Articles | Volume 9, issue 1
https://doi.org/10.5194/amt-9-215-2016
https://doi.org/10.5194/amt-9-215-2016
Research article
 | 
25 Jan 2016
Research article |  | 25 Jan 2016

Non-parametric and least squares Langley plot methods

P. W. Kiedron and J. J. Michalsky

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 = V0eτ ⋅ 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.

Download
Short summary
Langley plots are plots of the ln(V0) vs. air mass m used to calibrate sun radiometers, mainly, to determine the aerosol component of the atmosphere assuming attenuation follows V = V0·exp(−τ·m). This is simple if the atmosphere is stable since the intercept of this plot is ln(V0), which can then be used to solve for tau for any measured V. The atmosphere is not stable; therefore, 11 methods to measure V0s and determine robust estimates of V0s for interesting field sites are discussed.