- Email: [email protected]
A comparative study of turbidity in an urban and a rural environment at Toronto
A COMPARATIVE STUDY OF TURBIDITY IN AN URBAN AND A RURAL ENVIRONMENT AT TORONTO SHUJIYA~IASHITA* Atmospheric Environment Service. Toronto. Ontario. Canada (First receiced 27 February 1973 and injkalform
3 Ocroher 1973)
Abstract-Values of Linke’s turbidity factor in Toronto and at nearby rural locations were computed from the measurements ofglobal and diffuse sky solar radiation on cloudless days. The turbidity factor was larger in the city than in the rural area through the year except for some time in the summer. A comparison of turbidity factor for the same optical air masses and wind directions was also made.
I. INTRODUCTION
Munn (1966) has proposed the comparative study of rural and city observing stations using the published climatic summaries as one of the most useful methods in the study of urban climatology. As Landsberg (1961) points out, however, there are not many comparative records for city and rural environments. Especially, there are very few data of the radiation components observed in conjunction with other meteorological elements, such as temperature and precipitation. A few parallel records of the latter are available “by accident at an outlying airport” as stated by Landsberg (1961). On the other hand, radiation observation stations are generally moved to the rural outskirts with the expansion.of a city, so observations within a city are sparse and comparison between rural and urban measurements even more rare. For example, the radiation observation station at Toronto has moved twice so far; from Toronto Bloor Street (Toronto city) to Scarborough, and then to the Toronto Meteorological Research Station (TMRS). Unfortunately, in TorQnto, the periods of global and sky diffuse radation data in the city and in the rural environment do not overlap, so they are not comparable in a strict sense. 2. DATA
Global solar and diffuse sky radiation at Toronto city (43”40’ N, 79”33’ W), Scarborough (43’43’ N, 79’14’ W) and TMRS (43”48’ N, 79”33’ W) were obtained from the Monthly Radiation Summaries of the Canadian Atmospheric Environment Service. Data used in this study are as follows: Height above MSL(m) Toronto city Scarborough TMRS
Period
116 I57 194
Januaryl95b&lay1957 January 1960-July 1967 August 1967-December 1971
* Present address: Dept. of Geography. Aichi University, Toyohashi, Japan 507
508
SHi II ~a\L-\SFiII
4
During these periods the m~asur~men ts tvere mrtde by means of Eppiey 1SOp~ranometers except that during October 197sDecemhsr 1971. global solar radiation at TMRS V,XS measured by Eppley Model 2. Table
1. Frequency
LVinti‘r Toronto tit: (19x-19591 Scarborough (196G-196.;3
ofcloudlessd;tys
distribution
Spring
Summer
Autumn
13
2’
II
ib
IS
32
I0
20
1;
21
P
17
11
9
6
6
tII
Scarborough (3) ii9641967, TMRS 11967-19711
Only ctoudless hours were utilized in this study. The criterion for cloudless hours used here was zero sky cover and 100 per cent of possible sunshine during the daytime. The data that was used to select the cloudless days at Toronto city and Scarborough radiation stations were cloud amounts reported in the surface weather records at Toronto city (partly replaced by Toronto Island Airport when there were no data at Toronto city). Those at Downsview Airport were used to select cloudless days at TMRS. Sunshine hours at Toronto city were also used. Radiation data at Scarborough were divided into two periods; from January 1960 to December 1963 [Scarborough {1)J and from January 1964 to july 1967 [~ar~rou~h (2)-j; i.e. records of almost the same length as those for Toronto city. Figure I shows the location of these observation stations. The frequency distriblition of the cloudIess days selected in this way is shown in Table 1. 79’115‘W
7993O’W
T&S 43*45’N /
/
/
/
i 1 / / \1---_
r3 Downsview Airport
0
I
t-
IO
5
miles 1 0
I 5
I IO
I 15
km
Fig. I. The
location of the observation stations (dotrcd line shows boundary of predominantly urban land use).
Stud! ol’turbidit) 3
LISKE’S
at Toronto
TCRBIDITY
X9
FACTOR
Linke’s turbidity factor T is defined as T =
[log I0 - log I - log q
[IJIG(IJ~)]-~
= P(W) [log I, - log I - log S-j
(1) (2)
here n7 is the optical air mass, G(M)is the mean Rayleigh extinction coefficient of all wave length, I, is the solar constant (1.98 ly min- ’ was used here). 1 is the intensity of direct solar radiation at the normal incidence on the earth’s surface and S is the reduction factor for meansolardistance(S = (RIRm)‘. where R is the sun-earth distance corresponding to the date, Rm is the mean sun-earth distance and R/Rm was obtained from the American Ephemeris and Nautical Almanac. U.S. Naval Observatory. 1948). Values of P(m) ( = [/H4117)] - l ; are given by Csagi (1956). Optical air mass is assumed to be approximately equal to set Z = cosec h. where Z is the sun’s zenith angle and 11is the solar elevation. The solar elevation may be computed from solar declination (6). geographical latitude (4) and hour angle (H) by the following equation: sin h = sin d sin 4 + cos d cos $I cos H.
