A study of the aerosol of Santiago de Chile—II. Mass extinction coefficients, visibilities and Ångström exponents

Aerosol of Santiago de Chile---I 383

t -

UJ

0 , 5 -

o.4 - ~ % " ' ~ ~b \ ~"c~+% ',4o+++ "q

__ " ,7% \ ,,+v Houston ots + Gr; , ~ "ON. £ /

o.1 I I I I I I 8 10 12 14 16 18h

Time of day

Fig. 11. Comparison of the time dependence of extinction coefficients in San- tiago de Chile, Houston, TX, U.S.A. and Vienna, Austria. Telephotometric measurements are reported for the three cities. For Houston and Vienna sum curves of absorption coefficient plus scattering coefficient measured with a heated nephelometer are available and are also shown in order to bring out humidity effects. These are seen to be important in the case of Houston. In Santiago relative

humidity was generally low.

tion of the extinction coefficient found in Santiago differs markedly from that found in the other cities. Data reported for Berlin (light scattering) show no definite trend, with peak values in the morning, eve- ning, noon or no peak at all (Heits and Israel, 1982).

SUMMARY AND C O N C L U S i ~ . .

The atmosphere of Santiago de Chile has Been investigated by telephotometry in various parts of the city. Spectral extinction coefficients were measured during the time period between September 1988 and January 1989 (mid-spring to mid-summer) and again between February 1990 and June 1990 (autumn to mid-winter). The values measured show a distinct daily pattern with high values in the morning and a steady decrease in the afternoon. Due to the geo- graphic location and topography of the urban area the air basin has no exit; weak local winds move air masses around the basin. Furthermore, heating by the intense sunshine causes a steady increase of the height of the mixing layer during the daytime which permits an increased dilution of the aerosol, causing the observed decrease of extinction.

In contrast to the urban area, measurements at a remote site in the Andes Mountains showed a constant increase in light extinction during the daytime. In the morning the extinction coefficients are similar to the background values off the coast on the Pacific (Horvath, 1991). In the afternoon, after the mixing layer has expanded above 3000m, the extinction coefficients in the mountains are comparable to those in the urban area. The increase of the height of the

mixing layer in combination with a gradual decrease of the extinction coefficient within it may explain the phenomena of sudden appearance and disappearance of distant mountains.

Sight paths which were elevated above ground were reached later in the day by the mixing layer and show in fact a later maximum of light extinction coefficient in the morning. Dramatic changes in extinction coefficients of up to a factor of five have been observed within 1 h. In such cases the wavelength dependence of the aerosol undergoes changes with the advection or production of a different aerosol.

A comparison with the pollution of other large cities shows that the daily pattern of variation of the extinction coefficient in Santiago is different. Changes in the extinction coefficient by a factor of five are unusual in other cities. The values of the extinction coefficients observed in Santiago de Chile are comparable to European and Asian cities; due to the possibility of seeing mountains at distances of 40km the visual quality of the air is judged very critically by an observer. Once the large population (>~4,500,000) of Santiago de Chile is taken into account there is no large qualitative difference with other population centres.

With this study part of the complex behaviour of the atmosphere of Santiago de Chile has been understood; much further work is needed for a complete under- standing. Nevertheless the optical technique has pro- ven very useful in investigating the atmosphere in real time with good time resolution and the possibility of also making space-resolved measurements. Future work would greatly benefit from the availability of systematic vertical profile measurements of temper- ture and humidity.

384 H. HORVATH and A. TRIER

Acknowledgements~This work was in part supported by a grant of the Fundaci6n Andes and by the Universidad de Santiago de Chile. The cooperation of Dr Mario Mufioz and Ing. I Oleata of the Programa Contaminacifn Aire is greatly appreciated.

REFERENCES

,~ngstr6m A. (1929) On the atmospheric transmission of sun radiation II. Geogr. Analer 12, 130-159.

Berner A. (1984) Design principles of the AERAS low pressure impactor. In Aerosols: Science, Technology and Industrial Applications (edited by Liu B. Y. H., Pui D. Y. H. and Fissan H. J. F.), pp. 139-142. Elsevier, New York.

Daisey J. M., Kneipp T. J., Wang Ming-Xing, Ren Li-Xin and L/i Wei-Xiu (1983) Organic and elemental composition of airborne particulate matter in Beijing, Spring 1981. Aer- osol Sci. Technol. 2, 407-414.

de la Vega V., Fuentes H., Ortiz J. and Pr6ndez M. (1987) Balance de masa en aerosoles atmosf6ricos de Santiago de Chile. Bol. Soc. Chil. Qulm. 32, 187-198.

Dzubay T. G., Stevens R. K. and Lewis C. W. (1982) Visibility and aerosol composition in Houston, Texas. Envir. Sci. Technol. 16, 514-525.

