning at 12.30 p.m. on 24 January and continuing until noon on 25 January. FICRJRES 10 and 11 show the vertical profiles of temperature and sulfur dioxide over that period.

As the temperature inversion built up during the late afternoon and evening hours, sulfur dioxide concentrations increased first at levels above the ground. By 8 p.m. the ground level concentration had not changed appreciably, but the concentration at the 100 ft level had increased 5-fold while that at the 350 ft level had multiplied by 4. By 2 hr later the concentration at the 100 ft level had further doubled, though it increased but slightly after that during the remainder of the night and through the following morning.

Meanwhile, the ground levei concentrations increased much less during the night and reached a maximum at 10 a.m. when the inversion had been broken up in the lowest 100 ft. This indicates again the fu~gation effect or the mixing in the lower layers which brings down pollutants from above. By noon the sulfur dioxide profile showed the effect of mixing through a deeper layer.

The concentrations at the 350 ft level underwent wide fluctuations, the reasons for which are not clear. By late evening (10 p.m. to mid-night), however, highest con- centration of sulfur dioxide existed at 350 ft. Concentrations at that height then dropped to relatively low values for the next 6 hr, then rose to the absoIute maximum value of 0.31 ppm between 6.30 and 8 a.m.

Principal sulfur dioxide sources are southwest of the city, and the tower is located at the northwest edge. Since winds were from the southwest during the January episode, measured concentrations at the tower location would be expected to be lower than those measured at the CAMP station in center city. The actual differences in measured ground level concentrations are shown in TABLE 3. The measurements at 100 ft on the tower are also included, and a ratio of the ground level concentrations at the CAMP station to those at the tower. From these data, assuming the same ratio for concentrations at 100 ft, the concentrations at 100 ft above center city are com- puted and shown in the last column of TABLE 3. These values are closer in order of magnitude to those being measured aloft over New York City by DAVIDSON (196?), where measured values go higher than 1 .O ppm on occasion. It is, of course, recognized that the estimated ~n~ntrations at 100 ft over the city involve assumptions which may

Metauolo~ and Vedcd Distribution of PolM~ts in Phkklplh 57f

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Tine: 1230-1600 1430-1600 1630-1800 1830-2000

FEO, lh. Vertical distribution of temperature and su&r dioxide at the Phifpddphia tower- 24 January, 1967.

Tine: 2200 0000

Temptrtture ('F) - - - SO 60 70 50 60 70 !~',~IPI...I

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572 FRANCIS K. DAVIS and HERMAN NBW.STEIN

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FIG. lla. Vertical distribution of temperature and sulfur dioxide at the Philadelphia tower- 25 January, 1967.

not be true. For example, the ground level ratio may not correspond to the ratio at the 100 ft level since the tower site is surrounded by vegetation and this may act as a better sink for sulfur dioxide than the paved surface surrounding the city site.

TABLE 3. COMPARISON OF SULFUII DIOMDE MWSUREMENTS (ppm) AT THE TOWER AND AT THE CAMP STATION, PHILADELPHIA, PENNSYLVANU, 24-25 JANUARY 1967

Time

Tower

Concentrations (ppm) Ground loo ft

Time

CAMP station

Concentrations (ppm) Ground Ratio loo ft (meas.) (est.)

1230-1400 0.018 0.022 1300-1400 0.10 5 0.11 1430-1600 0.012 0.015 1500-1600 0.16 13 0.20 1630-1800 0.008 0.016 1700-1800 0.18 22 0.36 1830-2000 0.010 0.069 1900-2wO 0.22 22 1.5 2030-2200 0.031 0.14 2100-2200 0.10 3 0.42 2230-2400 0.038 0.15 2300-2400 0.20 5 0.75 cNx3&0200 0.11 0.17 010&0200 0.22 2 0.34 0230-0400 0.067 0.15 03OO-0400 0.30 4 0.60 0430-0600 0.050 0.18 05OO-O6W 0.22 4 0.72

:I’ -- ---

574 FRANCB K. DAVE and HERMAN NEWSTEIN

8. CONCLUSIONS

The basic large scale weather pattern associated with the two high air pollution periods discussed in this paper features a stagnating anticyclone along the east coast with the resulting extended period of very light winds and marked atmospheric stability over Philadelphia.

The atmospheric stability was reflected in strong temperature inversions in the lower 350 to 500 ft persisting from late afternoon until after sunrise daily throughout the period. Consistently low wind speeds were an even more persistent characteristic during the “episode” periods with the wind at the 40 ft level practically never exceeding 4 knots.

Peak values of sulfur dioxide concentrations at ground level seem to be associated with bringing the pollutant down to ground level during the period of inversion break- up by solar heating after sunrise. Highest concentrations, on the other hand, occur aloft but below 500 ft, perhaps near the 350 ft level. By inference from tower data, sulfur dioxide concentrations aloft over downtown Philadelphia may reach levels well above 1 ppm during an episode such as the one which occurred in January 1967.

The further indication here is that 200 ft towers in a network such as that described by MUNN and STEWART (1967) would not be sufficiently high to record some air

pollution and meteorological data of significance for analysis.

Acknowle&ements--We would like to express our appreciation to WFIL-TV and KYW-TV for permission to use the tower, to WFIL-TV for services at and on the tower, to WALTER JACKSON for acquiring and graphing sulfur dioxide measurements on the tower, and to the Public Health Service and the City of Philadelphia for the use of the CAMP Station data on sulfur dioxide concentrations and FIGS. 5 and 9.

REFERENCES

DAVIDXIN B. (1967) A summary of the new York urban air pollution dynamics research program. J. Air Poll. Control Ass. 17, 154-158.

DAVIS F. K. (1960) The atmosphere over Philadelphia-its behavior and its contamination. Air Poll. Control Section, Department of Public Health, City of Philadelphia, 55 pp.

DAVIS F. K. (1962) The air over Philadelphia. Air Over Cities. pp. 115-129, SEC Tech Rep. A62-5, PHS, Taft Center.

DEMARRAI~ G. A. (1961) Vertical temperature difference observed over an urban area. Bull. Am. Meteor. Sot. 42.548-554.

DEMARRAIS G. A. (1965) The temporal changes of vertical differences after sunrise. J. appl. Meteor. 4, 535-541.

GEIGER R. (1950) The climate near theground, (see p. 86). Harvard University Press, Cambridge, Mass. HOCHHEISER S. (1964) Methods for measuring and monitoring Sot. Bull. No. 999AP6,47 pp. HOLLAND J. Z. (1952) Time-sections of the lowest 5000 feet. BUN. Am. Meteor. Sot. 33, l-6. MUNN R. E. and STEWART 1. M. (1967) The use of meteorological towers in urban air pollution

programs. J. Air Poll. Control Ass. 17, 98-101. NEWSTEIN H. (1966) An automated meteorological instrumentation and observing system on a loo0 ft

television tower. Final Report Cwb-10608 US. Department of Commerce, ESSA, 128 pp. TURNER D. B. (1961) Relationship between 24-hour mean air quality measurements and meteorological

factors in Nashville, Tennessee. J. Air Poll. Control Ass. 11,483-489. WANTA R. C. (1962) Diffusion and stirring in the lower troposphere. In: AirPollution, p. 101, Vol. 1,

edited by A. STERN, Academic Press, New York.