A note on average vertical profiles of vehicular pollutant concentrations in urban street canyons

Pergamon

Atmospheric Environment Vol. 29, NO. 24, pp. 3719-3725, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1352-2310/95 $9.50 + 0,00

1352-2310(95)00105-0

SHORT C O M M U N I C A T I O N A NOTE ON AVERAGE VERTICAL PROFILES OF VEHICULAR POLLUTANT CONCENTRATIONS IN URBAN STREET CANYONS N. M. Z O U M A K I S Technological Educational Institution of Thessaloniki, Laboratory of Atmospheric Physics, P.O. Box 14561, TEl Sindos 54101, Thessaloniki, Greece (First received 20 July 1994 and in final form 25 December 1994)

Abstract--Recent observations of air pollutant concentrations measured within and above street canyons were used to study the average vertical profiles of vehicular pollutant concentrations in the urban environment. The idea of an exponential vertical concentration distribution, exp( - Bz q) , resulted from a near ground-level source diffusing over fiat terrain, was tentatively extended to the urban street canyons, where the empirical parameters B and q are generally dependent on the atmospheric stability and the aerodynamic characteristics of the canyon. Key word index: Street canyon dispersion, vertical concentration profile, automotive emissions.

1. I N T R O D U C T I O N

Air pollution has important impacts on the local scale as well as on the urban or regional scales. In Greece, as in other countries, automotive emissions is an important air pollution problem, particularly for persons living or working in the urban street canyons with high traffic loads~ Vehicle motors, in fact, emit various pollutants, such as carbon monoxide, nitric oxides, particles, and several organic compounds. Some of these pollutants concur in several chemical processes that take place in the atmosphere, thus contributing to the formation of other pollutants, such as the near surface ozone. Many of these pollutants have injurious effects on the health of human beings and animals, damage vegetation and materials, reduce visibility and solar radiation, alter temperature and wind distribution, and generally affect urban climates. In addition, there is much current interest in possible effects of air pollutants on global climate (e.g. Seinfeld, 1986). Because emissions from automobiles constitute a major contribution to the total atmospheric pollutant load in urban areas, a number of studies have been undertaken in recent years that have been concerned with the diispersion of air pollutants in the vicinity of city streets. Most of these studies, in general, have involved attempts to establish empirical or correlational relationships between concentration, meteorological, land..use and traffic characteristics (e.g. Zoumakis et al.. 1994). Such empirical models usually contain many arbitrary coefficients (tuning

parameters) that would inhibit applying them to a variety of street canyon geometries. On the other hand, most of the theoretical microscale models were developed for characterizing the dispersion of motor vehicle exhaust gas emitted along a highway in rural open terrain. However, the validity and practical utility of highway models developed for rural roadways are highly questionable as there are significant differences in the dispersive processes in urban vs rural areas (e.g. Kono and Ito, 1990). The complexity of the flow within the urban canyons and the lack of a sound theoretical basis for a street canyon turbulence model have perhaps hindered the development of theoretical microscale dispersion models applicable to the urban canyon environment, for estimating the concentration of air pollutants emitted from motor vehicles. The concentration of pollutants in the vicinity of a street is strongly dependent on the street canyon geometry, e.g. the canyon asymmetry, the shape and size of the buildings (e.g. see Hoydysh and Dabberdt, 1988; Dabberdt and Hoydysh, 1991), the vehicle induced turbulence caused by vehicle presence and speed (e.g. see Gronskei, 1988), the entrainment of emissions from adjacent streets, the ambient air flow, e.g. the transition layer between the top of the street canyon and the inertial boundary layer (e.g. Zoumakis, 1994), the atmospheric stability, the traffic characteristics, etc. Therefore, it can be concluded that air dispersion in urban street canyons is a very complex phenomenon. However, various semi-empirical models for the flow, turbulence and dispersion of