(3)
The value of d was also noted from the American Ephemeris and Nautical Almanac. The intensity of the direct solar radiation I can be obtained from the global solar radiation K1, the diffuse sky radiation D and the solar elevation 17 as follows; Ki-D
I=-..--.-
(3)
sin /I ’
Global radiation is the downward direct and diffuse solar radiation as received on a horizontal surface. Diffuse sky radiation is the downward solar radiation from the sky as received on a horizontal surface. The value of I is the intensity in ly min-’ averaged over 1 h because the published radiation values were integrated for each hour ending at local time. Thus Linke’s turbidity factor obtained here expresses the average turbidity between the hours of local time. Though Linke’s turbidity factor is less dependent than the extinction coefficient (tl = HI- ’ log(l,,f)) on optical air mass. T is still a function of HI. as pointed out by Robinson (1966) and Polavarapu (1972). Tis also affected by water vapour content and, therefore, T can not be considered as a strict measure of turbidity. Yamamoto rt al. (1968) developed a new method of estimating the turbidity coefficient which might be considered as a more reasonable measure of atmospheric turbidity than Linke’s turbidity factor, but it requires humidity measurements by radiosonde. These data are not available for most places. including Toronto. Therefore Linke’s turbidity factor. which can be estimated from the direct solar radiation measurements at climatological observing stations. was used as the measure of turbidity in this study. 1.
DIURNAL
AND
SEASONAL
VARIATION
Diurnal variations of Linke’s turbidity factor at the various stations during each season are shown in Figs. 2-5. In winter no evident variation of turbidity is found except at ThlRS which has a rather pronounced variation. The turbidity factor at Toronto city remains almost constant at a value of 2.5. The reason why the turbidity factor in winter has no
510
SHCJ~ ~AMxSHITA
evident daily variation is explained later. Daily mean values are 2.35. 241 and 2.31. for Scarborough (1). Scarborough (2) and TMRS, respectivety.
Fig. 2. Diurnal variation of Linke’s t~rbjdity factor in winter for Toronto city [ .---.I. Scarborough (1) ( x x ).Scarborough (2) ( x --- x ) and TMRS (O----O). Also given are standard error of the mean (SE.).
SPRING
Fig. 3. Diurnal variation of Linke’s turbidity factor in spring (legend is the same as Fig. 2).
Stud!
of turbidit)
jll
at Toronto
In spring there is a relatively large difference betueen Toronto city and Scarborough (1). (3). which have daily means of about 3.03. 245 and 2.75. respectively. In summer there are evident diurnal variations of the turbidity factor at all stations. The difference in turbidity between Toronto city and Scarborough becomes evident in the afternoon, tvhile it is not apparent in the morning. The values at TMRS are almost the same as those, or sometimes even larger than those. at Toronto city. To interpret these rather high values at TMRS in the summer three reasons may be considered: (i) rural aerosol production (soil blown from the dry fields and so on). (ii) Lvater vapour or other emissions by Jet Aircraft (TMRS is rather close to Toronto International Airport) and (iii) a time-delay in the production of aerosol from gaseous emissions in the city. At the present time, however. it is impossible to find out this reason mainly due to the small sample size at TMRS. In autumn a rather large difference between the urban and the rural stations is found. The values at TMRS are particularly low and the daily mean is 1.53. Dnii! mean values at stations during each season are given in Table 2. Table
1. Srasonal
average
values of Link’s
Winter Toronto city Scarborough (1) Scarborough (2) TMRS
2.53 2.36 24 1 7.3, _ J_
Spring 3.03 _ _ '._Ii
‘,lj I.89
turbidity Summer 3.30 3.19 3.09 3.24
factor Autumn 3.16 2.83 2.92 z.jj
An interesting feature is that the turbidity factor is slightly larger in the afternoon than in the morning. Annual mean ratios of the turbidity factor in the morning to that in the afternoon are 0.965, 0.977, 1400 and 0.996 for Toronto city, Scarborough (1). (2) and TMRS, respectively. Though the difference between the morning and afternoon atmosphere is dependent on season, prevailing bvind and the dust content (Piggin. 1972). the large difference between Toronto city and the other stations demonstrates the effect of a city on turbidity. In Fig. 6 in which the annual variation of turbidity factor is shown. the days that are partially cloudy in the morning or in the afternoon are added because the sample size of only cloudless days is neither homogeneous nor large enough to obtain a monthly average value. However. the daily mean value on a partly cloudy day is determined only from radiation data during the cloudless period. The frequency distribution of the sum of cloudless and partly cloudy day is shown in Table 3 and reveals that the distribution is still not homogeneous. In particular, bias is introduced by the fact that not a single day is found for Toronto city in November and for Scarborough (2) in August. However, in general the annual variation of the turbidity factor is the same as that for cloudless days. i.e. low during the winter season and high durin g the summer. Again the turbidity factors at Toronto city are larger than at any other station through almost the year. In order to test that the above-mentioned differences were statistically significant, the standard error of the mean was computed by the equation. S.E. = G,‘\ ~1.where G is the S.D. and !I is the sample size. Originally the S.E. was computed on the bases of hourly turbidity values, but the results given in Figs, 2-4 were obtained for daily values. Also the Student’s “t” values (Panofsky and Brier, 1963) were computed in using the daily values of turbidity factor. The results are shown in Fi g. 7. All r-values were computed on the basis
,
1
.
