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Trends - Air Quality Indicators

Annual Mean Trends for Nitrogen Dioxide

In Wales (and the rest of the UK) the most widely exceeded Air Quality Strategy Objective is for annual mean nitrogen dioxide (NO2) concentration. The graph below shows how concentrations of this pollutant have varied with time.

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Graph showing the annual mean trends for Nitrogen Dioxide

Traffic Urban sites
For Traffic Urban sites, i.e. those in urban areas within 10 m of the kerb of a road, there appears to be no clear trend in annual mean NO2 concentration. Data for years prior to 2004 should be treated with caution as they are based on less than four sites (recent research by the University of Glasgow has indicated this to be the minimum number for reliable assessment of trends*).

Background Urban sites

The graph shows a clear downward trend in this pollutant between 1993 and around 2005 (though again, data prior to 2002 should be treated with caution as the statistic is based on less than four sites). After 2005, the downward trend appears to have levelled off and concentrations may even have increased.

The UK's Air Quality Expert Group (AQEG) have reported that downward trends in this pollutant have levelled off in recent years. They believe this may be due to**:

  • An increase in the proportion of the total oxides of nitrogen (NOx) emitted directly to the atmosphere as NO2. This, in turn, is due to the increased market penetration of diesel cars and the retrofitting of pollution control devices, such as catalytically regenerative traps to buses.
  • Increasing background concentrations of O3, which promotes the oxidation of emitted NO to NO2.

* Draft report University of Glasgow, School of Mathematics and Statistics.

** Air Quality Expert Group (2007) Trends in primary nitrogen dioxide in the UK. Available at