The worst single type of vehicle for air pollution is the black cab. The cab is a London icon, but it's a vehicle that was designed in the last century when air pollution was less well-understood. Johnson has made a bad situation worse by cancelling the 6-monthly emissions check for black cabs.
However, he's now considering a new 'very low emission zone', which may apply to all vehicles, including black cabs. There wouldn't seem much point in it not applying to black cabs, given that they account for 35% of tailpipe emissions.
The fact is that the London taxi is the least 'green' transport mode. Each cab weighs around 2 tonnes, yet the typical load factor (number of passengers being carried) is around one passenger per cab. Consequently, emissions of all types (CO2, NOx, particulates) per passenger are an order of magnitude greater than other transport types except for private cars. In central London, black cabs account for around 70% of traffic.