.
Study of turbidity at Toronto
Fig. 4. Diurnal variation of Linke’s turbidity factor in summer (legend is the same as Fig. 2).
of the values at Toronto city. In the following discussions, therefore, to be significantly different means the existence of a difference in the turbidity between Toronto city and the other specified stations. The significant difference at 1 per cent level was found in spring at Scarborough (1) and in autumn at TMRS. At 5 per cent level, there were significant
LaL
1 9
1 10
1 II
1 !J_
8 83
1 14
0
e
i 16
TIME
Fig. 5. Diurnal variation of Linke’s turbidity factor in autumn (legend is the same as Fig. 21.
Fig. 6. Annual variation of Linkc’s turbidit? factor (various lines have same mranings as used in Fig. -7).
differences in spring at Scarborough (2) and in autumn at Scarborough (I). For the hourly r-values, the following hours besides the above-mentioned seasons became significantly different as expected from Figs. 2-4; at 5 per cent level at 18:OOin summer at Scarborough (l), at 13:OO and I4:OO in autumn at Scarborough (2) and at 15:OO in winter at TMRS. Student’s r-test was also extended to the annual values shown in Fig. 6 which includes those on the partly cloudy days. The difference from Toronto city was significant at the 1 per cent level at Scarborough (1) and at the 5 per cent level at Scarborough II). r-values obtained are 3.50, 2.38 and 1.4s for Scarborough (1). (2) and TMRS, respectiveI>.
,o T-TM
n WINTER
SPRING
SUMMER
AUTUNlN
Fig. 7. Student’s r-values computed on the basis of daily values at Toronto city (T:Toronto city. SI : Scarborough (l), SZScarborough (2) and TM: TARS).
515
Stud! of turbidit! at Toronto 5. REL.ATION
OF TURBIDITY
FhCTOR TO OPTIC.AL DIRECTION
AIR
M.-\SS
A%D
TO
Q’IND
5.1 Optical air mss Xs mentioned above and pointed out by many authors (for example. Hinzpeter. 1950). there can be a variation of the turbidity factor even when the kvater vapour and dust content of the atmosphere remain constant. Then the comparison of turbidity factor for optical air masses is required. Figure 8 indicates the variation of turbidity factor with optical air mass at the three stations. This was obtained from the cloudless days only. The optical air mass was divided into four ranges; lS2.0. 2%3.0. 30-4.0 and 40-50. The rather sharp decrease in turbidity factor betbveen nl = lS2.0 and ITI= 2.0-3.0 is identical with the result of Hinzpeter (1950). BetLveen 111= 2.0 and 4.0. the turbidity factor remains relatively unchanged. This is the reason why winter has less diurnal variation, that is. because optical air mass during this time is always larger than 3.0. The increase of the turbidit) factor at ITI= 4.0-50 at Toronto city may be related to the large attenuation of total incoming shortwave radiation in an urban environment at low elevation of the sun (Yamashita, 1973).
Fig. 8. Variation of turbidity with optical air mass (various lmes have same meanings as used in Fig. 2).
