Measurement of the Specific Surface Area of Snow with the Nitrogen Adsorption Technique

Abstract

Few data exist for the surface area of snow despite its importance for modeling wet deposition and migration of reversibly deposited contaminants in snow. A method for determining the surface area of snow is described that uses a commercial nitrogen adsorption instrument (Gemini III 2375, Micromeritics Instruments Inc.). The method uses a sample vial containing snow and a reference vial containing an equal amount of snow that has been melted and refrozen in order to minimize its surface area. The basic quantity measured is the difference in volumes of nitrogen delivered to the sample and reference vials. This volume difference must be corrected for the free space difference in the vials before the adsorbed volume of nitrogen can be inferred. Two methods for obtaining this correction are discussed, measured, and estimated, and the results of BET analysis are presented for both types of correction. Fresh snow samples collected in southern Ontario exhibit type II isotherms with moderate to small BET constant values. Snow surface area ranges from 0.06 to 0.37 m2/g. Comparison with literature data for the dimensions of snow crystals suggests that the BET surface area of snow is comparable to the surface area measured by microscopy techniques.