Analysis of linear polarization data obtained by the Pioneer Venus Orbiter Cloud Photopolarimeter experiment indicates that the visible clouds at low and mid-latitudes are composed predominantly of 1 gm radius H2SO4 droplets, an identification made previously by using earth-based observations. Mixed within and extending above this main visible cloud is an extensive haze of submicron-sized particles. These haze particles have a refractive index of 1.45 +/- 0.04 at lambda similar or equal to 550 nm, an effective radius of 0.23 +/- 0.04 am, and a size distribution with an effective variance of 0.18 +/- 0.1. The polarization of the bright regions poleward of about 55 latitude is produced almost entirely by this submicron haze. The sub micron haze has been found to exhibit large spatial and temporal variations. In January 1979 the haze vertical optical thickness in the polar region at lambda = 365 nm was tau(h) similar to 0.8 above the main cloud of 1-mu m particles. By comparison, the optical thickness of the haze above the main cloud at low latitudes was typically 1 order of magnitude smaller, "rh 0.06, however haze mixed within the cloud contributed similar to 18% of the total scattering cross section per unit volume at lambda = 365 nm. More recent observations indicate that there are major changes of the haze on time scales of hundreds of days (e.g., the optical thickness of the polar cap haze is smaller by a factor of 2-3 in October 1979 than in January 1979). Substantial diurnal variations exist at low latitudes, with a greater amount of haze near the morning terminator than near the noon meridian. The global distribution of haze will be monitored during the extended Pioneer Venus mission to permit analysis of long time scale variations as well as correlation with characteristics of the atmospheric dynamics deduced from ultraviolet images of the cloud tops.
