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Measurements of visibility by photographic photometry at Seville (Spain)
0004-6981/87 S3.00+0.00 0 1987 Rr+wxt loutush ud.
Atmospknc fmuonnunr Vol. 21. No 12, pp. 2727-2730, 1987 Printed I” chat Bntam.
TECHNICAL NOTE MEASUREMENTS
OF VISIBILITY BY PHOTOGRAPHIC AT SEVILLE (SPAIN)
PHOTOMETRY
C. BELLVER Departamento de Optica, Fact&ad de Fisica, Univcrsidad de Sevilla, Apartado 1065,41080_Sevi11c,Spain
( First receiued 13 October 1986 and receisedfor publication 23 June 1987) Abstract-Determinations of meteorological range V, werecarried out at the city of Seville (Spain) during 1 year by photographing black objects-on the typical Giralda tower-against the adjacent sky and measuring on the negative the relative optical densities of the objects and the sky images by means of an automatic recording microdensitometer. The contrast of the film was also determined by density measurements on the negatives. Empirical relationships between V, and atmospheric particulate contents M were also studied. Key word index: Visibility, meteorological
range, optical density, densitometric
1. INTRODUCTION
expression:
In visibility studies, the daytime meteorological range V, is defined as the diitance, under daylight conditions, at which the apparent contrast between a black object and its background (horizon sky) baomcs just equal to the threshold contrast of the observer, s, usually taken to be E = 0.02 (McCartney, 1976; Middleton, 1963). For this value of s, the simple Kochsmeider relationship (Kochsmeider, 1924) between V, and atmospheric extinction coefficient j3 is (Cbarlson, 1969):
“m2!z. B In dealing with experimental measurements of V,, Steffens (1949) (Cadle, 1975) developed a photographic method that enabled more reliable objective meteorological range determinations to be made than visual direct estimates. This method is based upon the following considerations: If a black object is viewed against the horizon sky at a distance s, and changes of light intensity in traversing this distance are evaluated, a general expression of V,,, can be derived, supposing that atmospheric extinction and scattering coefficients are constant along the light path:
slogc
v,= log
(I-$ > m
where I, is the intensity of light in the direction from the object at the observer position and 1, the intensity from an infinitely distant object in the same direction at the same position (in practice, the intensity from the horizon sky next to the object is used for I, ). Once s is known and letting E = 0.02, if 1,/I, is available, V, can be easily determined. If the black object and its adjacent sky are photographed, the ratio of intensities can be determined from negatives. The density D of a negative can be expressed as, D=g+ylogE
analysis, particulate
(3) where g and y are constants and E the exposure. Supposing that E = I f(t), where f(t) is some function of the time, and taking into account that only ratios of intensities of the same negative are involved, the mentioned ratio is given by the
+ 5 10(D,-&J/r m
(4)
whem D, and D, are the densities of the images produced by the fluxes 1, and I,, respectively. and the contrast y is calculated from other density measurements on the negative. Expression (4) together with (2) alIows the cakulation of V,, according to this method, by measuring the differena in density between the black object and the adjacent sky images. On the other hand, one of the most noticeable features of atmospheric particulate pollution is the reduction in visibility as pollutant concentration increases. Among a great number of papers concerned with pollutants contribution to visibility reduction, we shag only mention two, because of their definite influena on this work: Charlson et al. (1967) made the simple assumption that the mass concentration of pollutants M is directly~proportional to coefficient 8. This assumption together with Equation (1) leads to the relationship: K
v, = -
M
where K is a constant of proportionality, and M is the weight of particulate matter per unit volume. Subsequently, No11et a/. (1968) studii the applicability of (5)at three locations in the U.S.A. giving the value of K for the three places. Taking into account the above reports, this paper attempts to evaluate the presumable dependence V,,,(M) at Seville, by looking for an empirical relationship between V, and M which provides the best fit.
