Analysis of sources and partitioning of oxidant in the UK—Part 2: contributions of nitrogen dioxide emissions and background ozone at a kerbside location in London

Hourly mean concentration data for nitric oxide (NO), nitrogen dioxide (NO2) and ozone (O3) at Marylebone Rd (an urban kerbside site in London) are used to investigate the diurnal and seasonal dependence of sources of oxidant, and their origins. The observed concentrations of oxidant ([OX], defined as [NO2]+[O3]) are interpreted in terms of a the sum of a NOX-independent ‘regional’ contribution, and a linearly NOX-dependent ‘local’ contribution. The former is believed to equate to the background O3 concentration, whereas the latter is likely to be dominated by emissions of NO2 from road transport at the studied location. The derived regional OX concentrations display a significant seasonal variation, with a springtime maximum value of ca. 43 ppb in April, the results being broadly similar to those reported for background ozone at low altitude sites in northwest Europe. A strong diurnal variation in the local OX contribution is observed throughout the year, with the highest values (typically ca. 0.11 ppb ppb−1 NOX) during the daytime. Diurnal profiles averaged over the periods of the year when the UK operates under GMT and BST demonstrate a clear temporal shift, consistent with the local OX contribution being due to primary NO2 emissions from road transport. In conjunction with traffic flow statistics and associated NOX emissions estimates, the results suggest that primary NO2 from diesel-fuelled vehicles accounts for 0.996 v−0.6 of diesel NOX emissions, by volume, where v is the mean vehicle speed in km h−1 (valid range, 30–60 km h−1). This corresponds to (11.8±1.2)% of NOX emissions integrated over the average diurnal cycle for the conditions at Marylebone Rd. The results also suggest that primary NO2 emissions from petrol-fuelled vehicles are far less important, with an upper limit NO2/NOX emission ratio of <3%.