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

Wood, J; Smyth, TJ; Estelles, V. 2017 Autonomous marine hyperspectral radiometers for determining solar irradiances and aerosol optical properties. Atmospheric Measurement Techniques Discussions. 1-39. 10.5194/amt-2016-373

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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 – 1050nm at 6nm (Spectrometer 1) or 3.5nm (Spectrometer 2) resolution. Marine field-trials along a 100° transect (between 50°N and 50°S) of the Atlantic Ocean resulted in close agreement with existing commercially available instruments 5 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 R2 ~0.96; (2) irradiant energy, with R2~0.98 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 – 5%. Two long duration land based field campaigns of up to 18 months allowed both spectrometers 10 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 (~7.5° compared with ~1°). The need for this correction was also confirmed and independently quantified by atmospheric radiative transfer modelling and found to be a unction 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 ~0.02 – 0.03 with R2 15 > 0.95 at wavelengths 440, 500, 670 and 870nm. Corrections for the FOV as well as ship motion were applied to the data from the marine field trials. This resulted in AOD500nm ranging between 0.05 in the clear background marine aerosol regions to ~0.5 within the Saharan dust plume. The RMSE between the handheld Microtops sun photometer and Spectrometer 2 was between 0.047 – 0.057 with R2 > 0.94.

Item Type: Publication - Article
Subjects: Atmospheric Sciences
Earth Observation - Remote Sensing
Divisions: Plymouth Marine Laboratory > National Capability categories > Atlantic Meridional Transect
Depositing User: Tim Smyth
Date made live: 08 Mar 2017 17:01
Last Modified: 06 Jun 2017 16:18

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