The water vapour contents of the air in the urban and rural areas differ for three reasons; (i) the reduction in evaporation due to the rapid runoff and lack of vegetation in the city, (ii) water vapour produced in combustion and cooling process and (iii) the effect of the Lake Ontario. Generally speaking, the decrease of humidity in cities is 2-6 per cent (Landsberg, 1972). Table 4 shows what happens in Toronto. The values are the monthly means of water vapour pressure at 13:00 hour averaged from 1964 to 1968. The rather high values at Island Airport due to the proximity of the lake indicates that at_the screen levels water vapour is more influenced by the lake than by the urban structure. The values at Toronto city are lower from February to June and higher from July to October than
516
,/ ii
I I
I
‘1 I
Study of turbidity
517
at Toronto
at Downsview Airport, but these differences are quite small compared with those bettveen Island Airport and other two stations. Assuming that the environments at Scarborough and TMRS are similar to those at Island and Downsview Airport respectively. then Scarborough should show the maximum influence from the water vapour in the low atmosphere. At higher levels. a considerable amount of water vapour discharged into the urban air may drift over the other areas. With the present situation in Toronto it is not possible to estimate the effect of water vapour from the city on turbidity. The effect of Lake Ontario or the urban atmosphere on turbidity may be estimated by dividing the data into four groups according to NE-N-NW (N), E, SE-S (S) and SW’-W (W). The result is shown in Table 5. Roughly speakin g, turbidity factors with N winds are low and high with S winds at all stations. Of particular interest is that in the E sector the value at Scarborough is lower than at TMRS and in the W sector the one at Scarborough is higher than at TMRS. The difference in turbidity factor between Toronto city and other station is largest in the N sector which may indicate the southward shift of the urban atmosphere. Table 5. Turbidity
factor with the different
wind directions
Toronto city T X0. of cases Scarborough (I) T So. of cases Scarborough (2) T No. of cases T&IRS T No. of cases
6.
CONCLUDING
REMARKS
It would be useful if a metropolitan city like Toronto could have at least two radiation observation stations (including measurements of diffuse sky radiation); one in an urban area and another in a rural environment, because simultaneous measurements in and out of a city permit determination of the city’s effect on turbidity. The present paper examined Linke’s turbidity factor computed from the measurements of global and diffuse sky radiation in and out of Metropolitan Toronto. Although a careful selection limits the sampling number of cloudless days, the preliminary results are as follows: (ii) annual mean values of about 3.00, 2.71, 2.54 and 2.74 are obtained for Toronto city, Scarborough (l), (2) and TMRS, respectively, the last two stations being rural; (ii) mean monthly values for all three stations show higher turbidity during the summer season than during the winter; (iii) the difference in the turbidity factor between an urban and a rural environment is larger in spring and autumn than in winter and summer; (4) the turbidity factor is slightly larger in the afternoon than in the morning; (5) the statistics on wind direction show the effect of the urban atmosphere on turbidity. Acknowfe~~glgrmmts-The author vvishes to express his sincere gratitude to Drs. R. \V. Shaw and R. E. hfunn. Atmospheric Environment Service of Canada, for their invaluable suggestions and review of the manuscript. and to Dr. R. J. Polavarapu and J. R. Latimer, Atmospheric Environment Service of Canada, for their useful discussions. Also he gives thanks to the National Research Council of Canada for the award of a postdoctorate fcllowship and Atmospheric Environment Service of Canada for providing the facilities.
REFERENCES Angstrom A. K. 113iO) On determinations of the atmospheric turbidity and their relation to pyrheriometric measurements. .~limncr Gtioph,r. 14. 269-39-t. Csagi (1956) IGY fnsnucrion .~fanuul. Pt. IV. pp. 367-466; Radiation lnsrrtrmenrs and .Ifeaswtmenrs. .~nn. Intern Geophys. Year. Pergsmon Press. Oxford. Hinzpeter H. (19501 L’ber Truhungsbestimmungen in Potsdam in den Jahren 19-16und IY-li. Z. .\I,,r. 4. I Landsberg H. E. t 1961) City au-better or worse’! In Symposium: .4tr owr C~rirs. Tech. Rep. d62-5. I-X L S Public Health Service, Cincinnati. Landsberg H. E. (!972) Inadvertent atmospheric modification through urbanization. Tech. Note BN 731. Institute for Fluid Dynamics and Applied %tthematics. Univ. of Maryland Linke F. and Boda K. (1923~ Vorschlage zur Berechnung des Trubungsgrades der Atmosphare aus den Messungen der tinnenstrahlung. .Clereor. Z. 39. 161-166. Munn R. E. (1966) Descriptire .bJicrometeoroloy). Academic Press, New York. Munn R. E. (1973) Urban meteorology: some selected topics. Bull. ,-tnr. ;Mer. Sot. 54.90-93. Piggin I. G. (1972) Diurnal aqmmetries in global radiation. Arch. .Vrt. Geophs. Biokl. Ser. B 20.41-4S. Polavarapu R. J. I 1972) Atmospheric turbidity over Canada. Proceedings of rhr Inrernnrional Rndkzrinn .S!.mposium. 348-353. Sendai. Japan. Robinson N. (1966) Solar Rodhrrion. Elsevxr. New Y’ork. U.S. Naval Observatory (19-15)The America/z Ephemeris and .Vuuricol Almat~nc ofthe Year 1950. Yamamoto G.. Tanaka M. and ,Arao K. I 1968) Hemispherical distribution of turbidity coefficients as estimated from direct solar radiation measurements. J. ,Llrr Sot. Jrrpnrr 46, 257-299. Yamashita S. (19731 .&r pollution study from measurements of solar radiation. Arch. .LJrr. Geophs. Bioki.. SC,-. 8. 213-143.
