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Size segregated light absorption coefficient of the atmospheric aerosol
Pergamon
Atmospheric Environment Vol. 29, No. 8, pp. 875-883, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights r~erved 1352-2310/95 $9.50 + 0.00
1352-2310(95) 00025-9
SIZE SEGREGATED LIGHT ABSORPTION COEFFICIENT OF THE ATMOSPHERIC AEROSOL H.
HORVATH
Institute of Experimental Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria (First received 2 September 1994 and in final form 7 December 1994) Abstract--The light absorption coefficient of atmospheric aerosols in the visible can be determined by depositing the particles on a filter and measuring its "transmission" in a special optical arrangement. With an impactor with rotating impaction plates producing a homogeneous deposit, it is possible to extend this technique to size segregated aerosol samples. A simultaneous determination of the mass size distribution is possible. Test measurements with black carbon aerosol have shown the feasibility of this method. Samples of the atmosplaeric aerosol have been taken in and near Vienna, in Naples and near Bologna. The light absorption of the aerosol is always highest for particle diameters between 0.1 and 0.2 #m. Only in the humid environment of the Po valley it had a slightly larger peak size, whereas the size of the nonabsorbing particles increased con,;iderably. The light absorption of the atmospheric aerosol is always higher in an urban environment. 'The mass absorption coefficient of the aerosol at all four locations was very similar, and completely dilferent from values which could be. expected using effective refractive indices which are frequently used in models. Using the data measured in this work two alternate models for the effective refractive index and black carbon content of the aerosol are suggested: (a) a size-dependent refractive index, where the imaginary part varies from - 0.25 for particles smaller than 30 nm to - 0.003 for particles larger than 2/zm; this could especially be applied if an internal mixing of the aerosol is to be expected, or (2) a size-dependen~Lfraction of elemental carbon in the case of external mixing with 43% of carbon particles for sizes below 30 nm decreasing to 10% for sizes up to 0.4 #m.
INTRODUCTION The atmospheric aerosol particles consist of a mix of many substances. Although they are a minority component in the atmosphere, contributing to about 10-100 ppb by mass, they are the most important constituent of the atmosphere when dealing with the optical properties in the visible. The aerosol is responsible for 6 0 - 9 5 % of the visibility reduction, many atmospheric optical effects are dominated by the aerosol, and the aerosol is involved in the radiation balance of the globe, and thus climatic change considerations also have an aerosol aspect. The attenuation of light in the atmosphere is due to elastic scattering (the light is deflected by aerosol particles and molecules, but it is conserved) and absorption (the energy of the light is converted into internal energy of :the molecules and eventually to heat). Whereas all particles scatter light, only particles consisting of substances for which the complex refractive index has a nonzero imaginary part also show light absorption. A survey by Bohren and Huffman (1983, Fig. 14.1) of substances which could be found in the atmosphere poiints out that in the visible only elemental carbon and hematite (ct-Fe2Oa) are light absorbing: between 0.4 and 0.8 #m wavelength only carbon and hematiite have imaginary parts of the
refractive index which is larger than 0.5. For all other substances the imaginary part is < 10- 5. So these two substances can be considered as the major contributors. Except for iron ore mining areas hematite seems to be a very rare substance in the atmosphere. Recent measurements in Vienna and upwind V i e n n a have shown that iron contributes to less than 200 ng m - 3, whereas elemental carbon has a 20 times higher concentration, so even if all iron were ~t-Fe203 it would contribute to 7% of the light absorption which can be considered as the upper limit of contributions to light absorption by substances other than black carbon. Thus black carbon appears to be the only light absorbing substance in the atmosphere. If gases are considered there are ozone and nitrogen dioxide, but both have absorption coefficients which are at least one-tenth of the light absorption by black carbon. There might be substances in the atmosphere where the bulk material appears to be completely black, but still their light absorption as aerosol particles is negligible. This can be seen in a very simple way: the imaginary part k of the refractive index is related to the absorption coefficient ~t of the bulk material by ct = 4nk/;t. A substance which has a transmission of 10-10 in a 1 cm layer at 550 n m has an ~t = 2072 and definitely will be considered as black. Using the above relation-
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H. HORVATH
ship, k is obtained as k = - 0.000009. Materials with this imaginary part of the refractive index can be considered as nonabsorbing aerosol particles. This can also be seen by a simple calculation. A slab of 1 pm thickness of the above-mentioned substance has a transmission of (10-lo) I p m / l cm = 0.998 which is completely transparent. O n the other hand elemental carbon (with an imaginary part of the refractive index k = 1.0) has a bulk absorption coefficient of 0t = 2.28 x 10'; therefore the transmission of a 1 #m slab will be 1.19 x 10-10. The interested reader is also referred to Bohren and Huffman (1983, pp. 279, 280) where a paragraph with the title "A dirty silicate story" is devoted to a piece of hornblende, which was as black as coal, but still has no light absorption when assuming the material to be an aerosol particle. Elemental, graphitic or black carbon is produced by every combustion process, is the main constituent of soot and can be found everywhere on the globe. With respect to their mass, particles containing black carbon are very efficient in their interaction with light, and in many parts of the world black carbon is the second important substance contributing to the extinction budget. The light attenuation by black carbon is mainly due to absorption, whereas all other aerosol particles scatter the light. The light scattering of the global aerosol reduces the input of solar energy, and thus has a cooling effect on the climate, the light absorption of the soot particles transfer solar energy to the atmosphere and thus has a heating effect. The magnitude of these effects d e p e n d s - - a m o n g o t h e r s - - o n the size of the particles. The effectiveness of the light absorption by carbon can be enhanced considerably when small carbon particles (e.g. 50 nm in diameter) are included in a larger transparent particle. The combustion produced aerosol particles usually have sizes in the range of 30 nm to 1 #m with a peak near 0.1 #m. During their "life" in the atmosphere the particles can undergo various transitions, e.g. participating in the formation of mixed particles. During this obviously the particle size changes, which means that the particles have other optical properties. Therefore information on the particle size is desirable. Size segregated samples of aerosol particles can be obtained with cascade impactors; numerous designs exist, see, e.g. Lodge and Chan (1986) for an overview. The light absorption of the black carbon can be used to determine the size distribution of these particles, when being deposited in a special cascade impactor which produces a homogeneous deposit. The determination of light absorption on a filter is a standard technology by now; a very simple one is the integrating plate method ( L i n e t al., 1973). This method has been adapted and has been tested previously with success for laboratory generated aerosols (Horvath et al., 1986). Results obtained with an extended measuring range of the impactor and an improved measuring technique for atmospheric samples are presented here.
