Pergamon J. AerosolSci. Vol. 29. Suppl. 1, pp. Sl211-S1212, 1998

0 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain

0021.8502/98 $19.00 + 0.00

Longtime Monitoring of the Optical Properties and the Soot Content of

Atmospheric Boundary Layer Particles in Central Europe

Peter Herrrnann Institute for Meteorology and Geophysics, Johann-Wolfgang-Goethe-University, Frankfurt, Germany

Keywords: Atmospheric particles, Optical properties, Weather situation

The radiation budget of the atmosphere and thus the climate is influenced by particles in the atmospheric boundary layer, because of their ability to absorb and scatter solar radiation. On one hand heating of the particles due to absorption leads to a local warming of the atmosphere in the boundary layer (Hanel 1987) and on the other hand 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). 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 long-time measurements are available.

In this paper the results of a long-time study of the variations of the optical properties of particles from the atmospheric boundary layer and their soot content are presented. The particles’ properties were studied during a period of two years between April ‘95 and March ‘97. The sampling site was located at the summit of the mountain ‘Kleiner Feldberg’ (825m above MSL) which is situated north-west 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 (Hanel 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 pm. Consequently they are useful for model calculations (e.g. modelling of the atmospheric radiation budget) or other climate considerations.

The major results of this investigations are: 1. No distinct year-to-year cycle for the monthly means of the extinction, scattering and absorption coefficients and the soot content could be found. 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

s1211

s1212 Abstracts of the 5th International Aerosol Conference 1998

soot content increase with decreasing air temperature. 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 processes in the atmosphere and on the origin of the air masses arriving at the sampling site (see Fig.). For almost identical transport patterns the values of the particles’ properties do not scatter largely.

Norway/ North Sea Sweden

Extinction coefficient [l/km]

Figure: Dependence of the extinction coefficient on transport processes

As an example for possible applications to this results a simple model will be presented. With this model the local warming of the atmosphere due to absorption of solar radiation by atmospheric boundary layer particles can be estimated.

Referencies:

Bodhaine, B. A. (1995) J. Geophys. Res. 100,8967-8975 Charlock, T. P. and Sellers, W. 1). (1980) J. Atmos. Sci. 37, 1327-1341 HBnel, G. (1987) Atmosphiirische Spurenstoffe: Ergebnisse des gleichnamigen Sonderforschungsbereichs (Edited by Jaenicke, R.), VCH, Weinheim Hiinel, G. (1994)Appl. Opt. 33,7187-7199 Horvath, H. (1993) Atm. Environ. 27A, 293-3 17