One of the big contributors to poor urban quality, both for nitrogen oxides (NOx) and particulates has historically been bus fleets.
EMIS hearing, 20 June 2016 Questions to Emissions Analytics
Euro 6 on-road testing ICCT technical report 20160929
In 2015, the ICCT commissioned Emissions Analytics, a commercial test house specializing in testing real-world fuel consumption and emissions, to perform on-road testing of Euro 6-regulated gasoline and diesel vehicles. Emissions Analytics tested 7 vehicles between 2015 and 2016 for emissions of carbon monoxide (CO), carbon dioxide (CO2), nitric oxide (NO), nitrogen dioxide (NO2), and particulates.