Direct Measurement of pH in Individual Particles via Raman Microspectroscopy and Variation in Acidity with Relative Humidity

Rindelaub, Joel D.; Craig, R. L.; Nandy, Lucy; Bondy, Amy L.; Dutcher, Cari S.; Shepson, P. B.; Ault, Andrew P.

doi:10.1021/acs.jpca.5b12699

Cited by 119 publications

(181 citation statements)

References 59 publications

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“…However, the contribution of organosulfates and organonitrates to AMS-measured total sulfate and nitrate (Farmer et al, 2010;Dovrou et al, 2019) must be considered to provide robust inorganic aerosol composition for acidity predictions. In locations such as the eastern US in summer where organosulfates already account for 15 % of total sulfate (Riva et al, 2019), AMS-measured total sulfate, when used in a thermodynamic model as inorganic sulfate, can lead to erroneous predictions of particle composition and thus pH .…”

Section: Considerations For the Development Of Observationally Derivementioning

confidence: 99%

The acidity of atmospheric particles and clouds

et al. 2020

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Abstract. Acidity, defined as pH, is a central component of aqueous chemistry. In the atmosphere, the acidity of condensed phases (aerosol particles, cloud water, and fog droplets) governs the phase partitioning of semivolatile gases such as HNO3, NH3, HCl, and organic acids and bases as well as chemical reaction rates. It has implications for the atmospheric lifetime of pollutants, deposition, and human health. Despite its fundamental role in atmospheric processes, only recently has this field seen a growth in the number of studies on particle acidity. Even with this growth, many fine-particle pH estimates must be based on thermodynamic model calculations since no operational techniques exist for direct measurements. Current information indicates acidic fine particles are ubiquitous, but observationally constrained pH estimates are limited in spatial and temporal coverage. Clouds and fogs are also generally acidic, but to a lesser degree than particles, and have a range of pH that is quite sensitive to anthropogenic emissions of sulfur and nitrogen oxides, as well as ambient ammonia. Historical measurements indicate that cloud and fog droplet pH has changed in recent decades in response to controls on anthropogenic emissions, while the limited trend data for aerosol particles indicate acidity may be relatively constant due to the semivolatile nature of the key acids and bases and buffering in particles. This paper reviews and synthesizes the current state of knowledge on the acidity of atmospheric condensed phases, specifically particles and cloud droplets. It includes recommendations for estimating acidity and pH, standard nomenclature, a synthesis of current pH estimates based on observations, and new model calculations on the local and global scale.

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Section: Considerations For the Development Of Observationally Derivementioning

confidence: 99%

The acidity of atmospheric particles and clouds

et al. 2020

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“…It is well known that atmospheric aerosol particles can exist in two states of thermodynamic equilibrium, stable and metastable, depending on their chemical composition and RH history (Rood et al, 1989). Particles in the stable state may be solid, solid plus liquid, or liquid as ambient RH increases (liquid phase appears when ambient RH reaches the deliquescence RH).…”

Section: Ph Prediction By Thermodynamic Modelsmentioning

confidence: 99%

Fine-particle pH for Beijing winter haze as inferred from different thermodynamic equilibrium models

et al. 2018

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pH is an important property of aerosol particles but is difficult to measure directly. Several studies have estimated the pH values for fine particles in northern China winter haze using thermodynamic models (i.e., E-AIM and ISORROPIA) and ambient measurements. The reported pH values differ widely, ranging from close to 0 (highly acidic) to as high as 7 (neutral). In order to understand the reason for this discrepancy, we calculated pH values using these models with different assumptions with regard to model inputs and particle phase states. We find that the large discrepancy is due primarily to differences in the model assumptions adopted in previous studies. Calculations using only aerosol-phase composition as inputs (i.e., reverse mode) are sensitive to the measurement errors of ionic species, and inferred pH values exhibit a bimodal distribution, with peaks between −2 and 2 and between 7 and 10, depending on whether anions or cations are in excess. Calculations using total (gas plus aerosol phase) measurements as inputs (i.e., forward mode) are affected much less by these measurement errors. In future studies, the reverse mode should be avoided whereas the forward mode should be used. Forward-mode calculations in this and previous studies collectively indicate a moderately acidic condition (pH from about 4 to about 5) for fine particles in northern China winter haze, indicating further that ammonia plays an important role in determining this property. The assumed particle phase state, either stable (solid plus liquid) or metastable (only liquid), does not significantly impact pH predictions. The unrealistic pH values of about 7 in a few previous studies (using the standard ISORROPIA model and stable state assumption) resulted from coding errors in the model, which have been identified and fixed in this study.

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“…Despite its importance, the 5 inability to directly measure fine mode particle pH (e.g. Rindelaub et al (2016) presents an indirect method that infers particle H + activity for sizes above 10 µm and requires activity coefficient predicted by a thermodynamic modeling. This method reports the pH for a HSO4 -/SO4 2-aerosol system similar to the fine particle pH predicted by a thermodynamic modeling used in this study (Guo et al, 2015)) has led to the use of measurable aerosol properties as acidity proxies, such as aerosol ammoniumsulfate ratio or ion balances (e.g.…”

Section: Introductionmentioning

confidence: 99%

The underappreciated role of nonvolatile cations on aerosol ammonium-sulfate molar ratios

Weber

2017

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Abstract. Overprediction of fine particle ammonium-sulfate molar ratios (R) by thermodynamic models is suggested as evidence for an organic film that only inhibits the equilibration of gas phase ammonia (but not water or nitric acid) with aerosol sulfate and questions the equilibrium assumption long thought to apply for submicron aerosol. The ubiquity of such organic films implies significant impacts on aerosol chemistry. We test the organic film hypothesis by analyzing ambient observations with a 15 thermodynamic model and find that R and ammonia partitioning can be accurately reproduced when small amounts of nonvolatile cations (NVC), consistent with observations, are considered in the thermodynamic analysis. Exclusion of NVCs results in predicted R consistently near 2. The error in R is positively correlated with NVC and not organic aerosol mass fraction or concentration.These results strongly challenge the postulated ability of organic films to perturb aerosol acidity or prevent ammonia from achieving gas-particle equilibrium for the conditions considered. 20 1 Atmos. Chem. Phys. Discuss., https://doi

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Direct Measurement of pH in Individual Particles via Raman Microspectroscopy and Variation in Acidity with Relative Humidity

Cited by 119 publications

References 59 publications

The acidity of atmospheric particles and clouds

The acidity of atmospheric particles and clouds

Fine-particle pH for Beijing winter haze as inferred from different thermodynamic equilibrium models

The underappreciated role of nonvolatile cations on aerosol ammonium-sulfate molar ratios

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