Submillimeter-wave atmospheric transmission at El Leoncito, Argentina Andes

We present the results of one year of daytime atmosphere transmission measurements obtained at 212 and 405 GHz by a 1.5 m solar radio telescope located at El Leoncito site, 2550 m altitude in Argentina Andes. We used three different methods: 1) absolute derivation from strong solar signal at different elevation angles; 2) direct derivation from observed known solar brightness times the antenna coupling factor; and 3) fitting the observed sky emission plots as a function of elevation angle, also known as "tipping." Some differences were found for the transmission determined by the three methods, the most important is that methods 1) and 2) are restricted to smaller values of opacity. Method 1) is restricted to measurements at low elevation angles. Method 2) has the advantage to extend measurements to considerably higher values of attenuation. For the El Leoncito site the correlation of optical depth at 405 and 212 GHz was found (τ₄₀₅/τ₂₁₂)=4.43, smaller than model predictions (≈5.0). Measurements at both frequencies obtained for 319 days in one year indicate that 50% of time τ₂₁₂<0.3 (or >74% zenithal transmission) and τ₄₀₅<1.5 (or >22% transmission), with small seasonal dependence compared to other sites. The opacity related precipitable water vapor is smaller compared to model estimates, suggesting an upper atmosphere water vapor depression, characteristic to the region.