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.
Highlights
Large amounts of volatile organic compounds, such as isoprene, limonene, and α-pinene from biogenic sources and aliphatic and aromatic compounds from anthropogenic sources, are released into the atmosphere
We focused on brown limonene secondary organic material (LSOA) for the following reasons: first, brown LSOA contains light-absorbing molecules that are fluorescent and capable of rapid photobleaching (Lee et al, 2013)
Brown LSOA was produced at the University of California, Irvine (UCI) following the procedure outlined in Hinks et al (2016)
Summary
Large amounts of volatile organic compounds, such as isoprene, limonene, and α-pinene from biogenic sources and aliphatic and aromatic compounds from anthropogenic sources, are released into the atmosphere. Motivated primarily by the food industry, there have been a few tests of the Stokes–Einstein relation for predicting diffusion rates of organics in organic–water matrices, such as saccharide–water matrices (Bastelberger et al, 2017; Champion et al, 1997; Chenyakin et al, 2017; Corti et al, 2008; Price et al, 2016) In these cases, the matrices contained only two species (one organic and water), which is very different from SOA matrices that contain thousands of different species (Nozière et al, 2015). We measured diffusion coefficients of intrinsic fluorescent organic molecules within brown LSOA matrices using a technique called rectangular area fluorescence recovery after photobleaching These new data, combined with viscosity data that already exist in the literature for brown LSOA–water matrices, were used to test the accuracy of the Stokes–Einstein relation for predicting diffusion rates of organics within SOA particles. Limonene accounts for roughly 10 % of the global emissions of monoterpenes (and is an important source of SOA in the atmosphere) (Kanakidou et al, 2005; Sindelarova et al, 2014)
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