I M P A C T OF C H I N O O K S ON C A L G A R Y ' S A I R Q U A L I T Y :

A C O U S T I C S O U N D E R O B S E R V A T I O N S

OF A T M O S P H E R I C S T A B I L I T Y

R. B. HIC KS and T. M A T H E W S

Department of Physics, University of Calgary, Calgary, Alberta, Canada, T2N 1744

(Received 14 August, 1978; revised 15 November, 1978)

Abstract. An atmospheric acoustic sounder was operated at the University of Calgary for the period March 1976 to July 1977. The stability of the atmospheric boundary layer has been determined from the acoustic chart records. Comparison of the seasonal cycle of stability with changes in the diurnal variation of NO x concentrations in Calgary's downtown core indicates that the sounder measurements are relevant to the air quality problem. It is shown that stability criteria derived from sounder records can stratify hourly pollutant concentration data. In addition, the sounder identifies a characteristic state of the urban boundary layer linked with Chinook winds. Evidence is presented that the probability of pollutant build-up in this layer is particularly high ( ~ 90%) when surface winds are less than 30 km h - l and from the south quadrant.

1. Introduction

In the last decade, the acoustic echo sounder has emerged as a useful tool for probing the turbulent structure of the atmospheric boundary layer. The rapid growth in implementation and variety of applications of this remote-sensing device has been documented in a recent review paper (Brown and Hall, 1978). A particularly straight- forward and useful application of the sounder is the determination of the convective stability or instability of the atmospheric boundary layer at a particular instant. The heights to which ground-based inversions extend may also be reliably estimated from sounder records (Wyckoff et at., 1973; Hicks et al., 1977). This capability of the acoustic sounder indicates a potential for providing useful data for urban air pollution studies, in which boundary layer stability is a critical parameter (MacCready, 1978; Russell and Uthe, 1978). In the present work, we report results obtained from the operation of an acoustic sounder in the city of Calgary, Alberta. A comparison of acoustic records with hourly surface meteorological and air quality data has been made in order to demonstrate the unique input of the acoustic sounder in the interpretation of these data, particularly during occurrences of Chinook winds.

The city of Calgary is located on rolling grassland terrain some 70 km from the east slope of the Canadian Rocky Mountains. The downtown core of the city lies in the Bow River Valley; surrounding residential areas are elevated to 30 to 50 m above the floor of the valley. In winter months the city is susceptible to episodes of high air pollutant concentration, and several investigators have studied this air quality problem (Nkemdirim et al., 1975; Sandhu, 1976; Alberta Environment, 1976, 1977). None of these studies has involved an acoustic sounder. It has been concluded that

the main airborne pollutants participating in the formation of urban smog over

Water, Air, and SoilPollution 11 (1979) 159-172.0049-6979/79/0112-0159 $02.90 Copyright © 1979 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A.

160 R. B. HICKS A N D T. M A T H E W S

Calgary are NO x and O3. Levels of CO and certain reactive hydrocarbons may also be appreciable. Sulfur dioxide, although potentially photochemically active, is not present to any significant degree. The source of the active pollutants is considered principally to be vehicular traffic and residential and industrial combustion.

A consequence of the position of the city in the lee of the Rocky Mountains is the frequent occurrence of Chinook (foehn-type) winds. In winter the onset of these strong westerly Chinook winds is often accompanied by rapid ground-level warming. The effect of the Chinook on Calgary's air quality remains a matter of some controversy, mostly resulting from the difficulty of formulating a clear definition of the Chinook based on ground-level data alone (Brinkmann, 1969; Lester, 1976). A study by Alberta Environment (1977) concludes that air quality does not deteriorate significantly during Chinooks. Nkemdirim and Leggat (1978), however, find stronger heat island intensity and poorer air quality under Chinook conditions than prevail normally. We have found that the Chinook leaves a characteristic signature in the acoustic sounder records and can be shown to be linked to episodes of high air pollutant concentration.

