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

Information on liquid–liquid phase separation (LLPS) and viscosity (or diffusion) within secondary organic aerosol (SOA) is needed to improve predictions of particle size, mass, reactivity, and cloud nucleating properties in the atmosphere. Here we report on LLPS and viscosities within SOA generated by the photooxidation of diesel fuel vapors. Diesel fuel contains a wide range of volatile organic compounds, and SOA generated by the photooxidation of diesel fuel vapors may be a good proxy for SOA from anthropogenic emissions. In our experiments, LLPS occurred over the relative humidity (RH) range of inline-formula∼70 % to inline-formula∼100 %, resulting in an organic-rich outer phase and a water-rich inner phase. These results may have implications for predicting the cloud nucleating properties of anthropogenic SOA since the presence of an organic-rich outer phase at high-RH values can lower the supersaturation with respect to water required for cloud droplet formation. At inline-formula≤10 % RH, the viscosity was inline-formula $M4inlinescrollmathml\ge \mathrm{normal\; 1}\times {\mathrm{normal\; 10}}^{\mathrm{normal\; 8}}$ 46pt14ptsvg-formulamathimgd3becb04dea2544bfec575e486a5f107 acp-19-12515-2019-ie00001.svg46pt14ptacp-19-12515-2019-ie00001.png  Pa s, which corresponds to roughly the viscosity of tar pitch. At 38 %–50 % RH, the viscosity was in the range of inline-formula1×10⁸ to inline-formula3×10⁵ Pa s. These measured viscosities are consistent with predictions based on oxygen to carbon elemental ratio (inline-formulaO:C) and molar mass as well as predictions based on the number of carbon, hydrogen, and oxygen atoms. Based on the measured viscosities and the Stokes–Einstein relation, at inline-formula≤10 % RH diffusion coefficients of organics within diesel fuel SOA is inline-formula $M9inlinescrollmathml\le \mathrm{normal\; 5.4}\times {\mathrm{normal\; 10}}^{-\mathrm{normal\; 17}}$ 65pt14ptsvg-formulamathimg81f85a19804ee0bbb25c62f30c9dfc4e acp-19-12515-2019-ie00002.svg65pt14ptacp-19-12515-2019-ie00002.png  cminline-formula² sinline-formula⁻¹ and the mixing time of organics within 200 nm diesel fuel SOA particles (inline-formulaτ_mixing) is 50 h. These small diffusion coefficients and large mixing times may be important in laboratory experiments, where SOA is often generated and studied using low-RH conditions and on timescales of minutes to hours. At 38 %–50 % RH, the calculated organic diffusion coefficients are in the range of inline-formula $M13inlinescrollmathml\mathrm{normal\; 5.4}\times {\mathrm{normal\; 10}}^{-\mathrm{normal\; 17}}$ 55pt14ptsvg-formulamathimgb8aaddae8a73d2d23727b0f38112daf7 acp-19-12515-2019-ie00003.svg55pt14ptacp-19-12515-2019-ie00003.png to inline-formula $M14inlinescrollmathml\mathrm{normal\; 1.8}\times {\mathrm{normal\; 10}}^{-\mathrm{normal\; 13}}$ 55pt14ptsvg-formulamathimgbfcea12d119b18040d27e13eb2907160 acp-19-12515-2019-ie00004.svg55pt14ptacp-19-12515-2019-ie00004.png  cminline-formula² sinline-formula⁻¹ and calculated inline-formulaτ_mixing values are in the range of inline-formula∼0.01 h to inline-formula∼50 h. These values provide important constraints for the physicochemical properties of anthropogenic SOA.