3719

3720

N.M. ZOUMAKIS

pollutants within the urban street canyons have been developed and evaluated in different countries taking into consideration local conditions (e.g. Kono and Ito, 1990; Qin and Kot, 1993). On the other hand, the height variation of vehicular pollutant concentrations C(z) within a street canyon (e.g. carbon monoxide concentrations) is seen to be well approximated by an empirical simple exponential function (e.g. Georgii et al., 1967; Georgii, 1969):

model equations were derived from analytical solutions to the Fickian diffusion equation in which wind speed and diffusivities in the vertical and lateral directions are described as power-laws in height above the ground. Moreover, based on data obtained during SF6 dispersion experiments performed in Osaka, parameters within the analytical solutions were modified to construct the JEA model. As an example, for u / > l m s - 1 and 40 ° ~ < 0 < 9 0 °, the concentration of air pollutants may be estimated by using the following semi-empirical equation:

(1) W1 exp where A (in ppm) and B (non-dimensional) are regression coefficients, z is height (m), and h is the height of the buildings (m). A similar conclusion was also drawn from a series of fluid modelling experiments in the wind tunnel conducted by Hoydysh and Dabberdt (1988) and Dabberdt and Hoydysh (1991).

2. THEORETICAL BACKGROUND

By introducing power law approximations for wind profile and vertical eddy diffusivity, the vertical concentration profile (from a near ground-level continuous point source or an infinite crosswind continuous line source diffusing over flat terrain) can be represented by the following general exponential law (e.g. Huang, 1979): C(Z) ~ e -szq

(2)

where the parameters B and q depend on atmospheric stability and surface roughness. Equation (2) indicates that the vertical concentration distribution of gas follows the general exponential form (in general q ~ 2) rather than Gaussian distribution. These results are in excellent agreement with observations (e.g. Elliott, 1961; Huang, 1979). In this study, the idea of the general exponential function from equation (2) was also (tentatively) extended to the urban street canyon environment. Here, it is also assumed that the empirical parameters B and q are generally dependent on the atmospheric stability and the aerodynamic characteristics of the canyon. In practice, the semi-empirical formula (2) is also used often in urban street canyon applications of the microscale dispersion models. For example, the vertical concentration profile (at least for crosswind conditions) is approximated by a general exponential function [such as equation (2)] in the JEA (Japan Environmental Agency) and TOKYO line source models (e.g. see Kono and Ito, 1990). These microscale dispersion models were developed specifically to address the dispersion of pollutants emitted from motor vehicles, at locations within 200 m from the edge of a road. The JEA and TOKYO dispersion

-

zq

(3)

where u is the wind speed at 15 m above ground, 0 is the wind direction/roadway orientation (deg), Q1 is the line source emission rate (m 3 m - a s - 1), x is the perpendicular distance from line source to receptor (m), z is the receptor height above ground (m), and the empirical parameters A~, W1, Bt, s and q are functions of the length of the line source, wind speed, radiation balance, building size, building-to-land ratio, etc. The JEA dispersion model also includes two additional semi-empirical equations for parallel (u i> 1 ms -1 and0 ° ~< 0 < 40°) and calm (u < 1 ms -t ) wind conditions. JEA model is applicable for both rural open areas and urban areas for situations involving 2- to 4-store buildings. The TOKYO dispersion model is a modified version of the JEA dispersion model. The empirical parameters of the TOKYO dispersion model were determined based on concentration measurements of NOx near roadways in Tokyo. The measurements were performed in an area with 2- to 8-store buildings, and involved emissions from ground-level roads, depressed highways, and elevated highways. For a more detailed description of JEA and TOKYO dispersion models, see Kono and Ito (1990). As the authors pointed out, these models may be difficult to apply to cities with building sizes and street widths differing from those in Osaka and Tokyo. Although the parameters At, W~, B~, s and q are purely empirical, an important link between the vertical dispersive processes in rural open terrain and urban street canyon environment is provided by equation (3); i.e. the vertical concentration profile of vehicular pollutants in urban areas may be also represented (at least for crosswind conditions) by the same general exponential law [such as equation (2)]. Despite the simplifications and assumptions used in the derivation of equation (3), and the fundamental criticisms of the semi-empirical dispersion models (which tend to be site specific), it is reasonable to tentatively assume that instead of using an empirical simple exponential form [such as equation (1)] for describing the vertical average concentration profiles of vehicular pollutants in the urban street canyons; a semi-empirical general exponential form [such as equation (2)] is more suitable (at least for u 1> 1 m s-

