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

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.