Abstract. Hygroscopicity largely
affects environmental and climatic impacts of pollen grains, one important
type of primary biological aerosol particles in the troposphere. However, our
knowledge of pollen hygroscopicity is rather limited, and the effect of
temperature in particular has rarely been explored before. In this work three
different techniques, including a vapor sorption analyzer, diffusion
reflectance infrared Fourier transform spectroscopy (DRIFTS) and transmission
Fourier transform infrared spectroscopy (transmission FTIR) were employed to
characterize six anemophilous pollen species and to investigate their
hygroscopic properties as a function of relative humidity (RH, up to
95 %) and temperature (5 or 15, 25 and 37 ∘C). Substantial mass
increase due to water uptake was observed for all the six pollen species, and
at 25 ∘C the relative mass increase at 90 % RH, when compared
to that at <1 % RH, ranged from ∼30 % to ∼50 %,
varying with pollen species. It was found that the modified κ-Köhler equation can well approximate mass hygroscopic growth of all the
six pollen species, and the single hygroscopicity parameter (κ) was
determined to be in the range of 0.034±0.001 to 0.061±0.007 at
25 ∘C. In situ DRIFTS measurements suggested that water adsorption
by pollen species was mainly contributed to by OH groups of organic compounds
they contained, and good correlations were indeed found between
hygroscopicity of pollen species and the number of OH groups, as determined
using transmission FTIR. An increase in temperature would in general lead to
a decrease in hygroscopicity, except for pecan pollen. For example, κ
values decreased from 0.073±0.006 at 5 ∘C to 0.061±0.007
at 25 ∘C and to 0.057±0.004 at 37 ∘C for
Populus tremuloides pollen, and decreased from 0.060±0.001 at
15 ∘C to 0.054±0.001 at 25 ∘C
and 0.050±0.002 at
37 ∘C for paper mulberry pollen.
Supplementary material to "Water adsorption and hygroscopic growth of six anemophilous pollen species: the effect of temperature"
