  The polarization state of electromagnetic radiation scattered by atmospheric
particles such as aerosols, cloud droplets, or ice crystals contains much more
information about the optical and microphysical properties than the total
intensity alone. For this reason an increasing number of polarimetric
observations are performed from space, from the ground and from aircraft.
Polarized radiative transfer models are required to interpret and analyze these
measurements and to develop retrieval algorithms exploiting polarimetric
observations. In the last years a large number of new codes have been
developed, mostly for specific applications. Benchmark results are available
for specific cases, but not for more sophisticated scenarios including
polarized surface reflection and multi-layer atmospheres. The International
Polarized Radiative Transfer (IPRT) working group of the International
Radiation Commission (IRC) has initiated a model intercomparison project in
order to fill this gap. This paper presents the results of the first phase A of
the IPRT project which includes ten test cases, from simple setups with only
one layer and Rayleigh scattering to rather sophisticated setups with a cloud
embedded in a standard atmosphere above an ocean surface. All scenarios in the
first phase A of the intercomparison project are for a one-dimensional
plane-parallel model geometry. The commonly established benchmark results are
available at the IPRT website (http://www.meteo.physik.uni-muenchen.de/~iprt).
