Organic films on indoor surfaces serve as a medium for reactions and for partitioning of semi-volatile organic compounds and thus play an important role in indoor chemistry. However, the chemical...
Knowledge of diffusion coefficients as a function of temperature in secondary organic aerosol (SOA) or proxies of SOA is needed to predict atmospheric chemistry, climate, and air quality. We determined diffusion coefficients as a function of temperature of a fluorescent organic molecule in a sucrose matrix (a proxy for SOA). Diffusion coefficients were a strong function of temperature (e.g., at water activity = 0.43, diffusion coefficients decreased by a factor of ∼40 as the temperature decreased by 20 K). Interestingly, the apparent activation energy for diffusion of the fluorescent organic molecule was similar to the apparent activation for diffusion of water in the sucrose matrix. On the basis of these measurements, the mixing time of organic molecules by diffusion in some types of SOA particles will often be >1 h in the free troposphere, if a sucrose matrix is an accurate proxy for these types of SOA.
Biomass burning events emit large amounts of phenolic compounds, which are oxidized in the atmosphere and form secondary organic aerosol (SOA). Using the poke-flow technique, we measured relative humidity (RH)-dependent viscosities of SOA generated by the oxidation of three biomass burning phenolic compounds: catechol, guaiacol, and syringol. All systems had viscosity < 3 × 103 Pa s at RH ≳ 40% and > 2 × 108 Pa s at RH ≲ 3% at room temperature. At RH values of 0–10%, the viscosities of these SOA were at least 2 orders of magnitude higher than the viscosity of primary organic aerosol from biomass burning. We also developed a parameterization for predicting the viscosity of phenolic biomass burning SOA as a function of RH and temperature. Based on this parameterization, the viscosity of phenolic biomass burning SOA is strongly dependent on both RH and temperature. Under dry conditions, phenolic biomass burning SOA is highly viscous at room temperature (∼109 Pa s) and becomes a glass (viscosity > 1012 Pa s) when the temperature is < 280 K. For tropospheric temperature and RH values, phenolic biomass burning SOA is often in a liquid state (η < 102 Pa s) below ∼2 km altitude, a semi-solid state (102 < η < 1012 Pa s) between ∼2 and ∼9 km, and a glassy state (η > 1012 Pa s) above ∼9 km. Furthermore, the mixing time of organic molecules in a 200 nm phenolic biomass burning SOA particle exceeds 1 h above 3 km in the troposphere.
Abstract. The viscosity of secondary organic aerosol (SOA) is needed to improve predictions of air quality, climate, and atmospheric chemistry. Many techniques have been developed to measure the viscosity of micrometer-sized materials at room temperature; however, few techniques are able to measure viscosity as a function of temperature for these small sample sizes. SOA in the troposphere experience a wide range of temperatures, so measurement of viscosity as a function of temperature is needed. To address this need, a new method was developed based on hot-stage microscopy combined with fluid dynamics simulations. The current method can be used to determine viscosities in the range of roughly 104 to 108 Pa s at temperatures greater than room temperature. Higher viscosities may be measured if experiments are carried out over multiple days. To validate our technique, the viscosities of 1,3,5-tris(1-naphthyl)benzene and phenolphthalein dimethyl ether were measured and compared with values reported in the literature. Good agreement was found between our measurements and literature data. As an application to SOA, the viscosity as a function of temperature for lab-generated farnesene SOA material was measured, giving values ranging from 3.1×106 Pa s at 51 ∘C to 2.6×104 Pa s at 67 ∘C. We fit the temperature-dependent data to the Vogel–Fulcher–Tammann (VFT) equation and obtained a fragility parameter for the material of 7.29±0.03, which is very similar to the fragility parameter of 7 reported for α-pinene SOA by Petters and Kasparoglu (2020). These results demonstrate that the viscosity as a function of temperature can be measured for lab-generated SOA material using our hot-stage microscopy method.
Organosulfate compounds make up a substantial fraction of the particle mass concentration in some regions of the Earth's atmosphere, and organosulfate particles can have sufficiently high viscosity to limit rates of gas-particle interactions. Viscosity varies with relative humidity (RH). Herein, organosulfate particles were exposed to the gas-phase products of αpinene photooxidation. The gas-particle partitioning of these species was studied from 15 to 70% RH and <1 to 16 ppb NO at 299 K. The uptake of the α-pinene oxidation products increased with the increase in RH, and higher gas-phase NO concentrations resulted in increased particle-phase concentrations of nitrogen compounds. Particle hygroscopicity was examined by optical microscopy. Hygroscopic growth at elevated RH was sufficient to explain the RH-dependent uptake measurements, and kinetic limitations tied to particle viscosity were not observed. The lack of kinetic limitations combined with the Stokes−Einstein equation implied a viscosity much less than 1 × 10 6 Pa s. This value is consistent with estimated viscosities based on literature parameterizations for water mass fractions in the particles of at least 0.05 at 15% RH. Overall, these results suggest that organosulfate hygroscopicity plays a key role in their viscosity and hence in regulating gas-particle partitioning, thereby simplifying the treatment of atmospheric chemistry and transport of pollutants in models of the Earth's atmosphere. The role of organosulfates is expected to take on increasing importance for projected future emission trends.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.