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Aims: The present study was conducted to determine the optical extinctioncurve for Cerro Paranal under typical clear-sky observing conditions, with anaccuracy of 0.01 mag/airmass. Methods: The extinction curve of Paranal wasobtained through low-resolution spectroscopy of 8 spectrophotometric standardstars observed with FORS1 mounted at the 8.2 m Very Large Telescope, covering aspectral range 3300-8000 A. A total of 600 spectra were collected on more than40 nights distributed over six months, from October 2008 to March 2009. Theaverage extinction curve was derived using a global fit algorithm, whichallowed us to simultaneously combine all the available data. The mainatmospheric parameters were retrieved using the LBLRTM radiative transfer code,which was also utilised to study the impact of variability of the mainmolecular bands of O2, O3, and H2O, and to estimate their column densities.Results: In general, the extinction curve of Paranal appears to conform tothose derived for other astronomical sites in the Atacama desert, like La Sillaand Cerro Tololo. However, a systematic deficit with respect to the extinctioncurve derived for Cerro Tololo before the El Chichon eruption is detected below4000 A. We attribute this downturn to a non standard aerosol composition,probably revealing the presence of volcanic pollutants above the Atacamadesert. An analysis of all spectroscopic extinction curves obtained since 1974shows that the aerosol composition has been evolving during the last 35 years.The persistence of traces of non meteorologic haze suggests the effect ofvolcanic eruptions, like those of El Chichon and Pinatubo, lasts severaldecades. The usage of the standard CTIO and La Silla extinction curvesimplemented in IRAF and MIDAS produce systematic over/under-estimates of theabsolute flux.

Optical atmospheric extinction over Cerro Paranal