EXPERIMENTAL As already mentioned, in the visible the only light absorbing substance in the atmosphere is black carbon. Thus a method to measure the light absorption of a sample will also. permit the determination of the elemental carbon. There are more than a dozen methods to determine the light absorption coefficient of an aerosol sample; for an overview see Horvath (1993). Most of them require a homogeneous deposit on a flat substrate. In general this is done by sampiing the particles on a filter, such as Nuclepore or a fiber filter. Using one of the common methods to measure the light absorption of the sample obviously determines the value of all the particles deposited on the filter. Cascade impactors use inertial separation to obtain size fractionated deposits of aerosol samples. An impactor consists of a series of plates having small holes, through which the air containing the suspended particles is sucked. In the holes a jdt is formed, which is deflected by a plate positioned just behind the plate with the orifices. Those particles which have high inertia cannot follow the deflection of the air molecules and thus will hit the impaction plate and will be deposited there. Usually the impaction plate is covered by a foil or a filter, which is removed after a measurement has been performed and the particles deposited on it can be weighed or analyzed chemically. Particles having diameters larger than the cutoff diameter of the arrangement will be removed from the air stream. The cutoff diameter depends on the velocity of the air in the jet, the distance of the impaction plate and the gas pressure. By putting several impaction stages behind each other with decreasing diameters of the jet orifices the deposits on the impaction plates contain particles with sizes within a diameter interval de,ermined by the cutoff diameter of the stage (smallest size' and the previous stage (largest size). For this work mainly a 10stage low-pressure impactor (Berner, 1984) has been used. The interval of the diameters of the deposited particles always was a factor of 2, the smallest diameter collected was 15 nm, and the largest was 16 #m. A backup filter has not been used, since very few particles have sizes below 15 nm. For some measurements we have used an earlier design impactor with seven stages, depositing particles between 0.1 and 16 #m. In that case a backup filter was used. Normal cascade impactors form a dot-like deposit under each hole. This is sufficient for gravimetric and some kinds of chemical analysis, but unsuitable for XRF, PIXE and light absorption analyses. Therefore impactors with rotating deposition plates have been developed. The deposit of each hole then has the shape of a circle and by proper arrangement of the holes a quasi-homogeneous deposit on an annular ring is formed (Marple et al., 1981, Klaus and Berner, 1985; Horvath et al., 1986). With this deposit one of the light absorption techniques can be used. The flow rate through the impactor was 1.5 m 3 h- 1 and the homogeneous deposit was an annular ring with 58 mm outer and 42 nun inner diameter. It should be mentioned that light absorption measurements are also possible using the Kubelka Munk method (see e.g. Kort/im, 1969). In order to use this method a sample of the absorbing particles is needed which is large enough to mix it with white barium sulfate powder. The change in reflectivity can be used to deduce the light absorbing properties of the sample. This method has been applied by Lindberg and Gillespie (1977a). The only drawback of this method is the need for a fairly large sample. Four impactors were operated in parallel for several months in order to have sufficient material. We have applied the integrating plate technique to infer the size segregated light absorption coefficient aa of the atmospheric aerosol at a wavelength of 550 nm. For this purpose a deposit is formed on a Nuclepore filter. The integrating plate method (Linet al., 1973) is a simplification of the integrating sphere method developed by Fischer
Size segregated light absorption coefficient of the atmospheric aerosol (1973). In principle the "blackness" of a filter sample collected on a Nuclepore tilter is determined by measuring the transmittance T of the opaque filter with and without particle load. A special optical arrangement which integrates the light scattered by the particles and the filter (integrating sphere or integrating opal plate) takes care that only the light which is absorbed by the particles is missing from the transmitted light. The light absorption coefficient is determined using the following assumptions: (1) The aerosol particles deposited on the filter are contained in a column, the height x of which is determined by the sample volume V and the area of the deposit A by x = V/A. (2) With respect to light absorption the particle.,; deposited on the filter have the same optical properties as in the airborne state. (3) Any light scattered by the particles is integrated by the optical arrangement. Then the absorption coefficient aa is obtained by tr, = l / x In T. The method has some particularities and we have used an experimental calibration (Horvath and I-Iabenreich, 1989; Horvath and Metzig, 1990; Horvath, 1991) to derive the light absorption coefficient. The results of these calibrations can be summarized in the following way. For strongly light absorbing aerosol particles such as elemental carbon produced in a spark generator the values given by the integrating plate method agree with the values of absorption coefficients obtained from an absolute method. If the aerosol also contains nonabsorbing particles (either internally or externally mixed) the integrating plate method gives too high light absorption coefficients. The deviation is extremely large if the light absorption only gives a minor contribution to the extinction (e.g. wrong by a factor of 3 for aerosols where the absorption only contributes 5% to the extinction). For the atmospheric aerosol studied in this work the integrating plate method gives values too high by a factor of 30-40%. This agrees with estimates, e,g. given by Clarke (1982). But there is no need to form the deposit on a Nuclepore filter, since the filtering capabilities are not needed in the impactor. In principle any opaque material would serve the purpose. The Nuclepore filters have the advantage of being very homogeneous with respect to their optical properties. It is quite interesting to have the mass size distribution of the aerosol simultanecusly with the size distribution of the light absorbing ~erosel. Since only a small fraction (about 0.4 cm 2) of the total deposit (having an area of 12 cm 2) is needed for the determination of the light absorption coefficient, the remaining deposit can be used for other purposes. The following procedures have been used successfully to determine the mass size distribution and the size distribution of the light absorbing aerosol simultaneously: (1) A small pie-shaped piece having an angle of ~ 5° is cut off the foil which rests on the impaction plate. Underneath the foil a transparent paper is positioned, which just receives a deposit in the area, where the foil is cut away. The loss of "transmission" due to ]Jght absorbing particles can be measured by determining the light passing through the paper at a spot where a deposit has been formed and at a location where no deposition took place due to the overlying foil. (2) A 25 mm diameter Nuclepore filter is fixed to the foil by a low adhesion tape. The foil (in our case household aluminium foil), with the filter sticking to it, is weighed before and after the sampling After the second mass determination the Nuclepore filter is removed and the "transmission" is measured in an area with and without any deposit. This method is more elaborate and requires some skill, but it is more sensitive for light absorption measurements since the Nuclepore filters are very homogeneous. Also it has the advantage that changes of the optical properties of the Nuclepore filter due to contact with the air to be analyzed are the same on the area with and without deposit. For the atmospheric samples reported below the sampling time was between 10 and 24 h.
877
TEST MEASUREMENTS The feasibility of the method has been tested by comparing it with already established methods. Optical measurements of the size segregated light absorption coefficient is a new method, so the following has been done: (1) compare an integral value of this method with a value obtained by another method and (2) use the values derived from theoretical considerations to examine the technique. Both are not a perfect proof for the reliability of the method, but still they are an indication. Both have been done. (1) For the integrating plate method calibrations exist (Horvath, 1993; Horvath and Habenreich, 1989; Horvath and Metzig, 1990), and it is a simple task to sample with the impactor and on a polycarbonate filter simultaneously, add the light absorption coefficients of all the stages of the impactor and compare it to the value obtained from the sample containing all particles on one Nuclepore filter. This has been checked for several laboratory generated aerosols and agreement has been found. Obviously this just means that the use of the rotating impactor and the segregation of the sample in different size ranges does not introduce any additional errors and that the determination of the light absorption by this method is as good as the integrating plate method. (2) By Mie theory (Mie, 1908) it is possible to calculate the mass absorption coefficient of homogeneous spherical particles of any substance as a function of particle size. Input parameters are the wavelength (550 nm is used for all measurements and calculations), the refractive index of the bulk material and its density. Unfortunately neither refractive index nor density are well known for black carbon; thus one has to rely on educated guesses, which may differ by a factor of up to 2 (Horvath, 1993, Table 1). The soot particles consist of graphite which has a density of 2250 kg m - 3 and most likely a (complex) refractive index of m = 2 - i. The primary graphite particles are spherical with diameters around 25 nm and form fluffy and bizarre-shaped agglomerates. Consequently the average density and refractive index are less than those of the bulk material; the volume mixing rule is one of the frequently used ways to obtain a value for the effective refractive index of a mixture of substances, although it has no theoretical basis (Chylek et al., 1988), so it should only be considered as a guess. It also needs to be mentioned that soot particles produced by combustion processes may also contain unburnt fuel (mostly organic material) which is not light absorbing. For the calculations performed below, the following possibilities were considered: (1) pure graphite, which is unlikely; (2) graphite with about 50% empty space, having a density of 1 2 5 0 k g m -3 and a refractive index of m = 1.25 - 0 . 5 i ; and (3) graphite with 75% empty space having a density of 625 kg m - 3 and a refractive index of 1.25 - 0.25i.
878
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RESULTS: SIZE-DEPENDENT
LIGHT ABSORPTION
COEFFICIENT OF THE ATMOSPHERIC AEROSOL.
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Fig. 1. Mass absorption coefficient of various aerosols containing mainly black carbon. The lines give theoretical calculations for spherical particles (solid line: pure graphite; dashed line: graphite with 50% empty space; dotted line: graphite with 75% empty space). The data points give values obtained with various aerosols using the rotating cascade impactor. The squares are for a kerosene flame aerosol, the circles for emissions by a diesel motor, the diamonds for an aerosol obtained by spraying diluted India ink. The open symbols are for data points where either the mass and/or the light absorption coefficient were low; thus the data are insecure.
The ratio of the light absorption coefficient to the mass concentration of spherical soot particles has been calculated and is shown in Fig. 1 for the three above-mentioned assumptions as a function of particle size. Since a wide range of both densities and refractive indices has been considered, the reality can be assumed to lie in between. The light absorption of the particles is most effective in the size range from 0 to 0.2/~m, decreasing rapidly for larger particle sizes. The mass absorption coefficient is higher for the partitles containing more empty space due to the lower density. Using the rotating impactor this relationship has been checked. Several laboratory generated aerosols have been used for this purpose: aerosols produced by a kerosene flame, aerosols produced by a diesel engine and aerosols produced by atomizing diluted India ink. In all cases the mass absorption coefficient is in accordance with theory, so it can be assumed that this method gives reliable data. The data are closer to the curves represented by the aerosol particles containing empty space, which is realistic due to the structure of the agglomerates. With this relationship it is also possible to determine the content of elemental carbon of impactor samples. There might be some difficulties with internally mixed aerosol particles especially when they are larger than 1/~m, since in that case already a small amount of elemental carbon gives considerable light absorption.
Vienna: Sampling took place at the bottom part of the roof of the Stephansdom, the cathedral of Vienna. The impactor was located at an elevation of about 30 m above ground in June/July 1994. The weather could be characterized as fair and dry with mostly clear skies. Rural Vienna: Sampling took place at a small village (Breitenfurt) upwind of Vienna, ~ 20 km west of the town in June/July 1994. The inlet was located at an elevation of 6 m above ground. The measurements were taken at the same time as the ones in Vienna. Rural Bologna: Located in the Po valley near San Pietro de Capofiume, ~ 20 km north of Bologna. The sampling took place at a meteorological field. Samples were taken in December 1991. The Po valley has considerable air pollution. The climate in December is very humid, with many fogs occurring in the morning and in the afternoon. During the time the samples were taken it was fair weather with clear sky, but during the night fog occurred. Naples: Sampling took place in June 1992 at an elevation of ~ 15 m above ground on the roof of the Science building of the University of Naples; during the measuring period the weather was fair, with medium to high humidity and occasional rain. Sampling was stopped in the case of rain. All these samples were taken with a 10-stage rotating cascade impactor, where part of the deposit was used for light absorption analysis. For the 1991 and 1992 data light absorption measurements were done on transluscent paper, and due to inhomogeneities of the paper the sensitivity was not sufficient to measure light absorption coefficients on the stages having only a small change in transmission due to a black carbon deposit. Figure 2 a - d shows the average values of the mass size distribution, the light absorption coefficient and the mass absorption coefficient.
