ACE-1 Data Workshop June 1996 Report on Working Group on Air Mass Origin. Group Leader S. Whittlestone This report covers two groups entitled: Air Mas Characterisation (tuesday pm) and Baseline Criteria (Wednesday pm) The concept of air mass characterisation was agreed to be too broad. The term could validly embrace any aspect of the ACE-1 program, including characterisation of the turbulent dynamics of frontal systems, and study of the conditions accompanying high concentrations of ultrafine particles. It was decided to limit the scope of the group to application of air back trajectory calculations and the various tracers measured during ACE-1, to characterising the recent origin of air masses, on distance scales from tens to hundreds of kilometres. Accordingly, the group decided to call itself the "air mass origin" group. Species acceptable as general tracers had to be relevant to more than one platform, and preferably not too closely related to the species being studied. These terms of reference eliminated methyl iodide, or DMS, which would be used as tracers only by the aircraft platform for detecting vertical mixing of boundary layer air to the free troposphere. Three classes of air mass were defined: continental, urban and baseline. As usual in such discusions, "baseline" was criticised as a non-scientific concept. However, there is no harm in using it provided it is clear what is meant. In this case "baseline" will mean air that has concentrations of the continental and urban tracers below an agreed threshold. The group was more interested in baseline criteria than conditions in which an air mass could be considered strongly continental or urban. In practice this was important only for Cape Grim, because the other platforms did not have long periods of strongly non-baseline conditions. There were not enough data to divide the samples into more than two groups for each tracer. There was some discussion of the need to wait for a condition to persist for some time before regarding the origin as being established. For baseline, two hours was considered acceptable. For an indication of urban or continental air, the periods for which the particular origin persisted were so short that it was not useful to include a persistence requirement. The following table presents a summary of the present status of three tracers. Species Origin threshold comment radon continental CG 150 mBq m-3 Value derived by Covert after studying the ACE-1 data. MI 100 Value recommended by SS 100 Whittlestone. DI 100 elemental urban J. Gras will examine data and carbon (ec) advise group. condensation urban CG 1000 Suggested by Covert. nuclei (CN) Whittlestone undertook to examine the radon and CN data to find "interesting" events, and to obtain back trajectories. Radon can be used to verify the accuracy of the trajectory and whether passage over land was accompanied by mixing from ground level into the air mass. The trajectory indicates which land mass was the source of the radon. As new threshold levels are provided, an air mass origin data set will be updated, and the ACE-1 PI's notified by email.