On the water up-take of semisolid SOA particles

The dependence of aerosol particle hygroscopicity on particle composition is often represented with the single parameter κ commonly used in global models to describe the hygroscopic properties of atmospheric aerosol particles. From the theoretical formulation of κ 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. κ_HGF and κ_CCN) (Petters and Kreidenweis, 2007). Yet, a number of recent studies conducted on SOA indicate that the two measurements yield different κ values ( κ_HGF < κ_CCN). There are several studies discussing the behaviour but the underlying reasons are unresolved (Prenni et al, 2007, Petters et al., 2009, Wex et al., 2009).

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, Α-pinene, and longifolene precursors (Pajunoja et al., 2014). 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. O:C ratio of the formed particles was controlled by adjusting the OH exposure in the reactor. After PAM we measured hygroscopic growth factors (HGF) 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.

The HGF at RH ~90% ranged from 1.07 to 1.37 depending on the SOA type and O:C. Further, the HGF for each SOA type and RH increased with O:C, consistent with previous observations. To relate HGF to phase state, we investigated the RH at which BF approached 0 (the condition where particles behave mechanically as liquid particles). Our results demonstrate that, SOA particles that attain BF = 0 at RH > 90% have the smallest HGF values. On the other hand, SOA particles that attain BF = 0 already at relatively low humidity values have the highest HGF values. (Pajunoja et al., 2014)

To investigate the hygroscopic behaviour in more detail, we calculated the hygroscopicity parameter κ at sub- and supersaturation conditions. Interestingly, for less-oxidized, semisolid SOA κHGF decreased with increasing RH resulting a large discrepancy between the κ_HGF and κ_CCN values. To compare these observations for semisolid/solid SOA to the well-known adsorptive water uptake behaviour of solid insoluble particles, we measured the HGFs for SiO2 particles. According to our measurements κ_HGF measured for SiO₂ particles displayed similar decreasing trend with increasing RH. Because SiO₂ particles can take up water only via surface adsorption, this suggests that a similar mechanism occurs for semisolid or sparingly soluble SOA particles, where the apparent hygroscopic growth is due to surface adsorption of water rather than bulk water uptake. As a result, solubility and diffusion limitations in the particle bulk inhibit water uptake until dissolution occurs at very high RH. 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. (Pajunoja et al., 2014)

References:

Pajunoja, A. et al., Adsorptive uptake of water by semisolid secondary organic aerosols in the atmosphere, submitted.

Petters, M. & Kreidenweis, S. A single parameter representation of hygroscopic growth and cloud condensation nucleus activity. Atmos. Chem. Phys. 7, 1961–1971 (2007).

Petters, M. D. et al. Towards closing the gap between hygroscopic growth and activation for secondary organic aerosol – Part 2: Theoretical approaches. Atmos. Chem. Phys. 9, 3999–4009 (2009).

Prenni, A.J., Petters, M.D., Kreidenweis, S.M., DeMott, P.J. & Ziemann, P.J. Cloud droplet activation of secondary organic aerosol. J. Geophys. Res. Atmos. 112, D10223 (2007).

Wex, H. et al. Towards closing the gap between hygroscopic growth and activation for secondary organic aerosol: Part 1 - Evidence from measurements. Atmos. Chem. Phys. 9, 3987–3997 (2009).