3 Ocroher 1973)
Abstract-Values of Linke’s turbidity factor in Toronto and at nearby rural locations were computed from the measurements ofglobal and diffuse sky solar radiation on cloudless days. The turbidity factor was larger in the city than in the rural area through the year except for some time in the summer. A comparison of turbidity factor for the same optical air masses and wind directions was also made.
I. INTRODUCTION
Munn (1966) has proposed the comparative study of rural and city observing stations using the published climatic summaries as one of the most useful methods in the study of urban climatology. As Landsberg (1961) points out, however, there are not many comparative records for city and rural environments. Especially, there are very few data of the radiation components observed in conjunction with other meteorological elements, such as temperature and precipitation. A few parallel records of the latter are available “by accident at an outlying airport” as stated by Landsberg (1961). On the other hand, radiation observation stations are generally moved to the rural outskirts with the expansion.of a city, so observations within a city are sparse and comparison between rural and urban measurements even more rare. For example, the radiation observation station at Toronto has moved twice so far; from Toronto Bloor Street (Toronto city) to Scarborough, and then to the Toronto Meteorological Research Station (TMRS). Unfortunately, in TorQnto, the periods of global and sky diffuse radation data in the city and in the rural environment do not overlap, so they are not comparable in a strict sense. 2. DATA
Global solar and diffuse sky radiation at Toronto city (43”40’ N, 79”33’ W), Scarborough (43’43’ N, 79’14’ W) and TMRS (43”48’ N, 79”33’ W) were obtained from the Monthly Radiation Summaries of the Canadian Atmospheric Environment Service. Data used in this study are as follows: Height above MSL(m) Toronto city Scarborough TMRS
Period
116 I57 194
Januaryl95b&lay1957 January 1960-July 1967 August 1967-December 1971
* Present address: Dept. of Geography. Aichi University, Toyohashi, Japan 507
508
SHi II ~a\L-\SFiII
4
During these periods the m~asur~men ts tvere mrtde by means of Eppiey 1SOp~ranometers except that during October 197sDecemhsr 1971. global solar radiation at TMRS V,XS measured by Eppley Model 2. Table
1. Frequency
LVinti‘r Toronto tit: (19x-19591 Scarborough (196G-196.;3
ofcloudlessd;tys
distribution
Spring
Summer
Autumn
13
2’
II
ib
IS
32
I0
20
1;
21
P
17
11
9
6
6
tII
Scarborough (3) ii9641967, TMRS 11967-19711
Only ctoudless hours were utilized in this study. The criterion for cloudless hours used here was zero sky cover and 100 per cent of possible sunshine during the daytime. The data that was used to select the cloudless days at Toronto city and Scarborough radiation stations were cloud amounts reported in the surface weather records at Toronto city (partly replaced by Toronto Island Airport when there were no data at Toronto city). Those at Downsview Airport were used to select cloudless days at TMRS. Sunshine hours at Toronto city were also used. Radiation data at Scarborough were divided into two periods; from January 1960 to December 1963 [Scarborough {1)J and from January 1964 to july 1967 [~ar~rou~h (2)-j; i.e. records of almost the same length as those for Toronto city. Figure I shows the location of these observation stations. The frequency distriblition of the cloudIess days selected in this way is shown in Table 1. 79’115‘W
7993O’W
T&S 43*45’N /
/
/
/
i 1 / / \1---_
r3 Downsview Airport
0
I
t-
IO
5
miles 1 0
I 5
I IO
I 15
km
Fig. I. The
location of the observation stations (dotrcd line shows boundary of predominantly urban land use).
Stud! ol’turbidit) 3
LISKE’S
at Toronto
TCRBIDITY
X9
FACTOR
Linke’s turbidity factor T is defined as T =
[log I0 - log I - log q
[IJIG(IJ~)]-~
= P(W) [log I, - log I - log S-j
(1) (2)
here n7 is the optical air mass, G(M)is the mean Rayleigh extinction coefficient of all wave length, I, is the solar constant (1.98 ly min- ’ was used here). 1 is the intensity of direct solar radiation at the normal incidence on the earth’s surface and S is the reduction factor for meansolardistance(S = (RIRm)‘. where R is the sun-earth distance corresponding to the date, Rm is the mean sun-earth distance and R/Rm was obtained from the American Ephemeris and Nautical Almanac. U.S. Naval Observatory. 1948). Values of P(m) ( = [/H4117)] - l ; are given by Csagi (1956). Optical air mass is assumed to be approximately equal to set Z = cosec h. where Z is the sun’s zenith angle and 11is the solar elevation. The solar elevation may be computed from solar declination (6). geographical latitude (4) and hour angle (H) by the following equation: sin h = sin d sin 4 + cos d cos $I cos H.
