The water up-take of semisolid SOA particles

Aki Pajunoja1, Andrew T Lambe2, Jani P Hakala3, Narges Rastak4, Liqing Hao5, Mikhail Paramonov3, Juan Hong6, Ari J Laaksonen7, Markku Tapio Kulmala3, Paola Massoli8, Timothy Bruce Onasch2, Neil McPherson Donahue9, Ilona Riipinen10, Paul Davidovits11, Douglas R Worsnop2, Tuukka Petäjä12 and Annele Virtanen13, (1)Department of Applied Physics, University of Eastern Finland, Joensuu, Finland, (2)Aerodyne Research, Inc., Billerica, MA, United States, (3)University of Helsinki, Helsinki, Finland, (4)Stockholm University, Department of Applied Environmental Science, Stockholm, Sweden, (5)University of Eastern Finland, Joensuu, Finland, (6)University of Helsinki, Department of Physics, Helsinki, Finland, (7)Finnish Meteorological Institute, Helsinki, Finland, (8)Aerodyne Research Inc., Billerica, MA, United States, (9)Carnegie Mellon University, Center for Atmospheric Particle Studies, Pittsburgh, United States, (10)Stockholm University, Stockholm, Sweden, (11)Boston College, Chestnut Hill, MA, United States, (12)University of Helsinki, Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, Helsinki, Finland, (13)University of Eastern Finland, Department of Applied Physics, Kuopio, Finland
Abstract:
The dependence of aerosol particle hygroscopicity on particle composition is often represented with the single parameter k commonly used in global models to describe the hygroscopic properties of atmospheric aerosol particles. From the theoretical formulation of k the same value is expected for ideal solutes in both the sub- and supersaturated regimes as typically calculated from hygroscopicity tandem differential mobility analyser (HTDMA) and cloud condensation nuclei counter (CCNc) measurements respectively (i.e. k HGF and kCCN). Yet, a number of recent studies conducted on SOA indicate that the two measurements yield different k values (k HGF < kCCN). There are several studies discussing the behaviour but the underlying reasons are unresolved. To investigate this in more detailed, CCNc and HTDMA measurements were conducted to determine the effects of chemical composition, oxidation level, the phase state and RH on the associated water uptake properties of biogenic SOA particles formed from isoprene, a-pinene, and longifolene precursors. Pure SOA particles by OH and/or O3 oxidation of the gas-phase precursors were formed in a PAM (Potential Aerosol Mass) flow tube reactor. Hygroscopic growth factors (HGF) were measured by Hygroscopicity Tandem Differential Mobility Analyser (HTDMA) at RH range of 50-~95% and CCN activation by CCN counter. To investigate the physical phase of the particles the particle bounced fraction (BF) using an Aerosol Bounce Instrument (ABI) was also measured. SOA oxidation state and composition was measured by a c-ToF-AMS. Based on the measurements we suggest that at subsaturation conditions semi solid SOA particles take up water mostly via surface adsorption resulting a large discrepancy between the kHGF and kCCN values. By calculating the aerosol direct radiative effect (Wm-2) using our results we also show that ambiguity about the κ values has important implications for quantifying the climate effects of SOA in atmospheric models.
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