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Keywords:

  • scattering;
  • atmospheric effects;
  • instrumentation: miscellaneous;
  • methods: data analysis;
  • site testing

ABSTRACT

Because of the high quality of its sky, the Roque de los Muchachos Observatory (ORM), located on the island of La Palma in the Canary Islands, is home to many astronomical facilities. In the context of the Extremely Large Telescope Design Study, two intensive lidar campaigns were performed at the ORM near the Jacobus Kapteyn Telescope between 2007 July 9 and 11 and between 2008 May 26 and June 14. The goal of the campaign was to characterize the atmosphere in terms of the height of the planetary boundary layer (PBL) and the aerosol stratification versus synoptic conditions. Three typical synoptic situations were found, which supported the intrusion of aerosols from marine/oceanic, anthropogenic and Saharan origins, respectively. All measurements revealed a multilayer stratification with a mean PBL height of 546 ± 198 m agl and top layers as high as ∼8400 m asl. As a by-product, an estimate of the aerosol optical thickness was also obtained and compared to the total atmospheric extinction coefficient measured by the Carlsberg Meridian Telescope. Except in the presence of Saharan dust, the aerosol optical thickness is very low; the average values are 0.0405 at 532 nm and 0.0055 at 1064 nm. In the presence of Saharan dust, values of 0.233 and 0.157 were found at 532 and 1064 nm, respectively. The proportion of aerosol optical thickness contained in the layers above the PBL against that contained in the PBL is in all cases greater or equal to 50 per cent. This emphasizes the importance of the upper layers in the scattering and absorption of astronomical signals. Additionally, for the first time, spaceborne lidar measurements were also compared to those of a ground lidar, in order to evaluate the use of a spaceborne active instrument for aerosol content monitoring at an astronomical site.