Background
Airborne pollen may contribute to increased respiratory allergies. In Belgium, ~10% of people are sensitive to birch pollen and ~15% to grass pollen. Future climate and land-use changes may exacerbate this issue by increasing the amount of allergenic pollen in the air and prolonging pollen seasons. This study aims to disentangle the effects of changing meteorology and vegetation dynamics on long-term trends of birch and grass pollen levels near the surface in Belgium.
Materials & Methods
The pollen transport model SILAM (System for Integrated modeLling of Atmospheric coMposition) was applied using ECMWF ERA5 meteorological data with a bottom-up emission approach for the period 1982–2019 under two scenarios:
1. Reference Scenario (i): SILAM was run for 38 seasons with a fixed emission map of birch and grass pollen sources. Variations in airborne pollen levels result solely from changes in meteorological data.
2. Updated Scenario (ii): SILAM was run for 38 seasons with dynamically updated pollen emission maps derived from multi-decadal spaceborne NDVI data, forest inventory, and grass distribution maps. This scenario accounts for both changing emissions and meteorology.
The difference between the scenarios indicates the effect of vegetation dynamics on trends. Temporal trends (Theil-Sen) were computed for daily pollen levels and meteorological inputs (1982–2019). The association between trends in pollen and meteorology was estimated using Kendall correlation for each model grid cell.
Results
- Birch Pollen:
- A strong increase in birch pollen concentrations was observed, associated with increasing radiation, decreasing precipitation, and decreasing wind speed near the surface.
- Inter-seasonal variation in birch pollen production intensified the climate-induced increase in airborne birch pollen concentrations by ~6%.
- Grass Pollen:
- Grass pollen concentrations showed a strong reduction over time, primarily driven by a decreasing trend in grass pollen sources, which outweighed the impact of changing meteorology by a factor of ~10.
- When considering only meteorology, a significant increase in grass pollen emissions would be expected.
- The overall trend reduction was positively associated with changes in precipitation and wind, and negatively associated with radiation.
Conclusions
Between 1982 and 2019, airborne birch pollen levels near the surface in Belgium substantially increased due to changes in both meteorology and vegetation dynamics. In contrast, grass pollen levels decreased, driven by vegetation dynamics that overcompensated for the increasing effect of meteorology.
