Autonomous marine hyperspectral radiometers for determining solar irradiances and aerosol optical properties
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 direct and diffuse hyperspectral irradiance in the wa...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-05-01
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| Series: | Atmospheric Measurement Techniques |
| Online Access: | http://www.atmos-meas-tech.net/10/1723/2017/amt-10-1723-2017.pdf |
| Summary: | 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 direct and diffuse
hyperspectral irradiance in the wavelength range 350–1050 nm at 6 nm
(Spectrometer 1) or 3.5 nm (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 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 <i>R</i><sup>2</sup>
∼ 0.96; (2) irradiant energy, with
<i>R</i><sup>2</sup> ∼ 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 and 5 %. 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 ( ∼ 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 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 ∼ 0.02–0.03 with <i>R</i><sup>2</sup> > 0.95 at
wavelengths 440, 500, 670 and 870 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 nm</sub> ranging between 0.05 in the clear background marine aerosol
regions and ∼ 0.5 within the Saharan dust plume. The RMSE
between the handheld Microtops sun photometer and Spectrometer 2 was between
0.047 and 0.057 with <i>R</i><sup>2</sup> > 0.94. |
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| ISSN: | 1867-1381 1867-8548 |