J. Aerosol Sci., Vol. 24. Suppl. I. pp $577 $578. 1993 0021-8502/93 $6.00 +0.00 Printed in Great Britain Pergamon Press Ltd 45 O 01 Droplet Size Distributions and Liquid Water Contents in Stratus Clouds: Instrument Intercomparison between Holography, FSSP-100 Optical Particle Counter and Particulate Volume Monitor PVM-100 S. Borrmann(1),R. Jaenicke(2),R. Maser(3),B.G. Arends(') (1) NCAR, Division ATD, Boulder, Colorado, USA (2) Inst. f. Physik d. Atmosphere, Universit~t,Mainz, Germany (3) Zentrum ffir Umweltforschung, Universit~t Frankfurt, Germany (4) ECN, Petten, The Netherlands Keywords particulate volume monitor, optical particle counter, Fraunhofer in-line holography Background A recently developed, ground based HODAR (Holographic Droplet and Aerosol Recording)system was operated on the Taunus Mountain Observatory near Frankfurt, Germany along with a FSSP- 100 laser optical particle counter (Knollenberg, 19"/6) to measure cloud droplet size distributions during a stratus cloud event. Also a Gerber particulate volume monitor PVM-100 (Gerber, 1991) was utilized for liquid water content measurements. We present an intercomparison of the cloud droplet size distributions obtained from the HODAR and the FSSP-100 and also compare the derived liquid water contents with the data from the PVM-100. The HODAR implements the so-called Fraunhofer in-line holographic technique consisting of two steps: (1) recording of a hologram of a cloud volume of ~ 1 liter containing the droplets and (2) optical reconstruction of the individual droplet images with a suitably designed laboratory setup. By means of these images the droplet sizes, can be determined. The technical details of the HODAR recording and reconstruction devices, their capabilities, limits and errors are decribed in Borrmann and Jaenicke (1993). Results of the Intercomparison On November 13, 1990 three holograms were recorded in the early morning during an extended stratus cloud event. The HODAR was separated from the continuously running FSSP-100 and PVM-100 by a distance of 12 meters. Two of the size distributions extracted from the recon- structed holographic images are shown in the figures (solid lines with error bars) together with data obtained from the FSSP-100 (dashed lines). The recording of a hologram consisted of directing a pulse of intense laser light towards a high resolution photographic plate, while the data from the FSSP-100 were averaged over 10 minutes centered around the the exposure time of the HODAR. The error bars for the size distributions measured with HODAR are due to count statistics, cross sensitivity and an error inherent in the determination of the imaged cloud volume. The results obtained from the two different methods agree well. Also it is apparent that the lower particle size radius detection limit of the HODAR is around 3 #m, which is considerably above the limit of the FSSP-100. Smaller particles can not be imaged with HODAR because the wavelength of the recording beam (~ 0.7#m) is too close to the size of the droplets. The table gives the 10 minute averages of the liquid water contents obtained from the FSSP-100 and the PVM-100 together with those calculated from the size distributions measured by HODAR. AS 24. Suppl I MM $577