H~inel G. (1976) The properties of atmospheric particles as a function of relative humidity at thermodynamic equilibrium with the surrounding moist air. Adv. Geophys. 19, 73-188.

Heits B. and Israel G. W. (1982) Physikalische und chemische Charakterisierung des inner-stfidtischen Schwebstaubes. Staub--Reinhalt. Luft 42, 347-355.

Horvath H. (1981a) Atomospheric visibility. Atmospheric Environment 15, 1785-1796.

Horvath H. (1981b) The University of Vienna telephot- ometer. Atmospheric Environment 15, 2537-2546.

Horvath H. (1991) Spectral extinction coefficients of background aerosols in Europe, North and South America: a comparison. Atmospheric Environment 25A, 725-732.

Horvath H., Trier A. and Catalan L. (1989) The relationship between indoor and outdoor soot concentration in an urban environment. J. Aerosol Sci. 20, 1273-1276.

Husar R. B. and Wilson W. E. (1990) Trends of seasonal haziness and sulfur emissions over the eastern U.S. In Visibility and Fine Particles (edited by Mathai C. V.), pp.

318-327. Air and Waste Management Association, Pitts- burgh, PA.

Koschmieder H. (1924) Theorie der horizontalen Sichtweite. Beitr. Phys. Atm. 12, 33-53, 171-181.

Lewis C. W., Baumgardner R. E. and Stevens R. K. (1986) Receptor modelling study of Denver winter haze. Envir. Sci. Technol. 20, 1126-1136.

Lin C. I., Baker M. and Charlson R. J. (1973) Absorption coefficient of atmospheric aerosols: a method for measurement. Appl. Opt. 42, 1356-1363.

Molenar J. V., Persha G. and Maim W. C. (1990) Long path transmissometer for measuring ambient atmospheric extinction, In Visibility and Fine Particles (edited by Mathai C. V.), pp. 293-304. Air and Waste Management Associ- ation, Pittsburgh, PA.

Middleton K. E. W. (1968) Vision Through the Atmosphere. University of Toronto Press, Toronto, Canada.

Nilsson B. (1990) Correlation between atmospheric aerosol extinction and meteorologic parameters. In Aerosols: Science, Industry, Health and Environment (edited by Mas- uda S. and Takahashi K.), pp. 299-302. Pergamon Press, Oxford.

Okada K. and Isono K. (1982) Trends in visibility in the urban atmosphere--a case study in Nagoya, Japan. J. Met. Soc. Jap. 60, 777-786.

Pastuszka J. S. (1990) Studies on the relationship between visibility and mass size distribution of the aerosol in Katowice. In Aerosols: Science, Industry, Health and Envir- onment (edited by Masuda S. and Takahashi K.), pp. 290-293. Pergamon Press, Oxford.

Pr6ndez M., Ortiz J., Apablaza N. and Zolezzi S. (1989) Forma, tamafio y concentraci6n de aerosoles atmosf6ricos. XVIII Jornadas Chilenas de Qulmica 1989, Libro de res0menes, Tomo 2, pp. 502-504.

Richards L. W. (1988) Sight path measurements for visibility monitoring and research. J. Air PoUut. Control. Ass. 38, 784-791.

Seinfeld J. H. (1986) Atmospheric Chemistry and Physics of Air Pollution, p. 297. Wiley, New York.

Trier A. and Silva C. (1987) Inhalable urban atmospheric particulate matter in a semi-arid climate: the case of Santiago de Chile. Atmospheric Environment 21, 977-983.

Zak B. D., Einfeld W., Church H. W., Gay G. T., Jensen A. L., Trijonis, J., Ivey M. D., Homann P. S. and Tipton C. (1984) The Albuquerque winter visibility study. Volume 1. Over- view and data analysis. Sandia report Sand84-0173/1.

Atmospheric Environment Vol. 27A. No. 3, pp. 385 395, 1993. 0004 6981/93 $6.00+0.00 Printed in Greal Britain. ,~ 1993 Pergamon Press Ltd

A STUDY OF THE AEROSOL OF SANTIAGO DE CHILE--II. MASS EXTINCTION COEFFICIENTS, VISIBILITIES AND

,~NGSTR()M EXPONENTS

A. TRIER

Departamento de Fisica, Universidad de Santiago de Chile, Santiago, Chile

and

H. HORVATH

Institute of Experimental Physics, University of Vienna, Vienna, Austria

(First received 25 April 1991 and in final form 21 February 1992)