Average vertical properties of vehicular pollutant concentration and 40 ° ~ 0 < 90 °) : C(z) ~ A exp

[ (;)'] - B

3721 2.5

20

~=7Z:~mm ATHENS-PATISION ***l** z=lO.61m p ERPA (1989)r_,----J 18- ~ = = 1 3 . e 3 . ,

(4)

16-

2.0 ~c

.

,',tp..o'"

~Q.12-

o 3 o

....

1.5

v

*D/~"

~JlO-

0(.~ . 86-

3. EXPFRIMENTAL DATA

An extensive investigation (field study) on the vertical average CO concentration profile was also carried out in Greece. PERPA (1989) made a detailed measurement of CO vertical concentration profile in the Patision street canyon in Athens. The carbon monoxide concentr~:tion distribution within and above the street canyon was measured continuously day and night at nine measuring levels, for 20 weekdays during the May 1985 monitoring period. The Patision monitoring ,,;tation is located near the center of Athens at the commercial part. The buildings on both sides of the street are tall and nearly continuous. Such a geometry is quite common for the busy streets of the commercial center of Athens. The elevated sampling points were placed about 1 m away from the wall of the buildings and located along the same vertical straight line at eight measuring levels within the street canyon at heights of 2.25,7.38,10.61, 13.83,16.77, 19.71, 22.,55 and 25.62 m and one concentration sampling point above the roof-level at height of 29.03 m above the ground (where h,~ 29 m). Figure l a shows the observed average diurnal variation of CO at the nine sampling points discussed above (indicated by special smaU symbols for each level). Since carbon monoxide is emitted by automobiles, CO and traffic load (see broken solid line) tend to have a similar pattern variation during the day, as shown in Fig. la. The analysis of the measured concentrations shows a common CO-daily trend observed in many ambient air studies characterized by two relative maximum and minimum peaks (e.g. Bardeschi et al., 1991). The maximum concentrations occur in the morning between 0800 and 0900 h and in the evening between 2000 and 2200 h, the traffic rush hours in the commercial center of the cit~. The minimum concentrations are measured between 0400 and 0500 h, when the traffic flow is at its minimum; the second minimum of the concentrations occur in the daytime-hours characterized by a greater atmospheric turbulence, due to a greater solar irradiation and to a thicker mixing layer. As it is seen in Fig. l a carbon monoxide concentrations generally decrease with height. For example, CO concentrations measured at street level were considerably higher than those measured at the top of the street canyon. Moreover, the average evening COpeaks within the str(,et canyon were always higher than the average maximum CO concentrations observed during the morning traffic rush hours. How-

P+-H-+z = l S . S 2 m ~ O A D

r-~14-

where the empirical parameters (regression coefficients) A (ppm), B (n.d.), and q (n.d.) are strongly dependent on atmospheric stability and the urban street canyon particular aerodynamic characteristics.

L_

0,~00 •

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00'

"

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,

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.'+

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' '1'2' ' 't'6' ' '2'0' ' 2 0 ' 0

HOURS 2O

ATHENS - PATISION CO-VERTICAL PROFILE

"~'16:

z)=A,xp[-.(z/.)]

"~

u ::::: 01:o00

nO-12. ~

~

~

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~

22:00

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~

..., 0

4

.

.

