Observations and modelling of birch pollen emission and dispersion from an isolated source

Abstract

The occurrence of allergic diseases in western countries increased during the last decades due to greater awareness towards a hygienic lifestyle. The hygiene hypothesis relates the reduced expo- sure to microbial pollution to an underdevelopment of the immune system, which in turn favours the development of allergies. In order to provide information to affected individuals on adequate pre-emptive measures, numerous studies on the health impact of allergenic pollen focus on their atmospheric abundance and dispersion, including observations and simulation of emission and transport. Prognostic models for the spatial distribution and concentration of different pollen species on a regional scale are operational in many countries in order to identify highly affected regions and allow health offices to announce warnings to the affected population. These models are capable of predicting long-range transport in a full spatial resolution with respect to meteoro- logical conditions. However, the initial abundance of airborne pollen in the models is determined with empirically derived emission parameters, which are mostly based on long-term observation averages with respect to large areas. Field measurements and modelling work conducted in the framework of this thesis aimed at de- scribing the emission and dispersion characteristics of an isolated natural birch pollen source in the micro-scale, in order to improve the accuracy of the emission part in prognostic pollen transport models. The basic approach was to infer the emission of the pollen source from downwind obser- vations, with respect to meteorological conditions, by reproducing the observed pollen dispersion with numerical simulations. Birch pollen are used, because they are among the most important aeroallergens in Europe. In terms of quantifying the absolute pollen emission in speciffic cases, however, the field observations of pollen concentrations were subject to various difficulties related to sensor uncertainties and non-stationary conditions in the natural environment. Firstly, the detailed investigation of pollen transport up- and downwind of the isolated source relied on a large array of different instruments. In order to make the observations of birch pollen concentrations comparable among different used instruments, a substantial part of this thesis is dedicated to the description of performance and uncertainty of different pollen sampling methods. Secondly, since naturally emitted pollen are used for tracers, instead of a controlled release of artifficial particles, the observed pollen concentration can be biased by natural background con- centration, which relates to emission from unknown sources upwind of the experiment site. The wind ow directed towards the birch canopy is substantially disturbed by its roughness and, addi- tionally, a certain amount of airborne pollen is filtered by its vegetation elements. Observations of undisturbed concentrations upwind of the windbreak thus fall short of describing the complex pattern of downwind distribution. A computational uid dynamics model, therefore, is used to simulate Lagrangian-based trajectories of the pollen with respect to the disturbance of the wind field. The results indicate that the portion of background concentration in the observed downwind concentration is largely dependent on effects of accumulation due to deceleration of the wind ow. Deposition within the birch canopy is accounted for in a separate model, which is based on the optical porosity of the windbreak. A combination of the two model approaches allows to eliminate the portion of background concentration from the measured downwind concentrations, providing information on the emissivity of the isolated birch pollen source. Based on the corrected concentrations downwind of the windbreak, i.e. un-biased by background concentration, a method of estimating the source strength of the isolated pollen source with a Lagrangian particle model is assessed.