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We have developed two hyperspectral radiometer systems whichrequire no moving parts, shade rings or motorised tracking, making themideally suited for autonomous use in the inhospitable remote marineenvironment. Both systems are able to measure direct and diffusehyperspectral irradiance in the wavelength range 350–1050 nm at 6 nm(Spectrometer 1) or 3.5 nm (Spectrometer 2) resolution. Marine field trialsalong a 100° transect (between 50° N and 50° S)of the Atlantic Ocean resulted in close agreement with existing commerciallyavailable instruments in measuring (1) photosynthetically availableradiation (PAR), with both spectrometers giving regression slopes close tounity (Spectrometer 1: 0.960; Spectrometer 2: 1.006) and R2 ∼  0.96; (2) irradiant energy, withR2 ∼  0.98 and a regression slope of 0.75 which can beaccounted for by the difference in wavelength integration range; and(3) hyperspectral irradiance where the agreement on average wasbetween 2 and 5 %. Two long duration land-based field campaigns of up to 18months allowed both spectrometers to be well calibrated. This was alsoinvaluable for empirically correcting for the wider field of view (FOV) ofthe spectrometers in comparison with the current generation of sunphotometers ( ∼  7.5° compared with  ∼  1°). The need for this correction was also confirmed andindependently quantified by atmospheric radiative transfer modelling andfound 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, theRMSE in retrievals of AOD when compared with a CIMEL sun photometer werereduced to  ∼  0.02–0.03 with R2 > 0.95 atwavelengths 440, 500, 670 and 870 nm. Corrections for the FOV as well as shipmotion were applied to the data from the marine field trials. This resultedin AOD500 nm ranging between 0.05 in the clear background marine aerosolregions and  ∼  0.5 within the Saharan dust plume. The RMSEbetween the handheld Microtops sun photometer and Spectrometer 2 was between0.047 and 0.057 with R2 > 0.94

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