The problem comes with that it is a blunt tool and ignores that over 1/2 of particulates from vehicles are from brakes, tyres and the road itself. swapping out a car to one that produces slightly less particulates from the exhaust doesn't have as much an effect as is implied. Combined with the fact that a new car had to be created to take the place of a perfectly working vehicle with all the pollution and energy involved in that. My 12yr old car gets 50+mpg and has a cat and DPF filter but is euro 4 so i would get charged £9 going into Bristol (my nearest ULEZ). My dads car that is 15yrs old petrol that does about 35mpg can go in for free. If I bought a new diesel BMW X5 rolling on 315/35/20's would I actually be producing less particulates, than my older euro 4 car, given the larger tyres and weight? A Tesla model 3 weighs around 1700kgs my car is 1450kg, so is ~250kg lighter when I'm scrubbing my tyres around the city (besides the pollution the tesla is causing from elec generation).
Perhaps a better system would be based on both emission category and weight, as weight is going to have a significant effect on tyre, brake and road surface particulates.
None of the emission standards matter it’s all about road charging.