Abstract
Abstract. The physical phase state (solid, semi-solid, or liquid) of secondary organic aerosol (SOA) particles has important implications for a number of atmospheric processes. We report the phase state of SOA particles spanning a wide range of oxygen to carbon ratios (O / C), used here as a surrogate for SOA oxidation level, produced in a flow tube reactor by photo-oxidation of various atmospherically relevant surrogate anthropogenic and biogenic volatile organic compounds (VOCs). The phase state of laboratory-generated SOA was determined by the particle bounce behavior after inertial impaction on a polished steel substrate. The measured bounce fraction was evaluated as a function of relative humidity and SOA oxidation level (O / C) measured by an Aerodyne high resolution time of flight aerosol mass spectrometer (HR-ToF AMS). The main findings of the study are: (1) biogenic and anthropogenic SOA particles are found to be amorphous solid or semi-solid based on the measured bounced fraction (BF), which was typically higher than 0.6 on a 0 to 1 scale. A decrease in the BF is observed for most systems after the SOA is exposed to relative humidity of at least 80% RH, corresponding to a RH at impaction of 55%. (2) Long-chain alkanes have a low BF (indicating a "liquid-like", less viscous phase) particles at low oxidation levels (BF < 0.2 ± 0.05 for O / C = 0.1). However, BF increases substantially upon increasing oxidation. (3) Increasing the concentration of sulphuric acid (H2SO4) in solid SOA particles (here tested for longifolene SOA) causes a decrease in BF levels. (4) In the majority of cases the bounce behavior of the various SOA systems did not show correlation with the particle O / C. Rather, the molar mass of the gas-phase VOC precursor showed a positive correlation with the resistance to the RH-induced phase change of the formed SOA particles.
Highlights
The direct and indirect effects of aerosol particles on the Earth’s radiative budget remain the largest source of uncertainty in climate change modeling (IPCC, 2007, ch. 2)
In order to assess how general is the occurrence of the amorphous solid state of the secondary organic aerosol (SOA), and how other factors might affect particle phase state, we report a systematic characterization of the phase of laboratory SOA as a function of O / C and relative humidity (RH)
Our results suggest that most types of atmosphericallyrelevant SOA form amorphous solid or semi-solid particles at RHI 60 %
Summary
The direct and indirect effects of aerosol particles on the Earth’s radiative budget remain the largest source of uncertainty in climate change modeling (IPCC, 2007, ch. 2). E. Saukko et al.: Phase state of SOA formation process under natural conditions is complicated and it involves a multitude of gaseous precursors and a greater number of particle product compounds. The modeling of formation and aging of SOA has been mostly based on gas-particle equilibrium partitioning of volatile and semivolatile species (Pankow, 1994; Kanakidou et al, 2005). This implies fast enough condensed phase diffusion rates to keep the condensed phase in equilibrium with the gas phase as the particles’ size increases and the concentration of VOCs decreases
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