The atmospheric composition variability above Cerro Paranal/Chile

The Chilean Atacama desert hosts the largest astronomical observatories world-wide due to its unique dry meteorological conditions and high altitude of the Andes. One of the largest facilities is Cerro Paranal, among others hosting the Very Large Telescope, which is equipped with several spectrographs ranging from the ultraviolet to the mid-infrared. As ground-based telescopes have to observe through the Earth's atmosphere, the spectra taken from astronomical objects are affected by molecular absorption arising from the present various species. This imprint -called telluric contamination- varies in the same way as the composition of the Earth's atmosphere varies. Therefore it is of crucial importance for astronomers to know about the chemical components of the atmosphere at the time of observation to be able to correct for these contaminations.

In the past, mostly static atmospheric standard models were used to fit and correct the telluric contaminations. In the meanwhile, several sources of world-wide, time-dependent information of the chemical composition are available. We are currently investigating data from the Copernicus Atmospheric Monitoring Service (CAMS) Global Reanalysis (EAC4) and CAMS Global Greenhouse reanalysis (EGG4), which provide height profiles of various molecular species (e.g. NO, NO₂, O₃, CO, HNO₃,...) on a 3-hourly resolution ranging from 2003 through 2020 (EGG4) and 2024 (EAC4). This allows us a detailed analysis on the hourly, daily, seasonal, and yearly variability of the chemical composition of the Earth’s atmosphere above Cerro Paranal.

We found significant variations of nearly all species on various time scales, highly affecting the astronomical observations. In this presentation we show first results of our investigations. As astronomical observations are conducted during night, we focus on day/night-time differences and long-term trends to estimate the impact on telluric contamination.