. . . . . . . . . . . . . . . 8 12 16 20

C

24

HOURS Fig. 1. (a) The observed average diurnal variation of CO at nine sampling points in the Patision street canyon. The observed concentrations (PERPA, 1989) are indicated by special small symbols (for each level) and the traffic load by the broken solid line. (b) Typical examples of average carbon monoxide concentrations (at several hours of day) as a function of height in the Patision monitoring station. The exponential regression curves are illustrated as solid lines. (c) Diurnal variation of the regression coefficient B [see equation (1)].

ever, the average morning and evening maximum CO concentrations measured at the top of the street canyon remained at the same levels (about 4.0 ppm) and approached the ambient background values. Figure l b illustrates typical examples of hourly average carbon monoxide concentrations (at several hours of day) as a function of height. The observed concentrations are indicated by the data symbols and the exponential regression curves by the solid fines (where the dashed lines in Figs l a and b represent the street canyon-average concentrations). The height variation of CO con~ntration is seen to be reasonably wellapproximated by the simple exponential function [i.e. equation (1)] with the corresponding value of

3722

N.M. ZOUMAKIS

B ranging from a minimum of 1.18 to a maximum of 1.86, as shown in Fig. lc. Both A and B vary with wind direction in a complex manner (e.g. Hoydysh and Dabberdt, 1988; Dabberdt and Hoydysh, 1991). The exponential regression provides a reasonably good fit to the observed vertical concentration profile (with the correlation coefficient R ranging from 0.867 to 0.949).

1.0

~ ~k~, ATHENS-PATISION \, \~. \,

C(z)=Aexp[-B(z/h)q]

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-

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-

q=1.70 OODOO22:00

\ \~,~- " ' \, ~

,,~,0.6

a

ooooo o4:oo

~.~. ~,

-

- _ _ <,:,

~,,'~--

\~ ",~

q=2.16

~-,~pIrRPA(19S9)

4. RESULTS AND DISCUSSION

Assuming, as a working hypothesis, that the average vertical concentration distribution in the canyon can be approximated by the exponential function from equation (4), Fig. 2a illustrates the results of the regression analysis (solid lines) for three typical examples of the height variation of the observed CO concentrations (at 04:00,13:00 and 22:00 local time) in the Patision street canyon in Athens. The dashed lines in Fig. 2a represent the simple exponential regression curves (i.e. q = 1) from equation (1). The average diurnal variation of the correlation coefficient R (ranging from 0.891 to 0.992), and the regression coefficients q (ranging from 1.35 to 2.79) and B (ranging from 1.081 to 1.774) are presented in Figs 2b, c and d, respectively. It follows from Fig. 2b that the correlation coefficients for the general exponential function (i.e. q # 1) from equation (4) were always considerably higher than those for the simple exponential function (in which q = 1). On the other hand, Fig. 2e indicates a significant (almost linear) dependence of q on B. The shape factor q in the theoretical relationship (2) is related to the wind speed (Pw) and eddy diffusivity (Pd) profile power-law exponents by (e.g. Huang, 1979): q = 2 + Pw - Pd. In a laboratory experiment, Willis and Deardorff (1976) found q-values between 1.1 and 2.1 at some distance from an elevated source. A similar range is also given by Nieuwstadt and van Ulden (1978) in an analysis of Prairie Grass diffusion data (Barad, 1958). Here, in the analysis of the observed average vertical profiles of CO concentrations (illustrated in Fig. la), we found that the empirical parameter q ranges from 1.35 to 2.79 (see Fig. 2d). Furthermore, Figs 3, 4a and b illustrate the results of the regression analysis from equation (4) for the experimental data given in Table 1 of Hoydysh and Dabberdt (1988), and the observed vertical average concentration profiles of CO and NOx measured in East Huanshi Road street canyon in Guangzhou City (P.R. China) shown in Fig. 4 of Qin and Kot (1993). It now becomes obvious, from the comparison statistics, that the semi-empirical exponential profile from equation (4) provides a more precise and accurate representation of the average vertical profiles of air pollutant concentrations in the urban street canyons, than the simple exponential form from equation (1). In this discussion, the precision and accuracy of the estimates from equation (4) were also evaluated by exploring the quantitative nature of the vertical normalized concentration profiles (concentrations were

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B

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0

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*

, 1.~5

i

20

i

24 e

.

