On the sources of long-term trends of airborne birch and grass pollen

Airborne pollen may have a substantial contribution to respiratory allergies affecting the public health badly, especially in combination with long-term exposure by other air pollutants. In some European countries the prevalence of people with pollinosis is up to 40%. In Belgium, ~10% is sensitive to birch pollen and ~15% to grass pollen. In the future, even more people might be affected since climate change and land-use change elicit an increased amount of allergenic airborne pollen and prolonged pollen seasons.

In this study we have used the pollen transport model SILAM (System for Integrated modeLling of Atmospheric coMposition) for attributing the long-term changes in the releases of pollen by birches and grasses to meteorology and vegetation dynamics. The pollen transport model is applied for Belgium and is driven by ECMWF ERA5 meteorological data in a bottom-up emission approach for the period 1982-2019. The corresponding maps with grass and birch pollen emissions sources, i.e. the dynamic vegetation component, are based on merging multi-decadal datasets of spaceborne NDVI with forest inventory data and grass distribution maps for 1982-2019. For each model gridcell we compute temporal trends based on the Theil Sen slopes and the Area Under the Curve (AUC) of the seasonal birch and grass pollen cycles based on daily pollen levels, and of the daily meteorological model input for the period 1982-2019. The gridcell based association between trends in pollen and meteorology are derived based on the Kendall correlation coefficient.

Our findings indicate that the increasing radiation, the decreasing precipitation and the decreasing horizontal wind speed are associated with a strong increase in birch pollen levels for the period 1982-2019. The decreasing grass pollen levels in the air over the same period are associated with decreasing precipitation. This is, however, induced by the decreasing trend in grass pollen sources. The associations between meteorology and airborne birch pollen levels are much stronger compared to grass pollen. The specific contribution of birch pollen production dynamics to the levels of birch pollen in the air is highly associated with wind speed and precipitation. By introducing the inter-seasonal variation in birch pollen production the overall increase rate is dampened with ~7%. In contrast, the grass pollen production dynamics resulted into 3.5 times less grass pollen in the air over the studied period.