seems the existing ULEZ had produced some small mean 3% reduction in Nox/No2 .. and some sites started now increased or returned to pre-ulez levels ..
slower traffic flow being one reason. ... stupid sadiq barking up wrong tree.
There are three monitoring sites that showed a significant increase (at the 10% level) in roadside concentrations of either NOx (site CT6) or NO2 (sites HV3 and WA8). However, since increases in both NOx and NO2 were not observed at any sites, the results imply that the change in concentrations of NOx and NO2 were highly site-specific and could have been influenced by atmospheric chemistry, vehicle flows, changes in vehicle fleet (i.e. ULEZ compliance) and changes in traffic speeds, which affect vehicle NOx emissions factors and the fraction of NOx emitted as NO2
....
Specifically, the highest reduction in kerbside NO2 concentrations (13%) was observed at the only site within the ULEZ; the highest reduction in kerbside NOx concentrations (7%) was at a site that is outside the zone yet next to its boundary. However, significant concurrent decreases in both NO2 and NOx concentrations were not observed at either of these sites. We also observe a diminishing improvement in air pollution at some locations. For example, at site WM6 within the ULEZ, the normalised concentrations of NO2 and NOx both started to increase in July 2019 after an initial reduction, and by September 2019, their levels reached a plateau where NOx had returned to the pre-ULEZ levels while NO2 remained lower than pre-ULEZ (see SI section S14). For NO2 and NOx , this 'rebound' also occurred for NO2 at a roadside site within the ULEZ, but not at any other roadside sites within the ULEZ nor kerbside sites close to the ULEZ.