Journal of Atmospheric and Solar–Terrestrial Physics 199 (2019) 105214 Available online 27 January 2020 1364-6826/© 2020 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Journal of Atmospheric and Solar–Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp Research paper Optical depth measurements at 45 and 90 GHz in CASLEO J.F. Valle Silva a,∗ , C. Guillermo Giménez de Castro a,b , Celi Passarelli a , Deysi Cornejo Espinoza a , Marta Maria Cassiano c , Jean-Pierre Raulin a , A. Valio a a CRAAM, Universidade Presbiteriana Mackenzie, São Paulo, Brazil b IAFE, Universidad de Buenos Aires, Bs.As., Argentina c Rua Plínio de Morais, 152 - Sumaré, 01252-030 São Paulo, SP, Brazil ARTICLE INFO Keywords: Water vapor opacity Optical depth measurements Millimeter-to-IR astronomy Atmospheric modeling ABSTRACT The optical depth at 45 and 90 GHz have been obtained from calibration data of two solar radio telescopes located in El Leoncito Astronomical Complex (CASLEO), during the years 2012–2013. The observed period and the temporal resolution of the measurements is sufficient to show seasonal patterns. Using the precipitable water vapor content (PWV) obtained with a Sun-photometer for aerosols measurements, it was found a good correlation of PWV with the optical depth in both frequencies. With the water vapor content and data from a meteorological station, the atmosphere over CASLEO was modeled using ATM 2009, an Earth’s atmospheric model. The results of the model allows to evaluate the range of optical depth measurements and to predict the contributions from H 2O and O 2 , the main components of optical depth at these frequencies. It was found a reasonable agreement at 45 GHz when the PWV is lower than 5 mm, but notable discrepancies for the entire range of water vapor content at 90 GHz. An empirical relation is established between the water vapor contribution to the measured optical depth for the observed frequencies. Our results show that CASLEO, despite being a site with 2500 meters high, is appropriate for solar observations in the millimeter range and is still useful for observations in the submillimeter at the epoch of the year with low water vapor content. 1. Introduction The knowledge of Earth’s atmosphere emission and absorption spec- trum is of crucial importance for the absolute calibration of ground- based millimeter wave continuum observations. In clear weather con- ditions water vapor and molecular oxygen play a major role in the absorption process at wavelengths above 2 mm. Under drier conditions, the contribution of mesospheric ozone could become increasingly im- portant (Dryagin et al. 1966, Straiton 1975, Waters 1976). Pressure broadened transitions of these molecules limit observations to several spectral windows. Within these windows, the line wings both absorb incoming radiation and emit thermally. The H 2 O is the principal vari- able component of the atmosphere and the absorption at millimeter wavelengths is dominated by the transition of molecules from the lowest to higher pure rotational energy levels corresponding to reso- nance spectral lines: 22.2, 183.3 GHz and 325.1 GHz. The oxygen that occupy the 20.9% by volume of the Earth’s dry atmosphere presents a serious limitation to the propagation of electromagnetic waves at millimeter wavelengths, O 2 has a no permanent electric dipole moment but has a triplet electronic ground state, with the remarkable property that its two outer electrons are paired with parallel spins. This gives rise to an intrinsically weak magnetic dipole moment with magnetic ∗ Corresponding author. E-mail address: jfvallesilva@gmail.com (J.F. Valle). dipole transitions that are weaker than the electric dipole transitions of polar molecules like water. The weakness of this dipole moment is compensated by the large abundance, implying O 2 line intensities similar to H 2 O ones (Guélin, 2005). The microwave spectrum of the oxygen molecule  2 is a result of fine-structure transitions in which the magnetic moment (electronic spin) assumes various directions with re- spect to the rotational angular momentum of the molecule. Meeks and Lilley (1963) made computations to determine the center frequencies for the oxygen transitions and computed the opacity and the thermal emission produced by the millimeter-wavelength complex oxygen lines between 48.4 and 71.05 GHz and one line at 118.7 GHz. CASLEO (2550 m) (Spanish: Complejo Astronómico El Leoncito) located at (31.798 ◦ S, 69.295 ◦ W) is an astronomical observatory in the San Juan province, Argentina. The combination of altitude, topograph- ical and meteorological conditions made CASLEO a place with good conditions for observations at high frequencies. Since 2002 CASLEO is the site of solar-dedicated radio telescopes operating in the submillime- ter range: 212 GHz, 405 GHz. About 50% of the time, the zenith optical depth is lower than 0.3 neper (Np) at 212 GHz and lower than 1.5 Np at 405 GHz (Melo et al., 2005). More recently Cornejo Espinoza 2017 has https://doi.org/10.1016/j.jastp.2020.105214 Received 24 July 2019; Received in revised form 21 January 2020; Accepted 22 January 2020