2. Derivation of Atmospheric Stability from Acoustic Sounder Data

The University of Calgary acoustic sounder installation has been described elsewhere (Hicks et al., 1977). The sounder, located on the University campus in the northwest residential quadrant of the city, produces chart records which display the evolution of the atmospheric turbulence profile with time, the chart being darkened where acoustic backscatter from turbulence is detected. Some examples are shown in Figure 1. It is possible to identify periods of atmospheric convective stability and instability from the sounder records. When a ground-based temperature inversion exists, the sounder chart is darkened by horizontally-stratified acoustic echoes up to the height of the inversion layer. In convectively unstable situations, thermal plumes pass over the sounder, leaving a series of vertically-oriented features on the chart corresponding to the turbulent roots of the cells of rising air. These two stability situations may be readily distinguished using the chart records, as seen in Figure 1 a.

It is possible to track the transition from nocturnal boundary layer stability to daytime instability (and vice-versa) throughout the year. Monthly means of the transition times are plotted in Figure 2 for 2 yr of data, with error bars indicating the standard deviations for each month. These times show a seasonal variation that is tied to sunrise and sunset, as expected, and which indicates the increased overall stability of the boundary layer during winter months, when typical acoustic records exhibit little or no daytime plume activity (Figure lb). This trend can be shown in another way by plotting monthly distributions of duration times of inversions observed by the sounder for the same period (Figure 3). In winter months, inversions lasting longer than 24 h are frequently observed.

C H I N O O K S A N D C A L G A R Y ' S A I R Q U A L I T Y

500 , " : ' " ! ' ' i ' !b! ' i '

.,..- oo o6 ]2 18 MST 24

00 06 12 18 MST 24

(a ) MAY 20 -21 , 1977

161

E 00 06 12 18 / V ~ l 24 4--

-r-

UU 06 12 18 /V~b I 24

(b ) FEB 6 - 7 , 1977

Fig. 1. (a) Two days of summer acoustic chart records, May 20-21, 1977. The chart paper is darkened where acoustic backscatter is detected. The transition from stable nocturnal conditions (dark ground-based layers) to daytime instability (vertical striping of chart as turbulent convective plumes pass over the sounder) is clearly seen near 0800 MST on each day. The nocturnal inversion re-forms near 1900 MST. (b) Chart records for February 6-7, 1977. In contrast to summer conditions, the dark ground-based layer persists throughout, indicating long-lived winter inversion conditions in the atmospheric boundary layer up to 200 m

above ground.

Fig. 2.

24

18

M S T

12

oo

r I E I I I r I [ I

T 4--f TET I -

SUNRISE

I I I I I I I

• FORMATION OF INVERSION

o ONSET OF INSTABILITY I J I I i I I I I I I I I I I I 1

M A M J J A S 0 N D J F M A M J J 76 77

M O N T H

Seasonal variation of times of formation and dissipation of nocturnal inversion layers at Calgary. The curves give the times of sunset and sunrise throughout the year.

162

Fig. 3.

R . B . H I C K S A N D T. M A T H E W S

5 JUL 77 0 ~ 4rFL----~-~IFI ~

c1~ L + ] n i

I MAY ~ J

~APR ~ I - + + + --

I = .

ov

~ L sEP ~_~ - i r L m __n N I

L JUL

'~ MAY ; ~ . . . . . . . .

:I MARTB I (2nd holf~ O~ 5 n I ~ l q I 10 15 20 21

INVERSION DURATION (hours) Monthly distribution of inversion duration as determined from acoustic sounder records. Note the

preponderance of inversions lasting longer than 24 h in December-February.

Similar statistics on inversions may be compiled using conventional tower-mounted thermometers (e.g., Takle et al., 1975). However, we emphasize that the height regions accessed by the sounder (50 to 750 m) and by typical instrumented towers (0 to 100 m) are quite different, and this may lead to different definitions o f inversion conditions. The sounder results characterize overall stability of the boundary layer, while towers sense the near-ground region only.

