An improved technique to measure firn diffusivity Alden Adolph ⇑ , Mary R. Albert Thayer School of Engineering, Dartmouth College, 14 Engineering Drive, Hanover, NH 03755, USA article info Article history: Received 30 January 2012 Received in revised form 28 December 2012 Accepted 11 February 2013 Available online 15 March 2013 Keywords: Gas diffusivity Firn Measurement Transport properties abstract Gas diffusivity in snow and firn is a basic transport parameter that is important for snow-air exchange processes and for ice core interpretation of past atmospheric composition. Firn is snow that is more than one year old, and it exists over many thousands of kilometers of the polar ice sheets, comprising approx- imately the upper 60–120 m of the ice sheet. Because use of firn air for climate and environmental inves- tigations is a relatively new science, and because of the difficulties in working with firn as a temperature- dependent porous material, few direct measurements of the gas diffusivity through the firn column have been made. This paper describes the development, testing, validation, and initial use of a diffusion cham- ber specifically designed to measure inert gas diffusivity on firn cores. A brief review of previous methods for determining gas diffusivity for various porous media is discussed. The new technique and apparatus are described. Results of validation of the technique on multiple sizes of glass beads are presented, including comparison to experimental and theoretical results from the literature. After this verification, initial results obtained from using the technique to measure the SF 6 gas diffusivity of firn samples from Summit, Greenland are presented; results ranged from 0.005 cm 2 /s to 0.033 cm 2 /s. This set of measure- ments is compared to the diffusivity of other natural porous media and also to the few previous measure- ments of gas diffusivity of firn. The improved technique offers a more direct measurement of the transport parameter than other techniques previously used, and offers good potential for future investi- gations aimed at understanding processes of firnification and gas trapping for ice core climate change research. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Inert gas diffusivity is a basic transport parameter of porous media. In commonly encountered materials such as soils or insula- tion, techniques for measuring gas diffusivity have long been used to determine this important parameter. A natural porous media termed ‘firn’, which is snow more than one year old, is ubiquitous as the surface 60–120 m in depth across the vast expanses of the Greenland and Antarctic polar ice sheets. In the cold polar locations where snow has rarely melted, firn serves as a natural archive of old air, reflecting past atmospheric composition. An improved understanding of the processes of gas diffusion through firn will facilitate improved understanding of past atmospheric composi- tion and ice core interpretation. Yet few published measurements of the gas diffusivity of firn exist, and those that do exist make use of techniques which are not optimal for determination of the gas diffusivity. Because the crystal structure of firn is temperature dependent, any measurement of its gas diffusivity requires special instrumentation. In addition, because acquisition of firn samples involves coring the material from the polar ice sheets, the method for measurement should be amenable to cored material. Methods developed for other porous media, such as soils, cannot be directly used for firn samples. Ice core investigations have played a large role in determining atmospheric composition over the past hundreds of thousands of years. Composition of the ancient atmosphere is determined by measuring gas concentrations in the bubbles that are trapped in the ice, and this ice was originally formed as snow deposited on the surface of the ice sheet. In the very cold locations where past atmospheres are studied, the snow has not melted but has contin- ually accumulated with the layers of snow built upon previously deposited layers. The snow that is more than one year old is re- ferred to as ‘firn’, and it and has become compacted due to over- burden pressure. With increasing depth, this firn becomes more and more compacted. At the surface, firn is extremely porous and gases can diffuse through the pore space very easily, but deeper in the column, the firn is increasingly compacted, and the pore spaces become smaller and less connected. At a point in the col- umn known as pore close off, the pores are no longer connected to one another; they have become independent bubbles of air. These bubbles remain in the ice structure and preserve the air that was at that depth when the pore closed. The complication that 0017-9310/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2013.02.029 ⇑ Corresponding author. Tel.: +1 504 606 5824; fax: +1 603 646 3856. E-mail addresses: alden.c.adolph@dartmouth.edu (A. Adolph), mary.r.albert@- dartmouth.edu (M.R. Albert). International Journal of Heat and Mass Transfer 61 (2013) 598–604 Contents lists available at SciVerse ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt