H. Horvath

UV-VIS-NIR spectral optical properties of soot and soot-containing aerosols

Abstract The UV-VIS-NIR spectral optical properties of soot and soot containing aerosols were investigated in detail during the AIDA Soot Aerosol Campaign 1999. One aim of the campaign was a comprehensive comparison of the microphysical properties of Diesel and spark generator soot. The mass specific extinction cross section at λ=450 nm of Diesel soot is 10.6±0.5 m 2 g −1 which is almost a factor of two larger than the corresponding value of 5.7±0.3 m 2 g −1 measured for spark generator soot. Coagulation-induced particle growth does not affect the soot extinction cross section and has a weak influence on the scattering properties of the soot aggregates. Atmospheric processing of freshly emitted soot was simulated in mixing experiments. The formation of mixed Diesel soot and dry ammonium sulfate particles by coagulation has only a minor effect on the soot absorption cross section. The coating of spark generated soot with organic material results in a strong increase of the single scattering albedo. A significant increase of the absorption coefficient at λ=473 nm during the coating process can be attributed to an enhancement of the specific soot absorption cross section by more than 30%.

The size distribution and composition of the atmospheric aerosol at a rural and nearby urban location

Abstract At a suburban location near Vienna and in the center of Vienna the aerosol was sampled with a 10 stage rotating cascade impactor permitting the classification of the aerosol particle sizes between aerodynamic diameters of 15 nm and 16 um. Both sampling sites were at an elevated location. Gravimetric, light absorption and PIXE analysis have been performed for all samples. The light absorption analysis also gives the concentration of black carbon, the PIXE analysis yielded concentrations of Si, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Br, and Pb. The mass size distribution of the aerosol for most of the elements had two modes. One peak occurred between 0.2 and 0.8 um, the coarse particles had a slight peak usually above 3 μm. On occasions an indication of a third peak below 50 nm could be observed. The total mass of the submicrometer particles was more than half of the total mass, except for calcium and iron. Little difference between the mass of the submicrometer particles at the two locations was found, except for Ca, Pb, and black carbon. The particles were generally larger at the suburban location. The light absorbing particles mostly had only one peak at a diameter of 0.2 um, thus being smaller than the other particles. Soot at the suburban location was a factor of 2 less and slightly larger than at the urban site. The similarity of the size distribution for almost all elements between the urban and the suburban location, as well as similarities to the arctic haze and the aerosol in Northern Europe suggests that the aerosol predominately originates from non-local sources and might be considered the central and north European average aerosol. Differences to aerosol characteristics at other locations exist. On a day-to-day basis the aerosol shows a very high variability.

Experimental calibration for aerosol light absorption measurements using the integrating plate method—Summary of the data

Abstract The determination of the light absorption coefficient by means of the integrating plate method is a very simple and reproducible procedure frequently used in various configurations. In this work it was calibrated with laboratory generated aerosol of different absorption and scattering properties generated from internal and external mixtures of carbon and a transparent substance. The light absorption coefficient was determined by an independent method which can be considered free of systematic errors. The light absorption coefficient determined by the integrating plate method was always higher than the value obtained by the reference method. It was strongly dependent on the ratio of light absorption coefficient to extinction coefficient of the aerosol. For weakly absorbing aerosols the integrating plate method gave excessively high values. For example, when absorption contributed 5% to the extinction, the absorption coefficient determined by the integrating plate method was too high by a factor of 2.8. Calibration factors for a variety of aerosols are given. The deviations can be explained by considering the transmission, scattering and absorption of light in the slab of deposited particles. Radiative transfer calculations show the same trend as the experimental values. It turns out that this deviation is inherent to all methods which determine the light absorption coefficient by transmission with simultaneous integration of the scattered light.

On the applicability of the koschmieder visibility formula

Abstract The inverse proportionality between the visibility and the extinction coefficient, first derived by Koschmieder, is only applicable under very limited conditions: the atmosphere must be illuminated homogeneously, the extinction coefficient and the scattering function are not allowed to vary with space, the object must be ideally black and be viewed against the horizon, and the eye of the observer must have a constant contrast threshold. A general formula, taking these facts in account has been derived and used to calculate possible errors which might arise if the simple Koschmieder Formula is used instead. The following results were obtained: inhomogeneous illumination generates errors smaller than 5 per cent. An inhomogeneous distribution only due to different dilutions of the aerosol gives no error if the average of the extinction coefficient is used. Using non black objects as visibility markers can give errors up to 50 per cent if they are illuminated by the sun, but the errors will be below 5 per cent if they are in their own shadow. The errors due to a varying contrast threshold of the eye can be considerable, if the visibility markers are too small. By proper selection of the visibility markers it will be possible to use the Koschmieder Formula to calculate the extinction coefficient from observed visibilities with an error of less than about 10 per cent.