3. Chinook Observations

Chinook events have been observed to leave an identifiable signature in the sounder records (Mathews and Hicks, 1978). In these situations, a regime o f cold arctic air at

CHINOOKS AND CALGARY'S AIR QUALIT~ 163

ground level is invaded from above by warmer air which has passed eastward over the Rocky Mountains. The descending interface between the cold and warm air masses is turbulent, and, as it enters the range of sensitivity of the sounder, shows as a dark return at the top of the sounder chart which approaches ground level with time (Figure 4). The arrival of the acoustic return near ground level is well correlated with ground level warning and the onset of strong westerly winds. In many cases, when the ground level air temperature subsequently rises to the freezing point, a deep ground-based layer of turbulence is formed. As it appears in the acoustic chart records, this characteristic layer is distinguished from more normal inversion returns by its relatively greater thickness (200 to 500 m) and also by the diffuse and variable quality of its upper boundary. For the purposes of this study, this type of return will be referred to as the 'Chinook' layer.

The evolution of the Chinook layer is quite interesting, and points up a difference between acoustic sounder and conventional observations of the Chinook phenomenon. The Chinook layer as observed in sounder records can last for several days, long after surface westerly winds have subsided. The ground level temperature associated with the layer is in excess of 0°C; ground level winds may fall to near calm and change direction markedly. In the conventional description of the phenomenon by a ground-based observer, the Chinook event, with its associated strong westerly winds, may have been over for many hours. The acoustic sounder, however, records that the state of the boundary layer initiated by the Chinook still persists. The ability to identify the presence of this Chinook-related situation is unique to the acoustic sounder.

500

0 O0 06 12 18 /W~)I 24

E ¢-

10 )

"I- O0 06 12 18 / ~ , I 24

DEC 19 - 2 0 - 21, 1976

Fig. 4. Acoustic chart records for thq Chinook event of Dec. 19-21. 1976. Initially the surface air temperature on the 19th was - 10°C. The descending acoustic return on this day indicates the approach of the warm Chinook air mass near the ground. Temperatures rose sharply early on Dec. 20, until at 1200 MST the freezing point was exceeded, and str~ng westerly surface winds sprang up. The characteristic Chinook surface layer recorded by the sounder be gan at 1200 on Dec. 20, and persisted until the termination of the

Chinool ! surface warming at 0600 Dec. 21. I

164 R. B. HICKS AND T. MATHEWS

4. Comparison With Air Quality Data

Air quality data are routinely provided for Calgary by Alberta Environment, which maintains three monitoring stations in the city. We have examined hourly NO x readings from the station located in the downtown area of the city. Our acoustic sounder is located in a residential area, 3 km to the northwest of the pollutant monitoring station; this separation of sites has not adversely affected the present comparison.

4.1. DIURNAL VARIATION

The qualitative nature of the stability data provided by the acoustic sounder necessarily limits the types of comparisons that can be made with air quality data. It is possible to relate the seasonal variation in diurnal air pollutant behavior to the stability transition-time data presented in Section 2. Monthly mean diurnal profiles of NOx concentrations have been plotted for March 1976 to February 1977 in Figure 5. The curves have been normalized to the mean concentration for each month. The diurnal variation normalized in this way reflects fluctuations in the level of automobile engine combustion, which is rapidly varying, rather than more slowly

<

~ C

~ -5C

C3

~ o ~ -50

; MAR 7 6

APR

~ MAY

JUN

L I . ~ ~..,..0 , k ~ JUL

~ ' I~rJ~ SEP

- OCT

~ NOV

F -_

_ DEC

AUG ~ r , j i $~.j~.. ] FEB

k, , . O0 (~ 12 18 ;)4

MST Fig. 5. Seasonal dependence of the diurnal variation in NO~ concentration in downtown Calgary. Dashed lines indicate times of peal vehicle traffic. Arrows indicate monthly mean times of dissipation and onset of

nocturnal inversions as indicated by the acoustic sounder.

CHINOOKS AND CALGARY'S AIR QUALITY 165

varying forms of combustion. For reference, dashed lines indicate the typical times

of maximum vehicular rush traffic during the day. For each month, the mean time of transition from stable to unstable boundary layer (from the acoustic data) is shown as an upward arrow. Similarly a downward arrow indicates the mean onset of the nocturnal inversion for the month. No arrows are shown for December or January, when the boundary layer was predominantly stable.

The following interpretation of the diurnal behavior of NO x concentration

emerges from this comparison: (a) Low background concentrations exist in the early morning hours. Morning

rush hour traffic in the presence of the nocturnal inversion accounts for the rapid

rise in concentration near 0600. The leading edge of this morning peak, like other features tied to vehicular activity, shows a shift of 1 h to earlier times in summer, consistent with the transition to daylight saving time.

