NOx
NOx (mixture of NO and NO2) is dirty brown in colour; it is a key ingredient in smog that most of us have seen around cities in inversion conditions.
I would suppose (never read it definitively) that NOx is the main cause of brown exhaust trails. You can see the brown tint quite clearly even on NG jets if you stand in a position to look up the exhaust trail - looking up the length of the exhaust as it were.
Regarding the CO2 numbers quoted earlier - the numbers weren't quite right Specnut. 12t of C reacts with 32t of O2 to make 44t of CO2.
So 80t of C in Jet A(1) should react with air to give 293t of CO2.
cheers,
O8