Estimation of the average visibility in central Europe

Abstract Visibility has been obtained from spectral extinction coefficients measured with the University of Vienna Telephotometer or size distributions determined with an Aerosol Spectrometer. By measuring the extinction coefficient in different directions, possible influences of local sources could be determined easily. A region, undisturbed by local sources usually had a variation of extinction coefficient of less than 10% in different directions. Generally good visibility outside population centers in Europe is considered as 40–50 km. These values have been found independent of the location in central Europe, thus this represents the average European “clean” air. Under rare occasions (normally rapid change of air mass) the visibility can be 100–150 km. In towns, the visibility is a factor of approximately 2 lower. In comparison to this the visibility in remote regions of North and South America is larger by a factor of 2–4. Obviously the lower visibility in Europe is caused by its higher population density. Since the majority of visibility reducing particulate emissions come from small sources such as cars or heating, the emissions per unit area can be considered proportional to the population density. Using a simple box model and the visibility measured in central Europe and in Vienna, the difference in visibility inside and outside the town can be explained quantitatively. It thus is confirmed, that the generally low visibility in central Europe is a consequence of the emissions in connection with human activities and the low visibility (compared, e.g. to North or South America) in remote location such as the Alps is caused by the average European pollution.

Spectral extinction coefficients of rural aerosol in southern italy-a case study of cause and effect of variability of atmospheric aerosol

The spectral extinction coefficient of the atomospheric aerosol at a rural location in Southern Italy was determined by means of a telephotometer: the radiance of a target at a horizontal distance of 12 km was measured at nine different wavelengths in the visible. Therefore the extinction coefficient of the aerosol contained within a conical volume of about 450,000 m3 and a length of 12 km was measured. All measurements were performed in summer 1993 during a period of stagnant air, presumably always with the same air mass and thus a similar type of aerosol. The daily variation usually followed a similar pattern: decrease in extinction coefficients in the morning and early afternoon and an increase towards the evening. This variation correlated well with the change in humidity. The Angstrom exponent of the spectral extinction coefficient, was lowest at high humidity and highest at low humidity. Comparing the horizontal attenuation measurements and vertical transmission measurements done with a solar photometer, a considerable vertical extent of the aerosol was found. This is important for climate considerations, since additional light absorption and scattering by the aerosol can lead to an increase or a decrease of the temperature. Inversion of the spectral extinction coefficient data to obtain particle size distributions shows that the dry (< 50% r.h.) particles have a peak of the volume size distribution at 0.38 μm, which increases to 0.81 μm at 80% r.h. Light absorption by the aerosol varied little on a day to day basis, amounting to 20 to 40% of the light extiniction coefficient. Values this high are common in Europe, also outside densely populated areas. The average single scattering albedo of the dry aerosol was 0.76, thus the aerosol will have a heating effect due to its light absorption.

Diesel emissions in Vienna

Abstract The aerosol in a non-industrial town normally is dominated by emissions from vehicles. Whereas gasoline-powered cars normally only emit a small amount of particulates, the emission by diesel-powered cars is considerable. The aerosol particles produced by diesel engines consist of graphitic carbon (GC) with attached hydrocarbons (HCs) including also polyaromatic HCs. Therefore the diesel particles can be carcinogenic. Besides diesel vehicles, all other combustion processes are also a source for GC; thus source apportionment of diesel emissions to the GC in the town is difficult. A direct apportionment of diesel emissions has been made possible by marking all the diesel fuel used by the vehicles in Vienna by a normally not occurring and easily detectable substance. All emitted diesel particles thus were marked with the tracer and by analyzing the atmospheric samples for the marking substance we found that the mass concentrations of diesel particles in the atmosphere varied between 5 and 23 μg m−3. Busy streets and calm residential areas show less difference in mass concentration than expected. The deposition of diesel particles on the ground has been determined by collecting samples from the road surface. The concentration of the marking substance was below the detection limit before the marking period and a year after the period. During the period when marked diesel fuel was used, the concentrations of the diesel particles settling to the ground was 0.012–0.07 g g−1 of collected dust. A positive correlation between the diesel vehicle density and the sampled mass of diesel vehicles exists. In Vienna we have a background diesel particle concentration of 11 μg m−3. This value increases by 5.5 μg m−3 per 500 diesel vehicles h−1 passing near the sampling location. The mass fraction of diesel particles of the total aerosol mass varied between 12.2 and 33%; the higher values were found in more remote areas, since diesel particles apparently diffuse easily. Estimates of diesel particle concentration by emission inventory or by using lead concentrations as an indicator for vehicle emissions gave similar values to those obtained in this study. Using available cancer risk data and diesel particle concentration found in this study, 1–2.6 additional lung cancers per 100,000 persons yr−1 breathing diesel emissions in the measured concentration the whole lifetime can be expected.