(b) The onset of convective instability in the boundary layer, with consequent

increase in the mixing depth, reduces the NO x concentration near background levels during the day. In December and January, when instability is rarely established, pollutant levels remain high.

(c) Because it normally occurs while unstable conditions prevail, the evening rush

hour traffic has relatively little impact on NO x levels, although a small peak can be discerned near 1630 MST. Only in January, when the diurnal profile assumes the character of a double step-function, does the evening rush become a significant

factor. (d) The nocturnal peak in pollutant concentration occurs at much lower levels of

vehicular activity during stable conditions. Quiet traffic near midnight leads to the

decay of NO x levels to the morning background. The time of onset of the evening peak is tied to the formation of the nocturnal temperature inversion, and varies considerably during the year. It involves widely different source conditions in different seasons. As a result, the peak behaves in complex fashion from month to month, tending to be smallest in M a y - J u n e - J u l y when the overlap time between the nocturnal inversion and appreciable traffic activity is at most a few hours.

Although no startling conclusions have arisen from this interpretation, the

systematic and close correlation of the acoustic sounder results with features in the NO x diurnal variation indicates that the residentially-located sounder does provide meaningful data on the state of the urban boundary layer.

4.2. HOURLY COMPARISON

By examining acoustic sounder records and air pollutant concentrations on an hourly basis, it was determined to what degree the sounder could identify boundary layer situations which were particularly conducive to pollutant build-up. Attention was restricted to the winter months of the data collection period (November 1976 to February 1977) when significant episodes of air pollution tended to occur in Calgary.

166 R, B. HICKS AND Y. MATHEWS

This winter was characterized by unusually frequent occurrence of Chinooks, particularly

in the month of December, making it possible to assess the role of the Chinook phenomenon in the air pollution problem.

The acoustic sounder chart records were broadly classified hour by hour as stable or unstable according to the principles discussed in Section 2 above. For several hours, no discernible returns were present on the chart. These ambiguous cases were

classified as unstable. A further category could readily be identified among the stable cases, namely the characteristic 'Chinook' layer of turbulence as discussed in Section 3. Occurrences of this type of return are listed in Table I. As will be shown,

TABLE I

Occurrences of the "Chinook" layer in detected by the Acoustic Sounder

Calgary as

Interval Duration

1976 Nov. 29, 2300-Nov. 30, 1400 (MST) 15 h Dec. 6, 1500-Dec. 6, 1800 3 Dec. 10, 0600-Dec. 1 I. 0200 20 Dec. 13,0900-Dec. 14, 0100 16 Dec. 14, 0900-Dec. 16, 2200 61 Dec. 20, 1200-Dec. 21, 1000 22 Dec. 26, 0400-Dec. 26, 1100 7

1977 Jan. 17, 0500-Jan. 18, 0400 23 Feb. 12, 1600-Feb. 12, 2300 7 Feb. 15, 0600-Feb. 15, 1000 4 Feb. 16, 1600-Feb. 17, 0400 12

Total: 190h

this choice of categories (unstable, stable, 'Chinook') led to a successful stratification of the hourly air quality data. Attempts to further sub-classify the sounder records,

for example to distinguish between 'weak' or 'strong' stable returns by the degree of darkening of the facsimile chart paper, did not produce any significant results with respect to air pollution.

Distributions of hourly mean NOx concentrations for the three categories are shown in Figure 6. For a number of hours, NO x data were not available, and these hours are not included in the Figure. Although these distributions are peaked at relatively low concentrations of NO x (0 to 0.2 ppm), they are considerably skewed, and in some cases possess long tails representing adverse pollution episodes. To characterize these distributions in a simple way, we have chosen to use the average NO x concentration together with the fraction of hours in each distribution in excess of 0.2 ppm, corresponding to discernible air pollution. (In the Province of Alberta,

the maximum permissible level for NO2 (typically one half of the NOx concentration) is 0.21 ppm. No similar guideline has been given for NOx. ) This latter parameter will be designated P, and may be thought of roughly as the probability of exceeding 0.2 ppm in NO x when the sounder is recording the given type of return. The relevant values are listed in Table IIa.

