MACC-III supports French authorities on elevated SO2 values
In the middle of September several European countries were surprised by measurements of high sulphur dioxide concentrations at ground level. SO2 is known as a precursor for acid rain, but also acts as an irritant to the respiratory system as well as being a precursor of sulphate aerosol. Due to strong European efforts over the last decades to reduce SO2 emissions, high concentrations of SO2 are now quite rare in Western Europe -except in specific areas affected by industrial or shipping emissions. So what was going on?
French in-situ air quality stations observed high values of SO2, especially along the northwestern coast, as seen in the figure on the right. However, the hypothesis that these high values could be linked to ship emissions trapped in the lower atmosphere appeared unlikely because they were exceptionally high and observations in the United Kingdom and the Netherlands (Figure below) also showed high concentrations between 21 and 25 September. This all pointed towards an episode of large spatial extent.
The situation was explained by the MACC near-real-time forecasting system thanks to its use of satellite observations to constrain the model forecasts. The OMI satellite instrument observed concentrations of volcanic SO2, emitted by the Icelandic Bardarbunga volcano, and these observations were assimilated by the MACC system. The subsequent 5-day forecast then captured the transport of this plume of volcanic SO2 southward reaching the Channel on 23 September (Figure below, left). A parallel forecast (Figure below, right), for which no OMI data were used, further identified the volcanic nature of the plume. No elevated SO2 values were forecasted without assimilating the OMI data, which means that “normal” emissions of SO2 (including shipping and industrial activities) could not explain the observed situation.
Following the plume of SO2 further inland, the case is made clear again. Figure below (left) shows observed concentrations of SO2 at the Deutsche Wetterdienst stations at Hohenpeissenberg (blue) and Schneefernerhaus (red) on the 22 of September. Figure below (right) shows the MACC forecast with (red) and without (blue) assimilating OMI data in the form of a vertical profile of SO2 at Hohenpeissenberg. Without assimilation of satellite data the volcanic SO2 plume could not have been forecasted.
In the end, the French authorities used the output from the MACC global assimilation and forecasting system to assess the situation and decide upon local emission reduction measures. The knowledge that the elevated SO2 values were coming from the Bardarbunga in Iceland and therefore had a very temporary character helped to make an informed decision.