A review of experimental techniques for aerosol hygroscopicity studies

Abstract

<p><strong>Abstract.</strong> Hygroscopicity is one of the most important physicochemical properties of aerosol particles, and also plays indispensable roles in many other scientific and technical fields. A myriad of experimental techniques, which differ in principles, configurations and cost, are available for investigating aerosol hygroscopicity under subsaturated conditions (i.e., relative humidity below 100 %). A comprehensive review of these techniques is provided in this paper, in which experimental techniques are broadly classified into four categories, according to the way samples under investigation are prepared. For each technique, we describe its operation principle and typical configuration, use representative examples reported in previous work to illustrate how this technique can help better understand aerosol hygroscopicity, and discuss its advantages and disadvantages. In addition, future directions are outlined and discussed for further technical improvement and instrumental development.</p>

Pre-deliquescent water uptake in deposited nanoparticles observed with in situ ambient pressure X-ray photoelectron spectroscopy

“…A wide range of experimental techniques are available to study the hygroscopic properties of particles in sub-saturated (< 100 % RH) conditions, including humidity-tandem differential mobility analyzers (H-TDMAs), various optical extinction and scattering methods, physisorption analyzers, quartz crystal microbalances and various microscopic and spectroscopic methods (Kreidenweis and Asa-Awuku, 2014;Tang et al, 2019). A drawback of many of the otherwise highly valuable traditional aerosol techniques, such as the H-TDMA or optical methods, is that they are based on detection of growth in mass or volume.…”

“…A number of spectroscopic techniques have been applied to systems of environmental and atmospheric relevance (see, e.g., the reviews of Ault and Axson, 2016;Tang et al, 2019). Surface-sensitive techniques such as X-ray photoelectron spectroscopy (XPS) provide molecular-level information on the chemical composition and properties of surfaces.…”

“…Surface-sensitive techniques such as X-ray photoelectron spectroscopy (XPS) provide molecular-level information on the chemical composition and properties of surfaces. So far, most of these studies, in particular those employing XPS, have adopted a surface science approach using single crystal surfaces as simplified model systems (Tang et al, 2019). For example, XPS has been used to study the photochemistry of TiO 2 (110) (Lampimäki et al, 2015), ionic mobility on NaCl (001) surfaces induced by water adsorption (Verdaguer et al, 2008) and reaction of water vapor with MgO (001) surfaces (Kaya et al, 2011).…”

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

“…While hygroscopicity and CCN activity are closely related, studies have shown that measurements of hygroscopicity at subsaturated RH can deviate 45 from estimates of hygroscopicity derived from CCN activity measurements, attributable to solubility and surface tension effects (Petters and Kreidenweis, 2013Ruehl et al, 2010Ruehl et al, , 2016. Deriving a complete understanding of the CCN activity and physical properties of aerosol particles across the full range of RH requires a careful approach utilizing both experimental methods and models (Tang et al, 2019).…”

Reviewer's Comments

“…The authors may consider discussing how advances in orthogonal fields may be of future importance to atmospheric measurements. For example, although not currently relevant to aerosol, the production of highly sensitive humidity sensors should be considered (e.g., Liang et al, 2018 ). The Dash-P and HFIMS, use faster sizing instrumentation however faster RH technology may also advance studies.…”