~ ) Pergamon J. Aerosol Sci. Vol. 28, Suppl. I, pp. $237-$238, 1997 ©1997 Elsevier Science Ltd. All rights reserved

Printed in Great Britain PH:S0021-8502(97)00166-3 002l-S502/97 SlT.00+o.oo

Complete Sets of Optical Properties and the Soot Content of Atmospheric

Boundary Layer Particles : Longtime Monitoring in Central Europe

Peter Het+Jmann

Institut fur Meteorologie und Geophysik, Johann-Wolfgang-Goethe-Universit~it, Frankfurt, Germany

Particles in the atmospheric boundary layer influence the radiation budget of the atmosphere

and thus the climate, because of their ability to scatter and absorb solar radiation. Backscattering of

solar radiation to space by atmospheric particles may lead to a cooling in the lower part of the

atmosphere (Charlock and Sellers 1980) and heating of the particles due to absorption leads to a local

warming of the atmosphere in the boundary layer (H~inel 1987).

Because of the direct impact of atmospheric particles on climate, a thorough knowledge of

their optical properties (absorption, scattering and extinction coefficients, the asymmetry parameter of

the phase function of scattered light and the complex refractive index) and their soot content (Horvath

1993) is of great interest. But until now only a few complete parameter sets and especially no longtime

measurements are available.

To fill that gap the variations of the optical properties of particles from the atmospheric

boundary layer and their soot content are studied during a period of two years between April "95 and

March "97. During this period the particles were continuously sampled on Nuclepore-filters. The

sampling site was located at the summit of the mountain 'Kleiner Feldberg' (825m above MSL) which

is situated northwest of the city of Frankfurt am Main (Germany) at a distance of 15 Kilometres.

The optical properties and the soot content of the particles were determined using a polar photometer

and a subsequent mathematical inversion (H~inel 1994). During the measurements the particles are

nearly dry. Thus, the optical properties of dry particles are obtained. The method allows the derivation

of a complete set of the optical properties and the soot content from the same sample of particles. Due

to the setup of the measuring method the optical properties are spectral mean values for extraterrestrial

solar radiation or can be interpreted as spectral values for radiation at a wavelength of 0.7 lam.

$237

$238 Abstracts of the 1997 European Aerosol Conference

Consequently they are useful for model calculations (e.g. modelling of the atmospheric radiation

budget) or other climate considerations,

Until now the major results are:

1. A year-to-year cycle for the monthly means of the extinction, scattering and absorption coefficients

and the soot content was found. For the extinction and scattering coefficient the highest values are

found during summer and winter and the lowest values were found during spring. The year-to-year

cycle of the absorption coefficient and the soot content is of a sinusoidal form with highest values

during winter and lowest values during summer. The monthly means of the absorption coefficient are

one order of magnitude larger than those obtained at remote stations (Bodhaine 1995).

2. During wintertime the observed absorption coefficient and the soot content are significantly related

to the air temperature measured at the sampling site. The absorption coefficient and the soot content

increase with decreasing air temperature. Despite a considerable scatter of the data the correlations are

highly significant. For the remaining time of the year no significant correlation of the particles'

properties to air temperature could be obtained.

3. The particles' properties are influenced by the local weather situation. It was found that the

attenuation coefficients strongly depend on the stratification of the lower troposphere. They are

significantly higher during situations with a stable stratification or an inversion in the boundary layer

than during all the other situations. The difference is highest during wintertime (up to a factor of two).

4. The values of the extinction, scattering and absorption coefficients and the soot content largely

depend on large and mesoscale transport paths in the atmosphere and on the origin of the air masses

arriving at the sampling site. For almost identical transport patterns the values of the particles'

properties do not scatter largely (e.g. the lowest extinction coefficient was found in air masses

originated over the Northern Atlantic and travelled fast over Great Britain to the sampling site),

Secondly in maritime air masses the attenuation coefficients are smaller than in continental air masses.

This difference is highest during winter and less in summertime.

Bodhaine, B. A. (1995) J. Geophys. Res. 100, 8967-8975

Charlock, T. P. and Sellers, W. D. (1980) J. Atmos. Sci. 37, 1327-1341

Hand, G. (1987) Atmosphiirische Spurenstoffe: Ergebnisse des gleichnamigen Sonderforschungsbereichs (Edited by

Jaenicke, R.), VCH, Weinheim

H~inel, G. (1994) Appl. Opt. 33, 7187-7199

Horvath, H. (1993) Attn. Environ. 27A, 293-317