DISCUSSION
The mass size distribution (thin solid line) shows a peak of the accumulation mode around a diameter of 0.5/zm in rural and urban Vienna and in Naples. The Po valley data have a peak at ~ 1 pm which is most likely caused by the increase in particle size due to uptake of water at the high humidity present. The second peak at particle sizes above 5/tm is due to resuspension. It is missing at the rural Bologna data, since the soil was sufficiently humid due to dew and deposition by the fog droplets that a resuspension could not take place.
Size segregated light absorption coefficient of the atmospheric aerosol Vienna, June 1994 h I.fl
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Fig. 2. Mass size distribution (thin solid line), size-dependent light absorption coefficient (thick solid line) and mass absorption coefficient (dotted line) for the four measuring campaigns. Mass and light absorption coefficient are given per stage of the impactor in the following units: mass concentration #g m-3, light absorption coefficient M m- 1, mass absorption coefficient m 2 g-1. (a) Rural near-Vienna, June 1994; (b) Vienna, June 1994; (c) rural near-Bologna, December 1991; (d) Naples, June 1992.
The light absorption is most important at sizes between 0.1 and 0.2/~m, and a maximum light absorption is found in this size range in all the data sets. The highest light absorption coefficients have been found in Naples and rural Bologna, the Vienna values are approximately half, rural Vienna again is a factor of 2 less. There is an indication of increased light absorption at the last stage (15-30 nm diameter) at least for the Vienna data set. :Light absorbing aerosol particles this small occurred irregularly. The dark deposit was clearly visible on the foils/filters. It also occurred occassionally at the other locations. Particles in this size range are the primary graphitic particles produced, e.g. in a diesel engine. Most of the particles
coagulate, but some may be left over. Obviously their occurrence is an indication for a source dominated aerosol. Size distributions of elemental carbon measured at the desert regions of the SW of the United States are quite similar in shape but the light absorption coefficient is a factor of 7 lower than that for the rural near-Vienna data (McMurry and Zhang, 1989). The size distribution of the light absorption coefficient of the atmospheric aerosol can be compared with the size segregated light absorption coefficient of diesel particles, which are the major contributors of light absorption in an urban environment. Figure 3 shows the average of all atmospheric measurements
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Fig. 3. Average size distribution of the light absorption coefficient of the atmospheric aerosol (solid line), of diesel particles emitted during a standard driving cycle (dashed line) and diesel particles emitted by a diesel motor under low load conditions (dash-dot line).
(solid line, it has been normalized to 1 in the size range of 0.1-0.2/~m) and two size distributions of diesel emissions. The average size distribution of the light absorption of the diesel particles emitted during several driving cycles has been obtained from measured mass size distribution (Kreiner, 1985; Norek, 1985). In comparison to the atmospheric aerosol it contains smaller particles and especially particles above 2 #m are almost lacking. In addition emissions of a warmed-up diesel engine in idle and low load have also been measured. These emissions may be more representative for nowadays stop-and-go traffic in congested towns. It has a size distribution shifted towards larger particles (dash-dotted line in Fig. 3) and it very much resembles the average size distribution of the light absorption coefficient of the atmospheric aerosol, although the particles are somewhat larger. Emissions from a car moving at high speed for extended time periods and those from a car mainly in idle or low load are the two extremes; the size distribution of the source for the majority of the light absorbing particles will lie somewhere in between, just as that of the atmospheric aerosol, as can be seen in Fig. 3. But it should also be noted that for the smallest measured particle range of 15-30 nm the light absorption coefficient of the atmosphere is higher than expected from diesel emissions. Also for sizes above 4 #m the light absorption of the atmosphere is higher than expected from diesel emissions. This could be caused by mixed particles containing soot as a minority component. Particles of this kind can be resuspended particles which mainly contain nonabsorbing substances with a small fraction of soot, as is usually the case in road dust. By incorporating soot in particles consisting of transparent material the absorbing
properties of elemental carbon are greatly enhanced. Mixed particles of this size could also be formed by in-cloud transformation. The ratio of the light absorption coefficient to the mass concentration of the aerosol (mass absorption coefficient) is not too different for the four locations as can be seen in Fig. 4. So it appears that the aerosol at all four locations is very similar with respect to its relation between light absorbing and nonabsorbing parts. The Vienna aerosols show a very high mass absorption coefficient at the 15-30 nm range which is close to the one of pure carbon, suggesting a source dominated aerosol. This is not the case for the rural near-Vienna aerosol which on the other hand has a higher mass absorption (but lower absorption) coefficient than Vienna between diameters of 30-400 nm. The rural aerosol near Bologna has low values in mass absorption coefficient between 0.4 and 1.6/~m diameter, since at the high humidity the majority of the aerosol particles increased in size more than the carbon particles. For modeling of the atmosphere with respect to its radiative properties it is desirable to have some optical input parameters which describe the aerosol well. It has become a habit to use an apparent or effective refractive index, which can be used to calculate the scattering and absorption coefficient of the aerosol in various scenarios of the model. It should be pointed out that the effective refractive index is a hypothetical value which is used in calculations and it is not expected that the substance the aerosol particles are made of has this refractive index, but that the use of this hypothetical refractive index will give the right values, e.g. for mass absorption coefficient. H/inel (1989) calls this refractive index "apparent but not necessary real". Examples for this are, e.g. Fischer (1973), Patterson et al. (1977), H/inel and Dlugi (1977), Gerber (1982) and H/inel (1989). For the European strongly light absorbing aerosol an imaginary part of the refractive index of - 0.02 to - 0.1 is used. For comparison the calculated values for mass absorption coefficients for a pure carbon aerosol and aerosols having an effective refractive index of 1.5 - 0.1i and 1.5 - 0.05i are shown. One can see that the use of one effective refractive index will not adequately characterize the aerosol with respect to its absorptive properties. For all the model values of the complex refractive index the mass absorption coefficient should increase up to a diameter of ~ 0.3/~m and then decrease for larger diameters, whereas the measured mass absorption coefficient decreases steadfly with size. Maybe the rural near-Vienna data have some indication of such a behavior for sizes below 0.2/~m. Anyhow, using a measured mass size distribution and using a single value of the effective refractive index will not give an adequate value of the -light absorption of the aerosol. There are several possible explanations for this discrepancy. The aerosol consists of a black carbon component which has its origin in combustion processes
Size Segregated light absorption coefficient of the atmospheric aerosol (mainly diesel engim;s) and the size distribution has a peak at about 0.1 urn. The majority of the aerosol particles are made up of transparent particles which are larger in size, peaking at 0.3-0.6 #m. With these two different size distributions for the absorbing and the nonabsorbing aerosol a stronger decrease in mass absorption coefficient with increasing size than for pure carbon can be explained. The carbon particles could also be thought of as attached to some but not all of the transparent particles, which will give a very similar result. This will also explain the larger size of the light absorbing aerosol particle in the Po valley. But it is not feasible to assume that the aerosol particles consist of the same homogeneous mixture of light absorbing and transparent substance for all sizes. A solution to this can be achieved by either using a size-dependent effective refractive index, as has been
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Fig. 4. Mass absorption coefficient of the aerosol samples at the four locations. The thick solid and dashed step curves are the data of Vienna and rural near-Vienna, the thin solid and dashed step curves are Naples and rural near-Bologna. The dash-dot curve is the calculated relationship for graphite containing 50% empty space, the upper solid curve is for an aerosol with a refractiw~ index of 1.5 - 0. li and the lower one for a refractive index of 1.5 - 0.05i, both with a density of 1500 kgm -3 .
