A Dual-Droplet Approach for Measuring the Hygroscopicity of Aqueous Aerosol

Abstract

Abstract. Accurate characterization of the water activity and hygroscopicity of aqueous aerosol material allows us to predict the chemical and physical state of aerosol particles exposed to humid conditions in the environment. The hygroscopicity of aerosol determines the size, phase morphology, viscosity, chemical reactivity, and optical properties of constituent particles, and directly impacts their ability to form clouds in the atmosphere. In this work, we describe measurements of hygroscopicity using a linear quadrupole electrodynamic balance (LQ-EDB). We levitate two droplets, one droplet that acts as a relative humidity (RH) probe and one sample droplet, and expose them to controlled environmental conditions. We describe the development of a RH measurement using probe droplets of aqueous NaCl or LiCl, allowing for precise in-situ measurements of RH in the LQ-EDB chamber. We demonstrate that the RH may be determined with an accuracy of 0.5 % at 50 % RH and better than 0.1 % at 90 % RH using NaCl, and show that LiCl is effective at characterizing the RH from ~10 % RH up to ~90 %. We simultaneously measure the response of sample droplets containing aqueous material (including ammonium sulfate, citric acid, 1,2,6-hexanetriol, and tetraethylene glycol) and report hygroscopic growth via their radial growth factors. We use established thermodynamic models to validate the accuracy of the RH probe and to compare with the measured hygroscopicity of the samples. This approach shows significant advantages over other methods for accurately characterizing the hygroscopicity of samples with a range of characteristics, such as high viscosity and vapor pressure.

Exploring the hygroscopicity, water diffusivity, and viscosity of organic–inorganic aerosols – a case study on internally-mixed citric acid and ammonium sulfate particles

“…In these measurements, the dual-droplet approach, detailed in our earlier work, was used to establish the radial growth factor as a function of RH. 44 Sample droplets were exposed to various RH from dry conditions (0% RH) up to 50% This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. across all particle compositions, in addition to pure CA.…”

Measuring the Chemical Evolution of Levitated Particles: A Study on the Evaporation of Multicomponent Organic Aerosol

“…Aqueous particles containing n -EG will evaporate over extended timescales due to the reduced activity of evaporating species brought about by the addition of water. Assumptions of ideality in aqueous solutions break down, as evidenced by previous hygroscopic growth measurements on individual n -EG components and thermodynamic modeling . In an ideal solution, the activity of water in the solution, set by the RH, will equal the mole fraction of water in the mixture.…”

“…In real solutions of n -EG, the mole fraction of water is higher than predicted by ideality for most of the RH range, indicating the activity coefficient of water to be less than 1. Table shows the expected mole fraction of water in individual particles exposed to a range of RH conditions as predicted by the Aerosol Inorganic-Organic Mixtures Functional group Activity Coefficients (AIOMFAC) modelthese have been verified by experiments and shown to be accurate for the pure component cases. , These predictions allow the aqueous composition of mixtures to be estimated using the Zdanovskii–Stokes–Robinson (ZSR) mixing rule, which equates the sum of the water content in a mixture as the sum of the water content associated with each individual components at the same humidity. The AIOMFAC model may also be used to predict the water content of the mixtures; however, it was not possible to incorporate these predictions into the evaporation model for a continuously changing composition.…”

“…Assumptions of ideality in aqueous solutions break down, as evidenced by previous hygroscopic growth measurements on individual n-EG components and thermodynamic modeling. 70 In an ideal solution, the activity of water in the solution, set by the RH, will equal the mole fraction of water in the mixture. In real solutions of n-EG, the mole fraction of water is higher than predicted by ideality for most of the RH range, indicating the activity coefficient of water to be less than 1.…”

“…Inorganic-Organic Mixtures Functional group Activity Coefficients (AIOMFAC) modelthese have been verified by experiments and shown to be accurate for the pure component cases. 70,71 These predictions allow the aqueous composition of mixtures to be estimated using the Zdanovskii−Stokes− Robinson (ZSR) mixing rule, which equates the sum of the water content in a mixture as the sum of the water content associated with each individual components at the same humidity. The AIOMFAC model may also be used to predict the water content of the mixtures; however, it was not possible to incorporate these predictions into the evaporation model for a continuously changing composition.…”