Issue 23, 2023

Direct quantification of changes in pH within single levitated microdroplets and the kinetics of nitrate and chloride depletion

Abstract

The hygroscopicity and pH of aqueous microdroplets and smaller aerosols control their impacts on human health and the climate. Nitrate depletion and chloride depletion through the partitioning of HNO3 and HCl into the gas phase are processes that are enhanced in micron-sized and smaller aqueous droplets and this depletion influences both hygroscopicity and pH. Despite a number of studies, uncertainties remain about these processes. While acid evaporation and the loss of HCl or HNO3 have been observed during dehydration, there is a question as to the rate of acid evaporation and whether this can occur in fully hydrated droplets at higher relative humidity (RH). To directly elucidate the kinetics of nitrate and chloride depletion through evaporation of HNO3 and HCl, respectively at high RH, single levitated microdroplets are probed with cavity-enhanced Raman spectroscopy. Using glycine as a novel in situ pH probe, we are able to simultaneously measure changes in microdroplet composition and pH over timescales of hours. We find that the loss of chloride from the microdroplet is faster than that of nitrate, and the calculated rate constants infer that depletion is limited by the formation of HCl or HNO3 at the air–water interface and subsequent partitioning into the gas phase.

Graphical abstract: Direct quantification of changes in pH within single levitated microdroplets and the kinetics of nitrate and chloride depletion

Supplementary files

Article information

Article type
Edge Article
Submitted
21 Dec. 2022
Accepted
11 Apr. 2023
First published
11 Apr. 2023
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY license

Chem. Sci., 2023,14, 6259-6268

Direct quantification of changes in pH within single levitated microdroplets and the kinetics of nitrate and chloride depletion

K. J. Angle and V. H. Grassian, Chem. Sci., 2023, 14, 6259 DOI: 10.1039/D2SC06994F

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

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