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To improve predictions of air quality, visibility, and climate change,\udknowledge of the viscosities and diffusion rates within organic particulate\udmatter consisting of secondary organic material (SOM) is required. Most\udqualitative and quantitative measurements of viscosity and diffusion rates\udwithin organic particulate matter have focused on SOM particles generated\udfrom biogenic volatile organic compounds (VOCs) such as α-pinene and isoprene. In this study, we\udquantify the relative humidity (RH)-dependent viscosities at 295 ± 1 K\udof SOM produced by photo-oxidation of toluene, an anthropogenic VOC. The\udviscosities of toluene-derived SOM were 2  ×  10−1 to  ∼  6  ×  106 Pa s from 30 to 90 % RH, and greater than\ud ∼  2  ×  108 Pa s (similar to or greater than the\udviscosity of tar pitch) for RH  ≤  17 %. These viscosities\udcorrespond to Stokes–Einstein-equivalent diffusion coefficients for large\udorganic molecules of  ∼  2  ×  10−15 cm2 s−1 for\ud30 % RH, and lower than\ud ∼  3  ×  10−17 cm2 s−1 for RH  ≤  17 %.\udBased on these estimated diffusion coefficients, the mixing time of large\udorganic molecules within 200 nm toluene-derived SOM particles is 0.1–5 h\udfor 30 % RH, and higher than  ∼  100 h for RH  ≤  17 %. As\uda starting point for understanding the mixing times of large organic\udmolecules in organic particulate matter over cities, we applied the mixing\udtimes determined for toluene-derived SOM particles to the world's top 15 most\udpopulous megacities. If the organic particulate matter in these megacities is\udsimilar to the toluene-derived SOM in this study, in Istanbul, Tokyo,\udShanghai, and São Paulo, mixing times in organic particulate matter during\udcertain periods of the year may be very short, and the particles may be\udwell-mixed. On the other hand, the mixing times of large organic molecules in\udorganic particulate matter in Beijing, Mexico City, Cairo, and Karachi may be\udlong and the particles may not be well-mixed in the afternoon\ud(15:00–17:00 LT) during certain times of the year

atmospheric chemistry and physics

Relative humidity-dependent viscosity of secondary organic material from toluene photo-oxidation and possible implications for organic particulate matter over megacities