A ~ t r a c ~ T h e atmosphere of Santiago de Chile has been investigated by telephotometry. Spectral extinction coefficients were measured during the time period between September 1988 and January 1989 (mid-spring to mid-summer) and again from February 1990 to June 1990 (autumn to mid-winter). The measured data give a distinct daily pattern which on the average shows high values in the morning and a steady decrease in the afternoon. A similar pattern is found for the mass concentration of particulate matter but not for sulphur dioxide. The extinction coefficient in Santiago decreases generally with increasing wavelength; a satisfactory approximation for the wavelength dependence of the extinction coefficient on the average is a power law with an Angstr6m exponent of ct = 1.22. Measured extinction coefficients correlate well with mass concentration of suspended inhalable particulate matter. Mass extinction coefficients in Santiago are higher but comparable to other locations; a representative value of 5 m 2 g- 1 can be given. The measured extinction coefficients show strong statistical correlations with visibilities observed at three airports in and near Santiago. A comparison with atmospheric pollution indicators in other large cities shows that the daily pattern of variation of the extinction coefficient in Santiago de Chile is markedly different and that the values of the extinction coefficients and mass concentrations observed in Santiago are higher than or comparable to European and Asian cities. Pollutant dispersion from the Santiago air basin is dependent on thermal inversion phenomena, air pollution indicators being high in the morning and much lower in the afternoon.

Key word index: Atmospheric aerosol, mass extinction coefficient, ,~ngstr6m exponent, visibility, statistical correlations, linear models.

INTRODUCTION

In a previous paper (Horvath and Trier, 1993; hence- forth quoted as HT) we have reported light extinction measurements by telephotometry in the city of San- tiago de Chile. The telephotometric method offers advantages for the study of urban air pollution: tele- photometers can be built at reasonable cost, are easily transportable and simple to operate. Depending on the design a time resolution of a few seconds to minutes is possible, so that the time evolution of the aerosol can be studied. By measuring the wavelength dependence of the extinction coefficient (i.e. by deter- mining the Angstr6m exponent) information on size distribution changes of the aerosol can be obtained. By pointing the instrument in different directions a spatially inhomogeneous distribution of the aerosol can also be studied.

No routine measurements have been obtained by us; we have tried instead to cover as much time as possible in the period between September 1988 and January 1989 (mid-spring to mid-summer) and again between February 1990 and June 1990 (autumn to

mid-winter). During these periods average humidities were relatively low so that changes in the extinction coefficients were mainly caused by particle changes; effects due to uptake of water are generally unimportant. We have performed determinations of the extinction coefficient at different places and in different directions totalling over 1000 data runs. Since only one instrument was available we could not perform simultaneous measurements at different locations, but measurements from one location in different directions in short sequence (10 min) were possible. The data taken on different days at different locations are comparable on a statistical basis as reported below. Singular events have been reported in HT.

As in most cities around the world particles and gases suspended in the atmosphere are dominated in Santiago by local sources. For Santiago the two main sources are traffic, both by emission (mainly from buses) and resuspension due to the dry climate. Be- sides particles emitted directly, production by conversion from precursor gases in the atmosphere must alse be considered. Santiago is surrounded by mountains and there is little wind usually; atmospheric particles

385

386 A. TRIER and H. HORVATH

accumulate which leads to high particle loads. A high insolation causes vertical movement of the air during the day and possibly also chemical reactions. Signific- ant changes in the aerosol with the time of day have in fact been observed.

The optical properties of atmospheric particles are influenced by humidity since hygroscopic materials take up water as the humidity increases. In areas with high humidity much of the haze observed in the morning or late afternoon can be caused by humidity growth of particles; e.g. in Houston a change in extinction coefficient by a factor of 2 from 08:00 to 12:00 is associated with a change in relative humidity from 95 to 40% (Dzubay et al., 1982). However, humidity effects are negligible below relative humidities of 50% and small below 60%. Therefore optical investigations at low humidities will give information on the aerosol. Since many optical techniques are in situ techniques the aerosol is little or not at all disturbed by the measurement, and a short time resolution also gives the possibility of acquiring information on the dynamics of the aerosol.

Particles reduce visibility efficiently and the mountains near Santiago are usually invisible for at least part of the day, making the impression of pollution even more severe. Visibility is recorded routinely at airports and extensive records exist. They can be used to investigate the history of air pollution (Husar and Wilson, 1990). Since the obserwation procedure, measuring methods and selection of targets can have a significant influence on the measured values, a correlation of observed visibilities and extinction coefficients is an important means for obtaining "calibration factors" for different locations.

The extinction coefficient of atmospheric aerosol generally decreases with increasing wavelength lead- ing to a red or yellow colour when attenuation of white light through a long layer of haze occurs (e.g. at sunrise or sunset). We have used the ]lngstr6m exponent to characterize this wavelength dependence.

As will be shown below, the particle load, the mass extinction coefficient and the visibility are not too different in Santiago in comparison to other cities.

ANGSTROM EXPONENTS

The optical properties of the atmospheric aerosol are dominated by the particles in the size range of the accumulation mode. Depending on the location and conditions, the mass mean diameter of the particles lies between 0.3 and 0.6/~m. This size is comparable to or smaller than the wavelength and the extinction of light by the particles is higher in the blue than in the red. The wavelength dependence of the extinction coefficient yields information on the predominant size of the suspended particles.