881
already done by Lindberg and Gillespie (1977a), or subdividing the aerosol in a light absorbing fraction and a nonlight absorbing fraction. Both methods have their merits. If one can be sure that the light absorbing particles mainly consist of mixed particles, then an effective refractive index is most adequate, if the carbon particles and transparent particles exist separately in the atmosphere then the fractional approach is more adequate. The effective refractive index has been calculated for the average of the data of the atmospheric aerosol in the different size classes which were measured by the impactor. A density of the particles of 1500 kg m - 3 and a real part of the refractive index of 1.5 was assumed. As an alternative the mass fraction of elemental carbon in the different size classes was calculated, using a density of 1250 k g m -3 and a refractive index of m = 1.25 - 0.5i for elemental carbon particles. The results are presented in Table 1. Several conclusions can be drawn from this. Both the imaginary part of the refractive index and the mass fraction of elemental carbon are high for particles below 60 nm, i.e. black carbon is of considerable importance. For sizes up to 0.7 #m mean diameter, the contribution by elemental carbon is less, which can also be seen by considering the effective refractive index, where the imaginary part goes down to - 0 . 0 2 1 . Particles between 0.04 and 0.7/~m mean diameter can be considered as having their origin in the combustion process of diesel engines. For particles in the 15-30 nm range their origin is still unclear. For particles above a size of 2 #m the mass fraction of black carbon increases again, although the effective refractive index is small. This seems to be a paradox, but it can be explained. The light attenuation in carbon is so strong that with the previously determined absorption coefficient of the bulk elemental carbon of = 2.28 x 107 a slab of 5 / t m thickness has a transmission of 3 x 10- 50, i.e. all light is absorbed. If we now assume that the material contains only 1% of carbon finely dispersed in a transparent material we obtain c< = (1/100)2.28 x 107, which gives a transmission of 0.32, i.e. 68% of the light is adsorbed. This amount of absorption would be equivalent to the absorption of a 5/~m slab having "islands" of elemental carbon in
Table 1. Optical characteristics of the aerosol size fractions Impactor stage no. 1 2 3 4 5 6 7 8 9 10
Size range 15-30 nm 30-60 nm 60-120 nm 120-250 nm 0.25-0.5/~m 0.5-1/~m 1-2 #m 2-4 #m 4-8 #m 8-16/~m
Mean size
Effective refractive index
Mass fraction of carbon (%)
21.2 nm 42.4 nm 85 nm 176 nm 0.35 #m 0.71 #m 1.41 #m 2.83 ~m 5.66 #m 11.3 pm
1.5-0.25i 1.5-0.142i 1.5-0.083i 1.5-0.083i 1.5-0.021i 1.5-0.009i 1.5-0.0066i 1.5-0.0030i 1.5-0.0040i 1.5-0.0033i
42 24 14 13 10 11 21 17 36 51
882
H. HORVATH
a sea of transparent material, with the islands covering 68% of the area. Transferring this to the aerosolphase, it means that small amounts (1%) of carbon dispersed in a transparent material can absorb just as much as considerable amounts (68 %) of carbon mixed externally. The present author is aware that this reasoning contains many oversimplifications, since the mixing rule cannot be applied (see Chylek et al., 1988; Lindberg and Gillespie, 1977a), neither will atmospheric particles be a homogeneous mixture, nor is a slab an aerosol particle, but results by Fuller (1994) for carbon particles included eccentrically in sulfate show similar results. So the reasoning appears to be correct and it appears that particles above a size of a few micrometers may be a mixture of carbon and transparent materials produced either by aging in the atmosphere or coming from a different source. This is further supported by the fact that diesel emissions have marginal contributions both to mass and light absorption for particles larger than a few micrometers. The data on the effective refractive index which are shown in Table 2 can also be compared with the early work by Lindberg and Gillespie (1977a). They give imaginary parts of the refractive index for the desert aerosol in central New Mexico. It has values of - 0 . 1 1 at the smallest measured particle size of 0.43#m, - 0 . 0 1 4 at particles with diameters of 0.65 #m, decreasing to 0.002 at 11/~m. In the submicron range this value is about 10 times smaller than the value obtained from European samples (obviously caused by the many more sources in Europe) but comparable for particle diameters above a few micrometers. Lindberg and Gillespie (1977b) give the imaginary part of the apparent refractive index averaged over all particle sizes as - 0 . 0 0 8 for the New Mexico Desert and - 0.05 for central Europe, whereas H/inel (1989) gives values of - 0.018 for the summer values and - 0 . 0 3 8 for the winter values at Frankfurt, Germany. So one can see that a considerable variation of the data exists and clarification would be desirable. The size segregated determination of the light absorption coefficients will be a help. It will be possible to derive size distribution of the black carbon aerosol from the measured size-dependent light absorption coefficient. In that case the refractive index and the density of the black carbon particles have to be known. Most likely this will be difficult to find. On the other hand the present data suggest that empirical calibrations will permit one to do this, because although the aerosol considerably varies in quantity its composition seems to be stable, at least for the locations studied here.
SUMMARY A N D
CONCLUSION
With an impactor producing a homogeneous deposit a simultaneous determination of the mass size distribution and the size-dependent light absorption
coefficient is possible. Calibrations with black carbon aerosol have shown the feasibility of this method. Samples of the atmospheric aerosol in different locations of Europe have shown that the light absorption of the aerosol is always highest for particle sizes between 0.1 and 0.2 #m. Only in the humid environment it had a slightly larger peak size, whereas the size of the nonabsorbing particles increased considerably. The mass absorption coefficient of the aerosol at all four locations was very similar, and completely different from values which could be expected from models using a single value for an effective refractive index. Acknowledgements--Part of the measurements were supported by grants of the Fonds zur F6rderung der wissenschaftlichen Forschung in Osterreich, European Community, Bundesministerium fiir Wissenschaft und Forschung, Osterreichische Akademie der Wissenschaften, Wien, and Accademia dei Lincei, Roma.
REFERENCES
Berner A. (1984)Design principle of the AERASlow pressure impactor. In Aerosols: Science, Technology and Industrial Applications of Airborne Particles (edited by Liu B. Y. H., Pui D. Y. H. and Fissan H. J.) p. 20.[Elsevier, New York. Bobren C. F. and Huffman D. R. (1983) Absorption and Scattering of Light by Small Particles. WileyInterscience, New York. Clarke A. D. (1982) Effects of filter internal reflectioncoefficient on light absorption measurementsusing the integrating plate method. Appl. Optics 21, 3011-3020. Chylek P., Srivastava V., Pinnick R. G. and Wang R. T. (1988) Scattering of electromagnetic waves by composite spherical particle: experiments and effective medium approximations. Appl. Optics 27, 3011-3020. Fischer K. (1973) Mass absorption coefficients of natural aerosol particles in the 0.4 to 2.5/zm wavelengthinterval. Contr. atmos. Phys. 46, 89. Fuller K. A. (1994) Absorption cross section of aerosol particles. Paper presented at the Aerosols and Atmospheric Optics Conference, Snowbird, Utah, 26-30 September 1994; sponsored by AGU and AWMA. Gerber H. E. (1982) Absorption of light by atmospheric aerosol particles. Review of instrumentation and measurements. In Light Absorption by Aerosol Particles (edited by Gerber H. E. and Handyman E. E.). Spectrum Press, Hampton, Virginia H~inel G. (1989) Single scattering albedo, asymmetry parameter, apparent refractive index and apparent soot content of dry atmospheric particles. Appl. Optics 27, 2287-2295. H~inelG. and Dlugi R. (1977) Approximation of the absorption coefficientof airborne atmospheric aerosol particles in terms of measurable bulk properties. Tellus 29, 75-82. Horvath H. (1993) Atmospheric light absorption. Atmospheric Environment 27A, 293-317. Horvath H. (1993) Comparison of measurements of aerosol optical absorption by filter collection and a transmissometric method. Atmospheric Environment 27A, 319-326. Horvath H. and Habenreich T. A. H. (1989) The absorption coefficient of the Vienna aerosol: comparison of two methods. Aerosol Sci. Technol. 10, 506-514. Horvath H. and Metzig G. (1990) Experimental determination of the accuracy of light absorption measurements with the integration plate method. J. Aerosol Sci. 21, 525-528.
Size segregated light absorption coefficient of the atmospheric aerosol Horvath H., J/iger J., Kreiner I. and Norek C. (1986) Determination of tile size dependent light absorption coefficient of aerosols. J. Aerosol Sci. 17, 258-260. Klaus N. and Berner A. (1985) Ein Kaskadenimpaktor mit rotierenden Stauplatten. Staub Reinhalt. Luft 45, 168-170. Kortiim G, F. (1969) Reflexionsspektrometrie. Springer, Berlin. Kreiner I. (1985) Vergleicbende Massengrfl]enverteilungen, Massenkonzentrationen, Absorptionskoeffizienten sowie des Beitrages yon graphitischen Kohlenstoff zum atmosph~irischen Aerosol, 160 pp. Ph.D. dissertation, University of Vienna. Lin C. I., Baker M. and Charlson R.J. (1973) Absorption coefficient of atmospheric aerosols: a method of measurement. Appl. Optics ][2 1356-1363. Lindberg J. D. and Gillespie J. B. (1977a) Relationship between particle size and imaginary refractive index in atmospheric dust. Appl. Optics 16, 2628-2630. Lindberg J. D. and Gillespie J. B. (1977b) Kubelka Munk optical properties of a barium white reflectance standard: a comment. Appl. Optics 16, 2627-2628.
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Lodge J. P. and Chan T. L., eds (1986) Cascade lmpactor Sampling and Data Analysis. American Industrial Hygiene Association, Akron, Ohio. McMurry P. H. and Zhang Q. C. (1989) Size distributions of ambient organic and elemental carbon. Aerosol Sci. Technol. 10, 430-437. Marple V. A., Liu B. Y. H. and Kuhlmey G. A. (1981) A uniform deposit impactor. J. Aerosol Sci. 11, 333-337. Mie G, (1908) Beitr/ige zur Optik triiber Medien, speziell kolloidaler Metallrsungen. Ann. Phys. 25, 377-455. A modern treatment of the subject can be found, e.g. in Bohren C. F. and Huffman D. R. (1983) Absorption and Scattering of Light by Small Particles. Wiley Interscience, New York. Norek C. (1985) MassengrrBenverteilung der yon Dieselmotoren emittierten Partikel sowie Bestimmung des Dieselemissionsanteiles am atmosph/irischen Aerosol in Wien mittels eines aktivierbaren Tracers, 176 pp. Ph.D. dissertation, University of Vienna. Patterson E. M., Sheldon C. E. and Stockton B. H. (1977) Kubelka Munk optical properties of a barium sulfate white reflectance standard. Appl. Optics 16, 729-732.