******

1.~o B

1:>5

2.00

Fig. 2. (a) Typical examples of the hourly average vertical CO-concentration profiles in the Patision street canyon (PERPA, 1989). The observed concentrations are indicated by the small symbols and the regression curves rfrom equation (4)] by the solid (q # 1) and dashed (q = 1) lines. (b) Diurnal variation of the correlation coefficient R. (e) and (d) Diurnal variation of the regression coefficients q and B, respectively. (e) Variation of q with B (the solid line represents the linear regression fit to the data).

normalized by the value of the observed concentration at the lowest measurement height). Therefore, equation (4) leads directly to: C(zx) ~ exp

L \ zl /

,,,

3723

Average vertical properties of vehicular pollutant concentration

?-- \

and C ( z l ) is the concentration at the lowest measurement height (z,). In the special case of z, = 0, the normalized concentration profile can be estimated from the semi-empirical relationship:

HOYDYSH and DABBERDT (1988)

1.0

z) A+xp[ B(zlh)~l

0.8 \

ooaoo Windward \ . . . . . q=l ',\---q=1.8125 '~ 8=0.5738

.o6

~,

- B

Co

N

'~

0.2

o o o o o Leeward o~,

J .....

q=0.5875 B=0.3677

~', I

,

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]

,

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6 7 8

l

,

,

,

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l

9 10 11

~ ~

C(:z~ (1000m-l~

1.0

Fig. 3. Same as in Fig. 2a, but for the experimental data given in Table 1 of Hoydysh and Dabberdt (1988).

0.8

~\A~

ATHENS-PATISION

45

\\ O,~\ \~

q=3.2811 ~.=AA= 13:00 q=2.4839

\

j=0.6 8.

~30

- -

v

0.4

- -

0.2

......... q=l rlc~r~nn Middle (July) . . . . . q=l ~ q=0.1 East Huanshi Road Guangzhou City P.R. CHINA

I 0 w

"r15

0.0

k•

0

~CO i m g

~

,

,

i

9 ~ 12 m- )

,

,

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,~k'-

W15

0.85 |

q

q=0.21

0.'1'

20

0.'2 ' 0.'3.' 0.'4 ' 0.5

NO,~ (rag m -°)

or, equivalently,

EQUATION(5)

I

(6)

where 0 < z l < z, m = B ( z l / h ) ~, C . = C(zt) exp(m)

~

4

8

.......... 12 16

20

24

HOURS 5.00 4.00 q 3.00 1.00

C , , e x p [ - m(--z Y l \ z/ , j

="'c.'=o

5.00 4.00 3.00 2.00

3.00 2.50 I B 2.00 1.50 1.00 0

Fig. 4. Same as in Fig. 2a, but for the observed vertical concentration profiles of (a) CO and (b) NO= measured in East Huanshi Road street canyon in Guangzhou City (P.R. China) shown in Fig. 4 of Qin and Kot (1993).

C(z)~

'6

1.00

',\\ East Huanshi Road i',\ \ Guangzhou City ~ \. \~P.R.\ CHINA ,

(~.10

1'2

(pp~)

R 0.90

0IN and KOT (1993) b C ( z ) = A e x p [ - B ( z / h ) q] Z~AZ~AZ~South side (July) . . . . . . q=l --q=0.25 . . . . . . Noah side (July) ......... q = l ~X°°°°° q=lMiddle (July)

v

8 co

15

45

~30

4

1.00

. . . .

I

~ ~PA(1989)

N

~A~AASoufh side (July) q=l ---q=0.325 ..... North side (July)

a

\\ \~:(z)=C(z,)oxN-m[(z/z,)'-l]l kk o4,=22'm ooooo04.00

,o\ '\t

QIN and KOT (1993) C ( z ) ' . = A e x p [ - B ( z / h )q]

(7)

where Co = C ( z l = 0) . The results of applying the exponential regression fit from equation (5) to the data• (discussed in the previous figures) are shown in Figs 5-7. The average diurnal variation of the correla-

0.4

0.0

[(;)1

C(z) = exp

~ ~ , e * ** 1.0

. . . . . . . . . . . . 1.5 2.0 2.5 B

3.0

Fig. 5. Same as in Fig. 2, but the regression curves are estimated from equation (5).