A higher attenuation of light by the atmosphere in the blue than in the red has first been reported by

,~ngstr6m (1929) when performing extensive spectral sun photometry. He proposed a power law relationship between the wavelength 2 of the light and the extinction coefficient cre (2) of the atmosphere in the following way:

with 20 a reference wavelength, e.g. 1 ~m,° e and ~ being constants; ~ usually is called the Angstr6m exponent or coefficient. Its value is close to four for particle-free air and zero for fog. Generally high values of the Angstr6m exponent indicate small particles, Rayleigh scattering dominating, or clean air; low exponents usually indicate large particles.

From our data the *ngstr6m exponents have been obtained by means of a log-log regression of the measured extinction coeffÉcients versus wavelength. For measurements at nine spectral windows (i.e. data from September 1988 to January 1989) only results from regressions showing correlation coefficients r > 0.92 have been retained here. For measurements at four spectral windows (February 1990 to June 1990) only regressions with correlation coefficient r > 0.99 were retained.

In a number of cases an enhancement of the extinction was found at the wavelength of 700 nm; in such cases this wavelength datum was excluded from the regression, since this enhancement could have causes not related to the aerosol: (1) light absorption by water vapour between 650 and 700 nm may occur at higher humidities, and (2) most of the targets have a higher intrinsic radiance at 700 nm. If the value of the radiance of the target is underestimated an extinction coefficient is calculated which is too high.

The values of the ,~ngstr6m exponents estimated in the above manner ranged from 0.3 to 3.0 with an overall average of 1.22__.0.35 (840 data sets). This average value is lower than values reported for locations in southern California, U.S.A., and somewhat higher than values reported for locations in Europe (Hitzenberger, 1988). The results may indicate lesser ageing, on average, of the Santiago aerosol in comparison with the European cases reported. In this connection, however, the relative dryness of the San- tiago climate should be noted: the overall relative humidity during our measurements was 62.7_+9.8%. Central European relative humidities generally are more than 10% higher on the average.

Data from sun photometry can be used to obtain the optical density of the atmosphere, and as a bypro- duct the ,~ngstr6m exponent can be determined. It should be noted that sun photometers measure the attenuation of light of the whole air column, the aerosol contained between the ground level and the top of the atmosphere being thus characterized. A few data are shown in Table I. One can see that the exponents are comparable to the Santiago data, although in drier climates, e.g. Beijing, values smaller

Aerosol of Santiago de Chile--II 389

We wish to review in particular the following results.

(i) Statistical correlation of extinction coefficients with inhalable particle mass concentrations

For this purpose the extinction coefficients at wavelengths 400 and 550 nm were studied. Particle concentrations are those at MACAM station B, except for the case of the Matucana data, where the particle concentrations from station D (Plaza Ercilla) were taken. Using all available data the correlation coefficients were:

at 400 nm: r=0.50, 838 datum points

at 550 nm: r=0.51, 844 datum points.

This calculation does not involve any assumption about the distribution function of the data. There is considerable scatter as shown in Fig. 2 for the 550 nm data.

Linear regression models for the correlation of extinction coefficients and particle mass concentrations were examined. Linear and quadratic fits with intercept proved unsatisfactory in that the intercepts yielded values for the air (i.e. "no particles") extinction which were far too high. Linear no-intercept fits worked best. Calling the extinction coefficient a~ and the inhalable particle mass concentration mc linear fits with data uncorrected for air extinction yield:

at 400 nm: a~ = mc (0.0077 __+ 0.0001) km - ~ m 3/.tg- 1 (1)

at 550 nm: tr~ = mc (0.0053 + 0.0001) km - ~ m 3/tg- ~. (2)

The values of the regression coefficients were R 2 > 0.80. Correction of the data for air extinction due to Rayleigh scattering yielded no improvement of the fits. The corrections were calculated for an atmospheric pressure of 715 mb which is fairly typical for Santiago. This yields Rayleigh extinction coefficients of 0.040km -~ at 4 0 0 n m and 0.011 km -~ at 550nm.

As discussed in HT, extreme (i.e. very low and very high) values of the extinction coefficients can be subject to considerable experimental error. For this reason linear fits were also attempted excluding values of the extinction coefficients below 0.20 and above 1.20 km-~. Again these calculations yielded no significant improvement in the fits.

