Autonomous marine hyperspectral radiometers for determining solar irradiances and aerosol optical properties

Article, Other literature type English OPEN
Wood, John ; Smyth, Tim J. ; Estellés, Victor (2017)
  • Publisher: Copernicus Publications
  • Journal: (issn: 1867-8548, eissn: 1867-8548)
  • Related identifiers: doi: 10.5194/amt-2016-373, doi: 10.5194/amt-10-1723-2017
  • Subject: TA170-171 | TEC | Earthwork. Foundations | EA0 | EAS | Environmental engineering | ATM | MET | TA715-787 | MAR

We have developed two hyperspectral radiometer systems which require no moving parts, shade rings or motorised tracking making them ideally suited for autonomous use in the inhospitable remote marine environment. Both systems are able to measure the direct and diffuse hyperspectral irradiance fields in the wavelength range 350&ndash;1050&thinsp;nm at 6&thinsp;nm (Spectrometer 1) or 3.5&thinsp;nm (Spectrometer 2) resolution. Marine field-trials along a 100° transect (between 50°&thinsp;N and 50°&thinsp;S) of the Atlantic Ocean resulted in close agreement with existing commercially available instruments in measuring: (1) photosynthetically available radiation (PAR) with both spectrometers giving regression slopes close to unity (Spectrometer 1: 0.960; Spectrometer 2: 1.006) and R<sup>2</sup>&thinsp;~&thinsp;0.96; (2) irradiant energy, with R<sup>2</sup>&thinsp;~&thinsp;0.98 and a regression slope of 0.75 which can be accounted for by the difference in wavelength integration range and; (3) hyperspectral irradiance where the agreement on average was between 2&ndash;5&thinsp;%. Two long duration land based field campaigns of up to 18 months allowed both spectrometers to be well calibrated. This was also invaluable for empirically correcting for the wider field-of-view (FOV) of the spectrometers in comparison with the current generation of sun photometers (~&thinsp;7.5° compared with ~&thinsp;1°). The need for this correction was also confirmed and independently quantified by atmospheric radiative transfer modelling and found to be a function of aerosol optical depth (AOD) and solar zenith angle. Once Spectrometer 2 was well calibrated and the FOV effect corrected for, the RMSE in retrievals of AOD when compared with a CIMEL sun photometer were reduced to ~&thinsp;0.02&ndash;0.03 with R<sup>2</sup>&thinsp;>&thinsp;0.95 at wavelengths 440, 500, 670 and 870&thinsp;nm. Corrections for the FOV as well as ship motion were applied to the data from the marine field trials. This resulted in AOD<sub>500&thinsp;nm</sub> ranging between 0.05 in the clear background marine aerosol regions to ~&thinsp;0.5 within the Saharan dust plume. The RMSE between the handheld Microtops sun photometer and Spectrometer 2 was between 0.047&ndash;0.057 with R<sup>2</sup>&thinsp;>&thinsp;0.94.
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