Validation of millimetre and sub-millimetre atmospheric collision-induced absorption at Chajnantor

¹ Consejo Superior de Investigaciones Cientifícas, Instituto de Física Fundamental, Calle Serrano 121, 28006 Madrid, Spain
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ European Southern Observatory, Alonso de Córdova 3107, Vitacura Casilla 7630355, Santiago, Chile
⁵ Centre National de la Recherche Scientifique, Observatoire de Paris, Laboratoire d’Etudes du Rayonnement et la Matière en Astrophysique, 77 Avenue Denfert Rochereau, 75014 Paris, France
⁶ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁷ Centro de Astro-Ingeniería UC, Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Avda Vicuña Mackenna 4860, Macul, Santiago, Chile
⁸ Departamento de Física de la Tierra y Astrofísica e Instituto de Física de Partículas y del Cosmos (IPARCOS). Universidad Complutense de Madrid, Ciudad Universitaria, 28040 Madrid, Spain

^★ Corresponding author; jr.pardo@csic.es

Received: 6 September 2024
Accepted: 18 November 2024

Abstract

Due to the importance of a reference atmospheric radiative transfer model for both planning and calibrating ground-based observations at millimetre and sub-millimetre wavelengths, we have undertaken a validation campaign consisting of acquiring atmospheric spectra under different weather conditions, in different diurnal moments and seasons, with the Atacama Pathfinder EXperiment (APEX), due to the excellent stability of its receivers and the very high frequency resolution of its back-ends. As a result, a dataset consisting of 56 spectra within the 157.3–742.1 GHz frequency range, at kilohertz resolution (smoothed to ∼2–10 MHz for analysis), and spanning one order of magnitude (∼0.35–3.5 mm) in precipitable water vapour columns, has been gathered from October 2020 to September 2022. These data are unique for their quality and completeness and, due to the proximity of APEX to the Atacama Large Millimeter/Submillimeter Array (ALMA), they provide an excellent opportunity to validate the atmospheric radiative transfer model currently installed in the ALMA software. The main issues addressed in the study are possible missing lines in the model, line shapes, vertical profiles of atmospheric physical parameters and molecular abundances, seasonal and diurnal variations, and collision-induced absorption (CIA), to which this paper is devoted, in its N₂–N₂ + N₂–O₂ + O₂–O₂ (dry) and N₂–H₂O + O₂ –H₂O (‘foreign’ wet) mechanisms. All these CIA terms should remain unchanged in the above-mentioned ALMA atmospheric model as a result of this work.