Airborne system for fast measurements of upwelling and downwelling spectral actinic flux densities

Jäkel, Evelyn; Wendisch, Manfred; Kniffka, Anke; Trautmann, Thomas

doi:10.1364/ao.44.000434

Cited by 31 publications

(47 citation statements)

References 25 publications

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“…The dark measurements before and after the lamp measurements were routinely made to allow for checking that there was no significant drift in dark signals during the calibration measurements. The method to improve the calibration accuracy by using two lamp distances was recommended by the manufacturer and was already applied for diode-array-based spectroradiometers (PDA-SRs; Kanaya et al, 2003;Jäkel et al, 2005;Bohn et al, 2008) and CCDSRs (Jäkel et al, 2007). The procedures described in the following were elaborated to improve the determination of stray-light signals and to utilize several integration times to obtain optimized spectral sensitivities.…”

Section: Radiometric Laboratory Calibrationmentioning

confidence: 99%

Calibration and evaluation of CCD spectroradiometers for ground-based and airborne measurements of spectral actinic flux densities

Bohn¹,

Lohse

2017

Atmos. Meas. Tech.

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Abstract. The properties and performance of charge-coupled device (CCD) array spectroradiometers for the measurement of atmospheric spectral actinic flux densities (280-650 nm) and photolysis frequencies were investigated. These instruments are widely used in atmospheric research and are suitable for aircraft applications because of high time resolutions and high sensitivities in the UV range. The laboratory characterization included instrument-specific properties like the wavelength accuracy, dark signal, dark noise and signal-tonoise ratio (SNR). Spectral sensitivities were derived from measurements with spectral irradiance standards. The calibration procedure is described in detail, and a straightforward method to minimize the influence of stray light on spectral sensitivities is introduced. From instrument dark noise, minimum detection limits ≈ 1 × 10 10 cm −2 s −1 nm −1 were derived for spectral actinic flux densities at wavelengths around 300 nm (1 s integration time). As a prerequisite for the determination of stray light under field conditions, atmospheric cutoff wavelengths were defined using radiative transfer calculations as a function of the solar zenith angle (SZA) and total ozone column (TOC). The recommended analysis of field data relies on these cutoff wavelengths and is also described in detail taking data from a research flight on HALO (High Altitude and Long Range Research Aircraft) as an example. An evaluation of field data was performed by ground-based comparisons with a double-monochromatorbased, highly sensitive reference spectroradiometer. Spectral actinic flux densities were compared as well as photolysis frequencies j (NO 2 ) and j (O 1 D), representing UV-A and UV-B ranges, respectively. The spectra expectedly revealed increased daytime levels of stray-light-induced signals and noise below atmospheric cutoff wavelengths. The influence of instrument noise and stray-light-induced noise was found to be insignificant for j (NO 2 ) and rather limited for j (O 1 D), resulting in estimated detection limits of 5 × 10 −7 and 1 × 10 −7 s −1 , respectively, derived from nighttime measurements on the ground (0.3 s integration time, 10 s averages). For j (O 1 D) the detection limit could be further reduced by setting spectral actinic flux densities to zero below atmospheric cutoff wavelengths. The accuracies of photolysis frequencies were determined from linear regressions with data from the double-monochromator reference instrument. The agreement was typically within ±5 %. Because optical-receiver aspects are not specific for the CCD spectroradiometers, they were widely excluded in this work and will be treated in a separate paper, in particular with regard to airborne applications.

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Section: Radiometric Laboratory Calibrationmentioning

confidence: 99%

Calibration and evaluation of CCD spectroradiometers for ground-based and airborne measurements of spectral actinic flux densities

Bohn¹,

Lohse

2017

Atmos. Meas. Tech.

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“…In particular 4 π sr aircraft applications with two optical receivers covering opposite hemispheres need extended considerations for two reasons. Firstly, SZA and polar angles may differ during flight manoeuvres unless technical equipment ensures compensating movements of the receivers (Jäckel et al, 2005). Secondly, measures to minimise cross talks to the opposite hemispheres are complicated by restrictions to the size of horizontal shadow rings for aerodynamic reasons.…”

Section: Dm-sr and Reference Instrumentmentioning

confidence: 99%

“…Examples for aircraft applications of actinic flux receiver optics can be found elsewhere (e.g. VolzThomas et al, 1996;Hofzumahaus et al, 2002;Jäckel et al, 2005).…”

Section: Dm-sr and Reference Instrumentmentioning

confidence: 99%

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Photolysis frequency measurement techniques: results of a comparison within the ACCENT project