Atmospheric Environment Vol. 29, No. 8, pp. 875-883, 1995 Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights r~erved 1352-2310/95 $9.50 + 0.00
1352-2310(95) 00025-9
SIZE SEGREGATED LIGHT ABSORPTION COEFFICIENT OF THE ATMOSPHERIC AEROSOL H.
HORVATH
Institute of Experimental Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria (First received 2 September 1994 and in final form 7 December 1994) Abstract--The light absorption coefficient of atmospheric aerosols in the visible can be determined by depositing the particles on a filter and measuring its "transmission" in a special optical arrangement. With an impactor with rotating impaction plates producing a homogeneous deposit, it is possible to extend this technique to size segregated aerosol samples. A simultaneous determination of the mass size distribution is possible. Test measurements with black carbon aerosol have shown the feasibility of this method. Samples of the atmosplaeric aerosol have been taken in and near Vienna, in Naples and near Bologna. The light absorption of the aerosol is always highest for particle diameters between 0.1 and 0.2 #m. Only in the humid environment of the Po valley it had a slightly larger peak size, whereas the size of the nonabsorbing particles increased con,;iderably. The light absorption of the atmospheric aerosol is always higher in an urban environment. 'The mass absorption coefficient of the aerosol at all four locations was very similar, and completely dilferent from values which could be. expected using effective refractive indices which are frequently used in models. Using the data measured in this work two alternate models for the effective refractive index and black carbon content of the aerosol are suggested: (a) a size-dependent refractive index, where the imaginary part varies from - 0.25 for particles smaller than 30 nm to - 0.003 for particles larger than 2/zm; this could especially be applied if an internal mixing of the aerosol is to be expected, or (2) a size-dependen~Lfraction of elemental carbon in the case of external mixing with 43% of carbon particles for sizes below 30 nm decreasing to 10% for sizes up to 0.4 #m.
INTRODUCTION The atmospheric aerosol particles consist of a mix of many substances. Although they are a minority component in the atmosphere, contributing to about 10-100 ppb by mass, they are the most important constituent of the atmosphere when dealing with the optical properties in the visible. The aerosol is responsible for 6 0 - 9 5 % of the visibility reduction, many atmospheric optical effects are dominated by the aerosol, and the aerosol is involved in the radiation balance of the globe, and thus climatic change considerations also have an aerosol aspect. The attenuation of light in the atmosphere is due to elastic scattering (the light is deflected by aerosol particles and molecules, but it is conserved) and absorption (the energy of the light is converted into internal energy of :the molecules and eventually to heat). Whereas all particles scatter light, only particles consisting of substances for which the complex refractive index has a nonzero imaginary part also show light absorption. A survey by Bohren and Huffman (1983, Fig. 14.1) of substances which could be found in the atmosphere poiints out that in the visible only elemental carbon and hematite (ct-Fe2Oa) are light absorbing: between 0.4 and 0.8 #m wavelength only carbon and hematiite have imaginary parts of the
refractive index which is larger than 0.5. For all other substances the imaginary part is < 10- 5. So these two substances can be considered as the major contributors. Except for iron ore mining areas hematite seems to be a very rare substance in the atmosphere. Recent measurements in Vienna and upwind V i e n n a have shown that iron contributes to less than 200 ng m - 3, whereas elemental carbon has a 20 times higher concentration, so even if all iron were ~t-Fe203 it would contribute to 7% of the light absorption which can be considered as the upper limit of contributions to light absorption by substances other than black carbon. Thus black carbon appears to be the only light absorbing substance in the atmosphere. If gases are considered there are ozone and nitrogen dioxide, but both have absorption coefficients which are at least one-tenth of the light absorption by black carbon. There might be substances in the atmosphere where the bulk material appears to be completely black, but still their light absorption as aerosol particles is negligible. This can be seen in a very simple way: the imaginary part k of the refractive index is related to the absorption coefficient ~t of the bulk material by ct = 4nk/;t. A substance which has a transmission of 10-10 in a 1 cm layer at 550 n m has an ~t = 2072 and definitely will be considered as black. Using the above relation-
875 AE 29:8-B
876
H. HORVATH
ship, k is obtained as k = - 0.000009. Materials with this imaginary part of the refractive index can be considered as nonabsorbing aerosol particles. This can also be seen by a simple calculation. A slab of 1 pm thickness of the above-mentioned substance has a transmission of (10-lo) I p m / l cm = 0.998 which is completely transparent. O n the other hand elemental carbon (with an imaginary part of the refractive index k = 1.0) has a bulk absorption coefficient of 0t = 2.28 x 10'; therefore the transmission of a 1 #m slab will be 1.19 x 10-10. The interested reader is also referred to Bohren and Huffman (1983, pp. 279, 280) where a paragraph with the title "A dirty silicate story" is devoted to a piece of hornblende, which was as black as coal, but still has no light absorption when assuming the material to be an aerosol particle. Elemental, graphitic or black carbon is produced by every combustion process, is the main constituent of soot and can be found everywhere on the globe. With respect to their mass, particles containing black carbon are very efficient in their interaction with light, and in many parts of the world black carbon is the second important substance contributing to the extinction budget. The light attenuation by black carbon is mainly due to absorption, whereas all other aerosol particles scatter the light. The light scattering of the global aerosol reduces the input of solar energy, and thus has a cooling effect on the climate, the light absorption of the soot particles transfer solar energy to the atmosphere and thus has a heating effect. The magnitude of these effects d e p e n d s - - a m o n g o t h e r s - - o n the size of the particles. The effectiveness of the light absorption by carbon can be enhanced considerably when small carbon particles (e.g. 50 nm in diameter) are included in a larger transparent particle. The combustion produced aerosol particles usually have sizes in the range of 30 nm to 1 #m with a peak near 0.1 #m. During their "life" in the atmosphere the particles can undergo various transitions, e.g. participating in the formation of mixed particles. During this obviously the particle size changes, which means that the particles have other optical properties. Therefore information on the particle size is desirable. Size segregated samples of aerosol particles can be obtained with cascade impactors; numerous designs exist, see, e.g. Lodge and Chan (1986) for an overview. The light absorption of the black carbon can be used to determine the size distribution of these particles, when being deposited in a special cascade impactor which produces a homogeneous deposit. The determination of light absorption on a filter is a standard technology by now; a very simple one is the integrating plate method ( L i n e t al., 1973). This method has been adapted and has been tested previously with success for laboratory generated aerosols (Horvath et al., 1986). Results obtained with an extended measuring range of the impactor and an improved measuring technique for atmospheric samples are presented here.
