Cape Grim Air Archive
CAPE GRIM AIR ARCHIVE
Shortly after establishment of the Cape Grim Baseline Air Pollution Station in 1976, researchers started setting aside air from the site to preserve a record of atmospheric composition. This would make possible the future reconstruction of atmospheric histories of trace gases and isotopes that had not been previously measured or were yet to be discovered, or to better define records of previously measured species using improved analytical techniques.
The first Cape Grim Air Archive (CGAA) sample was collected on April 26, 1978. Regular sampling has continued ever since with typically 4-6 cylinders filled each year. At present there are about 170 CGAA cylinders in storage and available for future analyses.
The cylinders used in the early years (1978-1988) of the CGAA program have an interesting history that predates their use for scientific research. After being originally used as beverage containers, they were repurposed for storage of breathing oxygen on US military aircraft during World War II.
The upper photograph above is from February, 1942 showing the cylinders being cleaned and treated at the Firestone Rubber Company (Ohio, USA) in preparation for military use. During the war, the cylinders were used on some of the larger US aircraft of the period, including bombers such as the B-17 Flying Fortress and B-24 Liberator as oxygen cylinders for breathing masks (lower photograph above; historical information provided by John White, Senior Curator of Military Technology, Australian War Memorial). Decades later some of these cylinders were acquired by scientists for use in atmospheric research applications.
Since the late 1980s, cylinders used for the bulk of CGAA samples have been purpose-made and supplied by Essex Industries (Missouri, USA). Before being filled with air, the cylinders are internally electropolished to minimise interaction of the stainless steel surfaces with the contained air. The cylinder design currently in use allows air to be stored at 60 atmospheres pressure.
TRACE GAS MEASUREMENTS
The CGAA has been analysed for concentrations and/or isotopic ratios of more than 100 different trace gas species. Research activities conducted by CSIRO, often in collaboration with one of numerous organisations around the world, have generated over 100 scientific publications thus far (see list of publications). This work has helped define changing atmospheric composition over the past 40 years, and improved the understanding of processes affecting atmospheric levels of greenhouse, ozone-depleting and other miscellaneous trace gas species. CGAA records figure prominently in Ozone Assessment and Intergovernmental Panel on Climate Change (IPCC) scientific reports. They have also played an important role in linking direct atmospheric measurements with ancient records derived from air trapped in polar ice sheets.
The CGAA has been pivotal in defining the forcing of stratospheric ozone loss by halogenated trace gases since 1978. Many of these gases are man-made and used for a variety of industrial applications. In many cases, their emissions only commenced in recent years or decades, and their presence in the atmosphere was detected some time later. The CGAA has proved to be the optimal resource for high time-resolution reconstruction of the atmospheric histories of these gases.
One example shown here is the hydrochlorofluorocarbon HCFC-133a that was only recently discovered in the atmosphere. Direct measurements of its atmospheric concentration only commenced in 2014. The ability to analyse old air from the Cape Grim Air Archive has extended the record back to 1978 (Laube et al., Nature Geoscience, 2014).