Spectral signatures of submicron scale light-absorbing impurities in snow and ice using hyperspectral microscopy

Anna Dal Farra,Thomas D Bucheli,Susan Kaspari,Margit Schwikowski,Michael Schaepman,James Beach

doi:10.1017/jog.2018.29

Abstract

ABSTRACTLight-absorbing impurities (LAI) can darken snow and ice surfaces, reduce snow/ice albedo and accelerate melt. Efforts to allocate the relative contribution of different LAI to snow/ice albedo reductions have been limited by uncertainties in the optical properties of LAI. We developed a new method to measure LAI spectral reflectance at the submicron scale by modifying a Hyperspectral Imaging Microscope Spectrometer (HIMS). We present the instrument's internal calibration, and the overall small influence of a particle's orientation on its measured reflectance spectrum. We validated this new method through the comparison with a field spectroradiometer by measuring different standard materials. Measurements with HIMS at the submicron scale and the bulk measurements of the same standard materials with the field spectroradiometer are in good agreement with an average deviation between the spectra of 3.2% for the 400–1000 nm wavelength range. The new method was used (1) to identify BC (black carbon), mineral dust including hematite and the humic substances present in an environmental sample from Plaine Morte glacier and (2) to collect the individual reflectance spectra of each of these types of impurity. The results indicate that this method is applicable to heterogeneous samples such as the LAI found in snow and ice.

Highlights

Glaciers and the seasonal snowpack are important sources of water in many regions (Jansson and others, 2003; Barnett and others, 2005), and are essential components of the Earth’s climate system
We describe modifications made to Hyperspectral Imaging Microscope Spectrometer (HIMS) to allow particle reflectance to be measured at the submicron scale
We present the instrument’s internal calibration, a test done to identify the optimal sample substrate for measurement and the results comparing reflectance spectra of reflectance standards, Diesel soot, mineral standards and humic substances at the particle scale measured with the HIMS to spectra of the bulk materials obtained with a spectroradiometer that has previously been used for measuring light absorbing impurities (LAI) (FieldSpec 3, ASD Inc., PANanalytical) (Kaspari and others, 2015; Naegeli and others, 2015)

Summary

Introduction

Glaciers and the seasonal snowpack are important sources of water in many regions (Jansson and others, 2003; Barnett and others, 2005), and are essential components of the Earth’s climate system. Widespread decline in glacier size and snowpack depth as well as extent, has occurred in recent decades (IPCC, 2013). Previous studies have suggested that along with rising temperatures, a decrease in surface albedo plays a role in their decline (Hock, 2005; Flanner and others, 2007). Albedo is influenced by the ageing of snow, which affects snow grain size (Dozier and Painter, 2004); the presence of liquid water (Colbeck, 1979; Gardner and Sharp, 2010; Brun, 2017); the exposure of underlying bare ice on glaciers; and the presence of light absorbing impurities (LAI) (Warren and Wiscombe, 1980; Flanner and others, 2009). LAI cause darkening of the snow/ice surface (i.e., reduce the albedo), resulting in greater absorption of solar energy, heating of the snow/ice and accelerated snow and glacier melt. In regions with high LAI deposition, LAI can be a larger driver of snow/ice melt than temperature (Hansen and Nazarenko, 2004; Ramanathan and Carmichael, 2008)

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