Presumably the ban in cities will also include all buses, coaches, vans and lorries, nearly all of which use diesel engines. A much more intelligent approach would be to use the German model which only allow vehicles that meet the latest EU6 standard to got into large towns and cities.
If all diesel engine vehicles are banned and petrol is used instead what will happened to the C02 emissions? Diesel engines still produce less C02 than petrol engines. One day perhaps electric vehicles will replace both, but then we will need more power stations to cope with the demand and no doubt they will produce more C02. I don't think anyone has come up with a good solution as yet.