There are a few documents floating around about NOX emissions in london. I actually sat down and read them.
The TL;DR version is:
1: Cars used to account for about 2/3 of the NOX levels in inner london (inside the north/south circulars). They now account for about 1/2 - it should be lower if emissions statements were accurate and that's had people headscratching for a while.
1a: Lean-burn petrol engines emitted just as much nox (sometimes more) as diesels did (this is why they got banned in the USA by way of legislation mandating that petrol engines must always burn fuel stoiciometrically)
2: MOST of the rest of the NOX emissions are accounted for by gas and oil heating systems
3: Most of _that_ is oil burning systems and gas burning systems over 15 years old. All boilers built in the last 12 years have NOX limits they must comply with (so it's older unregulated installations making most of the mess. Some are documented as up to 30 years old)
4: Condensing boilers emit fuck-all NOX - they run cooler in the first place. (Lewis, you really are full of it)
5: NOX levels outside the north/south circular were never enough to be a health problem.
NOX levels outside the inner london ring road are not a health problem except along arterial routes.
NOX levels inside that road are problematic and used to be hazardous - they're still hazardous along some arterial routes where cars can move at high speed (hot engine, more NOX)
NOX levels outside the M25 are effectively nonexistent, even right next to the motorway.
IOW: NOX emission controls are a blunt instrument addressing a particular set of circumstances - urban areas (not suburban) with restricted air movement due to the buildings + high vehicle density and areas subject to inversion layer trapping (LA county, CA Central Valley, Paris)
There's a good argument that better control of NOX-emitting vehicles in such areas (eg: london's clean air zones) or having NOX sensors on vehicles which switch to low-emission mode when levels start to climb would be a better overall solution than one which unnecessarily penalises fuel consumption most of the time.
NOX doesn't rain out as acid rain, there's not enough of it to send it acid. It forms in lightning strikes, etc anyway. Soil bacteria fix what does rain out into nitrates.
Sulfur(now the official spelling) was another matter - acid rain from that was mainly coal-sourced, but sulfur had to be removed from petrol/diesel as it poisons catalytic converters.
That happened relatively recently with USA diesel - which really was filthy at over 100ppm sulfur. Japanese diesel was even worse at 200ppm.