2. EXPERIMENTAL Measurements of meteorological range, V,, were carried out from the flat roof of tbe Main Building of Seville University. The circular openings into the ‘Giralda’ tower belfry were found to bevery satisfactory black objects. This famous tower is situated 620m north of the observation site, and its silhouette was viewed against the adjacent sky. Its belfry was
2727
photographed with an ‘Exacta’ retlex camera provided with an objective of 122 cm focal length. Two pictures of the belfry and its adjacent sky were taken for each determina tion of V,: one of the right side and another of the left, centering systematically on the top half of the picture the opening closest to the right or left edge, respectively. The portion of the sky photographed was always cloudless (see Fig. 1). In order to find the contrast y of the Elm, a positive gray scale was introduced into the camera in such a way that it was superimposed on the bottom half of the picture. A ‘16 Din Kodak Panatomic-X Elm was always used. This Elm combines extremely line grain with very high sharpness and resolution. It has panchromatic sensitixing. In addition, a ‘Kodak Microdol-X’ developer was used. Film development was carried out at 22°C in a cuvette provided with a thermostat and the development, fixing and washing periods were accurately measured. A ‘Joyce-Loebl Model MK III Cs’ automatic recording microdensitometer was used. The densitometer included a motor-driven 6Im holder so that the film density could be
scanned along a pmselected beam. As the output of the system was fed to a recorder, the resulting chart was a trace along the path followed. In this work, the direction of scan was always the line joining the circular opening antres. These points corresponded to the minima in the density graph, and the quasismooth ‘plateau’ correspooded to the sky region close to the tower (see Fig 2). Diffczences in density between the openings and the sky were obtained by superimposing a transparent grid on each graph. On this grid, at regular intervals, three pamIle straight lines in a direction coincident with the vertical of the graph were traced in such a way that when the central line was tangent to the downing branch of the minimum closest to the sky, one of the remainder lines passed through the adjacent minimum and the other cut the sky region on a certain point. Measuring the vertical distance between these latter points and taking into accouot the chart scale, differences in density could be easily determined. As in most cases, two photographs were taken in order to determine V,,,, two values V,i and Vm2were obtained, and
Fig. 2. Densitometric tracing of the negative in Fig. 1 along the path joining the circular openings.
2129
Technical Note consequently
the average meteorological
three empirical relationships are considered:
range V, = l/2
(V,, + V,) wasconsidered.
V,=aM+b,
Simultaneously with determinations of V,, measurements of mean wind speed w, air relative humidity LI and concentration of pollutants M, were carried out at the same site. In order to measure M, samples were collected by the filtration of a certain volume of air (usually 500 L) through a ‘Whatman 1’ paper filter. Using an ‘EEL’ reflectometer, one could evaluate the total mass of particulate matter per unit volume. It was expressed in pg m- ‘. This method provided reasonably good information about local atmospheric particulate contents. The results obtained by its application agreed fairly well in our city with those obtained by weighing in an electrobalance the samples obtained by filtration of a measured volume of air through a paper filter for several hours (3-4 h). Relative humidity data were also used to restrict measurements of visibility to those days on which U was below 70%.
v,=
(‘3
KM”.
(8)
where a, b, A, B, K and n are adjustable constants. Equation (8) becomes Equation (5) when n = - 1. The different values of the correlation coefficient r for the 3 expressions (6), (7) and (8) are - 0.75, 0.65 and -0.70, respectively. So, highest correlation is found in the first case, by using relationship (6). Moreover, it is obvious that our experimental data do not conform to (5). Nevertheless this result is not surprising and it is not difficult to understand, because Noll et al. established some limitations in application of (5) and they considered V,as the prevailing meteorological range, while in this work V,,, corresponds to the meteorological range in one direction. On the other hand, expressions (7) and (8) make it possible to estimate, with a reasonable reliability, values of V,,, from M data. go, expression
3. RESULTS
During the period September 197~September 1980, a total of 137 determinations of V,,,were carried out together with simultaneous measurements of w, U and M. Although the same kind of film was always used, several determinations of y were made on each roll of film. The average y over 88 data points was found to be 0.70 & 0.03. Values of V, obtained by rigorously applying the above method ranged between 2.8 and 15 km. According to the International Visibility Code, these extreme vahres correspond to numbers 5 and 7 and to weather conditions of Haxe and Clear, respectively (McCartney, 1976). The plot of M VI V,,,for 137 data points is shown in Fig. 3. There seems to be a definite deuease in meteorological range as the particulate contents increase. In order to evahtate the dependence between V, and M,
V, = -0.085 M + 13.0 provides values of V,,, with a relative error of less than 25 % for 76 % of the collected data. For the same limit of error, this percentage is equal to 70 when the expression v, = +
+ 5.3
is used, and it is 74 for Vm = 490.
