Abstract
Abstract. 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 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 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 R2 > 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 AOD500 nm 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 R2 > 0.94.
Highlights
Tiny particles within the atmosphere, collectively known as aerosols, play a key role in the functioning of the Earth System as a whole
In this paper we describe a similar concept, but with the following important differences in construction: (1) use of a unique etched shadow design (Badosa et al, 2014), to remove the need for moving parts for splitting the irradiance into the global and diffuse components (2) use of hyperspectral radiometers to give finer spectral detail and aerosol optical characterisation
Following analysis of the orientation values after the AMT cruise, the VectorNav yaw values showed significant drift compared to the yaw values calculated from both the GPS track and the ship’s heading record
Summary
Tiny particles within the atmosphere, collectively known as aerosols, play a key role in the functioning of the Earth System as a whole. Aerosols produced in the marine environment by breaking waves, wind-driven spume and bubble bursting are generally large (up to 10 μm) but relatively low in number. They have a high single-scattering albedo (> 0.95) and absorb a relatively small proportion of incoming solar radiation. Just from these two simple examples it can be seen that aerosol type will have a large bearing on the local, regional
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