CHINOOKS AND CALGARY'S AIR QUALITY 167

4C

2(

0

400

~ :300

6 Z

100

0

40

0 ,

1. UNSTABLE (250 h)

, , r - [

2. STABLE (2013 h)

3, "CHINOOK" LAYER (140 h)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 HOURLY NO x CONCENTRATION, p p m

ql 0.8

Fig. 6. Distributions of hourly NO x concentrations recorded Nov. 1, 1976-Feb. 28, 1977. Hours have been classified into three boundary layer stability categories on the basis of acoustic records as described in the

text.

Given the relatively short data-collection period, it is difficult to assess the statistical significance of these values. If it is assumed, for example, that the P values are binomially distributed, a standard deviation o~p may be assigned taking into account the size of the data sample. Since it cannot be true that each hour is independent of neighboring hours, this will tend to underestimate the error in P. However, as an indication of the size of this error, o'p values have been included in Table II. It is

TABLE II

Mean NO x concentrations and P-values for the three categories of acoustic sounder record

Acoustic record type total average P (%) = orp (%) hours [NOx], ppm fraction with

[NOx] > 0.2 ppm

(a) All wind conditions: unstable 250 0.08 4 1 stable 2013 0.12 16 1 Chinook 140 0.20 36 4

(b) Surface winds < 10 km h- l : unstable 64 O. 12 13 4 stable 732 O. 15 23 2 Chinook 22 0.33 68 10

168 R. B. H I C K S A N D T. M A T H E W S

apparent that a statistically significant stratification of the air quality data has been

achieved by the use of sounder records. When the Chinook layer was present on the

sounder chart, 36% of the hourly NO x concentrations exceeded 0.2 ppm, compared with 16°70 for other stable layers, and 4% for unstable cases. This is also reflected in the mean values.

4.3. CONSIDERATION OF SURFACE WINDS AND SOURCE FLUCTUATION

In this initial comparison, we have neglected the important effect of surface winds and source variability. Clearly if the three categories of sounder records do not

sample similar conditions of wind speed and source strength, we are making an

unfair comparison. As an approximation to wind conditions in the downtown, we

have used readily-available hourly wind data f rom the Calgary International

Airport . The distribution of wind speeds for each of the three sounder record

categories is shown in Figure 7a. As can be seen, while 'stable ' and 'unstable '

categories have similar wind speed distributions, that for the Chinook layer has

relatively more frequent occurrences of winds in excess of 30 km h-~, as might be expected f rom the nature of the Chinook phenomenon. Source strength data were

not readily available; we instead assumed that the diurnal variation of source

strength for these winter months was approximately constant. In that case, it was

only necessary to sample similar times of day to ensure a fair comparison of sounder

record types. The distributions of the three categories over time of day are shown in

4O

3O

20

r~ 10

I

d 6 Z

~ 4

(a)

WIND SPEED, KM/H

L I

_j L r____J Ln

(2 I'8 24

HOUR OF DAY (MSTI

Fig. 7. Distributions of the three stability categories over (a) surface wind speed and (b) time of day. Dot- dash l i n e - unstable; thin solid l i n e - stable; heavy solid l i ne -Ch inook layer. The right-hand scale of the

lower graph applies to the unstable category only.

CHINOOKS AND CAlGARY'S AIR QUAL[TY 169

Figure 7b. Here both 'stable' and 'Chinook' categories sample more or less

uniformly over 24 h, whereas the unstable cases are peaked in the afternoon between

1100 and 1800 MST. The effect of introducing wind speed data into the comparison is shown in Figure 8.

The wind speed can be introduced in the form of an upper cut-off value (i.e., data are rejected if the wind exceeds a certain speed) as in Figure 8a. Here mean NOx concentrations and P values are plotted against cut-off wind speed and smooth curves have been drawn to indicate the trends. Alternatively, the data may be binned according to wind speed as plotted in Figure 8b. Whereas the first set of curves is affected by the distribution of wind speeds inherent in the data sample (Figure 7), the curves of Figure 8b show more explicitly the dependence of the NO X distributions on surface wind speed conditions.

