Cloud droplet activation of polymerized organic aerosol

Abstract

High‐molecular‐weight organic compounds represent an important fraction in atmospheric aerosols, but their interactions with atmospheric water vapour are not well understood. Specifically, the hygroscopicity of polymerized atmospheric aerosols is larger than expected for their high molecular weights and is not a strong function of the degree of polymerization, but their cloud‐nucleating activity declines as polymerization progresses. We apply a Flory–Huggins based theory for polymer–water interactions to describe water activity in aqueous solutions of atmospherically relevant oligomers and polymers, and show via experiments on particles composed of commercial polymers that it is consistent with observed particle hygroscopic growth and cloud formation behaviour.

Discontinuities in hygroscopic growth below and above water saturation for laboratory surrogates of oligomers in organic atmospheric aerosols

“…In agreement with results of previous studies of the hygroscopicity of polymers and HULIS, we observed little variability in hygroscopic growth across PEG systems with different molecular masses (Brooks et al, 2004;Petters et al, 2006;Ziese et al, 2008). HGFs measured here are within ∼ 5 % of those measured for 100 nm (dry size) particles comprised of PEG with average molecular masses of 600 and 3400 measured by Petters et al (2006). In general, results were similar for the PEG-AS systems; however, at RHs of 80 and 90 %, greater growth was observed for PEG200-AS particles (HGFs = 1.39 and 1.60) compared to PEG1000-AS (HGFs = 1.24 and 1.38) and PEG10000-AS particles (HGFs = 1.35 and 1.42).…”

“…These HGFs correspond to κ HGF,app values of 0.162 and 0.133. In agreement with results of previous studies of the hygroscopicity of polymers and HULIS, we observed little variability in hygroscopic growth across PEG systems with different molecular masses (Brooks et al, 2004;Petters et al, 2006;Ziese et al, 2008). HGFs measured here are within ∼ 5 % of those measured for 100 nm (dry size) particles comprised of PEG with average molecular masses of 600 and 3400 measured by Petters et al (2006).…”

“…While HGFs for the PEG10000-AS aerosols exceed those for the PEG1000-AS system at RHs of 80 and 90 %, this difference is within experimental uncertainty. The large degree of similarity in HGFs for the aerosol systems containing PEG1000 and PEG10000, despite the order of magnitude difference in molecular mass, is in agreement with previous hygroscopic-growth measurements showing that the influence of polymer chain length on water uptake displays a threshold behavior, becoming relatively constant for higher degrees of polymerization (Baltensperger et al, 2005;Petters et al, 2006). In contrast to the HGF measurements, measurements of CCN activity, as characterized by D crit at a constant supersaturation of 0.8 %, suggest that particle hygroscopicity at conditions of high RH increases with increasing molecular mass of the PEG oligomer (since the measurements suggest a decrease in D crit with increasing PEG molar mass).…”

Volatility and hygroscopicity of aging secondary organic aerosol in a smog chamber

“…Only small changes are expected for i s . Overall a decrease of κ is expected under the influence of oligomerization, though the effect becomes smaller with increasing M s (Petters et al, 2006).…”

Amorphous and crystalline aerosol particles interacting with water vapor: conceptual framework and experimental evidence for restructuring, phase transitions and kinetic limitations

“…Earlier studies have shown that the FH approach is suitable to describe the absorption of water by polymerized organic aerosol particles (Petters et al, 2006) and by water-miscible organic solids that are in a rubbery state (Hancock and Zografi, 1993;Zografi, 2000, 2001). For FH modeling, we assumed φ s = g −3 m and f = (M s /ρ s )/(M w /ρ w ) based on volume additivity, and we inserted a w from Eq.…”