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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-430
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 14 May 2018

Research article | 14 May 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Assessment of Sun photometer Langley calibration at the high-elevation sites Mauna Loa and Izaña

Carlos Toledano1, Ramiro González1, David Fuertes1,2, Emilio Cuevas3, Thomas F. Eck4,5, Stelios Kazadzis6, Natalia Kouremeti6, Julian Gröbner6, Philippe Goloub7, Luc Blarel7, Roberto Román1, África Barreto8,3,1, Brent N. Holben4, and Victoria E. Cachorro1 Carlos Toledano et al.
  • 1Group of Atmospheric Optics, University of Valladolid (GOA-UVa), Spain
  • 2GRASP-SAS, Lille, France
  • 3Izaña Atmospheric Research Center, Meteorological State Agency of Spain (AEMET), Tenerife, Spain
  • 4NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • 5Universities Space Research Association, Columbia, MD, USA
  • 6Physikalisch-Meteorologisches Observatorium Davos, World Radiation Center – PMOD/WRC, Davos, Switzerland
  • 7Laboratory of Atmospheric Optics, University of Lille, Villeneuve d’Ascq, France
  • 8Cimel Electronique, Paris, France

Abstract. The aim of this paper is to analyze the suitability of the high-mountain stations Mauna Loa and Izaña for Langley plot calibration of Sun photometers. Thus the aerosol optical depth (AOD) characteristics and seasonality, as well as the cloudiness, have been investigated in order to provide a robust estimation of the calibration accuracy, as well as the number of days that are suitable for Langley calibrations. The data used for the investigations belong to AERONET and GAW-PFR networks, which maintain reference Sun photometers at these stations with long measurement records: 22 years at Mauna Loa and 15 years at Izaña. In terms of clear sky and stable aerosol conditions, Mauna Loa (3397m a.s.l.) exhibits on average of 377 Langleys (243 morning and 134 afternoon) per year suitable for Langley plot calibration, whereas Izaña (2373m a.s.l.) shows 343 Langleys (187 morning and 155 afternoon) per year. The background AOD (500nm wavelength) values, on days that are favorable for Langley calibrations, are in the range 0.01–0.02 throughout the year, with well-defined seasonality that exhibits a spring maximum at both stations plus a slight summer increase at Izaña. The statistical analysis of the long-term determination of extraterrestrial signals yields to a calibration uncertainty of ~0.2–0.5%, being this uncertainty smaller in the near infrared and larger in the ultraviolet wavelengths. This is due to atmospheric variability that cannot be reduced based only on quality criteria of individual Langely plots.

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Short summary
Most of the ground-based radiometric networks have their reference instruments and/or calibrate them at Mauna Loa or Izaña. The suitability of these high-mountain stations for absolute radiometric calibrations is investigated with the support of 20 years of first-class Sun photometer data from the AERONET and GAW-PFR networks. We analyze the number of calibration days at each site in a climatological sense and investigate the accuracy of the calibrations based on long-term statistics.
Most of the ground-based radiometric networks have their reference instruments and/or calibrate...
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