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Volume 18, issue 19
Atmos. Chem. Phys., 18, 14555-14567, 2018
https://doi.org/10.5194/acp-18-14555-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Atmos. Chem. Phys., 18, 14555-14567, 2018
https://doi.org/10.5194/acp-18-14555-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 11 Oct 2018

Research article | 11 Oct 2018

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, Alberto Berjón1, Brent N. Holben4, and Victoria E. Cachorro1 Carlos Toledano et al.
  • 1Group of Atmospheric Optics, University of Valladolid (GOA-UVa), Valladolid, 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 Centre – 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 uncertainty as well as the number of days that are suitable for Langley calibrations. The data used for the investigations belong to the 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 (3397ma.s.l.) exhibits on average 377 Langley plots (243 morning and 134 afternoon) per year suitable for Langley plot calibration, whereas Izaña (2373ma.s.l.) shows 343 Langley plots (187 morning and 155 afternoon) per year. The background AOD (500nm) 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  ∼ 0.25–0.5%, this uncertainty being smaller in the visible and near-infrared wavelengths and larger in the ultraviolet wavelengths. This is due to atmospheric variability produced by changes in several factors, mainly the AOD. The uncertainty cannot be reduced based only on quality criteria of individual Langley plots and averaging over several days is shown to reduce the uncertainty to the needed levels for reference Sun photometers.

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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 uncertainty 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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