Ozone (O 3) is monitored routinely at three sites in Northern Ireland: Belfast Centre, Derry Brooke Park and Lough Navar. The latter is a rural background site, the other two are urban background. All have been in operation since 1998 or earlier. Figure 6 shows a trend plot of monthly mean O3 concentration for these three long–running sites.
Derry Brooke Park shows a highly significant downward trend year on year; Lough Navar shows a (less significant) upward trend. Belfast Centre shows no significant upward or downward trend in ozone concentrations over the period shown.
Ozone is a "secondary" pollutant, that is, it is formed from other pollutants by chemical reactions in the presence of sunlight. Ozone concentrations (and the number of exceedances in any given year) therefore depend to a large extent on meteorological conditions.
Figure 6: De-seasonalised Trend Plots for Ozone at Three Long-Running Background Sites, 1998-2016
(clicking on the graph will open a larger image in a new window)