M-0.4’.
In the three expressions, V, is in km and M in Pg m- 3.
16
. 14
.
.
l*9s
‘L
::
.. .
. l”
.. .
.d.
.
l*
. .
***a? ...“_..** .... ..
..
. :
.
l
* .
l
l
l.*.
.**-.
. .
l
. -
*
. l
.-
.
.
l.
0.
.
.
.
l.
.
:
.
l
c
am .
.. l.:
l
. .
. l
.*
.
c 2
25
50 PARTICULATE
75 POLLUTION (Ugf
100
tTt-3)
Fig. 3. particulate pollution vs meteorological range for 137 data points.
125
2730
Technical Note
Fig. 4. Resulting densitometric tracings of a double scanning along two parallel paths on the negative of a picture of the sky taken when a positive gray scale is superimposed on the bottom half of the film. Trace A corresponds to the sky region that is photographed through the gray scale. Finally, the correlation coefhcient between V,,, and relative humidity LI was r = - 0.60. The influence of wind speed, w, on the values of V,,, is even less marked. Acknowledgements-l am indebted to Prof. Dr. R. Mirquez and Prof. Dr. V. Hemlndez for their continuous help and advice.
If the same light flux, F. , falls upon 1 and 2. the transmitted fluxes F, and F2 are related to the densities and the incident illumination by the expressions, D, = log;
1 D2 -D,
REFERENCES
Cadle R. D. (1975) The Measurement of Airborne Particles. Wiley, Chichester. Charlson R. J. (1969) Atmospheric visibility related to aerosol mass concentration. Envir. Sci. Technol. 3, 913-918. Charlson R. J., Horvath H. and Pueschel R. F. (1967) The direct measurement of atmospheric light scattering coefficients for studies of visibility and pollution. Atmospheric Enoiroament 1,469-478. Kochsmeider H. (1924) Tbeorie der horizontalen Sichtweite. Beirr. Phys. Freien Atmos. 12, 3%53, 171-181. McCartney E. J. (1976) Optics ofthe Atmosphere. Scattering by Molecules and Particles. Wiley. Middleton W. E. K. (1963) Vision Through tke Atmosphere. University of Toronto Press, Toronto. No11K. E., Mueller P. K. and Imada M. (1968) Visibility and aerosol concentration in urban air. Atmospheric Environment
2,465475.
Steffens C. (1949) Measurement of visibility by photographic photometry. Ind. Engng Chem. 41, 2396-2399.
APPENDIX As mentioned previously, the contrast of the film y was determined with the aid of a positive gray scale. Suppose in the mentioned scale two consecutive bands called 1 and 2, respectively, with optical densities I), and D2.
and
DZ = log;
2
then, =
log;. 2
(9)
Consider now that the gray scale is superimposed on a film. Supposing that the arrangement is uniformly illuminated by F,, the fluxes transmitted by two adjacent gray scale bands, F, and F, , fall upon two consecutive zones of the film. Then if D,, and D,, are the densities of these film zones after development, it can be shown that, D,,-Dzp=ylog<.
F,
(10)
The ratio of the expresstons (9) and (10) allows y to be obtained from differences in density, ‘=
Dir-D,, Dr-D,
(11)
In practice, in order to measure y. a picture of the sky close to the belfry and above the tower vane was taken. As just mentioned, the gray scale is superimposed on the bottom half of the picture. Once the film is developed, it is scanned along two parallel paths close to the divisory line on both halves of the negative. Figure 4 shows theappearance of these graphs. The stepped one corresponds evidently to the sky region which is photographed through the gray scale. In order to evaluate DIP -D2v in expression (ll), differences in height between consecutive steps are measured, by referring these heights individually to the corresponding level of the top graph. If the positive gray scale is directly scanned, D1 - D, is easily determined, and (I 1) provides the value of y.