(3)
The value of d was also noted from the American Ephemeris and Nautical Almanac. The intensity of the direct solar radiation I can be obtained from the global solar radiation K1, the diffuse sky radiation D and the solar elevation 17 as follows; Ki-D
I=-..--.-
(3)
sin /I ’
Global radiation is the downward direct and diffuse solar radiation as received on a horizontal surface. Diffuse sky radiation is the downward solar radiation from the sky as received on a horizontal surface. The value of I is the intensity in ly min-’ averaged over 1 h because the published radiation values were integrated for each hour ending at local time. Thus Linke’s turbidity factor obtained here expresses the average turbidity between the hours of local time. Though Linke’s turbidity factor is less dependent than the extinction coefficient (tl = HI- ’ log(l,,f)) on optical air mass. T is still a function of HI. as pointed out by Robinson (1966) and Polavarapu (1972). Tis also affected by water vapour content and, therefore, T can not be considered as a strict measure of turbidity. Yamamoto rt al. (1968) developed a new method of estimating the turbidity coefficient which might be considered as a more reasonable measure of atmospheric turbidity than Linke’s turbidity factor, but it requires humidity measurements by radiosonde. These data are not available for most places. including Toronto. Therefore Linke’s turbidity factor. which can be estimated from the direct solar radiation measurements at climatological observing stations. was used as the measure of turbidity in this study. 1.
DIURNAL
AND
SEASONAL
VARIATION
Diurnal variations of Linke’s turbidity factor at the various stations during each season are shown in Figs. 2-5. In winter no evident variation of turbidity is found except at ThlRS which has a rather pronounced variation. The turbidity factor at Toronto city remains almost constant at a value of 2.5. The reason why the turbidity factor in winter has no
510
SHCJ~ ~AMxSHITA
evident daily variation is explained later. Daily mean values are 2.35. 241 and 2.31. for Scarborough (1). Scarborough (2) and TMRS, respectivety.
Fig. 2. Diurnal variation of Linke’s t~rbjdity factor in winter for Toronto city [ .---.I. Scarborough (1) ( x x ).Scarborough (2) ( x --- x ) and TMRS (O----O). Also given are standard error of the mean (SE.).
SPRING
Fig. 3. Diurnal variation of Linke’s turbidity factor in spring (legend is the same as Fig. 2).
Stud!
of turbidit)
jll
at Toronto
In spring there is a relatively large difference betueen Toronto city and Scarborough (1). (3). which have daily means of about 3.03. 245 and 2.75. respectively. In summer there are evident diurnal variations of the turbidity factor at all stations. The difference in turbidity between Toronto city and Scarborough becomes evident in the afternoon, tvhile it is not apparent in the morning. The values at TMRS are almost the same as those, or sometimes even larger than those. at Toronto city. To interpret these rather high values at TMRS in the summer three reasons may be considered: (i) rural aerosol production (soil blown from the dry fields and so on). (ii) Lvater vapour or other emissions by Jet Aircraft (TMRS is rather close to Toronto International Airport) and (iii) a time-delay in the production of aerosol from gaseous emissions in the city. At the present time, however. it is impossible to find out this reason mainly due to the small sample size at TMRS. In autumn a rather large difference between the urban and the rural stations is found. The values at TMRS are particularly low and the daily mean is 1.53. Dnii! mean values at stations during each season are given in Table 2. Table
1. Srasonal
average
values of Link’s
Winter Toronto city Scarborough (1) Scarborough (2) TMRS
2.53 2.36 24 1 7.3, _ J_
Spring 3.03 _ _ '._Ii
‘,lj I.89
turbidity Summer 3.30 3.19 3.09 3.24
factor Autumn 3.16 2.83 2.92 z.jj
An interesting feature is that the turbidity factor is slightly larger in the afternoon than in the morning. Annual mean ratios of the turbidity factor in the morning to that in the afternoon are 0.965, 0.977, 1400 and 0.996 for Toronto city, Scarborough (1). (2) and TMRS, respectively. Though the difference between the morning and afternoon atmosphere is dependent on season, prevailing bvind and the dust content (Piggin. 1972). the large difference between Toronto city and the other stations demonstrates the effect of a city on turbidity. In Fig. 6 in which the annual variation of turbidity factor is shown. the days that are partially cloudy in the morning or in the afternoon are added because the sample size of only cloudless days is neither homogeneous nor large enough to obtain a monthly average value. However. the daily mean value on a partly cloudy day is determined only from radiation data during the cloudless period. The frequency distribution of the sum of cloudless and partly cloudy day is shown in Table 3 and reveals that the distribution is still not homogeneous. In particular, bias is introduced by the fact that not a single day is found for Toronto city in November and for Scarborough (2) in August. However, in general the annual variation of the turbidity factor is the same as that for cloudless days. i.e. low during the winter season and high durin g the summer. Again the turbidity factors at Toronto city are larger than at any other station through almost the year. In order to test that the above-mentioned differences were statistically significant, the standard error of the mean was computed by the equation. S.E. = G,‘\ ~1.where G is the S.D. and !I is the sample size. Originally the S.E. was computed on the bases of hourly turbidity values, but the results given in Figs, 2-4 were obtained for daily values. Also the Student’s “t” values (Panofsky and Brier, 1963) were computed in using the daily values of turbidity factor. The results are shown in Fi g. 7. All r-values were computed on the basis
,
1
.
