Red and Green Snow Algae Surface Spectra

Surface spectra of red and green snow algae were collected at two sites on King George Island (KGI), the largest of the South Shetland Islands, and one site on northern Nelson Island (NI), southwest of KGI in January 2018. Optically thick (> 30cm) snow packs were prioritized for spectral albedo data acquisition and corresponding snow algae sampling in order to minimize the impact of the underlying ground on spectral albedo. Sites were also selected based on where it was possible to sample 1) a control site with relatively clean snow having no visible snow algae 2) green snow algae, 3) red snow algae and 4) mixed-phase green and red algae. At each site, duplicates of each snow type were measured with the spectrometer (except at Nelson Island where only one Mixed site was observed). All samples were collected around noon local Chilean time, when the seasonal snow pack was also receiving the most incoming solar radiation. Spectral reflectance measurements were collected with an Analytical Spectral Devices (ASD) FieldSpec® 4 hyperspectral spectroradiometer (Malvern Panalytical, USA) between 350 and 2500 nm. The sensor was equipped with a light-diffusing fore optic remote cosine receptor (RCR) to measure planar irradiance. We selected three different locations and collected spectral measurements for two samples each of green, red, and mixed snow algae patches, and two algae-free or “clean” snow areas, for a total of 24 measurement sites (2 of each of the 4 types across the 3 sites). Areas with snowmelt ponding were avoided. The RCR was placed upward to collect the downwelling planar irradiance incident upon the snow surface (Ed) and the upwelling planar irradiance reflected from the snow (Eu). Measurements were collected in triplicate. The operator was located in a direction 90 - 135º away from the sun to minimize solar glint and self-shadowing. Snow conditions did not allow for a tripod, so nadir orientation was determined by practice with a level and by visual assistance of an observer. Since the measurements were carried out under heavily overcast conditions where irradiance is dominated by the diffuse insolation with no solar azimuthal dependence, the influence of slight tilt when measuring the downwelling irradiance (i.e. the cosine error) is expected to be minor (<0.5%). The reflectance measurements were taken prior to excavation of snow sample for laboratory analysis. Post-processing of the data involved computing spectral reflectance, as the ratio of the upwelling flux normalized to the downwelling flux for each wavelength. The mean of the three measurements was calculated for each site. Ambient light conditions were too low in the short-wave infrared wavelengths for getting adequate signal-to-noise for our measurements. In post-processing, reflectance values were truncated at 1350 nm for this analysis. This value represents the limit often used for RF calculations in other studies. In addition, empirical correction coefficients were used to correct for temperature related radiometric inter-channel steps using the procedure and MATLAB code from Hueni et al. (2017). This removed the step function near 1000 nm for most of the spectra, although not fully for all spectra. However, this discontinuity does not significantly impact results or albedo calculations. Albedo was calculated as the integrated R in two different intervals: visible (400-700 nm) and infrared (700-1300 nm).