Atmospknc fmuonnunr Vol. 21. No 12, pp. 2727-2730, 1987 Printed I” chat Bntam.
TECHNICAL NOTE MEASUREMENTS
OF VISIBILITY BY PHOTOGRAPHIC AT SEVILLE (SPAIN)
PHOTOMETRY
C. BELLVER Departamento de Optica, Fact&ad de Fisica, Univcrsidad de Sevilla, Apartado 1065,41080_Sevi11c,Spain
( First receiued 13 October 1986 and receisedfor publication 23 June 1987) Abstract-Determinations of meteorological range V, werecarried out at the city of Seville (Spain) during 1 year by photographing black objects-on the typical Giralda tower-against the adjacent sky and measuring on the negative the relative optical densities of the objects and the sky images by means of an automatic recording microdensitometer. The contrast of the film was also determined by density measurements on the negatives. Empirical relationships between V, and atmospheric particulate contents M were also studied. Key word index: Visibility, meteorological
range, optical density, densitometric
1. INTRODUCTION
expression:
In visibility studies, the daytime meteorological range V, is defined as the diitance, under daylight conditions, at which the apparent contrast between a black object and its background (horizon sky) baomcs just equal to the threshold contrast of the observer, s, usually taken to be E = 0.02 (McCartney, 1976; Middleton, 1963). For this value of s, the simple Kochsmeider relationship (Kochsmeider, 1924) between V, and atmospheric extinction coefficient j3 is (Cbarlson, 1969):
“m2!z. B In dealing with experimental measurements of V,, Steffens (1949) (Cadle, 1975) developed a photographic method that enabled more reliable objective meteorological range determinations to be made than visual direct estimates. This method is based upon the following considerations: If a black object is viewed against the horizon sky at a distance s, and changes of light intensity in traversing this distance are evaluated, a general expression of V,,, can be derived, supposing that atmospheric extinction and scattering coefficients are constant along the light path:
slogc
v,= log
(I-$ > m
where I, is the intensity of light in the direction from the object at the observer position and 1, the intensity from an infinitely distant object in the same direction at the same position (in practice, the intensity from the horizon sky next to the object is used for I, ). Once s is known and letting E = 0.02, if 1,/I, is available, V, can be easily determined. If the black object and its adjacent sky are photographed, the ratio of intensities can be determined from negatives. The density D of a negative can be expressed as, D=g+ylogE
analysis, particulate
(3) where g and y are constants and E the exposure. Supposing that E = I f(t), where f(t) is some function of the time, and taking into account that only ratios of intensities of the same negative are involved, the mentioned ratio is given by the
+ 5 10(D,-&J/r m
(4)
whem D, and D, are the densities of the images produced by the fluxes 1, and I,, respectively. and the contrast y is calculated from other density measurements on the negative. Expression (4) together with (2) alIows the cakulation of V,, according to this method, by measuring the differena in density between the black object and the adjacent sky images. On the other hand, one of the most noticeable features of atmospheric particulate pollution is the reduction in visibility as pollutant concentration increases. Among a great number of papers concerned with pollutants contribution to visibility reduction, we shag only mention two, because of their definite influena on this work: Charlson et al. (1967) made the simple assumption that the mass concentration of pollutants M is directly~proportional to coefficient 8. This assumption together with Equation (1) leads to the relationship: K
v, = -
M
where K is a constant of proportionality, and M is the weight of particulate matter per unit volume. Subsequently, No11et a/. (1968) studii the applicability of (5)at three locations in the U.S.A. giving the value of K for the three places. Taking into account the above reports, this paper attempts to evaluate the presumable dependence V,,,(M) at Seville, by looking for an empirical relationship between V, and M which provides the best fit.
