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Trends

Sulphur Dioxide (SO2)

Sulphur dioxide is formed when fuels containing small amounts of sulphur (such as coal and oil) are burned. In the past, such fuels have been widely used for domestic heating in Northern Ireland, leading to relatively high concentrations of sulphur dioxide. However, as natural gas has become more widely available, sulphur dioxide concentrations have declined significantly. This is illustrated by Figure 1, which shows a smoothed trend graph for monthly mean SO 2 concentration as measured at Northern Ireland's two earliest long-running automatic SO2 monitoring sites: Belfast East (which ran from 1989 to 2007) and Belfast Centre (which has been monitoring SO2 since 1992). Figure 1 clearly shows how ambient concentrations of this pollutant in Belfast have decreased substantially since the early 1990s.

Figure 1: Smoothed Trend Plot of SO2 at two Belfast sites, 1989-2016 (clicking on the graph will open a larger image in a new window)

Four of Northern Ireland’s air quality monitoring sites have measured SO 2 for ten years or more. These are; Ballymena Ballykeel (in operation since 2003), Belfast Centre (operational since 1992), Derry Brooke Park (operational since 1997, although it closed in early 2016 and was replaced by nearby Derry Rosemount), and Strabane Springhill Park (since 2002). The Openair Theil-Sen function has been used here to quantify trends in SO2 over the years 2003-2016 for these four sites.

De-seasonalised trends in ambient SO 2 concentration are shown in Figure 2. The trend line is shown by a solid red line, with 95% confidence intervals shown by dotted red lines. The trend is given at the top of the plot in green, with confidence intervals shown in square brackets. The trend is given as units (i.e. µg m-3) per year, over the period shown. This may be followed by a symbol indicating the level of statistical significance; these are explained in the Introduction.

Figure 2: De-seasonalised Trend Plots for SO2 at Four Long Running Sites, 2003-2016 (clicking on the graph will open a larger image in a new window)

Figure 2 shows that three of these four sites have significant downward trends in concentrations of this pollutant (significant at the 0.05 level or greater). However, only Ballymena Ballykeel and Belfast Centre appear to be showing a consistent and continuing decrease: the plots for Derry Brooke Park and Strabane Springhill Park show a sharp decrease in the early years of the current millenium, after which the decreasing trends appear to have flattened off in recent years.