Scattering

Rayleigh scattering involves particles which are much smaller than the incident 
wavelength.  The cross-section is well known, and the effect of this 
transmission loss process will be included in our retrieval technique.  Mie 
scattering involves particles which are comparable or larger than the incident 
wavelength.  The size, shape and density of the scattering molecule all 
contribute to the effect.  A simple Mie scattering model will be included in our 
analysis.  A substantial increase to the aerosol layer at 10-15 km (due to major 
events such as volcanic eruption) may hinder our ability to obtain 
concentrations of trace gases in the lowest regions and should be avoided.

Polar stratospheric clouds (PSCs) are common in the arctic from late November to 
early March (JGR,99,13083,1994) where they are formed within the polar vortex at 
low temperatures.  Our experiment will likely be flown within this time frame, 
which affords us the opportunity to map any PSCs which we might encounter.  The 
destruction of ozone is due to the coupling effect of the heterogeneous 
reactions that occur on PSCs and the catalytic reactions that occur during the 
polar spring.  Detection and mapping of PSCs provides critical information about 
ozone depletion and the coupling between these formations and stratospheric 
trace gas concentrations.  

It has been proposed (Wayne?) that the two types of polar stratospheric clouds 
would each have unique scattering properties.  Analysis of our data may provide 
some insight into the extinction within a cloud formation should this event 
occur and depending on the intensity of the stars within the field of view at 
that time.  The advantage that our mission provides in order to map PSCs or 
determine the best Mie scattering model is that there will be more than one 
viewing angle for simultaneous radiance measurements.  The viewing angle 
dependence of the retrieved aerosol optical thickness has been suggested as 
evidence of particle nonsphericity and may also be used to reject unreasonable 
aerosol models (GRL,22,1077,1995 - practically word for word).  The substantial 
effect of PSCs on the abundance of trace gases in the stratosphere 
(GRL,20,2059,1993) may also be determined by our measurements.  Should a PSC 
occur, the ability of our instrument to determine other physical parameters of 
the formation is improved as the number of lines of sight is increased.  
Specifically, the best condition would be multiple stars above and below the 
horizontal.