Abstract
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 (3397 m a.s.l.) exhibits on average 377 Langley plots (243 morning and 134 afternoon) per year suitable for Langley plot calibration, whereas Izaña (2373 m a.s.l.) shows 343 Langley plots (187 morning and 155 afternoon) per year. The background AOD (500 nm) 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.
Highlights
The Langley plot method (Shaw, 1983) is widely used for absolute calibration of Sun photometers
We have considered that for a certain period within a particular day, the Langley calibration is possible if σ < 0.006, the number of valid points is > 33 % of the initial number of observations (Harrison and Michalsky, 1994) and aerosol optical depth (AOD) (500 nm) < 0.025
The main AOD characteristics of the high-elevation sites Mauna Loa and Izaña have been analyzed in detail in order to quantify the characteristics of these locations for Langley plot calibration of Sun photometers
Summary
The Langley plot method (Shaw, 1983) is widely used for absolute calibration of Sun photometers. Mauna Loa is a reference site for radiometric observations and calibrations It was very early considered an ideal place for calibration of Sun photometers using the Langley technique (Shaw, 1979); it hosts reference instruments of the main radiometric networks. After years of continuous Sun photometer observations at the Mauna Loa and Izaña observatories, long and highquality measurement records are available, and the quantification of the calibration performance can be accomplished with the support of robust data sets. The aim of this paper is to analyze the capability of the two high-mountain stations Mauna Loa and Izaña for Langley plot calibration, in terms of aerosol characteristics, seasonality and cloudiness, and provide statistically robust quantification of calibration uncertainty. Several factors and physical processes affecting the performance of the Langley plots are analyzed
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
