19 August 2006
Ambient concentrations of nitrogen dioxide (NO2) in London and other urban areas in the UK have not declined as expected despite significant reductions in annual mean NOx concentrations, according to a draft report commissioned by the UK’s Department for Environment, Food and Rural Affairs (Defra) and published by the Air Quality Expert Group (AQEG). This indicates that the ratio of NO2/NOx showed an increase, which has been linked to increased usage of diesel oxidation catalysts and particulate filters.
The partitioning in the atmosphere between NO and NO2 changes as the concentration of NOx falls, so that an increase in NO2/NOx would be expected—says the report—but the extent of the observed upward trend exceeded these expectations, indicating that additional changes were taking place. The possible causes that were identified include (1) an increase in emissions of primary NO2, associated with increased market penetration of diesel cars and (2) an increase in primary NO2 associated with the fitting of some emission control devices, such as catalytic particulate filters that had been retrofitted to London buses.
The increased nitrogen dioxide levels may present a problem, as NO2 can produce more adverse health effects than NO. However, the health effects from exposure to particulate matter are much more significant that those from NO2, said Defra. Therefore, the application of catalytic particulate filters and diesel oxidation catalysts is believed to produce a public health benefit, despite the increased NO2/NOx ratio. The observed changes in NO2/NOx ratio are localized and have no climate change implications.