Sequential outflow boundaries

Another noob question here......I've been watching the storms fire up on the Panhandle this evening and on the radar images you can see sequential outflow boundaries moving southward from the western portion of the line. In some radar frames I can make out as many as four distinct boundaries pushing south ahead of the advancing line.

I was under the impression from the answers posted to my previous question on outflow boundaries that there were two different air masses on each side of the boundary. Does that mean that in a sequential set of boundaries the air behind each boundary is slightly cooler/denser than the previous "pulse" that created the previous outflow? Or is the air the same, it's just that the ambient temps at the surface have weakened the air masses of the initial outflow boundaries?
 
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