2. EXPERIMENTAL Measurements of meteorological range, V,, were carried out from the flat roof of tbe Main Building of Seville University. The circular openings into the ‘Giralda’ tower belfry were found to bevery satisfactory black objects. This famous tower is situated 620m north of the observation site, and its silhouette was viewed against the adjacent sky. Its belfry was
2727
photographed with an ‘Exacta’ retlex camera provided with an objective of 122 cm focal length. Two pictures of the belfry and its adjacent sky were taken for each determina tion of V,: one of the right side and another of the left, centering systematically on the top half of the picture the opening closest to the right or left edge, respectively. The portion of the sky photographed was always cloudless (see Fig. 1). In order to find the contrast y of the Elm, a positive gray scale was introduced into the camera in such a way that it was superimposed on the bottom half of the picture. A ‘16 Din Kodak Panatomic-X Elm was always used. This Elm combines extremely line grain with very high sharpness and resolution. It has panchromatic sensitixing. In addition, a ‘Kodak Microdol-X’ developer was used. Film development was carried out at 22°C in a cuvette provided with a thermostat and the development, fixing and washing periods were accurately measured. A ‘Joyce-Loebl Model MK III Cs’ automatic recording microdensitometer was used. The densitometer included a motor-driven 6Im holder so that the film density could be
scanned along a pmselected beam. As the output of the system was fed to a recorder, the resulting chart was a trace along the path followed. In this work, the direction of scan was always the line joining the circular opening antres. These points corresponded to the minima in the density graph, and the quasismooth ‘plateau’ correspooded to the sky region close to the tower (see Fig 2). Diffczences in density between the openings and the sky were obtained by superimposing a transparent grid on each graph. On this grid, at regular intervals, three pamIle straight lines in a direction coincident with the vertical of the graph were traced in such a way that when the central line was tangent to the downing branch of the minimum closest to the sky, one of the remainder lines passed through the adjacent minimum and the other cut the sky region on a certain point. Measuring the vertical distance between these latter points and taking into accouot the chart scale, differences in density could be easily determined. As in most cases, two photographs were taken in order to determine V,,,, two values V,i and Vm2were obtained, and
Fig. 2. Densitometric tracing of the negative in Fig. 1 along the path joining the circular openings.
2129
Technical Note consequently
the average meteorological
three empirical relationships are considered:
range V, = l/2
(V,, + V,) wasconsidered.
V,=aM+b,
Simultaneously with determinations of V,, measurements of mean wind speed w, air relative humidity LI and concentration of pollutants M, were carried out at the same site. In order to measure M, samples were collected by the filtration of a certain volume of air (usually 500 L) through a ‘Whatman 1’ paper filter. Using an ‘EEL’ reflectometer, one could evaluate the total mass of particulate matter per unit volume. It was expressed in pg m- ‘. This method provided reasonably good information about local atmospheric particulate contents. The results obtained by its application agreed fairly well in our city with those obtained by weighing in an electrobalance the samples obtained by filtration of a measured volume of air through a paper filter for several hours (3-4 h). Relative humidity data were also used to restrict measurements of visibility to those days on which U was below 70%.
v,=
(‘3
KM”.
(8)
where a, b, A, B, K and n are adjustable constants. Equation (8) becomes Equation (5) when n = - 1. The different values of the correlation coefficient r for the 3 expressions (6), (7) and (8) are - 0.75, 0.65 and -0.70, respectively. So, highest correlation is found in the first case, by using relationship (6). Moreover, it is obvious that our experimental data do not conform to (5). Nevertheless this result is not surprising and it is not difficult to understand, because Noll et al. established some limitations in application of (5) and they considered V,as the prevailing meteorological range, while in this work V,,, corresponds to the meteorological range in one direction. On the other hand, expressions (7) and (8) make it possible to estimate, with a reasonable reliability, values of V,,, from M data. go, expression
3. RESULTS
During the period September 197~September 1980, a total of 137 determinations of V,,,were carried out together with simultaneous measurements of w, U and M. Although the same kind of film was always used, several determinations of y were made on each roll of film. The average y over 88 data points was found to be 0.70 & 0.03. Values of V, obtained by rigorously applying the above method ranged between 2.8 and 15 km. According to the International Visibility Code, these extreme vahres correspond to numbers 5 and 7 and to weather conditions of Haxe and Clear, respectively (McCartney, 1976). The plot of M VI V,,,for 137 data points is shown in Fig. 3. There seems to be a definite deuease in meteorological range as the particulate contents increase. In order to evahtate the dependence between V, and M,
V, = -0.085 M + 13.0 provides values of V,,, with a relative error of less than 25 % for 76 % of the collected data. For the same limit of error, this percentage is equal to 70 when the expression v, = +
+ 5.3
is used, and it is 74 for Vm = 490.
