Observation of viscosity transition in <i>α</i>-pinene secondary organic aerosol

Abstract

Published by Copernicus Publications on behalf of the European Geosciences Union. E. Järvinen et al.: Observation of viscosity transition in α-pinene secondary organic aerosolAbstract. 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 Cosmics Leaving Outdoor Droplets (CLOUD) experiment at The European Organisation for Nuclear Research (CERN), we deployed a new in situ optical method to detect the viscous state of α-pinene SOA particles and measured their transition from the amorphous highly viscous state to states of lower viscosity. The method is based on the depolarising properties of laboratory-produced non-spherical SOA particles and their transformation to non-depolarising spherical particles at relative humidities near the deliquescence point. We found that particles formed and grown in the chamber developed an asymmetric shape through coagulation. A transition to a 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 viscositytransition 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 icenucleation properties of SOA and SOA-coated particles in the atmosphere.

Mixing times of organic molecules within secondary organic aerosol particles: a global planetary boundary layer perspective

“…Due to the experimental conditions used by Grayson et al (2016) and Järvinen et al (2016), the parameterization developed here is applicable to SOA generated using a mass concentration of ∼ 1000 µg m −3 . We focused on ∼ 1000 µg m −3 because both low-temperature and roomtemperature viscosity measurements have been carried out using this mass concentration.…”

“…We use the viscosity measurements from Grayson et al (2016) determined with a mass concentration of 520 µg m −3 to be more consistent with the mass concentrations used by Järvi-nen et al (2016). Although there are other room-temperature measurements of the viscosity of α-pinene SOA Hosny et al, 2016;Kidd et al, 2014;Pajunoja et al, 2014;Renbaum-Wolff et al, 2013), we used the roomtemperature measurements from Grayson et al (2016) because (1) viscosity was measured over a range of relative humidities in this study, (2) the mass concentrations used by Grayson et al (2016) to generate the SOA were similar to the mass concentrations used by Järvinen et al (2016), and (3) Grayson et al (2016) measured the viscosity of the total SOA (both the water-soluble component and the waterinsoluble component).…”

“…Järvinen et al (2016) measured the temperature and RH values at which α-pinene SOA has a viscosity of approximately 10 7 Pa s. In these experiments, SOA was generated with a mass concentration of 707-1414 µg m −3 . Grayson et al (2016) measured viscosity of α-pinene SOA as a function of RH at 295 K. In these experiments, the SOA was generated with mass concentrations of 121 and 520 µg m −3 .…”

“…The following data were used to develop a parameterization of the viscosity α-pinene SOA as a function of temperature and RH: (a) room-temperature measurements of viscosity of SOA derived from α-pinene ozonolysis by Grayson et al (2016) (Table S1 in the Supplement), (b) lowtemperature measurements of viscosity for SOA derived from α-pinene ozonolysis by Järvinen et al (2016) (Table S2), and (c) temperature-dependent measurements of viscosity for water from Crittenden et al (2012) (Table S3). Järvinen et al (2016) measured the temperature and RH values at which α-pinene SOA has a viscosity of approximately 10 7 Pa s. In these experiments, SOA was generated with a mass concentration of 707-1414 µg m −3 .…”

“…To develop a parameterization for viscosity as function of temperature and RH, the following equation was fit to the measurements by Grayson et al (2016), Järvinen et al (2016), and Crittenden et al (2012) (Tables S1-S3):…”