A model study of the pollution effects of the first three months of the Holuhraun volcanic fissure

The volcanic fissure at Holuhraun, Iceland started at the end of August 2014 and continued for six months to the end of February 2015. Lava floated onto the Holuhraun plain associated with large SO2 emissions. In this paper we present results from EMEP/MSC-W model simulations where we added 750 kg/s SO2 emissions at the Holuhraun plain from September to November. The emission amounted to approximately 4.5 times the daily anthropogenic SO2 emitted from the 28 European Union countries, Norway, Switzerland and Iceland. Model results are compared to satellite observations and European surface measurements. The dispersion but also the ambiguity of the satellite data, due to what is assumed in the retrieval as a priori SO2 profile, is further explored with model sensitivity runs using different emission height distributions from the volcano. Satellite-comparable adjusted model vertical column densities are calculated for the different sensitivity runs where the SO2 mixing ratios from different vertical layers are weighted with the averaging kernel. The results show the importance of using the averaging kernel when comparing the model to satellite column loads, the maximum column densities over 10 DU in the original model data are reduced by around 50 % due to the weighting. For most days the satellite retrievals have higher mass burdens values than the adjusted model when summed up over the North Atlantic area. The discrepancies are explained by the unrealistic constant emission term in the model simulations, and because the area used for the summation is dependent on the satellite data detection limit, and the correct position of the model SO2 plume. Surface observations in Europe showed peak type increases of SO2 concentrations from volcanic plumes passing by and lasting only for a short time. Three well identified episodes are documented for more detail. For all the events the timing of the observed concentration peaks compared to the model quite well. For the first episode presented, the model concentrations are only about 10 % to 40 % of the observed concentrations. The transport of SO2 to Europe during this event is found to contribute to very high measured and modelled concentrations at the stations. For the later plumes, the observed and model concentrations at the stations compare better in magnitude. The overall changes in the European SO2 budget due to the volcanic fissure are estimated. SOX three monthly wet deposition in the 28 European Union countries, Norway and Switzerland is found to be more than 30 % higher in the control model simulation with Holuhraun emission compared to a model simulation with no Holuhraun emission. The biggest increases, apart from Iceland, are found on the coast of Northern Norway, a region with frequent precipitation during westerly winds. The total deposition levels in this region become equal to the most polluted regions over Europe and the average model deposition for Norway is doubled the level it was back in 1990. For SO2 and PM2.5 concentrations, there is only a ten and six percent increase over Europe between the two model simulations, respectively. Although the percent increase of PM2.5 concentration is highest over Scandinavia and Scotland, an increase in PM exceedance days is found over Ireland and the Benelux region. Especially the Benelux region is already very polluted, so that a small increase in pollution leads to an increase in exceedances days. Although there was a large increase in total daily emission of SO2 over Europe due to the eruption, Iceland is located too far away to make a large impact on average pollution levels in the European countries, except in Iceland itself.