Petrol cars are not necessarily less polluting than diesel. Both produce Nitrous Oxide. Euro 6 emissions standards: what do they mean for you? The new European limits I notice show petrol cars are allowed to produce a certain amount of pollutants like NOx. There are increasing limits on pollutants. That have been brought in over several years. So just getting rid of diseal buses will not solve the problem. The other issue I have with concentrating on air quality is that this ignores the contribution of traffic to climate change through CO2 emmisions. For a city like London what is needed is reduction of car use and increase in public transport.