We compute the circularly polarized signal from atmospheric molecular oxygen. Polarization of O2 rotational lines is caused by Zeeman effect in the Earth magnetic field. We evaluate the circularly polarized emission for various sites suitable for CMB measurements: South Pole and Dome C (Antarctica), Atacama (Chile) and Testa Grigia (Italy). An analysis of the polarized signal is presented and discussed in the framework of future CMB polarization experiments. We find a typical circularly polarized signal (V Stokes parameter) of ~ 50 - 300 {mu}K at 90 GHz looking at the zenith. Among the other sites Atacama shows the lower polarized signal at the zenith. We present maps of this signal for the various sites and show typical elevation and azimuth scans. We find that Dome C presents the lowest gradient in polarized temperature: ~ 0.3 {mu}K/circ at 90 GHz. We also study the frequency bands of observation: around { u} simeq 100 GHz and { u} simeq 160 GHz we find the best conditions because the polarized signal vanishes. Finally we evaluate the accuracy of the templates and the signal variability in relation with the knowledge and the variability of the Earth magnetic field and the atmospheric parameters.
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