EXPERIMENTAL As already mentioned, in the visible the only light absorbing substance in the atmosphere is black carbon. Thus a method to measure the light absorption of a sample will also. permit the determination of the elemental carbon. There are more than a dozen methods to determine the light absorption coefficient of an aerosol sample; for an overview see Horvath (1993). Most of them require a homogeneous deposit on a flat substrate. In general this is done by sampiing the particles on a filter, such as Nuclepore or a fiber filter. Using one of the common methods to measure the light absorption of the sample obviously determines the value of all the particles deposited on the filter. Cascade impactors use inertial separation to obtain size fractionated deposits of aerosol samples. An impactor consists of a series of plates having small holes, through which the air containing the suspended particles is sucked. In the holes a jdt is formed, which is deflected by a plate positioned just behind the plate with the orifices. Those particles which have high inertia cannot follow the deflection of the air molecules and thus will hit the impaction plate and will be deposited there. Usually the impaction plate is covered by a foil or a filter, which is removed after a measurement has been performed and the particles deposited on it can be weighed or analyzed chemically. Particles having diameters larger than the cutoff diameter of the arrangement will be removed from the air stream. The cutoff diameter depends on the velocity of the air in the jet, the distance of the impaction plate and the gas pressure. By putting several impaction stages behind each other with decreasing diameters of the jet orifices the deposits on the impaction plates contain particles with sizes within a diameter interval de,ermined by the cutoff diameter of the stage (smallest size' and the previous stage (largest size). For this work mainly a 10stage low-pressure impactor (Berner, 1984) has been used. The interval of the diameters of the deposited particles always was a factor of 2, the smallest diameter collected was 15 nm, and the largest was 16 #m. A backup filter has not been used, since very few particles have sizes below 15 nm. For some measurements we have used an earlier design impactor with seven stages, depositing particles between 0.1 and 16 #m. In that case a backup filter was used. Normal cascade impactors form a dot-like deposit under each hole. This is sufficient for gravimetric and some kinds of chemical analysis, but unsuitable for XRF, PIXE and light absorption analyses. Therefore impactors with rotating deposition plates have been developed. The deposit of each hole then has the shape of a circle and by proper arrangement of the holes a quasi-homogeneous deposit on an annular ring is formed (Marple et al., 1981, Klaus and Berner, 1985; Horvath et al., 1986). With this deposit one of the light absorption techniques can be used. The flow rate through the impactor was 1.5 m 3 h- 1 and the homogeneous deposit was an annular ring with 58 mm outer and 42 nun inner diameter. It should be mentioned that light absorption measurements are also possible using the Kubelka Munk method (see e.g. Kort/im, 1969). In order to use this method a sample of the absorbing particles is needed which is large enough to mix it with white barium sulfate powder. The change in reflectivity can be used to deduce the light absorbing properties of the sample. This method has been applied by Lindberg and Gillespie (1977a). The only drawback of this method is the need for a fairly large sample. Four impactors were operated in parallel for several months in order to have sufficient material. We have applied the integrating plate technique to infer the size segregated light absorption coefficient aa of the atmospheric aerosol at a wavelength of 550 nm. For this purpose a deposit is formed on a Nuclepore filter. The integrating plate method (Linet al., 1973) is a simplification of the integrating sphere method developed by Fischer
Size segregated light absorption coefficient of the atmospheric aerosol (1973). In principle the "blackness" of a filter sample collected on a Nuclepore tilter is determined by measuring the transmittance T of the opaque filter with and without particle load. A special optical arrangement which integrates the light scattered by the particles and the filter (integrating sphere or integrating opal plate) takes care that only the light which is absorbed by the particles is missing from the transmitted light. The light absorption coefficient is determined using the following assumptions: (1) The aerosol particles deposited on the filter are contained in a column, the height x of which is determined by the sample volume V and the area of the deposit A by x = V/A. (2) With respect to light absorption the particle.,; deposited on the filter have the same optical properties as in the airborne state. (3) Any light scattered by the particles is integrated by the optical arrangement. Then the absorption coefficient aa is obtained by tr, = l / x In T. The method has some particularities and we have used an experimental calibration (Horvath and I-Iabenreich, 1989; Horvath and Metzig, 1990; Horvath, 1991) to derive the light absorption coefficient. The results of these calibrations can be summarized in the following way. For strongly light absorbing aerosol particles such as elemental carbon produced in a spark generator the values given by the integrating plate method agree with the values of absorption coefficients obtained from an absolute method. If the aerosol also contains nonabsorbing particles (either internally or externally mixed) the integrating plate method gives too high light absorption coefficients. The deviation is extremely large if the light absorption only gives a minor contribution to the extinction (e.g. wrong by a factor of 3 for aerosols where the absorption only contributes 5% to the extinction). For the atmospheric aerosol studied in this work the integrating plate method gives values too high by a factor of 30-40%. This agrees with estimates, e,g. given by Clarke (1982). But there is no need to form the deposit on a Nuclepore filter, since the filtering capabilities are not needed in the impactor. In principle any opaque material would serve the purpose. The Nuclepore filters have the advantage of being very homogeneous with respect to their optical properties. It is quite interesting to have the mass size distribution of the aerosol simultanecusly with the size distribution of the light absorbing ~erosel. Since only a small fraction (about 0.4 cm 2) of the total deposit (having an area of 12 cm 2) is needed for the determination of the light absorption coefficient, the remaining deposit can be used for other purposes. The following procedures have been used successfully to determine the mass size distribution and the size distribution of the light absorbing aerosol simultaneously: (1) A small pie-shaped piece having an angle of ~ 5° is cut off the foil which rests on the impaction plate. Underneath the foil a transparent paper is positioned, which just receives a deposit in the area, where the foil is cut away. The loss of "transmission" due to ]Jght absorbing particles can be measured by determining the light passing through the paper at a spot where a deposit has been formed and at a location where no deposition took place due to the overlying foil. (2) A 25 mm diameter Nuclepore filter is fixed to the foil by a low adhesion tape. The foil (in our case household aluminium foil), with the filter sticking to it, is weighed before and after the sampling After the second mass determination the Nuclepore filter is removed and the "transmission" is measured in an area with and without any deposit. This method is more elaborate and requires some skill, but it is more sensitive for light absorption measurements since the Nuclepore filters are very homogeneous. Also it has the advantage that changes of the optical properties of the Nuclepore filter due to contact with the air to be analyzed are the same on the area with and without deposit. For the atmospheric samples reported below the sampling time was between 10 and 24 h.
877
TEST MEASUREMENTS The feasibility of the method has been tested by comparing it with already established methods. Optical measurements of the size segregated light absorption coefficient is a new method, so the following has been done: (1) compare an integral value of this method with a value obtained by another method and (2) use the values derived from theoretical considerations to examine the technique. Both are not a perfect proof for the reliability of the method, but still they are an indication. Both have been done. (1) For the integrating plate method calibrations exist (Horvath, 1993; Horvath and Habenreich, 1989; Horvath and Metzig, 1990), and it is a simple task to sample with the impactor and on a polycarbonate filter simultaneously, add the light absorption coefficients of all the stages of the impactor and compare it to the value obtained from the sample containing all particles on one Nuclepore filter. This has been checked for several laboratory generated aerosols and agreement has been found. Obviously this just means that the use of the rotating impactor and the segregation of the sample in different size ranges does not introduce any additional errors and that the determination of the light absorption by this method is as good as the integrating plate method. (2) By Mie theory (Mie, 1908) it is possible to calculate the mass absorption coefficient of homogeneous spherical particles of any substance as a function of particle size. Input parameters are the wavelength (550 nm is used for all measurements and calculations), the refractive index of the bulk material and its density. Unfortunately neither refractive index nor density are well known for black carbon; thus one has to rely on educated guesses, which may differ by a factor of up to 2 (Horvath, 1993, Table 1). The soot particles consist of graphite which has a density of 2250 kg m - 3 and most likely a (complex) refractive index of m = 2 - i. The primary graphite particles are spherical with diameters around 25 nm and form fluffy and bizarre-shaped agglomerates. Consequently the average density and refractive index are less than those of the bulk material; the volume mixing rule is one of the frequently used ways to obtain a value for the effective refractive index of a mixture of substances, although it has no theoretical basis (Chylek et al., 1988), so it should only be considered as a guess. It also needs to be mentioned that soot particles produced by combustion processes may also contain unburnt fuel (mostly organic material) which is not light absorbing. For the calculations performed below, the following possibilities were considered: (1) pure graphite, which is unlikely; (2) graphite with about 50% empty space, having a density of 1 2 5 0 k g m -3 and a refractive index of m = 1.25 - 0 . 5 i ; and (3) graphite with 75% empty space having a density of 625 kg m - 3 and a refractive index of 1.25 - 0.25i.
878
H. HORVATH labsimpe
RESULTS: SIZE-DEPENDENT
LIGHT ABSORPTION
COEFFICIENT OF THE ATMOSPHERIC AEROSOL.
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Fig. 1. Mass absorption coefficient of various aerosols containing mainly black carbon. The lines give theoretical calculations for spherical particles (solid line: pure graphite; dashed line: graphite with 50% empty space; dotted line: graphite with 75% empty space). The data points give values obtained with various aerosols using the rotating cascade impactor. The squares are for a kerosene flame aerosol, the circles for emissions by a diesel motor, the diamonds for an aerosol obtained by spraying diluted India ink. The open symbols are for data points where either the mass and/or the light absorption coefficient were low; thus the data are insecure.
The ratio of the light absorption coefficient to the mass concentration of spherical soot particles has been calculated and is shown in Fig. 1 for the three above-mentioned assumptions as a function of particle size. Since a wide range of both densities and refractive indices has been considered, the reality can be assumed to lie in between. The light absorption of the particles is most effective in the size range from 0 to 0.2/~m, decreasing rapidly for larger particle sizes. The mass absorption coefficient is higher for the partitles containing more empty space due to the lower density. Using the rotating impactor this relationship has been checked. Several laboratory generated aerosols have been used for this purpose: aerosols produced by a kerosene flame, aerosols produced by a diesel engine and aerosols produced by atomizing diluted India ink. In all cases the mass absorption coefficient is in accordance with theory, so it can be assumed that this method gives reliable data. The data are closer to the curves represented by the aerosol particles containing empty space, which is realistic due to the structure of the agglomerates. With this relationship it is also possible to determine the content of elemental carbon of impactor samples. There might be some difficulties with internally mixed aerosol particles especially when they are larger than 1/~m, since in that case already a small amount of elemental carbon gives considerable light absorption.
Vienna: Sampling took place at the bottom part of the roof of the Stephansdom, the cathedral of Vienna. The impactor was located at an elevation of about 30 m above ground in June/July 1994. The weather could be characterized as fair and dry with mostly clear skies. Rural Vienna: Sampling took place at a small village (Breitenfurt) upwind of Vienna, ~ 20 km west of the town in June/July 1994. The inlet was located at an elevation of 6 m above ground. The measurements were taken at the same time as the ones in Vienna. Rural Bologna: Located in the Po valley near San Pietro de Capofiume, ~ 20 km north of Bologna. The sampling took place at a meteorological field. Samples were taken in December 1991. The Po valley has considerable air pollution. The climate in December is very humid, with many fogs occurring in the morning and in the afternoon. During the time the samples were taken it was fair weather with clear sky, but during the night fog occurred. Naples: Sampling took place in June 1992 at an elevation of ~ 15 m above ground on the roof of the Science building of the University of Naples; during the measuring period the weather was fair, with medium to high humidity and occasional rain. Sampling was stopped in the case of rain. All these samples were taken with a 10-stage rotating cascade impactor, where part of the deposit was used for light absorption analysis. For the 1991 and 1992 data light absorption measurements were done on transluscent paper, and due to inhomogeneities of the paper the sensitivity was not sufficient to measure light absorption coefficients on the stages having only a small change in transmission due to a black carbon deposit. Figure 2 a - d shows the average values of the mass size distribution, the light absorption coefficient and the mass absorption coefficient.
