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" 

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 

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 

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 

The following table presents a summary of the present status of three 

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.