Optical properties from extinction cross-section of single pollen particles under laboratory-controlled relative humidity

Abstract

A growing body of research suggests that pollen suspended in the atmosphere have a major environmental and climatic impact. However, our current knowledge of pollen is rather limited with respect to its extinction capacity, its optical properties and how these vary with atmospheric water content. Understanding their water absorption capacity can improve our understanding of their radiative effects and, thus, improve climate models. In this work, an electrodynamic Paul trap was coupled to a cavity ring down spectroscopy (CRDS) to directly measure the ring down time (τ) of four individual types of pollen particles: Olea, Fraxinus, Populus and Salix exposed to changing relative humidity (RH). Resonant structures in τ values between ∼90 and 45 % RH indicated that pollen was wettable at high RHs. τ was used to calculate light extinction cross-section (σext) at 532 nm as a function of RH. Optical growth factor (fRH) was evaluated as the ratio between σext(∼80%RH) and σext(dry). From fRH, the semi-empirical single hygroscopicity parameter (κemp) was found to be 0.038–0.058 for the four pollen types. Under controllable treatment of the water content and an adequate selection of complex refractive index (m), CRDS-σext data was fitted to theoretical σext from Mie theory. The reasonable agreement achieved allowed for gaining knowledge about the m and how particle size shrugged during dehydration. As a result, a climate-lowering effect of Olea pollen particles, which contain a fraction of scattered aerosol, should be considered in the models.