The University of Vienna telephotometer

Abstract The measurement of the contrast reduction of distant targets is a useful tool in the determination of atmospheric visibility and atmospheric extinction coefficients. For this purpose an astronomic reflective telescope has been converted to a telephotometer. At the location of the real image of the target an adjustable stop with normal size of 1 mm is inserted, thus selecting a small portion of the target to be measured. The light emerging from the stop passes interchangeable interference filters and reaches a photodiode. The photocurrent is proportional to the luminance of the target. A plane parallel plate, which is inserted in the path of light, permits the measurement of the brightness of target and horizon repeatedly without realignment of the telescope. The extinction coefficient of the atmospheric aerosol can be calculated from the measured luminance of horizon, distant and close target and the distance of the target, using Middletons contrast reduction formula. Normally the intrinsic luminance of the target is determined from a similar target at short distance using calibration factors. The accuracy of the measured extinction coefficients depends on errors arising from the determination of the luminance of the distant target (apparent luminance), the inherent luminance, the distance of the target and from light scattered in the instrument. Model calculations have shown that the optimum distance of the target is 1 4 to 3 4 of the visibility. The light scattered in the instrument is less than 2 % at all wavelengths and thus can be neglected. Under conditions of inhomogeneous illumination (clouds) the extinction coefficient determined with the telephotometer can differ from the true value up to 30% with very specific cloud distributions in the sky. It can be shown that certain cloud distributions will give no error at all in the determination of the extinction coefficient, whereas for all possible cloud coverages the average of several measurements will have deviations from the true value of a few percent. Multiwavelength telephotometry also permits the calculation of the mass of the suspended particulates. Examples of measurements are presented.

A local optical closure experiment in Vienna

Abstract In a campaign in Vienna, the horizontal extinction over the central part of Vienna, the size dependent light absorption, and the mass size distribution have been measured and a size resolved PIXE analysis has been performed. PIXE did not give a complete chemical analysis; therefore the chemical compounds and the amount of nitrogen and organic carbon had to be guessed. The extinction coefficient has been determined by Mie calculation using the measured size distribution, density and refractive index from the chemical compounds. The closure was successful and a linear relationship between measured and calculated extinction coefficient was found, with the measured extinction coefficient about 20% higher than the calculated one. The Angstrom exponent was also compared, the averages agreed but the measured values were more variable than the calculated ones. A sensitivity analysis shows that the accuracy of the determination of the optical properties of the aerosol is governed by the accuracies of all input parameters and the generally unknown state of mixing. This leads to uncertainties of the optical parameters of at least ±20%. The accumulation mode makes a considerable contribution to the extinction coefficient; therefore knowledge of the size distribution, mass density and refractive index is critical. Lack of information in the coarse mode or the nucleation mode is less critical. Usually extinction coefficients are not measured on a routine basis, instead they are calculated using available aerosol data such as size distributions obtained from models, and densities and refractive indices obtained from chemical analyses or models. In that case larger uncertainties are to be expected.

Dry deposition of particles to building surfaces and soiling

Abstract A separated turbulent flow around a cube with slightly rough surfaces of 3×3 cm area was produced to simulate particle deposition on edges or other small scale structures of building walls. Polydisperse soot particles with a mass mean diameter of 0.8 μm produced by atomising a suspension of India ink and monodisperse fluorescent 0.6 μm latex spheres were used. The deposition velocity of the soot particles was determined by measuring the change in reflectance of the surrogate surfaces and the light absorption coefficient of the aerosol. The deposited fluorescent spheres were counted by means of a fluorescence microscope. Deposition velocities of the fluorescent spheres used in outdoor experiments were equal to deposition velocities of the soot particles used in a small wind tunnel. The density of the deposit was different on each side of the cube and also inhomogeneously distributed on each individual surface. The highest deposition was always found on the edges of the cube. This peculiar deposition pattern could be explained by the characteristics of the turbulent flow field around the surfaces which was measured by laser Doppler anemometry. Modelled soiling constants calculated with the help of the measured deposition velocities were up to a factor of 10 higher than values obtained for a flat plate in a simple boundary layer flow.