M-0.4’.
In the three expressions, V, is in km and M in Pg m- 3.
16
. 14
.
.
l*9s
‘L
::
.. .
. l”
.. .
.d.
.
l*
. .
***a? ...“_..** .... ..
..
. :
.
l
* .
l
l
l.*.
.**-.
. .
l
. -
*
. l
.-
.
.
l.
0.
.
.
.
l.
.
:
.
l
c
am .
.. l.:
l
. .
. l
.*
.
c 2
25
50 PARTICULATE
75 POLLUTION (Ugf
100
tTt-3)
Fig. 3. particulate pollution vs meteorological range for 137 data points.
125
2730
Technical Note
Fig. 4. Resulting densitometric tracings of a double scanning along two parallel paths on the negative of a picture of the sky taken when a positive gray scale is superimposed on the bottom half of the film. Trace A corresponds to the sky region that is photographed through the gray scale. Finally, the correlation coefhcient between V,,, and relative humidity LI was r = - 0.60. The influence of wind speed, w, on the values of V,,, is even less marked. Acknowledgements-l am indebted to Prof. Dr. R. Mirquez and Prof. Dr. V. Hemlndez for their continuous help and advice.
If the same light flux, F. , falls upon 1 and 2. the transmitted fluxes F, and F2 are related to the densities and the incident illumination by the expressions, D, = log;
1 D2 -D,
REFERENCES
Cadle R. D. (1975) The Measurement of Airborne Particles. Wiley, Chichester. Charlson R. J. (1969) Atmospheric visibility related to aerosol mass concentration. Envir. Sci. Technol. 3, 913-918. Charlson R. J., Horvath H. and Pueschel R. F. (1967) The direct measurement of atmospheric light scattering coefficients for studies of visibility and pollution. Atmospheric Enoiroament 1,469-478. Kochsmeider H. (1924) Tbeorie der horizontalen Sichtweite. Beirr. Phys. Freien Atmos. 12, 3%53, 171-181. McCartney E. J. (1976) Optics ofthe Atmosphere. Scattering by Molecules and Particles. Wiley. Middleton W. E. K. (1963) Vision Through tke Atmosphere. University of Toronto Press, Toronto. No11K. E., Mueller P. K. and Imada M. (1968) Visibility and aerosol concentration in urban air. Atmospheric Environment
2,465475.
Steffens C. (1949) Measurement of visibility by photographic photometry. Ind. Engng Chem. 41, 2396-2399.
APPENDIX As mentioned previously, the contrast of the film y was determined with the aid of a positive gray scale. Suppose in the mentioned scale two consecutive bands called 1 and 2, respectively, with optical densities I), and D2.
and
DZ = log;
2
then, =
log;. 2
(9)
Consider now that the gray scale is superimposed on a film. Supposing that the arrangement is uniformly illuminated by F,, the fluxes transmitted by two adjacent gray scale bands, F, and F, , fall upon two consecutive zones of the film. Then if D,, and D,, are the densities of these film zones after development, it can be shown that, D,,-Dzp=ylog<.
F,
(10)
The ratio of the expresstons (9) and (10) allows y to be obtained from differences in density, ‘=
Dir-D,, Dr-D,
(11)
In practice, in order to measure y. a picture of the sky close to the belfry and above the tower vane was taken. As just mentioned, the gray scale is superimposed on the bottom half of the picture. Once the film is developed, it is scanned along two parallel paths close to the divisory line on both halves of the negative. Figure 4 shows theappearance of these graphs. The stepped one corresponds evidently to the sky region which is photographed through the gray scale. In order to evaluate DIP -D2v in expression (ll), differences in height between consecutive steps are measured, by referring these heights individually to the corresponding level of the top graph. If the positive gray scale is directly scanned, D1 - D, is easily determined, and (I 1) provides the value of y.