ALMA Memo #384
Atmospheric Transparency at Chajnantor and Pampa la Bola

Simon J. E. Radford and Bryan J. Butler
National Radio Astronomy Observatory

Seiichi Sakamoto
National Astronomical Observatory of Japan

Kotaro Kohno
Nobeyama Radio Observatory

2001 Aug 02

Measurements with 220 and 225 GHz tipping radiometers demonstrate the atmosphere over both Chajnantor and Pampa la Bola has exceptional transparency, better than observed over Mauna Kea. These measurements indicate, however, the atmosphere is more transparent at Chajnantor than at Pampa la Bola. For hourly median optical depths from 1999 June through 2000 December, the median at Pampa la Bola is 0.011 larger than at Chajnator and the median normalized difference is [tau_220 (Bola) - tau_225 (Chaj.)]/tau_225 = +0.23. Side-by-side measurements at Chajnantor in 2001 April--May confirm the data from the two instruments can be directly compared. The measured transparency differences imply Chajnantor enjoys a significant advantage in observing time or sensitivity, especially for observations at submillimeter wavelengths.

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