Experimental Studies on Ambient Aerosol Hygroscopicity

Sammanfattning: The air around us is full of aerosol particles which originate from a myriad of sources and which have vastly different chemical and physical properties. These particles span size ranges from 1 nm to 100 μm, equivalent to a factor 100 000 in diameter difference and a factor 10^15 in volume difference. They affect our health by depositing in our airways, worsening cardiovascular deceases, and they affect our climate, by scattering electromagnetic radiation and by acting as cloud condensation nuclei. The Intergovernmental Panel on Climate Change (IPCC) has estimated that a large part of the uncertainty concerning climate modeling today is related to the water vapour – particle interaction, with regards both to hygroscopic growth at subsaturated conditions and to cloud dynamics and microphysics. In this work, particle water uptake both at sub- and supersaturation has been studied. A fraction of the water soluble organic carbon of ambient aerosol, named humic-like substances (HULIS) was investigated in laboratory measurements. It was found that HULIS in itself has low hygroscopic growth, but is a strong surfactant, which indicates that it could potentially have a strong influence on the critical supersaturation of the aerosol particles. In addition to this, a new instrument designed for long term measurements of hygroscopic growth was designed and deployed at the Swedish background aerosol monitoring station Vavihill; a Hygroscopic Tandem Differential Mobility Analyser (H-TDMA). More than two years of measurements were carried out in combination with size distribution and cloud condensation nuclei (CCN) measurements, representing the longest consecutive dataset collected so far. Parameterizations of the CCN concentrations for different seasons was derived and compared to modeled concentrations predicted from H-TDMA and size distribution data. Finally the H-TDMA and the size distribution data sets were combined to calculate boundary layer cloud supersaturation ratios, by investigating cloud processing of the particles, assuming that a single cloud passage has potential to significantly alter the chemical and physical properties of the particle. This work gave a mean cloud supersaturation (s) ratio of ~0.15%, with s values frequently up to 0.25%, but rarely >0.3%.