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Trends

Nitrogen Dioxide (NO2)

Within Northern Ireland (and throughout the UK) the most widely exceeded Air Quality Strategy Objective and EU limit value is that applying to annual mean nitrogen dioxide (NO 2) 40 µg m-3. Many other Member States of the European Union also have zones where this limit value is exceeded. It is therefore important to understand how ambient concentrations of this pollutant are changing over time.

NO2 at Urban Background Sites

There are only two long-running urban background sites in Northern Ireland: Belfast Centre (in operation since 1992) and Derry Brooke Park (in operation since 1997 to early 2016 when it was replaced by nearby Derry Rosemount). Figure 7 shows a smoothed trend plot for these two sites over their complete periods of operation. (Both these plots are based upon monthly averages of daily mean values.) Belfast Centre shows a consistent downward trend until the middle years of the last decade:  concentrations then rose slightly before the downward trend resumed.  At Derry Brooke Park (where concentrations are substantially lower), there is no clear trend.

Figure 7 : Smoothed Trend Plot for NO2 at Belfast Centre and Derry Brooke Park, 1992-2016
(clicking on the graph will open a larger image in a new window)

Figure 8 quantifies the trends for the same two datasets: whilst for Belfast Centre there is a highly significant downward trend, there is no significant trend in the case of Derry Brooke Park.

Figure 8 : De-seasonalised Trend Plots for NO2 at Belfast Centre and Derry Brooke Park, 1992-2016
(clicking on the graph will open a larger image in a new window)

NO2 at Urban Traffic Sites

Figure 9 shows de-seasonalised trend plots for six long-running urban traffic-related sites in Northern Ireland. These are; Armagh Lonsdale Road (2005-), Belfast Newtownards Road (2000 -), Belfast Stockman’s Lane (2005-), Derry Dale’s Corner (2003-), Newry Trevor Hill (2001- 2014) and North Down Holywood A2 (2003-). All of the above have monitored NOx for ten years or more.

The majority of these sites show statistically significant downward trends. However, there are two exceptions: Newry Trevor Hill and North Down Holywood A2.) Both of these show slight upward trends (statistically significant in the case of North Down Holywood A2). Trends therefore appear to vary from site to site, perhaps reflecting changes in traffic emissions on the nearby roads in each case.

Figure 9 : De-seasonalised Trend Plots for NO2 at Four Urban Traffic Sites, 2001-2016
(clicking on the graph will open a larger image in a new window)

The observation that not all roadside sites show a clear downward trend is consistent with the findings of a 2007 report by the Air Quality Expert Group (AQEG). This reported that roadside NO 2 concentrations in the UK (which had previously been falling) showed "little indication of a downward trend after 1997" a. AQEG attributed this to:

  • An increase in the proportion of total NOx emitted directly to the atmosphere as NO2. This 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 hemispheric background concentrations of ozone, which promotes the oxidation of emitted NO to NO2.

a Air Quality Expert Group (2007) Trends in primary nitrogen dioxide in the UK (PDF). [from national archives, accessed 23 November 2017].