Langley extrapolation

Abstract: High altitude balloon based facilities can make solar cell calibration measurements above 99.5% of the atmosphere to use for adjusting laboratory solar simulators. While close to on-orbit illumination, the small attenuation to the spectra may result in under measurements of solar cell parameters. Variations of stratospheric weather, may produce flight-to-flight measurement variations. To support the NSCAP effort, this work quantifies some of the effects on solar cell short circuit current (Isc) measurements on triple junction sub-cells. This work looks at several types of high altitude methods, direct high altitude measurements near 120 kft, and lower stratospheric Langley plots from aircraft. It also looks at Langley extrapolation from altitudes above most of the ozone, for potential small balloon payloads. A convolution of the sub-cell spectral response with the standard solar spectrum modified by several absorption processes is used to determine the relative change from AM0, Isc/Isc(AM0). Rayleigh scattering, molecular scattering from uniformly mixed gases, Ozone, and water vapor, are included in this analysis. A range of atmospheric pressures are examined, from 0.05 to 0.25 Atm to cover the range of atmospheric altitudes where solar cell calibrations are performed. Generally these errors and uncertainties are less than 0.2%.

Abstract: The polarization sensitivity of a Brewer MKIII double spectrophotometer was measured in the laboratory. We found two major sources of polarization sensitivity. 1) The flat quartz plate as the first optical element alters the polarization state of the transmitted light by Fresnel reflection at oblique incident angles. 2) The internal grating produces almost 100% polarization of the incident light perpendicular to the direction of the ruled grating. The combination of both effects results in a zenith angle (ZA) dependence of the instrument’s sensitivity for unpolarized input such as from Direct Sun measurements. The Brewer is 2% more sensitive at ZA=0° and 10% less sensitive at ZA=80° with respect to normal incidence (ZA=35°). Since the ZA-dependence is independent of wavelength this effect cancels out when calculating wavelength-ratios as used for total ozone retrieval. However the ZA-dependence causes errors when absolute signals at single wavelengths are needed as for aerosol optical depth (AOD) retrievals. Based on our laboratory measurements an overestimation of the Langley extrapolation between 3% and 5% is estimated even at best atmospheric conditions. The ZA-dependence causes 0.025-0.045 overestimation of AOD and an underestimation of the Angstrom exponent. We believe that this effect has not been detected from Brewer AOD-measurements since it is masked by larger uncertainty sources of other nature and AOD-comparisons to other instruments in the short UV-region are rare. Knowing the ZA-dependence it is possible to correct for the ZA-effect. We modified our Brewer by incorporating a depolarizer in its optical path and replacing the flat quartz window by a curved one, so that the input is always at normal incidence, which reduces the ZA-effect.

Abstract: The well-known Langley extrapolation technique produces measurements of atmospheric optical depth (AOD) by collecting direct sun irradiance at multiple zenith angles. One common application of this technique is used by sun photometers such as in NASA’s AErosol Robotic Network (AERONET). This large, spatially distributed network collects time averaging data from across the globe and applying Beer’s Law, produces hourly estimates of AOD. While this technique has produced excellent data, the dependence on direct sun irradiance requires cloudless skies and line-ofsight to the sun. Atmospheric LIDARs, on the other hand, can operate in the presence of clouds and can also produce range-resolved measurements of AOD by applying the same Langley technique. For aerosol LIDARs, this technique requires that the LIDAR be capable of producing high quality waveforms within the atmospheric coherence time and also be capable of taking measurements off zenith. At least two unique angles are required to produce data, although 3+ are recommended. This paper will present an overview of the Langley technique applied with a 1064 nm atmospheric aerosol LIDAR, an overview of the LIDAR hardware and capabilities, sample data collected by the LIDAR, and challenges associated with this technique. It will be shown that while this technique is useful, it requires measurements at all three angles to be made when the atmosphere is reasonably horizontally homogenous. Furthermore, the system optics, alignment, and laser power must be kept constant (keeping the LIDAR’s system constant the same for all measurements) for the data to be useful in a Langley analysis.

Abstract: To avoid the unnecessary needs to travel to high altitude for sunphotometers calibration, Perez-Dumotier calibration algorithm has been used as an objective means to select the right intensity data so that the calibration can be performed at any altitude levels. The governing theory of the algorithm was discussed in the previous chapter. This chapter presents information on how to implement the Perez-Dumotier calibration algorithm using actual field measurement. The implementation of the filtration procedure in step-by-step is discussed to render better framework of the proposed calibration algorithm. The aerosol retrieval inversion uses the extraterrestrial constant obtained from the final Langley plot to calculate retrieved AOD. The implementation example uses irradiance-matched technique by i-SMARTS radiative transfer code to derive corresponding reference AOD for validation purposes. The reliability of the technique was substantiated by radiative closure experiment to verify the promising direct solar irradiance to accurately derive the reference AOD values.