In considering these results one must bear in mind (a) that light extinction by particles depends on the nature and size distribution of the suspended particles in a complex way, (b) that both these aerosol properties are somewhat dependent on relative ambient humidity and (c) that the particle mass concentration data pertain to street-level monitoring stations which are removed from the actual sighting paths used in our observations. Thus local pollution incidents at the monitoring stations due, for example, to vehicular traffic, can result in high particle loads not reflected by the measured extinction coefficients. Inspection of Fig. 2 indicates that such situations may well have occur- red. This will lead directly to high values of intercept in

1 . 0 0

4

0.80

z

~ 0.60

0

u 0.40 z o

g 0.20

0 . 0 0

I ' ' ' ' I ' ' ' ' i ' ' ' ' I ' ' ' ' ~

o: o.: y o c o •

o o o o 8 0 0 0 " / /

o ° e ~mml#LO 2 ~ ~

50 100 150 200

M A S S C O N C E N T R A T I O N [ t~9 n ~ ]

Fig. 2. Relation between extinction coefficient at 550 nm and mass concentration of inhalable particles for Santiago de Chile. All data sets are included. The line gives regression Equation (2). Standard deviations for extinction coeffi-

cient values larger than 0.2 are less than 10%.

linear regression calculations such as those performed here.

In the above context the correlations and linear fits which have been found for our data can be considered as satisfactory. It should be noted that other authors were also unable to obtain linear fits with an intercept which gives the Rayleigh scattering coefficient of air, see e.g. Charlson et al. (1968) or Dzubay et al. (1982).

From the linear regression the mass extinction coefficient (coeffÉcient of mc) can be found. It amounts to 5.3 m 2 g - ~ at 550 nm. Using the measured mass size distribution of HT and calculating the extinction coefficient by Mie theory a value of 4.8m2g l at 550 nm can be obtained, which compares well with the measured value. Therefore a mass extinction coefficient of ~ e / m c = 5 m 2 g -1 at 550nm appears to be a representative value for the Santiago aerosol. This is a value frequently found in urban atmospheres.

A comparison with other locations is shown in Table 4. High mass extinction coefficients have been found in areas where the atmosphere contains a considerable amount of soot. This is the case for Santiago, Vienna, Berlin and also for Nagoya. The Nagoya data may be too high; the extinction coefficients have been obtained in that paper by converting observed visibilities to cre values using the standard visibility (see Equation 3). As will be discussed below, Equation (5) is better suited for this purpose since physically black objects are not usually available, and for the estimation of airport visibility, for example, an object defined to be visible should be well seen rather than only 50% of the time. Using Equation (5) below to obtain the mass extinction coefficients, the Nagoya values have to be multiplied by 0.56, giving 3.2, 5.2 and 4.8m2g -~ for the years 1970, 1973 and 1975. With these corrections the mass extinction coefficients are in the range of other cities; the change over the years may


Table 4. Mass extinction coefficients found during different studies

~o (mc)- ' Location (m 2 g- 1) Remark, reference

Santiago, Chile Vienna, Austria

Berlin, Germany

Nagoya, Japan

New York city, U.S.A.

Seattle, WA, U.S.A.

Houston, TX, U.S.A.

Denver, CO, U.S.A.

Spirit Mtn., NV, rural U.S.A.

5 this study 4.6 extinction measured summer 1990,

humidity < 55% 6.0 scattering measured, 25% absorption estimated,

during smog episode, Heits and Israel (1982) 5.61 1970; humidity ~< 70%, aerosol mainly from coal combustion 9.1 1973: decreasing importance over the years 8.4 1975: tr~ from visibility, Okada and Isono (1982) 3.3 scattering measured, 10% absorption estimated,

Charlson et al. (1968) 3.9 scattering measured, 10% absorption estimated,

Charlson et al. (1968) 4.08 scattering of dry aerosol measured,

absorption (16%) measured, Dzubay et al. (1982) 4.3 scattering and absorption measured, only particles < 2.5/am 2.1 scattering and absorption measured, all particles< 15 #m, Lewis

et al. (1986) 3.3 scattering measured, only fine particles 1.56 fine and coarse particles, White and Maoias (1990)

be explained by the change of the aerosol from mainly coal combustion to oil and gas.

The relation between ~ro and m c is also influenced by the cut offofthe sampling device for the determination of the mass of particulate matter. This is well demon- strated by the two values given for Denver (see Table 4). In the U.S.A., the mass mean diameter of the accumulation mode particles being smaller than in Europe, the particles attenuate light less efficiently; the smaller values for the mass extinction coefficient that have been found were therefore to be expected.

Since optical measurements can be performed easily and very rapidly, e.g. by telephotometry or sun photometry, a mass concentration of the aerosol can be derived simply from the mass extinction coefficient. Unfortunately there is no universal "calibration constant" since the relationship between the two variables depends very much on the aerosol and also on the sampling method for particles. For a single location it can give very reproducible results. The daily or weekly variation for Santiago is not large as can be seen from Table 5. There is a decrease at the end of the day and on weekends, which may indicate a slightly different aerosol. The extinction coefficients might be used to estimate mass concentrations to + 50%.

