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We present the 3.5-yr monitoring results of 225 GHz opacity at the summit ofthe Greenland ice sheet (Greenland Summit Camp) at an altitude of 3200 m usinga tipping radiometer. We chose this site as our submillimeter telescope(Greenland Telescope; GLT) site, because its location offers favorablebaselines to existing submillimeter telescopes for global-scale VLBI. The siteshows a clear seasonal variation with the average opacity lower by a factor oftwo during winter. For the winter quartiles of 25% and 50%, the Greenland siteis about 10%-30% worse than the ALMA or the South Pole sites. Estimatedatmospheric transmission spectra in winter season are similar to the ALMA siteat lower frequencies (<450 GHz), which are transparent enough to performastronomical observations almost all of the winter time with opacities <0.5,but 10%-25% higher opacities at higher frequencies (>450 GHz) than those at theALMA site. This is due to the lower altitude of the Greenland site.Nevertheless, half of the winter time at the Greenland site can be used forastronomical observations at frequencies between 450 GHz and 1000 GHz withopacities <1.2, and 10% of the time show >10% transmittance in the THz (1035GHz, 1350 GHz, and 1500 GHz) windows. One major advantage of the Greenland sitein winter is that there is no diurnal variation due to the polar nightcondition, and therefore the durations of low-opacity conditions aresignificantly longer than at the ALMA site. Opacities lower than 0.05 or 0.04can continue for more than 100 hours. Such long stable opacity conditions donot occur as often even at the South Pole; it happens only for the opacitylower than 0.05. Since the opacity variation is directly related to the skytemperature (background) variation, the Greenland site is suitable forastronomical observations that need unusually stable sky background.

3.5-Year Monitoring of 225 GHz Opacity at the Summit of Greenland