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The influence of organic surface coatings on the optical properties of aerosols
J. Aerosol Sci., Vol. 26. Suppl 1, pp. $197~.~198, 1995 Elsevier Science Ltd Printed in Great Britaila 0021-8502/95 $9.50 + 0.00
Pergamon
THE INFLUENCE OF ORGANIC SURFACE COATINGS ON THE OPTICAL PROPERTIES OF AEROSOLS
I. Colbeck and B. Atkinson Institute for Environmental Research, University of Essex, Colchester, CO4 3SQ KEYWORDS scattering, optical properties
Owing to their interaction with visible and infrared radiation, aerosols play a significant role in the earth's energy budget and climate. Thus, the determination of the optical properties of the aerosol is a critical factor in assessing climate change. The optical properties are functions of composition of the aerosol, of the morphology of individual particles and of the size distribution of the aerosol. Atmospheric aerosols are rarely spherical in shape and may well be coated by water or organic films. Such a coating may not only alter the shape of the panicle but could cause an agglomerate to break up into its component spheroids. In both cases the optical properties would change. A transmissometer-integrating nephelometer was used to study the bulk optical properties of aerosols. Measurements were made of the scattered signal and optical depth in order to calculate specific scattering and absorption coefficients. Butane smoke was produced by the combustion of liquefied butane in air in a chamber (volume 0.6 m3) and the smoke then passed into the nephelometer. Measurements were taken as the aerosol concentration fell, typically over a period of several hours. Similar measurements were taken using ammonium sulphate aerosol generated from solution using an atomizer. Ammonium sulphate is used as a calibration because it is a non-absorbing aerosol for which the extinction is equal to the scattering. Plots of scattered signal versus extinction coefficient and of scattered signal versus mass concentration for butane smoke are shown in Figures 1 and 2. The calibration plot for ammonium sulphate is shown in Figure 3. Provided the aerosol concentration is not so high that multiple scattering takes place the scattered signal would be expected to be proportional to the extinction coefficient and this is exhibited by these plots. Comparison of the gradients of the plot for butane smoke and ammonium sulphate yield a single scattering albedo of approximately 0.29, comparable with previous results (Colbeck et al., 1989). Comparison with the mass concentration plot yields a specific extinction coefficient, Bo, of approximately 12 meg"1. This implies a specific absorption coefficient of approximately 8.5 m2g-1. Butane smoke was coated with hexane by passing the smoke through a 1 m long glass tube lined with needlefelt which was soaked in hexane. This method of coating the smoke aggregates was confirmed to be successful by microscopic examination which revealed coated aggregates to be far more densely clumped than uncoated aggregates. Figure 4 shows a plot of scattered signal against extinction coefficient for coated butane smoke. This demonstrates that coating the smoke causes the single scattering albedo to fall by about half to around 0.14. Earlier work has suggested that aerosols comprising a solid particle within a liquid droplet have enhanced scattering characterisitics (Chylek et al., 1984). AS 26/9 suPP 1-I
S197
$198
I. COLBECKand B. ATKINSON
References Chen Shinn-Hwa (1994) MSc Thesis, University of Essex. Johnston A.M., Vincent J.H. and JOnes A.D. (1987)Aerosol Sci. Technol., 6, 115-127. Liu B.Y.H., Piu D.Y.H. and Szymanski W. (1985) Ann. Occup. Hyg., 29, 251'269.
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Pergamon
THE INFLUENCE OF ORGANIC SURFACE COATINGS ON THE OPTICAL PROPERTIES OF AEROSOLS
I. Colbeck and B. Atkinson Institute for Environmental Research, University of Essex, Colchester, CO4 3SQ KEYWORDS scattering, optical properties
Owing to their interaction with visible and infrared radiation, aerosols play a significant role in the earth's energy budget and climate. Thus, the determination of the optical properties of the aerosol is a critical factor in assessing climate change. The optical properties are functions of composition of the aerosol, of the morphology of individual particles and of the size distribution of the aerosol. Atmospheric aerosols are rarely spherical in shape and may well be coated by water or organic films. Such a coating may not only alter the shape of the panicle but could cause an agglomerate to break up into its component spheroids. In both cases the optical properties would change. A transmissometer-integrating nephelometer was used to study the bulk optical properties of aerosols. Measurements were made of the scattered signal and optical depth in order to calculate specific scattering and absorption coefficients. Butane smoke was produced by the combustion of liquefied butane in air in a chamber (volume 0.6 m3) and the smoke then passed into the nephelometer. Measurements were taken as the aerosol concentration fell, typically over a period of several hours. Similar measurements were taken using ammonium sulphate aerosol generated from solution using an atomizer. Ammonium sulphate is used as a calibration because it is a non-absorbing aerosol for which the extinction is equal to the scattering. Plots of scattered signal versus extinction coefficient and of scattered signal versus mass concentration for butane smoke are shown in Figures 1 and 2. The calibration plot for ammonium sulphate is shown in Figure 3. Provided the aerosol concentration is not so high that multiple scattering takes place the scattered signal would be expected to be proportional to the extinction coefficient and this is exhibited by these plots. Comparison of the gradients of the plot for butane smoke and ammonium sulphate yield a single scattering albedo of approximately 0.29, comparable with previous results (Colbeck et al., 1989). Comparison with the mass concentration plot yields a specific extinction coefficient, Bo, of approximately 12 meg"1. This implies a specific absorption coefficient of approximately 8.5 m2g-1. Butane smoke was coated with hexane by passing the smoke through a 1 m long glass tube lined with needlefelt which was soaked in hexane. This method of coating the smoke aggregates was confirmed to be successful by microscopic examination which revealed coated aggregates to be far more densely clumped than uncoated aggregates. Figure 4 shows a plot of scattered signal against extinction coefficient for coated butane smoke. This demonstrates that coating the smoke causes the single scattering albedo to fall by about half to around 0.14. Earlier work has suggested that aerosols comprising a solid particle within a liquid droplet have enhanced scattering characterisitics (Chylek et al., 1984). AS 26/9 suPP 1-I
S197
$198
I. COLBECKand B. ATKINSON
References Chen Shinn-Hwa (1994) MSc Thesis, University of Essex. Johnston A.M., Vincent J.H. and JOnes A.D. (1987)Aerosol Sci. Technol., 6, 115-127. Liu B.Y.H., Piu D.Y.H. and Szymanski W. (1985) Ann. Occup. Hyg., 29, 251'269.
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