Estimations of road vehicle primary NO2 exhaust emission fractions using monitoring data in London

Hourly mean concentration data for nitrogen oxides (NOX), nitrogen dioxide (NO2) and ozone (O3) have been used to derive a method for estimating the mean primary NO2 fraction from vehicle exhausts in London. A set of simple chemical equations has been used to explain the differences in NO2 concentrations between background and roadside site pairs and to estimate the proportion of NO2 that is likely to be derived from primary NO2 vehicle emissions and the NO2 formed through the reaction of nitric oxide (NO) with O3. These results suggest that there is a wide range of primary NO2 emission fractions observed on different roads in London. The estimates ranged from 3.2 to 23.5 vol% with a median value of 10.6 vol% for the 43 measurement sites considered. A median primary NO2 fraction of 10.6% accounts for an average of 21% of the observed NO2 concentration at roadside sites. The range in calculated primary NO2 fractions explains much of the variation in annual mean NO2 concentrations between monitoring sites in London. The estimated primary NO2 fraction is considerably higher than the 5.0 vol% typically used in modelling studies that aim to predict ambient NO2 concentrations. These results have implications for the assessment of NO2 concentrations in London and whether London will meet the limit values set by the European Union. Further work is required to explain the wide range of primary NO2 fractions estimated in terms of road traffic composition and activity.