DISCUSSION
The mass size distribution (thin solid line) shows a peak of the accumulation mode around a diameter of 0.5/zm in rural and urban Vienna and in Naples. The Po valley data have a peak at ~ 1 pm which is most likely caused by the increase in particle size due to uptake of water at the high humidity present. The second peak at particle sizes above 5/tm is due to resuspension. It is missing at the rural Bologna data, since the soil was sufficiently humid due to dew and deposition by the fog droplets that a resuspension could not take place.
Size segregated light absorption coefficient of the atmospheric aerosol Vienna, June 1994 h I.fl
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Rural near Vienna, June 1994
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Fig. 2. Mass size distribution (thin solid line), size-dependent light absorption coefficient (thick solid line) and mass absorption coefficient (dotted line) for the four measuring campaigns. Mass and light absorption coefficient are given per stage of the impactor in the following units: mass concentration #g m-3, light absorption coefficient M m- 1, mass absorption coefficient m 2 g-1. (a) Rural near-Vienna, June 1994; (b) Vienna, June 1994; (c) rural near-Bologna, December 1991; (d) Naples, June 1992.
The light absorption is most important at sizes between 0.1 and 0.2/~m, and a maximum light absorption is found in this size range in all the data sets. The highest light absorption coefficients have been found in Naples and rural Bologna, the Vienna values are approximately half, rural Vienna again is a factor of 2 less. There is an indication of increased light absorption at the last stage (15-30 nm diameter) at least for the Vienna data set. :Light absorbing aerosol particles this small occurred irregularly. The dark deposit was clearly visible on the foils/filters. It also occurred occassionally at the other locations. Particles in this size range are the primary graphitic particles produced, e.g. in a diesel engine. Most of the particles
coagulate, but some may be left over. Obviously their occurrence is an indication for a source dominated aerosol. Size distributions of elemental carbon measured at the desert regions of the SW of the United States are quite similar in shape but the light absorption coefficient is a factor of 7 lower than that for the rural near-Vienna data (McMurry and Zhang, 1989). The size distribution of the light absorption coefficient of the atmospheric aerosol can be compared with the size segregated light absorption coefficient of diesel particles, which are the major contributors of light absorption in an urban environment. Figure 3 shows the average of all atmospheric measurements
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Fig. 3. Average size distribution of the light absorption coefficient of the atmospheric aerosol (solid line), of diesel particles emitted during a standard driving cycle (dashed line) and diesel particles emitted by a diesel motor under low load conditions (dash-dot line).
(solid line, it has been normalized to 1 in the size range of 0.1-0.2/~m) and two size distributions of diesel emissions. The average size distribution of the light absorption of the diesel particles emitted during several driving cycles has been obtained from measured mass size distribution (Kreiner, 1985; Norek, 1985). In comparison to the atmospheric aerosol it contains smaller particles and especially particles above 2 #m are almost lacking. In addition emissions of a warmed-up diesel engine in idle and low load have also been measured. These emissions may be more representative for nowadays stop-and-go traffic in congested towns. It has a size distribution shifted towards larger particles (dash-dotted line in Fig. 3) and it very much resembles the average size distribution of the light absorption coefficient of the atmospheric aerosol, although the particles are somewhat larger. Emissions from a car moving at high speed for extended time periods and those from a car mainly in idle or low load are the two extremes; the size distribution of the source for the majority of the light absorbing particles will lie somewhere in between, just as that of the atmospheric aerosol, as can be seen in Fig. 3. But it should also be noted that for the smallest measured particle range of 15-30 nm the light absorption coefficient of the atmosphere is higher than expected from diesel emissions. Also for sizes above 4 #m the light absorption of the atmosphere is higher than expected from diesel emissions. This could be caused by mixed particles containing soot as a minority component. Particles of this kind can be resuspended particles which mainly contain nonabsorbing substances with a small fraction of soot, as is usually the case in road dust. By incorporating soot in particles consisting of transparent material the absorbing
properties of elemental carbon are greatly enhanced. Mixed particles of this size could also be formed by in-cloud transformation. The ratio of the light absorption coefficient to the mass concentration of the aerosol (mass absorption coefficient) is not too different for the four locations as can be seen in Fig. 4. So it appears that the aerosol at all four locations is very similar with respect to its relation between light absorbing and nonabsorbing parts. The Vienna aerosols show a very high mass absorption coefficient at the 15-30 nm range which is close to the one of pure carbon, suggesting a source dominated aerosol. This is not the case for the rural near-Vienna aerosol which on the other hand has a higher mass absorption (but lower absorption) coefficient than Vienna between diameters of 30-400 nm. The rural aerosol near Bologna has low values in mass absorption coefficient between 0.4 and 1.6/~m diameter, since at the high humidity the majority of the aerosol particles increased in size more than the carbon particles. For modeling of the atmosphere with respect to its radiative properties it is desirable to have some optical input parameters which describe the aerosol well. It has become a habit to use an apparent or effective refractive index, which can be used to calculate the scattering and absorption coefficient of the aerosol in various scenarios of the model. It should be pointed out that the effective refractive index is a hypothetical value which is used in calculations and it is not expected that the substance the aerosol particles are made of has this refractive index, but that the use of this hypothetical refractive index will give the right values, e.g. for mass absorption coefficient. H/inel (1989) calls this refractive index "apparent but not necessary real". Examples for this are, e.g. Fischer (1973), Patterson et al. (1977), H/inel and Dlugi (1977), Gerber (1982) and H/inel (1989). For the European strongly light absorbing aerosol an imaginary part of the refractive index of - 0.02 to - 0.1 is used. For comparison the calculated values for mass absorption coefficients for a pure carbon aerosol and aerosols having an effective refractive index of 1.5 - 0.1i and 1.5 - 0.05i are shown. One can see that the use of one effective refractive index will not adequately characterize the aerosol with respect to its absorptive properties. For all the model values of the complex refractive index the mass absorption coefficient should increase up to a diameter of ~ 0.3/~m and then decrease for larger diameters, whereas the measured mass absorption coefficient decreases steadfly with size. Maybe the rural near-Vienna data have some indication of such a behavior for sizes below 0.2/~m. Anyhow, using a measured mass size distribution and using a single value of the effective refractive index will not give an adequate value of the -light absorption of the aerosol. There are several possible explanations for this discrepancy. The aerosol consists of a black carbon component which has its origin in combustion processes
Size Segregated light absorption coefficient of the atmospheric aerosol (mainly diesel engim;s) and the size distribution has a peak at about 0.1 urn. The majority of the aerosol particles are made up of transparent particles which are larger in size, peaking at 0.3-0.6 #m. With these two different size distributions for the absorbing and the nonabsorbing aerosol a stronger decrease in mass absorption coefficient with increasing size than for pure carbon can be explained. The carbon particles could also be thought of as attached to some but not all of the transparent particles, which will give a very similar result. This will also explain the larger size of the light absorbing aerosol particle in the Po valley. But it is not feasible to assume that the aerosol particles consist of the same homogeneous mixture of light absorbing and transparent substance for all sizes. A solution to this can be achieved by either using a size-dependent effective refractive index, as has been
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Fig. 4. Mass absorption coefficient of the aerosol samples at the four locations. The thick solid and dashed step curves are the data of Vienna and rural near-Vienna, the thin solid and dashed step curves are Naples and rural near-Bologna. The dash-dot curve is the calculated relationship for graphite containing 50% empty space, the upper solid curve is for an aerosol with a refractiw~ index of 1.5 - 0. li and the lower one for a refractive index of 1.5 - 0.05i, both with a density of 1500 kgm -3 .
