Abstract
Abstract. Langley plots are used to calibrate sun radiometers primarily for the measurement of the aerosol component of the atmosphere that attenuates (scatters and absorbs) incoming direct solar radiation. In principle, the calibration of a sun radiometer is a straightforward application of the Bouguer–Lambert–Beer law V = V0e−τ ⋅ m, where a plot of ln(V) voltage vs. m air mass yields a straight line with intercept ln(V0). This ln(V0) subsequently can be used to solve for τ for any measurement of V and calculation of m. This calibration works well on some high mountain sites, but the application of the Langley plot calibration technique is more complicated at other, more interesting, locales. This paper is concerned with ferreting out calibrations at difficult sites and examining and comparing a number of conventional and non-conventional methods for obtaining successful Langley plots. The 11 techniques discussed indicate that both least squares and various non-parametric techniques produce satisfactory calibrations with no significant differences among them when the time series of ln(V0)'s are smoothed and interpolated with median and mean moving window filters.
Highlights
Langley plots are used to determine the instrumental constant V0, i.e., to calibrate, sun radiometers from a series of measurements Vi at various air masses mi
We describe some aspects of the objective algorithm (OA) because (a) its development is an excellent example of how a mathematical method was stimulated by the human eye-andmind approach, (b) it is based on physical phenomena that are responsible for the curve shape and the outliers, and (c) it is basically a non-parametric method despite the fact that least-square fit (LSF) is used for the final filtering
When Harrison and Michalsky (1994) developed the OA they tested it by comparing a set of cases from 384 days using 500 nm channel data where α and β were obtained by the eye-and-mind method of Michalsky, who disqualified the non-viable cases and identified the ones that, after the removal of outliers, produced Langley plots
Summary
Langley plots are used to determine the instrumental constant V0, i.e., to calibrate, sun radiometers from a series of measurements Vi at various air masses mi. The BLB law implies that the slope of the straight line fitted to points τi, 1/mi estimates the calibration constant correction factor (Cachorro et al, 2004, 2008) Finding this slope is a mathematically equivalent approach to finding the intercept in the Langley plot method. Photometers that measure aureole radiance simultaneously with direct solar flux can be calibrated when optical depth is not constant (Tanaka et al, 1986; Nieke et al, 1999; Zieger et al, 2007) These methods can identify anomalous Langley events and estimate the value of ε. The main objective of the paper is to analyze the efficacy of non-parametric and least squares methods of straight-line fitting to identify Langley plots useful for calibration.
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
