The use of the SILAM model in the interpretation of strong Betula airborne pollen episodes in Catalonia (ne of Spain)

The System for Integrated modeLling of Atmospheric coMposition (SILAM) dispersion model (Sofiev et al., 2006a) (http://silam.fmi.fi) was applied to the interpretation of the Betula (birch) airborne pollen dynamics at eight aerobiological stations of the Xarxa Aerobiològica de Catalunya (Aerobiological Network of Catalonia) during the pollination periods (1st March to 15th June) of the years 2006, 2007 and 2008. Birch trees are abundant in Central and North of Europe, but are scarce in Spain were they only grow in Northern regions under certain environmental conditions of height. The airborne birch pollen patterns in Catalonia show abrupt high concentrations in areas with usually low local influence. The use of modeling is a good tool to study and understand the atmospheric mechanisms that cause these peaks. The pollen grains incorporated into the atmosphere by anemophilous plants can produce allergic symptoms in a part of the population sensitive to it and birch pollen is one of the important causes of respiratory allergy in Northern and CentralEurope. In Catalonia the allergy to birch is not frequent, but it is occurring and the intensity of the derived health problems can be increased by the long range transport episodes. SILAM model was applied in its Eulerian mode to the domain 12°W -18°E and 35°N - 60°N, comprising Western Europe and the Mediterranean Sea, with a grid resolution of 15 km, using ECMWF fields as meteorological input data. The model incorporates a total amount of pollen emitted from a cell with 100% of area occupied by birch trees ( 2 109 grains/m2). The percentage of area occupied by birch trees was supplied with SILAM model in a base map of Europe. An update for the Iberian Peninsula was done into the base map with data from the National Forestry Inventory 3 (IFN3) from Spain. Simulations were performed for each year using both, the base map and the IFN3 map. A comparison of the model results with the airborne pollenmeasurements at the stations showed that the model reproduced better the measurements with the IFN3 map. The application of SILAM to southern Europe required also the adjustment of the estimates of the accumulated temperature thresholds provided with the model to the mountain areas, mainly the Pyrenees and the Alps, because these alpine regions are the main areas at our latitude where birches grow. Once introduced these new parameters into the model, the SILAM simulations became much more in agreement with the aerobiological measurements. The model application to the dates with high pollen levels showed that besides the regional origin, long range transport of Betula pollen from Europe to Catalonia is not rare and that the most common source area is Eastern France and central Europe.