Rising temperature leads to substantial changes in flowering season of European trees releasing large amount of allergenic pollen into the air. Some pollen proteins can exacerbate allergenic rhinoconjunctivitis (pollinosis) and allergic asthma by acting as antigens for the immune system. The most consistent signal is visible at the start of the season, the target variable of the current indicator. The starting of the pollen season is relevant also because patients are alerted in many regions by warning systems and mass media.
Pollen season has many definitions, depending on the target use. The definition relevant in the current context is a so-called “Clinically-relevant” pollen season, i.e., the period when concentrations of the specific pollen are sufficiently high to cause allergy symptoms. The general definition and the concentration thresholds for several aeroallergens were identified as a result of a Task Force of European Academy of Allergology and Clinical Immunology (EAACI).
Calculation of the clinically-relevant seasons are performed in several steps by SILAM atmospheric composition model (http://silam.fmi.fi, open-source code http://github.com/fmidev/silam-model):
1. phenological season of trees is computed by SILAM based on the concept of accumulated heat as a period when a specific plant releases pollen into the atmosphere;
2. atmospheric dispersion of the released pollen is computed by SILAM, thus obtaining pollen concentration as a function of time all over Europe;
3. clinically-relevant seasons are calculated based on the SILAM-computed pollen concentrations as input;
4. the calculations are repeated independently for each year through 1980-2020 and each type of trees;
5. differences of the last/first decadal medians of the clinically-relevant season start day are calculated for each grid cell.
Release into the air and atmospheric dispersion of pollen was computed by SILAM. Model computations have been made for 1980-2020 using the European meteorological Reanalysis ERA5. Each year is computed independently, with the same amount of pollen released as a seasonal integral. Spatial resolution was 0.1 degree lat-lon for the domain covering the whole Europe.
Computations of clinically-relevant season follows the EAACI definition: start of season is the 1st day of 5 days – out of 7 consecutive days – each of these five days with concentration exceeding "daily mean concentration threshold" and with a sum of these five days exceeding "5-days pollen concentration threshold". The numerical constants for concentration thresholds were used as follows ("pollen" - "1day cnc_threshold" / "5-days cnc_threshold" in [pollen/m3] ): Alder - 3/30; Birch - 10/100; Olive - 20/200.
Upon completion, the clinically-relevant season start, end, and duration (in calendar days) were computed from daily pollen concentrations for each grid cell independently.
Dataset contains 3 netCDF files (Alder, Birch, Olive) with multiyear pollen season characteristics computed over Europe for 1980-2020.