.
Study of turbidity at Toronto
Fig. 4. Diurnal variation of Linke’s turbidity factor in summer (legend is the same as Fig. 2).
of the values at Toronto city. In the following discussions, therefore, to be significantly different means the existence of a difference in the turbidity between Toronto city and the other specified stations. The significant difference at 1 per cent level was found in spring at Scarborough (1) and in autumn at TMRS. At 5 per cent level, there were significant
LaL
1 9
1 10
1 II
1 !J_
8 83
1 14
0
e
i 16
TIME
Fig. 5. Diurnal variation of Linke’s turbidity factor in autumn (legend is the same as Fig. 21.
Fig. 6. Annual variation of Linkc’s turbidit? factor (various lines have same mranings as used in Fig. -7).
differences in spring at Scarborough (2) and in autumn at Scarborough (I). For the hourly r-values, the following hours besides the above-mentioned seasons became significantly different as expected from Figs. 2-4; at 5 per cent level at 18:OOin summer at Scarborough (l), at 13:OO and I4:OO in autumn at Scarborough (2) and at 15:OO in winter at TMRS. Student’s r-test was also extended to the annual values shown in Fig. 6 which includes those on the partly cloudy days. The difference from Toronto city was significant at the 1 per cent level at Scarborough (1) and at the 5 per cent level at Scarborough II). r-values obtained are 3.50, 2.38 and 1.4s for Scarborough (1). (2) and TMRS, respectiveI>.
,o T-TM
n WINTER
SPRING
SUMMER
AUTUNlN
Fig. 7. Student’s r-values computed on the basis of daily values at Toronto city (T:Toronto city. SI : Scarborough (l), SZScarborough (2) and TM: TARS).
515
Stud! of turbidit! at Toronto 5. REL.ATION
OF TURBIDITY
FhCTOR TO OPTIC.AL DIRECTION
AIR
M.-\SS
A%D
TO
Q’IND
5.1 Optical air mss Xs mentioned above and pointed out by many authors (for example. Hinzpeter. 1950). there can be a variation of the turbidity factor even when the kvater vapour and dust content of the atmosphere remain constant. Then the comparison of turbidity factor for optical air masses is required. Figure 8 indicates the variation of turbidity factor with optical air mass at the three stations. This was obtained from the cloudless days only. The optical air mass was divided into four ranges; lS2.0. 2%3.0. 30-4.0 and 40-50. The rather sharp decrease in turbidity factor betbveen nl = lS2.0 and ITI= 2.0-3.0 is identical with the result of Hinzpeter (1950). BetLveen 111= 2.0 and 4.0. the turbidity factor remains relatively unchanged. This is the reason why winter has less diurnal variation, that is. because optical air mass during this time is always larger than 3.0. The increase of the turbidit) factor at ITI= 4.0-50 at Toronto city may be related to the large attenuation of total incoming shortwave radiation in an urban environment at low elevation of the sun (Yamashita, 1973).
Fig. 8. Variation of turbidity with optical air mass (various lmes have same meanings as used in Fig. 2).
The water vapour contents of the air in the urban and rural areas differ for three reasons; (i) the reduction in evaporation due to the rapid runoff and lack of vegetation in the city, (ii) water vapour produced in combustion and cooling process and (iii) the effect of the Lake Ontario. Generally speaking, the decrease of humidity in cities is 2-6 per cent (Landsberg, 1972). Table 4 shows what happens in Toronto. The values are the monthly means of water vapour pressure at 13:00 hour averaged from 1964 to 1968. The rather high values at Island Airport due to the proximity of the lake indicates that at_the screen levels water vapour is more influenced by the lake than by the urban structure. The values at Toronto city are lower from February to June and higher from July to October than
516
,/ ii
I I
I
‘1 I
Study of turbidity
517
at Toronto
at Downsview Airport, but these differences are quite small compared with those bettveen Island Airport and other two stations. Assuming that the environments at Scarborough and TMRS are similar to those at Island and Downsview Airport respectively. then Scarborough should show the maximum influence from the water vapour in the low atmosphere. At higher levels. a considerable amount of water vapour discharged into the urban air may drift over the other areas. With the present situation in Toronto it is not possible to estimate the effect of water vapour from the city on turbidity. The effect of Lake Ontario or the urban atmosphere on turbidity may be estimated by dividing the data into four groups according to NE-N-NW (N), E, SE-S (S) and SW’-W (W). The result is shown in Table 5. Roughly speakin g, turbidity factors with N winds are low and high with S winds at all stations. Of particular interest is that in the E sector the value at Scarborough is lower than at TMRS and in the W sector the one at Scarborough is higher than at TMRS. The difference in turbidity factor between Toronto city and other station is largest in the N sector which may indicate the southward shift of the urban atmosphere. Table 5. Turbidity
factor with the different
wind directions
Toronto city T X0. of cases Scarborough (I) T So. of cases Scarborough (2) T No. of cases T&IRS T No. of cases
6.