881
already done by Lindberg and Gillespie (1977a), or subdividing the aerosol in a light absorbing fraction and a nonlight absorbing fraction. Both methods have their merits. If one can be sure that the light absorbing particles mainly consist of mixed particles, then an effective refractive index is most adequate, if the carbon particles and transparent particles exist separately in the atmosphere then the fractional approach is more adequate. The effective refractive index has been calculated for the average of the data of the atmospheric aerosol in the different size classes which were measured by the impactor. A density of the particles of 1500 kg m - 3 and a real part of the refractive index of 1.5 was assumed. As an alternative the mass fraction of elemental carbon in the different size classes was calculated, using a density of 1250 k g m -3 and a refractive index of m = 1.25 - 0.5i for elemental carbon particles. The results are presented in Table 1. Several conclusions can be drawn from this. Both the imaginary part of the refractive index and the mass fraction of elemental carbon are high for particles below 60 nm, i.e. black carbon is of considerable importance. For sizes up to 0.7 #m mean diameter, the contribution by elemental carbon is less, which can also be seen by considering the effective refractive index, where the imaginary part goes down to - 0 . 0 2 1 . Particles between 0.04 and 0.7/~m mean diameter can be considered as having their origin in the combustion process of diesel engines. For particles in the 15-30 nm range their origin is still unclear. For particles above a size of 2 #m the mass fraction of black carbon increases again, although the effective refractive index is small. This seems to be a paradox, but it can be explained. The light attenuation in carbon is so strong that with the previously determined absorption coefficient of the bulk elemental carbon of = 2.28 x 107 a slab of 5 / t m thickness has a transmission of 3 x 10- 50, i.e. all light is absorbed. If we now assume that the material contains only 1% of carbon finely dispersed in a transparent material we obtain c< = (1/100)2.28 x 107, which gives a transmission of 0.32, i.e. 68% of the light is adsorbed. This amount of absorption would be equivalent to the absorption of a 5/~m slab having "islands" of elemental carbon in
Table 1. Optical characteristics of the aerosol size fractions Impactor stage no. 1 2 3 4 5 6 7 8 9 10
Size range 15-30 nm 30-60 nm 60-120 nm 120-250 nm 0.25-0.5/~m 0.5-1/~m 1-2 #m 2-4 #m 4-8 #m 8-16/~m
Mean size
Effective refractive index
Mass fraction of carbon (%)
21.2 nm 42.4 nm 85 nm 176 nm 0.35 #m 0.71 #m 1.41 #m 2.83 ~m 5.66 #m 11.3 pm
1.5-0.25i 1.5-0.142i 1.5-0.083i 1.5-0.083i 1.5-0.021i 1.5-0.009i 1.5-0.0066i 1.5-0.0030i 1.5-0.0040i 1.5-0.0033i
42 24 14 13 10 11 21 17 36 51
882
H. HORVATH
a sea of transparent material, with the islands covering 68% of the area. Transferring this to the aerosolphase, it means that small amounts (1%) of carbon dispersed in a transparent material can absorb just as much as considerable amounts (68 %) of carbon mixed externally. The present author is aware that this reasoning contains many oversimplifications, since the mixing rule cannot be applied (see Chylek et al., 1988; Lindberg and Gillespie, 1977a), neither will atmospheric particles be a homogeneous mixture, nor is a slab an aerosol particle, but results by Fuller (1994) for carbon particles included eccentrically in sulfate show similar results. So the reasoning appears to be correct and it appears that particles above a size of a few micrometers may be a mixture of carbon and transparent materials produced either by aging in the atmosphere or coming from a different source. This is further supported by the fact that diesel emissions have marginal contributions both to mass and light absorption for particles larger than a few micrometers. The data on the effective refractive index which are shown in Table 2 can also be compared with the early work by Lindberg and Gillespie (1977a). They give imaginary parts of the refractive index for the desert aerosol in central New Mexico. It has values of - 0 . 1 1 at the smallest measured particle size of 0.43#m, - 0 . 0 1 4 at particles with diameters of 0.65 #m, decreasing to 0.002 at 11/~m. In the submicron range this value is about 10 times smaller than the value obtained from European samples (obviously caused by the many more sources in Europe) but comparable for particle diameters above a few micrometers. Lindberg and Gillespie (1977b) give the imaginary part of the apparent refractive index averaged over all particle sizes as - 0 . 0 0 8 for the New Mexico Desert and - 0.05 for central Europe, whereas H/inel (1989) gives values of - 0.018 for the summer values and - 0 . 0 3 8 for the winter values at Frankfurt, Germany. So one can see that a considerable variation of the data exists and clarification would be desirable. The size segregated determination of the light absorption coefficients will be a help. It will be possible to derive size distribution of the black carbon aerosol from the measured size-dependent light absorption coefficient. In that case the refractive index and the density of the black carbon particles have to be known. Most likely this will be difficult to find. On the other hand the present data suggest that empirical calibrations will permit one to do this, because although the aerosol considerably varies in quantity its composition seems to be stable, at least for the locations studied here.
SUMMARY A N D
CONCLUSION
With an impactor producing a homogeneous deposit a simultaneous determination of the mass size distribution and the size-dependent light absorption
coefficient is possible. Calibrations with black carbon aerosol have shown the feasibility of this method. Samples of the atmospheric aerosol in different locations of Europe have shown that the light absorption of the aerosol is always highest for particle sizes between 0.1 and 0.2 #m. Only in the humid environment it had a slightly larger peak size, whereas the size of the nonabsorbing particles increased considerably. The mass absorption coefficient of the aerosol at all four locations was very similar, and completely different from values which could be expected from models using a single value for an effective refractive index. Acknowledgements--Part of the measurements were supported by grants of the Fonds zur F6rderung der wissenschaftlichen Forschung in Osterreich, European Community, Bundesministerium fiir Wissenschaft und Forschung, Osterreichische Akademie der Wissenschaften, Wien, and Accademia dei Lincei, Roma.
REFERENCES
Berner A. (1984)Design principle of the AERASlow pressure impactor. In Aerosols: Science, Technology and Industrial Applications of Airborne Particles (edited by Liu B. Y. H., Pui D. Y. H. and Fissan H. J.) p. 20.[Elsevier, New York. Bobren C. F. and Huffman D. R. (1983) Absorption and Scattering of Light by Small Particles. WileyInterscience, New York. Clarke A. D. (1982) Effects of filter internal reflectioncoefficient on light absorption measurementsusing the integrating plate method. Appl. Optics 21, 3011-3020. Chylek P., Srivastava V., Pinnick R. G. and Wang R. T. (1988) Scattering of electromagnetic waves by composite spherical particle: experiments and effective medium approximations. Appl. Optics 27, 3011-3020. Fischer K. (1973) Mass absorption coefficients of natural aerosol particles in the 0.4 to 2.5/zm wavelengthinterval. Contr. atmos. Phys. 46, 89. Fuller K. A. (1994) Absorption cross section of aerosol particles. Paper presented at the Aerosols and Atmospheric Optics Conference, Snowbird, Utah, 26-30 September 1994; sponsored by AGU and AWMA. Gerber H. E. (1982) Absorption of light by atmospheric aerosol particles. Review of instrumentation and measurements. In Light Absorption by Aerosol Particles (edited by Gerber H. E. and Handyman E. E.). Spectrum Press, Hampton, Virginia H~inel G. (1989) Single scattering albedo, asymmetry parameter, apparent refractive index and apparent soot content of dry atmospheric particles. Appl. Optics 27, 2287-2295. H~inelG. and Dlugi R. (1977) Approximation of the absorption coefficientof airborne atmospheric aerosol particles in terms of measurable bulk properties. Tellus 29, 75-82. Horvath H. (1993) Atmospheric light absorption. Atmospheric Environment 27A, 293-317. Horvath H. (1993) Comparison of measurements of aerosol optical absorption by filter collection and a transmissometric method. Atmospheric Environment 27A, 319-326. Horvath H. and Habenreich T. A. H. (1989) The absorption coefficient of the Vienna aerosol: comparison of two methods. Aerosol Sci. Technol. 10, 506-514. Horvath H. and Metzig G. (1990) Experimental determination of the accuracy of light absorption measurements with the integration plate method. J. Aerosol Sci. 21, 525-528.
Size segregated light absorption coefficient of the atmospheric aerosol Horvath H., J/iger J., Kreiner I. and Norek C. (1986) Determination of tile size dependent light absorption coefficient of aerosols. J. Aerosol Sci. 17, 258-260. Klaus N. and Berner A. (1985) Ein Kaskadenimpaktor mit rotierenden Stauplatten. Staub Reinhalt. Luft 45, 168-170. Kortiim G, F. (1969) Reflexionsspektrometrie. Springer, Berlin. Kreiner I. (1985) Vergleicbende Massengrfl]enverteilungen, Massenkonzentrationen, Absorptionskoeffizienten sowie des Beitrages yon graphitischen Kohlenstoff zum atmosph~irischen Aerosol, 160 pp. Ph.D. dissertation, University of Vienna. Lin C. I., Baker M. and Charlson R.J. (1973) Absorption coefficient of atmospheric aerosols: a method of measurement. Appl. Optics ][2 1356-1363. Lindberg J. D. and Gillespie J. B. (1977a) Relationship between particle size and imaginary refractive index in atmospheric dust. Appl. Optics 16, 2628-2630. Lindberg J. D. and Gillespie J. B. (1977b) Kubelka Munk optical properties of a barium white reflectance standard: a comment. Appl. Optics 16, 2627-2628.
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Lodge J. P. and Chan T. L., eds (1986) Cascade lmpactor Sampling and Data Analysis. American Industrial Hygiene Association, Akron, Ohio. McMurry P. H. and Zhang Q. C. (1989) Size distributions of ambient organic and elemental carbon. Aerosol Sci. Technol. 10, 430-437. Marple V. A., Liu B. Y. H. and Kuhlmey G. A. (1981) A uniform deposit impactor. J. Aerosol Sci. 11, 333-337. Mie G, (1908) Beitr/ige zur Optik triiber Medien, speziell kolloidaler Metallrsungen. Ann. Phys. 25, 377-455. A modern treatment of the subject can be found, e.g. in Bohren C. F. and Huffman D. R. (1983) Absorption and Scattering of Light by Small Particles. Wiley Interscience, New York. Norek C. (1985) MassengrrBenverteilung der yon Dieselmotoren emittierten Partikel sowie Bestimmung des Dieselemissionsanteiles am atmosph/irischen Aerosol in Wien mittels eines aktivierbaren Tracers, 176 pp. Ph.D. dissertation, University of Vienna. Patterson E. M., Sheldon C. E. and Stockton B. H. (1977) Kubelka Munk optical properties of a barium sulfate white reflectance standard. Appl. Optics 16, 729-732.