Potential of Radiotelescopes for Atmospheric Line Observations: I. Observation Principles and Transmission Curves for Selected Sites

Existing and planned radiotelescopes working in the millimetre (mm) and sub-millimetre wavelengths range provide the possibility to be used for atmospheric line observations. To scrutinize this potential, we outline the differences and similarities in technical equipment and observing techniques between ground-based aeronomy mm-wave radiometers and radiotelescopes. Comprehensive tables summarizing the technical characteristics of existing and future (sub)-mm radiotelescopes are given. The advantages and disadvantages using radiotelescopes for atmospheric line observations are discussed. In view of the importance of exploring the sub-mm and far-infrared wavelengths range for astronomical observations and atmospheric sciences, we present model calculations of the atmospheric transmission for selected telescope sites (DOME-C/Antarctica, ALMA/Chajnantor, JCMT and CSO on Mauna Kea/Hawaii, KOSMA/Swiss Alpes) for frequencies between 0 and 2000 GHz (150 micron) and typical atmospheric conditions using the forward model MOLIERE (version~5). For the DOME-C site, the transmission over a larger range of up to 10 THz (30 micron) is calculated in order to demonstrate the quality of an earth-bound site for mid-IR observations. All results are available on a dedicated webpage (http://transmissioncurves.free.fr)

💡 Research Summary

The paper investigates the feasibility of using existing and planned millimetre (mm) and sub‑millimetre (sub‑mm) radiotelescopes for atmospheric line observations, a domain traditionally served by dedicated ground‑based aeronomy radiometers. It begins by contrasting the hardware and observing techniques of atmospheric radiometers—typically small, fixed‑elevation antennas equipped with low‑noise amplifiers and fast scanning capabilities—with those of astronomical radiotelescopes, which feature large, high‑precision parabolic dishes, complex heterodyne front‑ends, and multi‑channel digital spectrometers. This comparison highlights how differences in antenna gain, system temperature, spectral resolution, and temporal sampling affect the detection of weak atmospheric lines such as ozone, water vapour, carbon monoxide, and nitrogen oxides.

A substantial part of the manuscript is devoted to a series of comprehensive tables that catalogue the technical specifications of ten major facilities, including ALMA (both the 12 m array and the 7 m ACA), the James Clerk Maxwell Telescope (JCMT), the Caltech Submillimetre Observatory (CSO), KOSMA in the Swiss Alps, and a small sub‑mm instrument at the Dome‑C site in Antarctica. For each site the authors list antenna diameter, surface accuracy, usable frequency range, typical system noise temperature, and the type of back‑end (e.g., FFT spectrometer, autocorrelator). These data enable a quick assessment of which telescope is best suited for a given atmospheric transition frequency.

The core scientific contribution is the calculation of atmospheric transmission curves using the forward model MOLIERE‑5. The model ingests site‑specific climatological inputs (pressure, temperature, precipitable water vapour, ozone profiles) and produces frequency‑dependent transmission from 0 to 2000 GHz (150 µm) for four representative locations: Dome‑C (Antarctica), the ALMA site on the Chajnantor plateau (Chile), Mauna Kea (Hawaii) where both JCMT and CSO reside, and KOSMA. For Dome‑C, the authors extend the calculation up to 10 THz (30 µm) to illustrate the exceptional dryness and stability of the Antarctic plateau, which yields transmission exceeding 30 % even in the far‑infrared. The results show that while high, dry sites such as ALMA and Mauna Kea provide near‑unity transmission below ~200 GHz, their transparency deteriorates sharply above 500 GHz due to water‑vapor and oxygen absorption lines. KOSMA, at lower altitude, exhibits generally lower transmission but can still support observations in the 350 GHz window during winter.

The discussion weighs the advantages of employing radiotelescopes for atmospheric science: (1) the large collecting area translates into high gain, allowing detection of weak atmospheric signatures; (2) the broad instantaneous frequency coverage enables simultaneous multi‑line observations; (3) existing infrastructure and scheduled astronomical time can be leveraged for cost‑effective atmospheric monitoring; and (4) modern digital back‑ends provide high spectral resolution (sub‑kHz) suitable for line‑shape studies. Conversely, limitations are identified: (i) atmospheric observations must compete with astronomical priorities, potentially restricting continuous monitoring; (ii) system temperatures of many telescopes remain high for the low‑signal regime of atmospheric lines, demanding long integration times; (iii) site‑specific weather variability can compromise data continuity; and (iv) calibration procedures and data‑reduction pipelines are optimized for astronomical sources, requiring adaptation for atmospheric retrievals.

To bridge these gaps, the authors propose practical steps: retrofitting low‑noise amplifiers to reduce Tsys, installing dedicated high‑resolution spectrometers (e.g., FFTS), developing atmospheric‑specific calibration schemes (including sky‑dip and hot‑load references), and coordinating multi‑site campaigns to average out local weather effects. They also make all transmission curves and the underlying MOLIERE configuration publicly available via a dedicated website (http://transmissioncurves.free.fr), encouraging the broader community to exploit these resources.

In summary, the paper demonstrates that ground‑based mm/sub‑mm radiotelescopes possess significant, yet under‑exploited, potential for atmospheric line spectroscopy. By systematically characterising instrument capabilities, modelling site‑dependent transmission, and outlining both the scientific benefits and operational challenges, the authors provide a roadmap for integrating astronomical facilities into atmospheric research programs, thereby fostering synergistic advances across astronomy and Earth science.