Abstract

Although knowledge of the physical state of aerosol particles is essential to understand atmospheric chemistry model and measurements, information on the viscosity and physical state of aerosol particles consisting of organic and inorganic salts are still rare. Herein, we quantified viscosities at 293 ± 1 K upon dehydration for the binary systems, sucrose/H2O and ammonium sulfate (AS)/H2O, and the ternary systems, sucrose/AS/H2O for organic-to-inorganic dry mass ratios (OIRs) = 4 : 1, 1 : 1, and 1 : 4. For binary systems, the viscosity of sucrose/H2O particles gradually increased from ~6 × 10−1 to > ~1 × 108 Pa‧s when the relative humidity (RH) decreased from ~83 % to ~24 %, which agrees with previous studies. The viscosity of AS/H2O particles remained in the liquid state (< 102 Pa‧s) for RH > ~50 %, and for RH ≤ ~50 %, the particles showed viscosity of > ~1 × 1012 Pa‧s, corresponding to a solid state. For ternary systems, the viscosity of organic-rich particles (OIR = 4 : 1) gradually increased from ~4 × 10−2 to ~1 × 108 Pa‧s for a RH decrease from ~80 % to ~18 %, similar to the sucrose/H2O particles. In the particles for OIR = 1 : 1, the viscosities ranged smaller than ~4 × 101 for RH > 34 %, and > ~1 × 108 Pa‧s at ~27 % RH. Compared to the organic-rich particles, in the inorganic-rich particles (OIR = 1 : 4), drastic enhancement in viscosity was observed as RH decreased; the viscosity enhanced approximately 8 orders of magnitude in RH from 43 % to 25 %. Based on viscosity data, all particles studied in this work were observed to exist as a liquid, semi-solid or solid depending on the RH. Furthermore, we compared the measured viscosities of ternary systems with OIRs of 4 : 1, 1 : 1, and 1 : 4 to the predicted viscosities using the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients Viscosity model (AIOMFAC-VISC) predictions with the Zdanovskii–Stokes–Robinson (ZSR)-style organic–inorganic mixing model, with excellent model–measurement agreement for all OIRs.

Highlights

  • Aerosol particles can play an important role in air quality, climate change, and human health (Kulmala et al, 2011; Zhang et al, 2015; IPCC 2019; Bhattarai et al, 2020)

  • The viscosity of sucrose/H2O particles gradually increased from ~6 × 10-1 to > ~1 × 108 Pa‧s when the relative humidity (RH) decreased from ~83% to ~24%, which agrees with previous studies

  • From the values of viscosity, sucrose/H2O particles were determined to be in a liquid state for RH greater than ~69%, a semisolid state for RH values between ~36% and ~69%, and semisolid or even a solid state for RH values less than ~24%

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Summary

Introduction

Aerosol particles can play an important role in air quality, climate change, and human health (Kulmala et al, 2011; Zhang et al, 2015; IPCC 2019; Bhattarai et al, 2020). Numerous laboratory studies on viscosities and physical states undertaken so far have focused on organic aerosols with different types of secondary organic aerosols investigated (Virtanen et al, 2010; Kuwata and Martin, 2012; Perraud et al, 2012; Renbaum-Wolff et al, 2013a; O'brien et al, 2014; Bateman et al, 2015; Song et al, 2015; Athanasiadis et al, 2016; Grayson et al, 2016; Hosny et al, 2016; Song et al, 2016a; Yli‐Juuti et al, 2017; Ham et al, 2019; Petters et al, 2019; Song et al, 2019; Gervasi et al, 2020; Song et al, 2021) These studies showed that the viscosities of 50 organic aerosol particles can vary depending on RH and chemical composition leading to different physical states, including amorphous solid (glassy), (semi-)solid, as well as liquid-like phases. The measured viscosities for sucrose/AS/H2O droplets with OIRs of 4:1, 1:1, and 1:4 are compared to predictions by the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients Viscosity model (AIOMFAC-VISC) when employing a Zdanovskii–Stokes–Robinson (ZSR)-style mixing model for viscosity contributions in aqueous organic–inorganic mixtures

Experimental
Optical observation of particles during dehydration
Thermodynamic calculations
Viscosities in binary systems of sucrose/H2O and AS/H2O
Summary

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