Effect of varying experimental conditions on the viscosity of <i>α</i>-pinene derived secondary organic material

Abstract

Abstract. Knowledge of the viscosity of particles containing secondary organic material (SOM) is useful for predicting reaction rates and diffusion in SOM particles. In this study we investigate the viscosity of SOM particles as a function of relative humidity and SOM particle mass concentration, during SOM synthesis. The SOM was generated via the ozonolysis of α-pinene at < 5 % relative humidity (RH). Experiments were carried out using the poke-and-flow technique, which measures the experimental flow time (τexp, flow) of SOM after poking the material with a needle. In the first set of experiments, we show that τexp, flow increased by a factor of 3600 as the RH increased from < 0.5 RH to 50 % RH, for SOM with a production mass concentration of 121 µg m−3. Based on simulations, the viscosities of the particles were between 6 × 105 and 5 × 107 Pa s at < 0.5 % RH and between 3 × 102 and 9 × 103 Pa s at 50 % RH. In the second set of experiments we show that under dry conditions τexp, flow decreased by a factor of 45 as the production mass concentration increased from 121 to 14 000 µg m−3. From simulations of the poke-and-flow experiments, the viscosity of SOM with a production mass concentration of 14 000 µg m−3 was determined to be between 4 × 104 and 1.5 × 106 Pa s compared to between 6 × 105 and 5 × 107 Pa s for SOM with a production mass concentration of 121 µg m−3. The results can be rationalized by a dependence of the chemical composition of SOM on production conditions. These results emphasize the shifting characteristics of SOM, not just with RH and precursor type, but also with the production conditions, and suggest that production mass concentration and the RH at which the viscosity was determined should be considered both when comparing laboratory results and when extrapolating these results to the atmosphere.

Particle size dependence of biogenic secondary organic aerosol molecular composition

“…Oligomer decomposition could be an artifact of the sample preparation and analysis steps after particle collection, or it could be an intrinsic aspect of accretion chemistry that occurs prior to particle collection and analysis. If reversibility is an intrinsic aspect of accretion chemistry, then the higher amounts of low-OSc monomers in large particles suggest that diffusion within the particle phase is hindered (Faulhaber et al, 2009;Grayson et al, 2016;Koop et al, 2011;Renbaum-Wolff et al, 2013) and/or phase separation has occurred (Laskina et al, 2015;Veghte et al, 2013;Virtanen et al, 2011;Werner et al, 2016), effectively trapping the released monomers within the particle and making them unable to re-equilibrate with the gas phase. Reversibility of the oligomerization process provides a reasonable explanation why β-pinene SOA yields are so strongly dependent on temperature and relative humidity (von Hessberg et al, 2009).…”

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

“…If SVOCs are not well mixed within SOA particles on this timescale, then chemical transport models could incorrectly predict SOA mass concentrations by up to an order of magnitude (Shiraiwa and Seinfeld, 2012) and incorrectly predict the size of SOA particles (Zaveri et al, 2014), with implications for air quality and climate predictions (Seinfeld and Pandis, 2006). Recent research has shown that mixing times of organic molecules within SOA particles can be > 1 h at roomtemperature and at low RHs (Abramson et al, 2013;Grayson et al, 2016;Liu et al, 2016;Perraud et al, 2012;RenbaumWolff et al, 2013;Song et al, 2016;Ye et al, 2016;Zhang et al, 2015). In addition, studies have shown that proxies of SOA particles can form glass at low RHs and low temperatures Zobrist et al, 2008).…”

“…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 . 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).…”

“…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 . 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).…”

“…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 .…”