Aerosol pH and Ion Activities of HSO4– and SO42– in Supersaturated Single Droplets

Li, Meng; Su, Hang; Zheng, Guangjie; Kühn, U.; Kim, Najin; Guo, Li; Ma, Nan; Pöschl, Ulrich; Cheng, Yafang

doi:10.1021/acs.est.2c01378

Cited by 17 publications

(12 citation statements)

References 69 publications

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“…Our research suggests that in real atmospheric conditions where many ambient aerosol particles exhibit high acidity (Angle et al, 2021;Song et al, 2018;Liu et al, 2019;Li et al, 2022), droplets encounter heightened impediments to phase separation and tend to display a homogeneous structure. Although we used bulk solution pH as an indicator of pH at droplet phase transition, previous studies (Craig et al, 2018;Coddens et al, 2019;Li et al, 2023) have shown that the pH deviation between single microdroplet measurements and bulk solution measurements is small.…”

Section: Effect Of Ph On Srh and Mrh Of Different Systemsmentioning

confidence: 85%

Influence of acidity on liquid−liquid phase transitions of mixed SOA proxy–inorganic aerosol droplets

Chen

Pei

Liu

et al. 2023

Preprint

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Abstract. Phase state and morphology of aerosol particles play a critical role in determining their effect on climate. While aerosol acidity has been identified as a key factor affecting the multiphase chemistry and phase transitions, the impact of acidity on phase transition of multicomponent aerosol particles has not been extensively studied in situ. In this work, we employ an aerosol optical tweezer (AOT) to probe the impact of acidity on the phase transition behavior of levitated aerosol particles. Our results reveal that higher acidity decreases the separation relative humidity (SRH) of aerosol droplets mixed with ammonium sulfate (AS) and secondary organic aerosol (SOA) proxy, such as 3-methylglutaric acid (3-MGA), 1,2,6-hexanetriol (HEXT) and 2,5-hexanediol (HEXD) across aerosol pH in atmospheric condition. Phase separation of organic acids was more sensitive to acidity compared to organic alcohols. We found the mixing relative humidity (MRH) was consistently higher than the SRH in several systems. Phase-separating systems, including 3-MGA/AS, HEXT/AS, and HEXD/AS, exhibited oxygen-to-carbon ratios (O:C) of 0.67, 0.50, and 0.33, respectively. In contrast, liquid-liquid phase separation (LLPS) did not occur in the high O:C system of glycerol/AS, which had an O:C of 1.00. Additionally, the morphology of 38 out of the 40 aerosol particles that underwent LLPS was observed to be a core-shell. Our findings provide a comprehensive understanding of the pH-dependent LLPS in individual suspended aerosol droplets and pave the way for future research on phase separation of atmospheric aerosol particles.

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Section: Effect Of Ph On Srh and Mrh Of Different Systemsmentioning

confidence: 85%

Influence of acidity on liquid−liquid phase transitions of mixed SOA proxy–inorganic aerosol droplets

Chen

Pei

Liu

et al. 2023

Preprint

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“…As a result, scientists still extensively used indirect approaches to retrieve aerosol pH, , such as thermodynamic models including E-AIM and ISORROPIA-II, which require well-defined compositions of the aerosols and precursors. Recently, many advancements are achieved in direct measurement of aerosol pH, which can be loosely categorized into two types, one type employs the thermodynamic equilibrium states within aerosols − and the other type utilizes various tracers. − Here, we use a ratiometric Raman spectroscopy method to measure the equilibrium concentration ratio of sulfate (SO 4 2– ) and bisulfate (HSO 4 – ) in aerosols and then retrieve the pH. To avoid the variation in compartments of different phases, the pH measurements are conducted at the RH values just above the SRHs.…”

Section: Resultsmentioning

confidence: 99%

Liquid–Liquid Phase Separation in Single Suspended Aerosol Microdroplets

Tong,

2023

Anal. Chem.

