A new hot-stage microscopy technique for measuring temperature-dependent viscosities of aerosol particles and its application to farnesene secondary organic aerosol

Abstract

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.

Boiling of Catechol Secondary Organic Aerosol When Heated to Mild Temperatures (36–52 °C) Due to Carbon Dioxide Formation and High Viscosity

“…Also related to the previous paragraph, thermal decomposition of SOA components could lead to a change in SOA viscosity since composition and viscosity are tightly connected. Many studies have investigated the viscosities of SOA at room temperature, − but less attention has focused on the viscosity of SOA upon heating, − and no studies have investigated the viscosity of phenolic SOA upon heating. In the studies that focus on the viscosity of SOA upon heating, researchers often assume that thermal decomposition of SOA components does not occur when aerosols are heated to mild temperatures.…”

Boiling of catechol secondary organic aerosol when heated to mild temperatures (36-52 °C) due to acetone and carbon dioxide formation and high viscosity

“…16 Information on the viscosities of SOA at temperatures commonly used in TD measurements is needed to test these assumptions. Many studies have investigated the viscosities of SOA at room temperature, [30][31][32][33][34][35][36][37][38] but less attention has focused on the viscosity of SOA upon heating, [39][40][41][42][43] and no studies have investigated the viscosity of phenolic SOA upon heating.…”

“…Recently, we developed a hot-stage microscopy technique for measuring the temperaturedependent viscosity of SOA. 41 This technique involves heating an SOA sample with non-spherical geometries and quantifying the change in shape of the SOA sample due to heating with an optical microscope. From the shape change, viscosities can be calculated.…”

“…This method worked as expected for farnesene SOA. 41 However, when attempting to measure the temperature-dependent viscosity of SOA generated by the ozonolysis of catechol, surprisingly, the volume of the SOA increased when heated to mild temperatures (36-52 °C).…”

Direct measurement of the viscosity of ternary aerosol mixtures

“…Viscosity is then obtained by measurement of the mobility of insoluble beads embedded in the organic matrix, 6 measurement of the restoration of shape in response to poking the sample with a needle, 7 measurement of the fluorescence lifetime of molecular rotors embedded in the organic matrix, 8 measurement of particle coalescence inside an optical trap or under an electron microscope, 9,10 or measurement of particle roundness as a function of temperature. 11 These offline techniques generally require the generation of several milligrams of material. Sample modification after collection via evaporation of semi-volatile compounds or uptake of moisture or reaction with atmospheric oxidants may occur.…”