It is interesting to compare the extinction coefficients and mass concentrations found in Santiago with values in other cities. A few values for comparison are given in Table 6. The values reported in the referenced publications are given. Where possible extinction data are listed with the influence of humidity having been eliminated. Since solar photometers only give the vertical optical density z, a standard scale height (height h to which an idealized atmosphere containing particles with constant concentration extends) of h = 1 km has been used to obtain the extinction coefficient tre by the relation tre h = z. For the conversion of

visibilities to extinction coefficients the formula for the standard visibility (3) was used. Under conditions of airport visibility, this may give an extinction coefficient too high by a factor of up to 1.75, cf. Equation (5) (e.g. for the data of Nagoya); see also the following paragraphs.

(ii) S t a t i s t i c a l c o r r e l a t i o n s w i t h a i rpor t v is ibi l i t ies

According to visibility theory (Koschmieder, 1924; Middleton, 1968; Horvath, 1981) the meteorological range L v or standard visibilty can be calculated from the extinction coefficient at 550 nm by

IlnO.021 Lv (3)

tre (550 nm)

For purposes of the analysis of the present data we will define the "Koschmieder visibility" K (2) according to

I ln 0.021 K (2)- - - (4)

ao(,l) We calculated K (2) for wavelengths of 400 and 550 nm in order to compare the statistical features of these visibilities with those of the available airport visibilities. The averages can be found in Table 7. It is to be noted that the data show considerable scatter.

It is seen that the airport visibility averages are rather consistent. The K (2) values show a wide scatter and at first glance only the K (400 nm) data appear to be meaningfully comparable to the airport data. It may be surprising that the visibility at the airports is lower on average compared to the visibility obtained from extinction measurements in downtown Santiago de Chile.

There may be two reasons for this discrepancy: (1) The Koschmieder visibility, using a contrast threshold

A e r o s o l o f S a n t i a g o de C h i l e I I 391

8 ._e e-

O

e,,

g . / "~

• ..- ~,.~ e,~ ~.q

[.,

I I

2 °°

of e=0.02, gives the distance at which a physically black object is just visible (actually in 50% of the cases). This definition is not particularly useful for aviation purposes where completely dark objects are uncommon and an object has to be seen clearly. Depending on the definition of ' ' just seeing" a target smaller visibilities are usually obtained; for this reason contrast thresholds of 0.05 to 0.1 are suggested. For an object which has a radiance of half of that of the horizon, and using the higher thresholds, a visibility lower than the Koschmieder visibility by a factor of 0.59 to 0.41 is calculated. For example, the National Bureau of Standards suggests the use of a 5.5% contrast threshold to convert transmissometer data to airport visibilities (Douglas, 1977). (2) There is a lack of visibility records > 25 km at the Santiago airports, possibly because at large visibility its precise value is unimportant for aviation purposes. Furthermore local topography limits the selection of distant objects for the determination of visibility. Therefore high values for visibility, mostly pertaining to the later hours of the day, are not recorded; this leads to moderate values for the standard deviation. On the other hand during the later part of the day the extinction coefficient in the central part of the city of Santiago usually drops to values below 0.15 km-1, which gives a Koschmieder visibility of > 26.1 km. These high values increase the average, and due to the large daily variation also the standard deviation. (3) With visibility usually increasing with the time of day the time of observation is very important.

The strongest statistical correlations (r>0.8) were found between the K (2) and the Los Cerrillos visibility. Cerrillos is the closest to the general area where the extinction measurements were made. Los Cerrillos visibility data correlated well (correlation coefficient r = 0.79) with those of El Bosque but more weakly with those of Pudahuel.

Assuming a contrast threshold of ~= 5.5% as suggested by the National Bureau of Standards and an object with an intrinsic radiance half the radiance of the horizon, the visibility V is obtained from

1 e. 2.21 v = - - )In ~. . I = - - - (5)

(7 e O . 3 . O e

Using the average extinction coefficient at 550 nm of all sight paths located in the downtown area of Santiago, a visibility of 9.2 km is calculated, which is very close to the observed average visibility of 9.4 km at the Los Cerrillos airport.

Linear regression models for the relationship of the extinction coefficients at 550 nm to airport visibilities were also investigated. Extinction data pertaining to the downtown Santiago area and the Los Cerrillos and El Bosque airports were included. We quote results for the Los Cerrillos data (385 datum points; visibilities exceeding 1 km are included only). For the visibility V and the extinction coefficient ~r~ over

AE(A) 27:3-H

t~

Tab

le 6

. M

ass

conc

entr

atio

ns a

nd e

xtin

ctio

n co

effi

cien

ts a

t 55

0nm

in

diff

eren

t ci

ties

Loc

atio

n

Sant

iago

, C

hile

Bei

jing,

Chi

na

Vie

nna,

Aus

tria

Ber

lin,

Ger

man

y

Nag

oya,

Jap

an

Rio

de

Jane

iro,

Bra

zil

Ath

ens,

Gre

ece

Val

lado

lid,

Spa

in

Kat

owic

e, P

olan

d

Hou

ston

, T

X,

U.S

.A.

Los

Ang

eles

, C

A,

U.S

.A.

