Here's the figures from the National Environmental Technical Centre.
Code:
2.6 Emissions for road vehicles (per vehicle kilometre) in urban conditions
Index car without three-way catalyst: pre 1993 = 100 1
Carbon Hydro- Oxides of Particu- Carbon
monoxide carbons nitrogen 2 lates 3 Dioxide
Petrol car without three-way catalyst: pre 1993 100 100 100 11 100
Petrol car with three-way catalyst: 1993-1996 15 9 19 5 108
Petrol car with three-way catalyst: 1997- 10 4 9 5 108
DERV car: pre 1993 7 10 43 100 111
DERV car: 1993-1996 4 4 29 25 111
DERV car: 1997- 3 3 21 11 111
Petrol light goods vehicle without three way catalyst: pre 1994 151 120 114 22 216
Petrol light goods vehicle with three way catalyst: 1994-1997 30 6 21 11 216
Petrol light goods vehicle with three way catalyst: 1998- 21 3 9 11 216
DERV light goods vehicle: pre 1994 10 20 82 209 192
DERV light goods vehicle: 1994-1997 8 15 40 66 192
DERV light goods vehicle: 1998- 6 9 30 33 192
Heavy goods vehicle - Rigid: pre 1993 38 192 640 484 406
Heavy goods vehicle - Rigid: 1993-1996 21 113 440 318 406
Heavy goods vehicle - Rigid: 1997- 17 105 316 168 406
Heavy goods vehicle - Artic: pre 1993 44 183 1,704 700 661
Heavy goods vehicle - Artic: 1993-1996 22 87 893 482 661
Heavy goods vehicle - Artic: 1997- 18 78 650 185 661
Buses: pre 1993 63 83 795 458 596
Buses: 1993-1996 28 90 859 304 596
Buses: 1997- 22 84 614 187 596
Motorcycle (less than 50cc): two stroke 37 135 2 - -
Motorcycle (greater than 50cc): two stroke 74 338 4 - -
Motorcycle (greater than 50cc): four stroke 67 68 13 - -
1 For diesel particulates index is DERV car: pre 1993 = 100. 020-7944 6399
2 Figures based on non methane hydrocarbons. The figures in this table are outside the
3 Legislative standards exist only for diesel vehicles. scope of National Statistics.
Petrol figures included for comparison. Source - National Environmental Technology Centre
As you can see, buses pump out hugely larger amounts of NOx and particulates than modern petrol cars.
IIRC average bus occupancy in London runs at around 36%, which sounds about right.
Bus occupancy can only average 100% if it's filled to capacity, terminus to terminus, on both the outward and inward journey, on every scheduled journey throughout the day.
Cheers
Peter