Roughly uniform source conditions may now be introduced at the expense of sample size by rejecting all times of day not commonly represented in the three sounder categories. Dashed lines in Figure 8 show the results when attention is

restricted to the hours of 11 to 18 MST for the 'stable' and 'Chinook' categories. (There is no appreciable change in the 'unstable' results.)

The systematic trends evident in these data only serve to reinforce the earlier

conclusion that the Chinook layer is relatively more efficient in trapping pollutants, and that it must represent particularly hard temperature inversion conditions in the boundary layer. The increased trapping efficiency of this layer relative to typical inversions is manifested at all surface wind speeds (as measured at the Calgary International Airport) below 30 km h -1. Above 30 km h -1 the surface winds effectively drain the downtown core of excess NOx regardless of the state of the atmospheric boundary layer. Table IIb summarizes the results for near-calm conditions; the Chinook layer is associated with P values near 70%, compared with typically 25 % for other inversion situations.

We emphasize that it is the characteristic Chinook-related situation identified by the acoustic sounder that stands out when air quality is examined. Other hours associated with Chinooks, for example, the hours of rising surface temperatures following the arrival of descending turbulent layers near the ground and prior to the onset of the 'Chinook' layer, produce results with respect to air pollutant build-up similar to those of typical non-Chinook inversions.

4.4. DEPENDENCE ON WIND DIRECTION

Alberta Environment (1976, 1977) confirms that southerly winds produce higher pollutant levels in the Calgary downtown than do winds from other quadrants. This directional anisotropy probably results from the topography of the Bow river valley in conjunction with industrial sources in southeast Calgary. The present data (winter of 1976-77) also showed this trend. Table III contains results for the distributions of hourly downtown NOx concentrations obtained when the data sample was sub- divided according to surface wind direction. Hours with wind speed above 40 km

170 R. B. HICKS AND T. MATHEWS

10C

P ( % )

5O

0 E 0.4

o z

(a)

o

2 [ ~ _ - .o- - .-o..

i i n I r

8o ~°~ ~--~- ~ ZO.2

§o.1 r ~ Z

~o 2o 30 4o 5o UPPER WIND SPEED CUT-OFF, km/h

(b) 10C

. . . . ii

50 - - ~ 3 _ _ ..

° I 0 " 4 ~ 3

0.2

o.1 - -q

0 O 10 20 30 40 50

WIND SPEED, km/h

Fig. 8. Wind speed dependence of P-values and mean hourly NO x ceoncentrations for the three stability cateogires: 1. unstable; 2. stable; 3. Chinook layer. (a) The result of rejecting all hours with wind speed above the abscissa value. Smooth curves indicate trends. Dashed lines for categories 2 and 3 result when only afternoon hours (11-18 MST) are considered. (b) Results as functions of wind speed (averaged over 10 km

h - l intervals). Dashed curves as before.

h -1 were rejected as they contributed no information regarding the directional

dependence. The results for the 'unstable ' category in Table III show no evidence of a south

quadrant effect. The 'stable ' hours, on the other hand, clearly show this direc-

TABLE III

Dependence of mean NO x concentrations and P values on wind direction (wind speed < 30 km h -~)

Type Winddirection Totalh Average [NOx] P (%) o'p(%) (Quadrant) ppm

Unstable N 70 0.05 1 1 E 21 0.08 0 S 82 0.10 2 2 W 42 0.10 10 5

Stable

Chinook

N 499 0.08 6 1 E 79 0.12 18 4 S 478 0.18 33 2 W 457 0.10 10 1

N 10 0.07 0 E 5 0.28 60 22 S 24 0.43 92 6 W 32 0.25 44 9

CHINOOKS AND CALGARY'S AIR QUALITY 171

tional anisotropy. For the 'Chinook' layer, too few hours occurred with winds from the north or east quadrants to draw conclusions regarding these directions. The south quadrant, however, is associated with an extremely high mean hourly NO x concentration (0.43 ppm), with -~ 90°70 of hours exceeding 0.2 ppm. These results are significantly higher than those for the west quadrant. These 24 h of south winds in the Chinook layer are drawn from six separate Chinook episodes occurring in December, January, and February. This indicates a recurring boundary layer situation (Chinook layer returns on sounder, light winds from south quadrant) which has a high danger of undesirable air pollutant build-up.