Den

ver,

CO

, U

.S.A

.

Alb

uque

rque

, N

M,

U.S

.A.

Ran

ge o

f va

lues

R

ange

of

valu

es

for

ae (

km-1

) fo

r m

e (/~

g m-3

)

0.15

-0.4

3 40

-68

0.24

45

.7

0.2-

0.5

33

172

0.15

-0.4

0 0.

2-0.

5 0.

3-2.

5 50

-376

0.78

13

9 (1

970)

0.

59

65 (

1973

) 0.

36

43 (

1975

) 30

22

0 --

27

-124

0.

15-0

.22-

0.47

0.15

-0.5

2 10

-65

0.21

0.07

5/0.

102

0.12

/0.3

1

42/3

0 (2

7/21

)

~<18

7

19/2

3 (3

5/37

)

19/5

4 (2

6/20

)

Rem

ark,

ref

eren

ce

tele

phot

omet

er,/

~ ab

sorp

tion

, th

is s

tudy

av

erag

e su

n ph

otom

eter

, L

i Je

and

M.

Jie

Tai

(19

90)

grav

imet

ric,

sum

mer

dat

a, H

orva

th e

t al

. (1

989)

te

leph

otom

eter

, su

mm

er d

ata,

Hab

enre

ich

(198

3)

tele

phot

omet

er,

win

ter

data

, H

aben

reic

h (1

983)

sc

atte

ring

mea

sure

d, 2

5% a

bsor

ptio

n es

tim

ated

; gr

avim

etri

c du

ring

win

ter

smog

epi

sode

, H

eits

and

Isr

ael

(198

2)

hum

idit

y ~<

70%

, ae

roso

l m

ainl

y fr

om c

oal

com

bust

ion,

dec

reas

ing

impo

rtan

ce o

ver

the

year

s ae

fro

m s

tand

ard

visi

bili

ty, O

kada

and

Iso

no (

1982

) A

ugus

t-S

epte

mbe

r, a

vera

ge ~

80/

~g m

- 3

Dai

sey

et a

l. (

1987

) av

erag

e 87

pg

m

3, s

umm

er 1

987,

Sch

eff a

nd V

alio

zis

(199

0)

low

-med

ium

-hig

h tu

rbid

ity,

fro

m s

un p

hoto

met

ry,

Cac

horr

o an

d C

asan

ova

(198

9)

from

sta

ndar

d vi

sibi

lity

and

gra

vim

etri

call

y, h

umid

ity

~< 50

%,

Pas

tusz

ka,

(199

0)

as o

f dr

y ae

roso

l +

aa;

day/

nigh

t fi

ne m

ass,

(da

y/ni

ght

coar

se m

ass)

, D

zuba

y et

al.

(19

82)

aver

age

60.2

#g m

-3,

Sol

omon

et

al.

(198

9)

scat

teri

ng a

nd a

bsor

ptio

n m

easu

red,

da

y/ni

ght

fine

mas

s, (

day/

nigh

t co

arse

mas

s),

Lew

is 'e

t al

. (1

986)

day/

nigh

t fi

ne m

ass,

(da

y/ni

ght

coar

se m

ass)

, Z

ak (

1986

), da

y/ni

ght

valu

e of

~r s +

a a

.>

~a

ca.

r~

o -I


Table 7. Visibility at airports and Koschmieder visibility in downtown Santiago. Average values in km (standard deviation given in

parentheses)

Airport visibility Cerrillos 9.4 (3.9) Bosque 11.2 (5.2) Pudahuel 11.1 (4.2)

Koschmieder visibility K (400 nm) 10.8 (7.7) K (550 nm) 16.3 (21.0)

downtown Santiago one finds a no-intercept fit:

2.751 ___ 0.048 a~ (6)

VCerrillos

with a regression coefficient R z =0.90. This fit is better than a log-log fit and much better

than a fit with intercept. The correlation coefficient between extinction coefficient and Cerrillos visibility was found to be -0.86. Correlations with E1 Bosque data were somewhat weaker.

It would appear that visibilities observed at Los Cerrillos yield a good indicator of horizontal atmospheric extinction at 550 nm over downtown San- tiago.

Using the mass extinction coefficient of 5 m 2 g - ' and the relation between Los Cerrillos visibility V and the extinction coefficient at 550 nm given by Equation (6) a relation between the mass concentration mc and the visibility V is obtained as

0.55[g m- 2] m c = (7 )

V

(iii) Rela t ion o f ex t inc t ion to other variables

Weak but significant correlations have been found to other parameters, of which we shall discuss humidity.

