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.