The 492 GHz Atmospheric Opacity at the Geographic South Pole

We present narrow-bandwidth submillimeter-wave sky opacity measurements made from the South Pole between 1995 February 9 and November 17, a period that includes an entire Austral winter. These measurements were made with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) using a heterodyne receiver tuned to a band that includes the 492 GHz fine-structure line of neutral atomic carbon. The zenith optical depth was below 0.72 half the time during the Austral winter and spring, and it reached values as low as 0.34 on day 232. The stability was also remarkably good: the opacity remained below 1.0 for weeks at a time. The South Pole is therefore an excellent site for submillimeter astronomy throughout the Austral winter and spring. The functional relationship between 492 GHz opacity and measured precipitable water vapor shows that a significant fraction of the opacity is caused by atmospheric constituents other than water vapor, indicating the need for accurate, site-dependent atmospheric modeling when opacity measurements at lower frequencies are extrapolated into the submillimeter.