Smokey engines

Date:         25 Sep 97 01:39:44 
From:         "Ken O'Riordan" <>
Organization: Union Bank of Switzerland
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On a recent visit to the U.S.A I was surprised to notice how smokey the
exhaust of jet engines were.  Taking the 737 300/400/500 series as an
example, I could distinctly see a brown trail coming out of each engine
as the aircraft climbed away from the airport.

In Europe I don't see this.  Can anyone explain the difference?  Is it a
slightly different fuel mix or atmospherics or just higher use?