TITLE: Twin Otter Particle size distributions as measured by the CAPS scatter probe. Submitted 10 Dec 2002 Version 1.0 AUTHOR: Haflidi H Jonsson CIRPAS/Naval Postgraduate School 3240 Imjin Road, Hangar 510 Marina Municipal Airport Marina, CA 93933 Tel: 831-384-2776 Fax: 831-384-3277 e-mail: hjonsson@nps.navy.mil 1.0 DATASET OVERVIEW The dataset contains the location, as well as per channel concentration of particles for 20 channels in forward scatter and backscatter. Particle diameters are give also as geometric mean diameters for each channel. In case of the forward scatter data, the mean diameter per channel reported is detertermined in the following manner: Probe response is calculated for various refractive indexes likely to be present in the atmosphere (1.33 <= RI <= 1.7). The curve cluster is lined up on calibration data. The cluster of curves thus defines the envelope of possible particle sizes that could generate the measured pulse. An upper and lower envelope diameter value is thus determined for each channel boundary. Reported with the data are channel diameter based on the geometric mean boundary diameters. The envelope values, whic define the uncertainty in the inversion are not presented in the data file. If interested in discussing them, please contact the POC above. In the backscatter section, the uncertainty envelope is much larger than in the forward scatter data. As a result the inversion from pulse heights to diameters has no physical meaning. Therefore, a set of diameters is presented with the data that represent particles having a particular refractive index (1.45), and the resultand "size distribution" is therefore only an equivalent one. When measuring particles having refractive index larger than 1.45, the diameters presented in association with the backscatter channels are too large, and when measuring particles having smaller refractive index, they are to small. A comparison between a forward scatter size distribution and a backscatter size distribution using the channel mean diameters presented thus may give an indication of particle refractive index. Note, however, that in all said it is assumed that partiles are spherical. The effect particle shape might have on the probe response is largely unexplored. 2.0 INSTRUMENT OVERVIEW The scatter section of te CAPS probe collects light scattered through a 2-12 degree angle out of a laser beam in the forward direction and 168-178 degrees in the backward direction. The resultant pulse gererated by the photodetectors are classified by height for each section into 20 channels, resulting in two pulse height spectra every second. The probe is calibrated by use of monodisperse spherical polystyrine and glass spheres, a,d the resultant mean pulse heights combined with calculated probe response used to generate an inversion scheme that converts measured pulse height spectra to size distributions. 3.0 DATA COLLECTION AND PROCESSING Data were logged to the CIRPAS Twin Otter data system, which is based on National Instruments computers and LabView instrument drivers. The CPAS probe is polled for its pulse height spectra once every second.