Bohn¹,

et al. 2008

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Abstract. An intercomparison of different radiometric techniques measuring atmospheric photolysis frequencies j(NO2), j(HCHO) and j(O1D) was carried out in a two-week field campaign in June 2005 at Jülich, Germany. Three double-monochromator based spectroradiometers (DM-SR), three single-monochromator based spectroradiometers with diode-array detectors (SM-SR) and seventeen filter radiometers (FR) (ten j(NO2)-FR, seven j(O1D)-FR) took part in this comparison. For j(NO2), all spectroradiometer results agreed within ±3%. For j(HCHO), agreement was slightly poorer between −8% and +4% of the DM-SR reference result. For the SM-SR deviations were explained by poorer spectral resolutions and lower accuracies caused by decreased sensitivities of the photodiode arrays in a wavelength range below 350 nm. For j(O1D), the results were more complex within +8% and −4% with increasing deviations towards larger solar zenith angles for the SM-SR. The direction and the magnitude of the deviations were dependent on the technique of background determination. All j(NO2)-FR showed good linearity with single calibration factors being sufficient to convert from output voltages to j(NO2). Measurements were feasible until sunset and comparison with previous calibrations showed good long-term stability. For the j(O1D)-FR, conversion from output voltages to j(O1D) needed calibration factors and correction functions considering the influences of total ozone column and elevation of the sun. All instruments showed good linearity at photolysis frequencies exceeding about 10% of maximum values. At larger solar zenith angles, the agreement was non-uniform with deviations explainable by insufficient correction functions. Comparison with previous calibrations for some j(O1D)-FR indicated drifts of calibration factors.

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“…The Spectral Modular Airborne Radiation measurement sysTem (SMART)-Albedometer (Wendisch et al, 2001;Jäkel et al, 2005;Ehrlich et al, 2008) was installed on a Partenavia P68B aircraft to measure upward and downward irradiances (350-1000 nm) with an accuracy of 5 %. The entrance optics of the SMART-Albedometer were leveled during the flight using a horizontal stabilization system (Wendisch et al, 2001), which assures a clear separation between photons from the upper and the lower hemispheres.…”

Section: Airborne Radiation Measurementsmentioning

confidence: 99%

Influence of spatial heterogeneity of local surface albedo on the area-averaged surface albedo retrieved from airborne irradiance measurements

2013

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Spectral airborne upward and downward irradiance measurements are used to derive the area-averaged surface albedo. Real surfaces are not homogeneous in their reflectivity. Therefore, this work studies the effects of the heterogeneity of surface reflectivity on the area-averaged surface albedo to quantify how well aircraft measurements can resolve the small-scale variability of the local surface albedo. For that purpose spatially heterogeneous surface albedo maps were input into a 3-dimensional (3-D) Monte Carlo radiative transfer model to simulate 3-D irradiance fields. The calculated up- and downward irradiances in altitudes between 0.1 and 5 km are used to derive the area-averaged surface albedo using an iterative retrieval method that removes the effects due to atmospheric scattering and absorption within the layer beneath the considered level. For the case of adjacent land and sea surfaces, parametrizations are presented which quantify the horizontal distance from the coastline that is required to reduce surface heterogeneity effects on the area-averaged surface albedo to a given limit. The parametrization which is a function of altitude, aerosol optical depth, single scattering albedo, and the ratio of local land and sea albedo was applied for airborne spectral measurements. In addition, the deviation between area-averaged and local surface albedo is determined for more complex surface albedo maps. For moderate aerosol conditions (optical depth less than 0.4) and a wavelength range between 400 and 1000 nm, the altitude and the heterogeneity of the surface albedo are the dominant factors determining the mean deviation between local and area-averaged surface albedo. A parametrization of the mean deviation is applied to an albedo map that was derived from a Landsat image of an area in East Anglia (UK). Parametrization and direct comparison of local and area-averaged surface albedo show similar mean deviations (20% vs. 25%) over land

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Airborne system for fast measurements of upwelling and downwelling spectral actinic flux densities

Cited by 31 publications

References 25 publications

Calibration and evaluation of CCD spectroradiometers for ground-based and airborne measurements of spectral actinic flux densities

Calibration and evaluation of CCD spectroradiometers for ground-based and airborne measurements of spectral actinic flux densities

Photolysis frequency measurement techniques: results of a comparison within the ACCENT project

Influence of spatial heterogeneity of local surface albedo on the area-averaged surface albedo retrieved from airborne irradiance measurements

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