3724

N.M. ZOUMAKIS HOYDYSH and DABBERDT (1988)

1.0 ooooo Windward

'~ Normalized\ Z,//h=O ~) B=0.408764 \ q=1.53209

- -

0.8

\

\

_\ C(z)=Coexp[-B(z/h)"]

0.6

\

N 0.4 \o

ooooo Leeward \ Normalized \ zl/h=0 o

,:o.;O18,, o\

0.2 0.0

q=0.88532

4

5

6

C(z~

8 9 10 11 12 (1000m-1~

7

Fig. 6. Same as in Fig. 3, but the regression curves are estimated from equation (7).

45

a OlN and KOT (1993)

C(z)=C(z,)e×pl-m[(z/z,)"-l]l "~'30 v I,,--

-r"

LM

~,~z~r.~ - ..... - -

T15

South side (July) q=0.31 44 North side (July) q=0.58745

East Huanshi Road Guongzhou City R. CHINA 3

6 CO ( m g

9 312 m-)

concluded that the semi-empirical exponential curve from equation (5) [or from equation (7) for the case of zl = 0] again provides a consistently good fit to the observations. It would be wise to note some of the obvious limitations of the proposed methodology: (i) Among the limitations, the most noteworthy fact is that, the shape factor q in equation (4) is a purely empirical parameter (dependent in general on the street canyon configuration and atmospheric stability). (ii) The rapid variations of the coefficients q and B with time indicate that most probably they are not related simply with the micro-meterological parameters. As it is seen in Figs 2c, d, 5c and d, there is no simple correlation (e.g. linear correlation) between the values of the best-fit parameters of relations (6) and those of relations (4) when they are fitted to the same series of data. (iii) The roof-level measurements were performed at just 30 cm above the roof-top (see PERPA, 1989). Thus, it would be desirable to extend the observed vertical profiles of vehicular pollutant concentrations to some higher levels (at least a few meters above the roof top) in order to distinguish between local and above-canyon background contributions to streetcanyon air quality levels. In addition, measurements above the roof top (at two or more points) provide the height where shear-free region starts. Notwithstanding these limitations, it becomes obvious (from the comparison statistics) that the height variation of vehicular pollutant concentrations within a street canyon environment can be well represented by the general exponential law: C ( z ) ~ e -B(Z/h)° where the empirical parameters B and q are generally dependent on the atmospheric stability and the aerodynamic characteristics of the canyon.

45 QIN and KOT (1993) C(z)=C(z,)exp}-m[(z/z,)"-I]~

~'30 -r (.9 Ld

-"15

b 5. CONCLUSIONS

zxzxzxzxzxSouth side (July) - q=3.32914 =-=== North side (July) ..... q=0.1004 ooooo Middle (July) -- q=0.2243 .X East Huanshi Road ~ \ ~.pG ~Ru a n g z h o u City . CHINA

0.1 0.2 0.3 3 0.4 NOx ( m g m - )

0.5

Fig. 7. Same as in Fig. 4, but the regression curves are estimated from equation (5).

tion coefficient R (ranging from 0.890 to 0.991), and the regression coefficients q (ranging from 1.094 to 4.497) and B (ranging from 1.134 to 2.887) are also presented in Fig. 5b, c and d, respectively. Moreover, Fig. 5e indicates the significant (almost linear) dependence of q on B. Finally, from Figs 5-7, it can be

