Abstract
Climate impacts on the pollen season are well-described however less is known on how frequently atmospheric transport influences the start of the pollen season. Based on long-term phenological flowering and airborne pollen data (1987–2017) for six stations and seven taxa across Bavaria, Germany, we studied changes in the pollen season, compared pollen and flowering season start dates to determine pollen sources, and analyzed the likelihood of pollen transport by HYSPLIT back trajectories. Species advanced their pollen season more in early spring (e.g., Corylus and Alnus by up to 2 days yr−1) than in mid spring (Betula, Fraxinus, Pinus); Poaceae and Artemisia exhibited mixed trends in summer. Annual pollen sums mainly increased for Corylus and decreased for Poaceae and Artemisia. Start of pollen season trends largely deviated from flowering trends, especially for Corylus and Alnus. Transport phenomena, which rely on comparisons between flowering and pollen dates, were determined for 2005–2015 at three stations. Pre-season pollen was a common phenomenon: airborne pollen was predominantly observed earlier than flowering (median 17 days) and in general, in 63% of the cases (except for Artemisia and Poaceae, and the alpine location) the pollen sources were non-local (transported). In 54% (35%) of these cases, back trajectories confirmed (partly confirmed) the pre-season transport, only in 11% of the cases transport modeling failed to explain the records. Even within the main pollen season, 70% of pollen season start dates were linked to transport. At the alpine station, non-local pollen sources (both from outside Bavaria as well as Bavarian lowlands) predominated, in only 13% of these cases transport could not be confirmed by back trajectories. This prominent role of pollen transport has important implications for the length, the timing, and the severity of the allergenic pollen season, indicating only a weak dependency on flowering of local pollen sources.
Highlights
Allergen pollen is a major health burden worldwide which is aggravated with anthropogenic climate change [1, 2]: Spring warming leads to earlier flowering of species, an advance of the start of the pollen season; higher atmospheric CO2 concentrations reinforce flower— pollen—production; invasive species promoted by warming fill last gaps in the pollen calendar; and interactions with other air pollutants may intensify pollen allergenicity [3, 4]
Poaceae and Artemisia, flowering in late spring and full summer, exhibited uneven, i.e., advancing and delaying, trends with the exception of Poaceae peak dates strongly advancing in Munich, Erlangen and Bamberg
The choice of alternative approaches to the percentage method had no noticeable effect on the trends of key parameters of the pollen season (Figure 2) except SOSP of Corylus and Alnus which were stronger at some stations and EOSP which were weaker than by the alternative methods
Summary
Allergen pollen is a major health burden worldwide which is aggravated with anthropogenic climate change [1, 2]: Spring warming leads to earlier flowering of species, an advance of the start of the pollen season; higher atmospheric CO2 concentrations reinforce flower— pollen—production; invasive species promoted by warming fill last gaps in the pollen calendar; and interactions with other air pollutants may intensify pollen allergenicity [3, 4]. It is likely that long- and medium-range transport is directly or indirectly altered by weather patterns and climate change, which would in turn affect the apparent pollen season at a specific site. Lightweight pollen grains of anemophilous species are built to fly and after their release in large amounts, they can be dispersed and transported over several hundreds of kilometers, even into the Arctic [6]. There is no generally applicable definition of medium- and long-range transport. Sofiev et al [7] proposed transport with the wind, mixing inside the atmospheric boundary layer as well as dry and wet removal at the regional scale as key processes for medium-range transport, and dispersion with synoptic-scale wind plus exchange between the boundary layer and the free troposphere as key processes for longrange transport
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