The observed relationship between the south pole 225-GHz atmospheric opacity and the water vapor column density

We compare our previously reported measurements of South Pole 225 GHz atmospheric opacity,, to the column of precipitable water vapor (PWV) which was derived from concurrent upper air soundings. From this comparison we found that=(2.8±0.1)×10^–2+(6.9±0.2)×10^–2×PWV with in units of nepers/airmass andPWV in units of mm of precipitable H₂O. We compared our results to predictions from Grossman's AT atmospheric transparency model which is widely used in the radio astronomy community. The coefficient of the second term of the above relation, 0.069, was consistent with the predictions from the model; however, the first term, 0.028, which represents the dry air opacity, was about five to ten times larger than expected. Most of this discrepancy between the observed and the predicted dry air opacity can be accounted for by including contributions from continuum emission from N₂ and O₂ as is done in Liebe's MPM atmospheric model.