5. Summary

The acoustic sounder is a convenient remote-sensing device for monitoring the atmospheric boundary layer turbulence profile on a continuous basis. The chart records produced by this device may be interpreted to determine convective stability or instability of the boundary layer at any time. Winter-time Chinook events at Calgary leave a characteristic signature in the sounder records: descending turbulent layers whose arrival within 200 m of the ground is associated with ground-level warming. When the near-surface air temperature reaches the freezing point, a characteristic deep layer of turbulence is often formed which is uniquely identifiable from sounder records. This characteristic state of the boundary layer has been designated the 'Chinook' layer.

Comparison of the atmospheric stability, as revealed by the acoustic sounder, with air quality data shows that the diurnal variation of pollutant concentrations in the downtown core can be interpreted on a seasonal basis, even though the acoustic sounder site is remote from the downtown. Comparison of the two data sets on an hour-by-hour basis for the winter months of 1976-77 shows that the classification of the urban boundary layer as 'stable' or 'unstable' by the acoustic sounder leads to significant stratification of the air pollutant concentration data. In addition, the characteristic 'Chinook' layer is markedly more effective in trapping air pollution in the downtown core than other stable situations, presumably due to the intensification of the urban inversion during Chinooks reported by Nkemdirim and Leggat.

The role of surface winds has to some extent emerged from the hourly analysis. It is evident that pollution cannot build up to significant levels when the surface winds exceed 30 km h- 1. Further, winds below this speed from the south (and possibly east) quadrants lead to relatively higher NOx levels in the stable boundary layer than other directions, probably due to advection of pollutants from industrial sites in southeast Calgary. These conclusions must be evaluated bearing in mind that the wind data were obtained from a remote site (airport). A particularly striking result is that for 24 h of Chinook layer sounder returns in the presence of south quadrant winds,

172 k.B. HlCKS AND X. MATHEWS

drawn from six different Chinook events, the mean hourly NO x level was 0.43 ppm, indicating unusually high air pollution.

Acknowledgments

This work was supported in part by Alberta Environment Research Secretariat and

by the National Research Council of Canada.

References

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Alberta Environment: 1977, A Preliminary Study on the Effect o f the Chinook on Air Pollutant Levels in Calgary.

Brinkmann, W. A.: 1969, The Definition of the Chinook in the Calgary Area, Unpublished MSc. Thesis, University of Calgary.

Brown, E. H. and Hall, F. F., Jr.: 1978, Rev. Geophys. and Space Phys., 16, 47. Hicks, R. B., Smith, D., Irwin, P. J., and Mathews, T.: 1977, Bound. Layer Meteor., 12, 201. Lester, P. F.: 1976, A Quantitative Definition o f the Alberta Chinook, Rept. No. 77, Environmental

Sciences Centre, Univ. of Calgary. Macready, P. B., Jr.: 1978, Monostatic Acoustic Radar Observations Plus in situ Surface Measurements: A

Total System of lnputs to Air-Quality Models, 4th A.M.S. Symp. on Meteor. Obs. and Instr. (Denver, April 10-14, 1978); Preprint Volume, p. 321-325.

Mathews, T. and Hicks, R. B.: 1978, Typical Features of Atmospheric Turbulence Associated with Atmosphere-Ocean Chinooks, (in press).

Nkemdirim, L. C., Lunn, G. R., and Rowe, R. D.: 1975, Water, Air, and Soil Pollut. 4, 99. Nkemdirim, L. C., and Leggat, K.: 1978, Water, Air, andSoilPollut. 9, 53. Russell, P. B. and Uthe, E. E.: 1975, Sodar Network Measurements ofRegionalMixing Depth and Stability

Patterns for an Air Quality Model 4th A.M.S. Symp. on Meteor, Obs. and Instr. (Denver, April 10-14, 1978), Preprint Volume, p. 490-497.

Sandhu, H. S.: 1976, Atmosphere, 14, 137. Takle, E. S., Shaw, R. H., and V aughan, H. C.: 1975, J. Appl. Meteor., 15, 36. Wyckoff, R. J., Beran, D. W., and Hall, F. F., Jr.: 1973, J. Appl. Meteor, 12, 1196.