Abstract
Abstract. Cloud formation characteristics of the water-soluble organic fraction (WSOC) of secondary organic aerosol (SOA) formed from the ozonolysis of alkene hydrocarbons (terpinolene, 1-methlycycloheptene and cycloheptene) are studied. Based on size-resolved measurements of CCN activity (of the pure and salted WSOC samples) we estimate the average molar volume and surface tension depression associated with the WSOC using Köhler Theory Analysis (KTA). Consistent with known speciation, the results suggest that the WSOC are composed of low molecular weight species, with an effective molar mass below 200 g mol−1. The water-soluble carbon is also surface-active, depressing surface tension 10–15% from that of pure water (at CCN-relevant concentrations). The inherent hygroscopicity parameter, κ, of the WSOC ranges between 0.17 and 0.25; if surface tension depression and molar volume effects are considered in κ, a remarkably constant "apparent" hygroscopicity ~0.3 emerges for all samples considered. This implies that the volume fraction of soluble material in the parent aerosol is the key composition parameter required for prediction of the SOA hygroscopicity, as shifts in molar volume across samples are compensated by changes in surface tension. Finally, using "threshold droplet growth analysis", the water-soluble organics in all samples considered do not affect CCN activation kinetics.
Highlights
Aerosols, by acting as cloud condensation nuclei (CCN), have a profound impact on the hydrological cycle and climate
In this study we report the experimental investigation of the CCN activity of the water soluble fraction of secondary organic aerosol (SOA) generated in laboratory chamber ozonolysis of alkenes; these measurements are used to constrain the hygroscopicity of the water-soluble organic fraction, using κ-Kohler theory (Petters and Kreidenweis, 2007)
For all three parent hydrocarbons, the original SOA samples activate at diameters larger than that of (NH4)2SO4; this is expected as organics are generally less hygroscopic than soluble electrolytes
Summary
By acting as cloud condensation nuclei (CCN), have a profound impact on the hydrological cycle and climate. Attempts to speciate SOA (Alfarra et al, 2006; Aschmann et al, 2002; Dommen et al, 2006; Gao et al, 2004a; Kalberer et al, 2006) have been met with limited success, as 80 to 90% of the aerosol mass
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