Estimations of primary nitrogen dioxide exhaust emissions from chemiluminescence NOx measurements in a UK road tunnel

► UK tunnel study determining fraction of NO2 in NOx emissions, f-NO2 using extensive dataset. ► Tunnel characterisation via static and mobile measurements; f-NO2 by two approaches. ► f-NO2 shows a significant diurnal profile linked to variations in fleet composition. ► Calculated variation in f-NO2 from traffic counts and emission inventories does not match observations. ► Early morning low values of f-NO2 are below calculations from emissions inventories.