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Liquid−liquid phase separation (LLPS) is ubiquitous in ambient aerosols. This specific morphology exerts substantial impacts on the physicochemical properties and atmospheric processes of aerosols, particularly on the gas−particle mass transfer, the interfacial heterogeneous reaction, and the surface albedo. Although there are many studies on the LLPS of aerosols, a clear picture of LLPS in individual aerosols is scarce due to the experimental difficulties of trapping a single particle and mimicking the suspended state of real aerosols. Here, we investigate the phase separation in individual contactless microdroplets by a self-constructed laser tweezer/Raman spectroscopy system. The dynamic transformation of the morphology of optically trapped droplets over the course of humidity cycles is detected by the time-resolved cavity-enhanced Raman spectra. The impacts of pH and inorganic components on LLPS in aerosols are discussed. The results show that the increasing acidity can enhance the miscibility between the hydrophilic and hydrophobic phases and decrease the separation relative humidity of aerosols. Moreover, the inorganic components also have various impacts on the aerosol phase state, whose influence depends on their different salting-out capabilities. It brings possible implications on the morphology of actual atmospheric particles, particularly for those dominated by internal mixtures of inorganic and organic components.

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“…Such a technique is highly precise (error: 0.09 pH units) and only uses water as a pH-sensing probe . Besides, pH was measured not only in deposited microdroplets but in levitated onesfor example, microdroplets that are optically levitated with Aerosol optical tweezers (AOTs). , Such a contact-free method enabled experimentalists to measure the acidity of aerosols under supersaturated conditions. , Finally, another state-of-the-art applies surface-enhanced Raman spectroscopy (SERS) to measure the microdroplet pH. ,, This SERS technique was pioneered by Vikesland’s team at Virginia Tech, and its application was later extended to studying the ionic distribution within ammonium droplets , and to measuring the kinetics of ammonium depletion from aerosols …”

Section: Introductionmentioning

confidence: 99%

“…23 Besides, pH was measured not only in deposited microdroplets but in levitated ones�for example, microdroplets that are optically levitated with Aerosol optical tweezers (AOTs). 26,27 Such a contact-free method enabled experimentalists to measure the acidity of aerosols under supersaturated conditions. 26,27 Finally, another state-of-the-art applies surface-enhanced Raman spectroscopy (SERS) to measure the microdroplet pH.…”

Section: ■ Introductionmentioning

confidence: 99%

“…26,27 Such a contact-free method enabled experimentalists to measure the acidity of aerosols under supersaturated conditions. 26,27 Finally, another state-of-the-art applies surface-enhanced Raman spectroscopy (SERS) to measure the microdroplet pH. 5,24,25 This SERS technique was pioneered by Vikesland's team at Virginia Tech, 5 and its application was later extended to studying the ionic distribution within ammonium droplets 24,25 and to measuring the kinetics of ammonium depletion from aerosols.…”

Section: ■ Introductionmentioning

confidence: 99%

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Spatiotemporally Resolved pH Measurement in Aerosol Microdroplets Undergoing Chloride Depletion: An Application of In Situ Raman Microspectrometry

et al. 2022

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Acidity is a defining property of atmospheric aerosols that profoundly affects environmental systems, human health, and climate. However, directly measuring the pH of aerosol microdroplets remains a challenge, especially when the microdroplets' composition is nonhomogeneous or dynamically evolving or both. As a result, a pH measurement technique with high spatiotemporal resolution is needed. Here, we report a spatiotemporally resolved pH measurement technique in microdroplets using spontaneous Raman spectroscopy. Our target sample was the microdroplets comprising sodium chloride and oxalic acid�laboratory surrogates of sea spray aerosols and watersoluble organic compounds, respectively. Our measurements show that the chloride depletion from the microdroplets caused a continuous increase in pH by ∼0.5 units in 2 hours. Meanwhile, the surface propensity of chloride anions triggers a stable pH gradient inside a single droplet, with the pH at the droplet surface lower than that at the core by ∼ 0.4 units. The uncertainties arising from the Raman detection limit (±0.08 pH units) and from the nonideal solution conditions (−0.06 pH units) are constrained. Our findings indicate that spontaneous Raman spectroscopy is a simple yet robust technique for precise pH measurement in aerosols with high spatiotemporal resolution.

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Aerosol pH and Ion Activities of HSO₄^– and SO₄^2– in Supersaturated Single Droplets

Cited by 17 publications

References 69 publications

Influence of acidity on liquid−liquid phase transitions of mixed SOA proxy–inorganic aerosol droplets

Influence of acidity on liquid−liquid phase transitions of mixed SOA proxy–inorganic aerosol droplets

Liquid–Liquid Phase Separation in Single Suspended Aerosol Microdroplets

Spatiotemporally Resolved pH Measurement in Aerosol Microdroplets Undergoing Chloride Depletion: An Application of In Situ Raman Microspectrometry

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