Observation of viscosity transition in α-pinene secondary organic aerosol

Abstract

Abstract. Under certain conditions, secondary organic aerosol (SOA) particles can exist in the atmosphere in an amorphous solid or semi-solid state. To determine their relevance to processes such as ice nucleation or chemistry occurring within particles requires knowledge of the temperature and relative humidity (RH) range for SOA to exist in these states. In the CLOUD experiment at CERN, we deployed a new in-situ optical method to detect the viscosity of α-pinene SOA particles and measured their transition from the amorphous viscous to liquid state. The method is based on the depolarising properties of laboratory-produced non-spherical SOA particles and their transformation to non-depolarising spherical liquid particles during deliquescence. We found that particles formed and grown in the chamber developed an asymmetric shape through coagulation. A transition to spherical shape was observed as the RH was increased to between 35 % at −10 °C and 80 % at −38 °C, confirming previous calculations of the viscosity transition conditions. Consequently, α-pinene SOA particles exist in a viscous state over a wide range of ambient conditions, including the cirrus region of the free troposphere. This has implications for the physical, chemical and ice-nucleation properties of SOA and SOA-coated particles in the atmosphere.

Global long-range transport and lung cancer risk from polycyclic aromatic hydrocarbons shielded by coatings of organic aerosol

“…At cool and/or dry conditions, however, we assume that highly viscous OA effectively shields BaP from heterogeneous oxidation, similar to that observed for eicosane (a highly viscous solid organic) coatings (18) (SI Appendix, Shielding of BaP by OA in the New Modeling Formulation). This approach is consistent with highly viscous SOA observed at dry conditions (27,28), persistence of highly viscous SOA at cooler temperatures in the free troposphere (26), and also higher viscosity of complex SOA mixtures (29) that are likely more atmospherically relevant compared with laboratory-generated SOA (SI Appendix, Shielding of BaP by OA in the New Modeling Formulation). We show that including the shielding of BaP by OA results in better agreement with BaP measurements in the field, promotes stronger long-range transport, and also produces greater global incremental lung cancer risk (ILCR) estimates due to exposure to PAHs, compared with the default model that neglects this shielding by OA.…”

“…S4B). This result suggests that more complex and aged atmospheric SOA, which is highly viscous at cool/dry conditions (24,26,29), is expected to be highly effective in shielding BaP from oxidation, as is assumed in our new modeling formulation.…”

“…Highly viscous SOA coatings limit the diffusion of PAH molecules from particle bulk to particle surface (24,25), effectively shielding PAHs from chemical degradation (24). In addition, OA is more viscous in cool/dry conditions (24,26), so the effectiveness of shielding by OA is likely higher at cool locations in the atmosphere compared with laboratory measurements, which are made at room temperature [i.e., around 296°K (18,19,25)]. Thus, temperature-and RH-dependent changes in the viscosity of OA coatings can strongly affect heterogeneous oxidation kinetics of BaP.…”

Heterogeneous ice nucleation of α‐pinene SOA particles before and after ice cloud processing

“…Below 258 K, the ABI had to be removed from the cold housing, meaning that it was not possible to measure the bounced fraction of the particles in the temperature regime covered by the ice nucleation experiments. Also, the specific demands with respect to particle number concentration and size for depolarization measurements to be sensitive to probe the phase state of the SOA particles, as estimated to be 10,000 cm −3 particles with a diameter of at least 600 nm [ Järvinen et al ., ], were not met in our study.…”

“…This could mean (i) that the particles had completely been liquefied and frozen homogeneously; (ii) that the particles had a core‐shell morphology at the freezing onset, with the glassy core just being inefficient in triggering ice nucleation by immersion freezing before the homogeneous freezing limit was reached and the surrounding liquid layer nucleated ice; and (iii) that the particles had remained entirely in the glassy state during humidification, and the onset of ice nucleation via deposition nucleation on the glassy surface just accidentally coincided with the homogeneous freezing threshold. Recently, the use of in situ, near‐backscattering depolarization measurements was explored to detect temperature‐ and humidity‐induced viscosity transitions in α ‐pinene SOA particles [ Järvinen et al ., ]. The applicability of this technique, however, is limited to laboratory experiments with very high number concentrations and particle sizes above 0.6 μm, where nonspherical SOA particle aggregates can be formed, whose transition to less viscous and thereby spherical particle morphologies can be detected by a decrease in the depolarization ratio.…”