  We present the detection and characterization of fluctuations in linearly
polarized emission from the atmosphere above the South Pole. These measurements
make use of Austral winter survey data from the SPT-3G receiver on the South
Pole Telescope in three frequency bands centered at 95, 150, and 220 GHz. We
use the cross-correlation between detectors to produce an unbiased estimate of
the power in Stokes I, Q, and U parameters on large angular scales. Our results
are consistent with the polarized signal being produced by the combination of
Rayleigh scattering of thermal radiation from the ground and thermal emission
from a population of horizontally aligned ice crystals with an anisotropic
distribution described by Kolmogorov turbulence. The signal is most significant
at large angular scales, high observing frequency, and low elevation angle.
Polarized atmospheric emission has the potential to significantly impact
observations on the large angular scales being targeted by searches for
inflationary B-mode CMB polarization. We present the distribution of measured
angular power spectrum amplitudes in Stokes Q and I for 4 years of winter
observations, which can be used to simulate the impact of atmospheric
polarization and intensity fluctuations at the South Pole on a specified
experiment and observation strategy. For the SPT-3G data, downweighting the
small fraction of significantly contaminated observations is an effective
mitigation strategy. In addition, we present a strategy for further improving
sensitivity on large angular scales where maps made in the 220 GHz band are
used to measure and subtract the polarized atmosphere signal from the 150 GHz
band maps. In observations with the SPT-3G instrument at the South Pole, the
polarized atmospheric signal is a well-understood and sub-dominant contribution
to the measured noise after implementing the mitigation strategies described
here.
