Water adsorption and hygroscopic growth of six anemophilous pollen species: the effect of temperature

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.