From the analysis of air pollution data measured within and above street canyons, it is shown that the average vertical profile of vehicular pollutant concentrations follows the general exponential form [from equation (2)] rather than the simple exponential function (in which q = 1) or the Gaussian distribution (in which q = 2). Even though, our findings of the form of concentration profile (in the vicinity of urban streets) are purely semi-empirical due to the lack of a theoretical basis for the value of q, however, the suggested methodology represents a preliminary-crude step in a more basic parameterization of the vertical dispersive process in the urban street canyon environment. It is realized that this idea needs further verification. In the future, research will be performed to examine and compare against wind tunnel and atmospheric observations, at different micro-meteorological conditions, over representative street canyon geometries, the vertical concentration profiles, as predicted from the semi-empirical general exponential form (4), before the validity of the new approach can be fully

Average vertical properties of vehicular pollutant concentration established. Hopefully, future theoretical and experimental work will help us to improve our understanding and predictability of the impacts of urban automotive emissions on air quality, both in the microscale of the canyon as well as on larger scales, as a consequence of emffting pollutants within the confines of urban street canyons. Acknowledgements--The author wishes to thank the three anonymous reviewers fo~:their helpful comments. This work has been partly supported by the Research Committee of the Technological Educational Institution (TEI) of Thessaloniki.

REFERENCES

Barad M. L. (ed.) (1958) Project Prairie Grass. A Field Prooram in Diffusion, Vol. 1. Geophysics Research Paper No. 59, Geophysics Research Directorate, Air Force Cambridge Research Center. Bardeschi A., Colucci A., Gianelle V., Gnagnetti M., Tamponi M. and Tebaldi G. 0991) Analysis of the impact on air quality of motor vehicle traffic in the Milan urban area. Atmospheric Environment 25B, 415-428. Dabberdt W. F. and Hoydysh W. G. (1991) Street canyon dispersion: sensitivity to block shape and entrainment. Atmospheric Enviromnent 25A, 1143-1153. Elliot W. P. (1961) The vertical diffusion of gas from a continuous source. Int. J Air Wat. Pollut. 4, 33-46. Georgii H. W. (1969) T~e effects of air pollution on urban climates. Bull. World Health Org. 40, 624-635. Georgii H. W., Busch E. and Weber E. (1967) Investigation of the temporal and spatial distribution of the emission concentration of carbon monoxide in Frankfurt/Main Re-

3725

port No. 11 of the Institute for Meteorology and Geophysics, University of Frankfurt/Main. Gronskei K. E. (1988) The influence of car speed in dispersion of exhaust gases. Atmospheric Environment 22, 273-281. Hoydysh W. G. and Dabberdt W. F. (1988) Kinematics and dispersion characteristics of flows in asymmetric street canyons. Atmospheric Environment 22, 2677-2689. Huang C. H. (1979) A theory of dispersion in turbulent shear flow. Atmospheric Environment 13, 453-463. Kono H. and Ito S. (1990) A comparison of concentration estimates by the OMG VOLUME-SOURCE dispersion model with three line dispersion models. Atmospheric Environment 24B, 253-260. Nieuwstadt F. T. M. and van Ulden A. P. (1978) A numerical study of the vertical dispersion of passive contaminants from a continuous source in the atmospheric surface layer. Atmospheric Environment 12, 2119-2124. PERPA (1989) The atmospheric pollution in the area of Athens. Tech. Report No. 3, Ministry of Environment, Physical Planning and Public Works, Greece. Qin Y. and Kot S. C. (1993) Dispersion of vehicular emission in street canyons, Guangzhou City, South China (P.R.C). Atmospheric Environment 27B, 283-291. Seinfeld J. H. (1986) Atmospheric Chemistry and Physics of Air Pollution. Wiley, New York. Willis G. E. and Deardorff J. W. (1976) A laboratory model of diffusion into the convective planetary boundary layer. Q. Jl R. Met. Soc. 102, 427--445. Zoumakis N. M. (1994) Determination of the mean wind speed and momentum diffusivity profiles above tall vegetation and forest canopies using a mass conservation assumption. J. appl. Met. 33, 295-303. Zoumakis N. M., Kelessis A. G., Petrakakis M., Nikolaou K. and Kozyraki T. I. (1994) A microscale dispersion model for carbon monoxide emitted from motor vehicles. Sci. Total Envir. 147, 401-407.