NO2 is an irritant and oxidant which can damage cell membranes and proteins. It has been linked to a range of adverse health effects, including asthma and cancer, but the most consistent association has been found with respiratory outcomes.
NO2 is predominantly a secondary pollutant, its major atmospheric source being the oxidation of NO emitted from combustion sources – notably road vehicle exhaust. However, some NO2 is emitted directly from vehicles, and this is commonly referred to as ‘primary NO2’. Emissions of NOx from vehicle exhaust are regulated at type approval, but NO2 emissions per se are not.
Analyses have indicated that a significant proportion of ambient NO2 must be emitted directly from vehicle exhaust, and that the direct road traffic contribution to ambient NO2 has increased in recent years.
Two contributing factors have been cited
(i) Diesel vehicles emit more NOx than petrol vehicles, and with a larger proportion of NO2 in NOx.
(ii) The average value of NO2 in diesel exhaust is increasing. This appears to be linked to the growth in the use of specific after-treatment technologies in modern diesel vehicles which involve in situ generation of NO2, such as catalytically regenerative particle filters.
Background concentrations of ozone are also increasing. As the ozone concentration increases the amount of NO converted to NO2 increases.
Furthermore, it seems likely that real-world NOx emissions from road vehicles are not decreasing as rapidly as models are predicting. Whilst this does not, in itself, affect actual NO2 concentrations, it does suggest that NOx controls have not been sufficiently stringent, or that vehicles are not performing as expected.
The overall consequence is that there is now a great deal of interest in the tighter regulation of NOx and NO2 emissions from diesel vehicles and the effects of different after-treatment devices. Direct-injection petrol engines with after-treatment technologies will also have an important impact on NOx emissions in the future.