Viscosities, diffusion coefficients, and mixing times of intrinsic fluorescent organic molecules in brown limonene secondary organic aerosol and tests of the Stokes–Einstein equation

Abstract

Abstract. Viscosities and diffusion rates of organics within secondary organic aerosol (SOA) remain uncertain. Using the bead-mobility technique, we measured viscosities as a function of water activity (aw) of SOA generated by the ozonolysis of limonene followed by browning by exposure to NH3 (referred to as brown limonene SOA or brown LSOA). These measurements together with viscosity measurements reported in the literature show that the viscosity of brown LSOA increases by 3–5 orders of magnitude as the aw decreases from 0.9 to approximately 0.05. In addition, we measured diffusion coefficients of intrinsic fluorescent organic molecules within brown LSOA matrices using rectangular area fluorescence recovery after photobleaching. Based on the diffusion measurements, as the aw decreases from 0.9 to 0.33, the average diffusion coefficient of the intrinsic fluorescent organic molecules decreases from 5.5×10-9 to 7.1×10-13 cm2 s−1 and the mixing times of intrinsic fluorescent organic molecules within 200 nm brown LSOA particles increases from 0.002 to 14 s. These results suggest that the mixing times of large organics in the brown LSOA studied here are short (<1 h) for aw and temperatures often found in the planetary boundary layer (PBL). Since the diffusion coefficients and mixing times reported here correspond to SOA generated using a high mass loading (∼1000 µg m−3), biogenic SOA particles found in the atmosphere with mass loadings ≤10 µg m−3 are likely to have higher viscosities and longer mixing times (possibly 3 orders of magnitude longer). These new measurements of viscosity and diffusion were used to test the accuracy of the Stokes–Einstein relation for predicting diffusion rates of organics within brown LSOA matrices. The results show that the Stokes–Einstein equation gives accurate predictions of diffusion coefficients of large organics within brown LSOA matrices when the viscosity of the matrix is as high as 102 to 104 Pa s. These results have important implications for predicting diffusion and mixing within SOA particles in the atmosphere.

Supplementary material to "Predictions of diffusion rates of organic molecules in secondary organic aerosols using the Stokes-Einstein and fractional Stokes-Einstein relations"

“…Diffusion coefficients in brown limonene SOA are reported as a function of aw in Ullmann et al (2019). The viscosity of brown limonene SOA matrices as a function of aw were also reported in that work.…”

“…Organic Molecule Radius (Å) Reference Diffusing Fluorescein 5.02 (Mustafa et al, 1993) Rhodamine 6G 5.89 (Müller and Loman, 2008) Calcein 7.4 (Tamba et al, 2010) Cresyl violet 3.7 Molecular radius calculated using Van der Waals theory of atomic increments (Edward, 1970) Diffusing and matrix Sucrose 4.5 Based on the density of amorphous sucrose (Chenyakin et al, 2017) Brown limonene SOA components 5.4 ± 0.9 (Ullmann et al, 2019) Matrix Citric acid 3.7 (Müller and Stokes, 1956) Sorbitol 3.6 (Comper, 1996)…”

Liquid–liquid phase separation and viscosity within secondary organic aerosol generated from diesel fuel vapors

“…One physicochemical property of SOA that remains insufficiently understood is liquid-liquid phase separation (LLPS) (Pankow, 2003;Marcolli and Krieger, 2006;Ciobanu et al, 2009;Bertram et al, 2011;Krieger et al, 2012;Song et al, 2012a;Zuend and Seinfeld, 2012;Veghte et al, 2013;You et al, 2014;O'Brien et al, 2015;Freedman, 2017). Very recent work has shown that SOA particles free of inorganic salts can undergo LLPS at a high relative humidity (RH) with implications for predicting the cloud nucleating properties of SOA (Petters et al, 2006;Hodas et al, 2016;Renbaum-Wolff et al, 2016;Ovadnevaite et al, 2017;Rastak et al, 2017;Song et al, 2017Song et al, , 2018Altaft et al, 2018;Liu et al, 2018;Davies et al, 2019;Ham et al, 2019).…”

Predicting Secondary Organic Aerosol Phase State and Viscosity and its Effect on Multiphase Chemistry in a Regional Scale Air Quality Model

“…The effective diffusion coefficient for IEPOX through the organic coating (Dorg) was then calculated using the Stokes-Einstein Equation (refer to Eq. (1)), assuming that = 1 nm (Evoy et al, 2019;Ullmann et al, 2019). = 0 .…”

“…When aerosols form a core-shell morphology, experimentally observed viscosities of the outer organic-rich shell and inner electrolyte-rich core have been shown to differ by up to three orders of magnitude resulting in possible diffusion limitations on reactive uptake (Ullmann et al, 2019).…”