Where can I find good data on volume of air traffic?
For $799 you can buy IATA's World Air Transport Statistics (WATS)

IATA publish a few summary data values in various reports


Scheduled Passenger - Kilometres Flown
Scheduled Passengers Carried
IATA Annual Review 2013
IATA Annual Review 2014
IATA Statistics


Note that I think this is data is only collected from IATA members and much of the data is for western-built jets (figures for other manufacturers are much worse, according to comments in IATA reports)

Passenger Km
Total (International + Domestic)

Rank  Airline                   Millions
1     United Airlines           286,802
2     Delta Air Lines           277,560
3     Emirates                  209,377
4     American Airlines         206,551
5     China Southern Airlines   147,841
6     Southwest Airlines        145,124
7     Lufthansa                 144,236
8     Air France                136,405
9     British Airways           130,129
10    Qantas Airways            110,203