CONCLUDING
REMARKS
It would be useful if a metropolitan city like Toronto could have at least two radiation observation stations (including measurements of diffuse sky radiation); one in an urban area and another in a rural environment, because simultaneous measurements in and out of a city permit determination of the city’s effect on turbidity. The present paper examined Linke’s turbidity factor computed from the measurements of global and diffuse sky radiation in and out of Metropolitan Toronto. Although a careful selection limits the sampling number of cloudless days, the preliminary results are as follows: (ii) annual mean values of about 3.00, 2.71, 2.54 and 2.74 are obtained for Toronto city, Scarborough (l), (2) and TMRS, respectively, the last two stations being rural; (ii) mean monthly values for all three stations show higher turbidity during the summer season than during the winter; (iii) the difference in the turbidity factor between an urban and a rural environment is larger in spring and autumn than in winter and summer; (4) the turbidity factor is slightly larger in the afternoon than in the morning; (5) the statistics on wind direction show the effect of the urban atmosphere on turbidity. Acknowfe~~glgrmmts-The author vvishes to express his sincere gratitude to Drs. R. \V. Shaw and R. E. hfunn. Atmospheric Environment Service of Canada, for their invaluable suggestions and review of the manuscript. and to Dr. R. J. Polavarapu and J. R. Latimer, Atmospheric Environment Service of Canada, for their useful discussions. Also he gives thanks to the National Research Council of Canada for the award of a postdoctorate fcllowship and Atmospheric Environment Service of Canada for providing the facilities.
REFERENCES Angstrom A. K. 113iO) On determinations of the atmospheric turbidity and their relation to pyrheriometric measurements. .~limncr Gtioph,r. 14. 269-39-t. Csagi (1956) IGY fnsnucrion .~fanuul. Pt. IV. pp. 367-466; Radiation lnsrrtrmenrs and .Ifeaswtmenrs. .~nn. Intern Geophys. Year. Pergsmon Press. Oxford. Hinzpeter H. (19501 L’ber Truhungsbestimmungen in Potsdam in den Jahren 19-16und IY-li. Z. .\I,,r. 4. I Landsberg H. E. t 1961) City au-better or worse’! In Symposium: .4tr owr C~rirs. Tech. Rep. d62-5. I-X L S Public Health Service, Cincinnati. Landsberg H. E. (!972) Inadvertent atmospheric modification through urbanization. Tech. Note BN 731. Institute for Fluid Dynamics and Applied %tthematics. Univ. of Maryland Linke F. and Boda K. (1923~ Vorschlage zur Berechnung des Trubungsgrades der Atmosphare aus den Messungen der tinnenstrahlung. .Clereor. Z. 39. 161-166. Munn R. E. (1966) Descriptire .bJicrometeoroloy). Academic Press, New York. Munn R. E. (1973) Urban meteorology: some selected topics. Bull. ,-tnr. ;Mer. Sot. 54.90-93. Piggin I. G. (1972) Diurnal aqmmetries in global radiation. Arch. .Vrt. Geophs. Biokl. Ser. B 20.41-4S. Polavarapu R. J. I 1972) Atmospheric turbidity over Canada. Proceedings of rhr Inrernnrional Rndkzrinn .S!.mposium. 348-353. Sendai. Japan. Robinson N. (1966) Solar Rodhrrion. Elsevxr. New Y’ork. U.S. Naval Observatory (19-15)The America/z Ephemeris and .Vuuricol Almat~nc ofthe Year 1950. Yamamoto G.. Tanaka M. and ,Arao K. I 1968) Hemispherical distribution of turbidity coefficients as estimated from direct solar radiation measurements. J. ,Llrr Sot. Jrrpnrr 46, 257-299. Yamashita S. (19731 .&r pollution study from measurements of solar radiation. Arch. .LJrr. Geophs. Bioki.. SC,-. 8. 213-143.