Timely information on birch and grass pollen levels in Belgium

Air pollution has tremendous effects on mortality and the quality of life. Air pollution is not restricted to anthropogenic contaminants only, since also natural sources (soils, lakes, marshes, vegetation, volcanoes, etc) may emit substantial amounts of unhealthy pollutants (VOCs, SO_X, NO_X, aerosols, etc). Releases of biogenic aerosols such as pollen affect the public health badly, putting additional distress on people already suffering from cardiovascular and respiratory diseases. In Belgium, ~10% of the people is estimated to suffer from allergies due to pollen released by the birch family trees and ~15% due to pollen emitted by grasses. In some European countries the prevalence is up to 40%.

To date, the only available airborne pollen level data in Belgium are retrieved by Sciensano at five stations that monitor off-line daily concentrations of grass and birch pollen among other species. Patients suffering from rhinitis have therefore no access to detailed real-time spatial information and warnings on forthcoming exposures.

Chemistry Transport Models (CTM) can both quantify as well as forecast the spatial and temporal distribution of airborne birch and grass pollen concentrations if accurate and updated maps of birch and grass pollen emission sources are available, and if the large inter-seasonal variability in emissions is considered.

Here we show the results of the modelled spatio-temporal distributions of grass and birch pollen over Brussels and other locations in Belgium using the CTM SILAM. This CTM is driven by ERA5 meteorological reanalysis data from ECMWF, an updated MACC-III birch tree fraction map, based on local information, and a grass pollen emission map showing the spatial distribution of the potential pollen sources. Pollen release is based on the temperature degree days approach. Inter-seasonal variability in birch pollen release was taken into account by using spaceborne MODIS vegetation activity (Gross Primary Productivity, GPP). For grass pollen this approach does not fit, therefore we use average temperature and precipitation of the previous year in a first approach.

SILAM modelled and observed time series of daily birch pollen levels of 50 birch pollen seasons at multiple sites in Belgium correlate well for the period 2008-2018 with an increased R² of up to ~50% compared to the reference run. What is more, SILAM is able to capture the allergy thresholds of 50 and 80 pollen grains m^-³ exposure from the observations for birch trees. Grass pollen simulations are in progress.