Working Group Report — Irreversible Species

Abstract

This group focused on aerosol species that deposit irreversibly on an ice sheet. Irreversibly depositing species include both primary aerosols emitted directly from sources and also secondary aerosols formed in the atmosphere from precursor gases. These species can be conveniently divided into several categories:

• 1. Soil-derived inorganic aerosols, including those containing Al, Ti, Si, Ca, Fe and several other elements. Al, Ti and Si are considered to be the best tracers of soil dust, although some of the Si in the polar regions may originate from coal combustion.

• 2. Seasalt inorganic aerosols, including those containing Na, CI and other elements. Na is considered to be one of the better tracers for this category. CI is prone to chemical reactions and subsequent losses, and a fraction of the CI in the polar regions may be from anthropogenic sources.

• 3. Anthropogenic inorganic aerosols, including those containing S, Pb, V, In, Zn, As, Cd and several other elements. The seven elements listed are all suitable tracers under many conditions.

• 4. Sulphate in older snow, and in Antarctica, derives mainly from marine biogenic emissions of DMS (producing also MSA), and from volcanic emissions.

• 5. Organic aerosols, including natural organics such as biogenically emitted aerosols as well as anthropogenic organics such as combustion products. However, not all organics can be considered irreversible.

The group considered three steps in the overall transfer of chemical species from source regions to their final resting place in the deep ice. The first step involves long-range atmospheric transport to the air over the ice sheet. This is followed by the second step in which chemical species reach the ice sheet surface. Finally, the third step involves physical and chemical changes that occur as the chemical species become buried by successive snowfall, and the snow changes to firn and ultimately to ice.