Calibration of Knollenberg FSSP Light-Scattering Counters for Measurement of Cloud Droplets
ARMY LAB COMMAND WHITE SANDS MISSILE RANGE NM ATMOSPHERIC SCIENCES LAB
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It is well known that atmospheric fog degrades the performance of DOD electroptic sensors. A quantitative assessment of these effects generally requires knowledge of droplet size distributions, which can have considerable spacial and temporal variations. An attractive approach to measuring fog or cloud drop sizes is the use of light-scattering counters. In this paper we evaluate the response characteristics of the Knollenberg model FSSP-100 light- scattering counter, which is well-suited for measurement of particles as large as fog droplets. This instrument has been widely used throughout DOD for aerosol measurement, without adequate understanding of its response characteristics. In this paper we find that measurement of cloud drop size distributions using the manufacturers calibration can lead to artificial bumps or knees in the distributions at about 0.6 micrometers, and sometimes at 2 micrometers to 4 micrometers radius. These artifacts are a consequence of the instrument having multivalued or slowly changing response in these regions of particle size. We have developed a modified calibration procedure that removes these artifacts, so that the true droplet size distribution can be obtained. We also have found that measurement of slightly nonspherical particles with refractive indexes characteristic of those of atmospheric aerosols will generally lead to under- sizing if the FSSP manufacture-supplied calibration is used, but likely by not more than a factor 2.
- Test Facilities, Equipment and Methods