Investigating the potential importance of primary NO2 emissions in a street canyon

Monitoring data from a busy street canyon location in central London and a background site have been used to estimate the potential importance of primary NO2 emissions from road vehicles travelling along the street canyon. By considering the difference in the total ‘oxidant’, OX (NO2+O3) between the two sites insight has been gained into the potential magnitude of primary NO2 emissions from road vehicles. It is shown that the day of the week and hour of the day variation in OX closely matches some attributes of the variation in road traffic flows and vehicle composition. In particular, the variation in OX compares well the variation in estimated NOX emissions from diesel vehicles by hour of the day and day of the week. A multiple regression is used to estimate the fraction of NO2 in petrol and diesel exhausts that is directly emitted. These results suggest that petrol and diesel vehicles in the street canyon emit 0.6±0.2 and 12.7±0.1 vol% NO2, respectively. Primary emissions of this magnitude appear to explain much of the variation observed from the measurements. These results have implications for modelling studies in terms of source apportionment and how NO2 concentrations respond to NOX control. It is recommended that detailed primary NO2 emissions inventories are developed and used by dispersion modellers for concentration predictions and comparisons with UK national and international limits for NO2.