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

<p><strong>Abstract.</strong> Viscosities and diffusion rates of organics within secondary organic aerosol (SOA) remain uncertain. Using the bead-mobility technique, we measured the viscosities as a function of water activity (a<sub>w</sub>) of SOA generated by the ozonolysis of limonene followed by browning by exposure to NH<sub>3</sub> (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 a<sub>w</sub> 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 a<sub>w</sub> decreases from 0.9 to 0.33, the average diffusion coefficient of the intrinsic fluorescent organic molecules decreases from 5.5∙10<sup>-9</sup> cm<sup>2</sup> s<sup>-1</sup> to 7.1∙10<sup>-13</sup> cm<sup>2</sup> s<sup>-1</sup> and the mixing times of intrinsic fluorescent organic molecules within 200 nm brown LSOA particles increases from 0.002 s to 14 s. These results suggest that the mixing times of large organics in the brown LSOA studied here are short (< 1 hr) for a<sub>w</sub> and temperatures often found in the PBL. Since the diffusion coefficients and mixing times reported here correspond to SOA generated using a high mass loading (~ 1,000 µg m<sup>-3</sup>), biogenic SOA particles found in the atmosphere with mass loadings ≤ 10 µg m<sup>-3</sup> are likely to have higher viscosities and longer mixing times. 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 10<sup>2</sup> to 10<sup>4</sup> Pa s. These results have important implications for predicting diffusion and mixing with SOA particles in the atmosphere.</p>

Kinetic modelling of formation and evaporation of SOA from NO<sub>3</sub> oxidation of pure and mixed monoterpenes

“…Furthermore, while the Stokes-Einstein relation seems to hold for similar systems at viscosities of up to 10 4 Pas (Ullmann et al, 2019), it is not clear whether it also holds for viscosities of 10 7 Pas derived in this study (Evoy et al, 2019).…”

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

“…From the measured viscosities, we calculated diffusion coefficients of the organic molecules within the diesel fuel SOA using the Stokes-Einstein relation: when the size of the diffusing organics is similar to the size of the matrix molecules and the temperature is not too close to the Tg of the matrix (Champion et al, 2000;Marshall et al, 2016;Price et al, 2015Price et al, , 2016Bastelberger et al, 2017;Chenyakin et al, 2017;Ullmann et al, 2019).…”

“…The viscosity of diesel fuel SOA may be higher if generated using lower particle mass concentrations (Grayson et al, 2016;Jain et al, 2018). Second, τmixing-values may be overestimated at low RH values due to the possible breakdown of the Stokes-Einstein relation near the glass transition RH (Champion et al, 2000;Bastelberger et al, 2017;Chenyakin et al, 2017;Evoy et al, 2019;Ullmann et al, 2019).…”

“…Recently, a number of studies have investigated viscosity or diffusion rates within SOA particles generated in the laboratory (Virtanen et al, 2010;Cappa et al, 2011;Perraud et al, 2012;Saukko et al, 2012;Abramson et al, 2013;Robinson et al, 2013;Renbaum-Wolff et al, 2013;Loza et al, 2013;Kidd et al, 2014;Pajunoja et al, 2014;Bateman et al, 2015;Li et al, 2015;Song et al, 2015;Wang et al, 2015;Zhang et al, 2015;Grayson et al, 2016;Liu et al, 2016;Song et al, 2016a;Ullmann et al, 2019;Ye et al, 2018). Almost all of these studies focused on SOA generated from a single VOC.…”

Evaluation of Biogenic Organic Aerosols in the Amazon Rainforest Using WRF‐Chem With MOSAIC

“…The reactive uptake of IEPOX is strongly limited by the IEPOX diffusion process. within SOA is influenced by the SOA composition, temperature, RH, and phase state (Chenyakin, 2015; Grayson et al., 2016, 2017; Huang et al., 2018; Maclean et al., 2017; Pajunoja et al., 2014; Schmedding et al., 2019, 2020; Song et al., 2016; Ullmann et al., 2019; Zhang et al., 2011, 2018). Our study sets to a fixed value of 1.0 × 10 −5 cm 2 s −1 for simplicity, which can introduce uncertainties into the calculation of the uptake coefficients.…”