Rapid and accurate measurement of the specific surface area of snow using infrared reflectance at 1310 and 1550 nm

Abstract

Even though the specific surface area (SSA) of snow is a crucial variable to determine the chemical and climatic impact of the snow cover, few data are available on snow SSA because current measurement methods are not simple to use in the field or do not have a sufficient accuracy. We propose here a novel determination method based on the measurement of the hemispherical reflectance of snow in the infrared using the DUFISSS instrument (DUal Frequency Integrating Sphere for Snow SSA measurement). DUFISSS uses 1310 and 1550 nm radiation provided by laser diodes, an integrating sphere 15 cm in diameter, and InGaAs photodiodes. For SSA<60 m2 kg−1, we use the 1310 nm radiation, reflectance is in the range 15 to 50% and the accuracy is 10%. For SSA>60 m2 kg−1, snow is usually of low to very low density (typically 30 to 100 kg m−3) and this produces artifacts caused by the e-folding length of light in snow being too long. We therefore use 1550 nm radiation for SSA>60 m2 kg−1. Reflectance is then in the range 5 to 12%, and the accuracy is 12%. No effect of crystal shape on reflectance was detected. We propose empirical equations to determine SSA from reflectance at both wavelengths, with that for 1310 nm taking into account the snow density. DUFISSS has been tested in the Alps to measure the snow area index (SAI) of the Alpine snowpack in a south facing area at 2100 m elevation. This was done by measuring the SSA, thickness and density of the seven main layers of the snowpack in just 30 min, and a value of 5350 was found, significantly greater than in Arctic and subarctic regions. DUFISSS can now be used to help study issues related to polar and Alpine atmospheric chemistry and climate.