Aerosol particles may increase their size due to uptake of water with increasing humidity. Typical examples for materials having these properties are NaC1, (NH4)2SO 4 and NH4NO 3. The phase trans- ition from the solid material to a solution occurs at a well-determined relative humidity. The relative humidities for equilibrium between water vapour and the saturated solution for the above-mentioned pure substances are 75, 80 and 62%. The overall average relative humidity for our data was 62.7% with a coefficient of variation of 15.6%. Since particles can contain trace substances which may take up water at lower humidities a slight increase in size below the mentioned humidities is possible. Particles which are for some time in the airborne state, even organic materials or soot, can be "etched" by the atmospheric trace substances, so that growth with increasing

humidity is observed. For example, for the Denver aerosol (Countess et al., 1981) an increase in particle size for a change in humidity from 35 to 75% of 1.48, 1.68, 1.16 and 1.23 has been measured for sulphates. nitrates, organic and elemental carbon. Under these circumstances the light extinction of the urban atmosphere can increase by more than a factor of 1.7 without changing the particle number.

Small humidity effects can be expected for our measurements. Changes in humidity should shift the values of the Angstr6m exponent, ct. Excluding the few

values larger than 2.5, which correspond to events on two singular days, a no-intercept fit yields

75.6 _ 0.8 ~: (8)

RH%

with a regression coefficient of R: = 0.96 for 719 datum points. The regression gives the expected trend: increasing humidity yields smaller exponents corres- ponding to increased particle size, and vice versa.

Some model calculations will illustrate humidity- dependent particle behaviour. We have used an average mass size distribution measured with a seven stage impactor (Pr6ndez, 1988) to develop a model size distribution for calculations of the extinction coefficient: an accumulation mode with a mass mean diameter of 0.35/~m consisting of 26% sulphates, 13% nitrates, 17% soot and 44% other materials, and a coarse mode consisting of crustal materials with a mass mean diameter of 10pm. Using the growth factors reported by Countess et al. (1981) and starting with a humidity of 35% an increase in extinction coefficient by a factor of 1.11 will take place if the humidity increases to 55%. A further increase to 75% relative humidity increases the extinction coefficient by a factor of 1.71. Since high humidities were infre- quent only changes in the range of 10% would norm- ally be expected. Humidity growth may have been present in the early morning hours during the San- tiago measurements.

With an increase in particle size the Angstr6m exponent decreases. For the same model, the Angs- tr6m exponent changes from 1.5 to 1.48 and 1.33 for humidities going from 35 to 55 and 75%. The smaller average value of the ~,ngstr6m exponent in the morning may be explained in this way.

SUMMARY AND CONCLUSIONS

The atmosphere of Santiago de Chile has been investigated by telephotometry in various parts of the city. Spectral extinction coefficients were measured during the time period between September 1988 and January 1989 (mid-spring to mid-summer) and again from February 1990 to June 1990 (autumn to mid- winter). The values measured show a distinct daily pattern which on average has high values in the morning and a steady decrease in the afternoon. A


similar pattern is observed for the mass concentration of the particles but not for sulphur dioxide.

The extinction coefficient decreases with increasing wavelength. A good approximation formula for the average spectral extinction coefficient in Santiago is a power law relationship with an exponent (~ngstr6m exponent) of ~ = 1.22. This value is less than values found in the U.S. and somewhat higher than in Europe. Mass extinction coefficients in Santiago are higher but still comparable to other locations. Since a considerable fraction of the mass of particulate matter is in the coarse mode (Trier and Silva, 1987), which has little influence on optical parameters, the cut-off diameter of any sampling device as well as the size distribution of the aerosol under observation are significant for the calculation of the mass extinction coefficient. Nevertheless a useful empirical relation between the mass concentrat ion and the extinction coefficient was found for Santiago.

The optical data measured by us show strong statistical correlations with visibilities observed at three airports in and near Santiago. Although the airport visibility correlates well with the Koschmieder visibility, the latter is consistently higher than the visibility observed at the airports. Assuming objects with a radiance half that of the horizon and a contrast threshold of 5.5% (the usual value for airport visibility) very good agreement between derived and measured visibility was found. Airport visibilities determined in Santiago appear to be good indicators of atmospheric extinction and thus of atmospheric pollution in the downtown areas.

Correlat ion between measured extinction coefficients and mass of suspended particulate matter is satisfactory; with other indicators for air pollution correlations are weak but significant.

A comparison with the pollution of other large cities shows that the daily pattern of variation of the extinction coefficient in Santiago is markedly different. The values of the extinction coefficients and mass concentrations observed in Santiago de Chile are higher than or comparable with those of European and Asian cities. The air basin of the greater Santiago area is restricted by mountains on all sides and only vertical exchange allows the spreading of the pollution, which usually takes place in the afternoon after intense insolation. In fact indicators of air pollution are high in the morning and low in the afternoon.

Acknowledgements--This work was in part supported by Fundaci6n Andes and by Universidad de Santiago de Chile. The authors wish to thank students Sandra Pino, W. Folch and R. Saavedra for help with the taking of data.

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A study of the aerosol of Santiago de Chile—II. Mass extinction coefficients, visibilities and Ångström exponents