Imaging of pH distribution inside individual microdroplet by stimulated Raman microscopy

Abstract

Aerosol microdroplets as microreactors for many important atmospheric reactions are ubiquitous in the atmosphere. pH largely regulates the chemical processes within them; however, how pH and chemical species spatially distribute within an atmospheric microdroplet is still under intense debate. The challenge is to measure pH distribution within a tiny volume without affecting the chemical species distribution. We demonstrate a method based on stimulated Raman scattering microscopy to visualize the three-dimensional pH distribution inside single microdroplets of varying sizes. We find that the surface of all microdroplets is more acidic, and a monotonic trend of pH decreasing is observed in the 2.9-μm aerosol microdroplet from center to edge, which is well supported by molecular dynamics simulation. However, bigger cloud microdroplet differs from small aerosol for pH distribution. This size-dependent pH distribution in microdroplets can be related to the surface-to-volume ratio. This work presents noncontact measurement and chemical imaging of pH distribution in microdroplets, filling the gap in our understanding of spatial pH in atmospheric aerosol.

Key Factors Determining the Formation of Sulfate Aerosols Through Multiphase Chemistry—A Kinetic Modeling Study Based on Beijing Conditions

“…As documented, decreased R p not only increases the surface area-to-volume ratio favoring the gas-particle interaction (Angle et al, 2021), but may also initiate an array of reaction advantages at the air/water interface, including the enrichment of reactant and oxidant (Anglada et al, 2020), the enhancement of interior pressure (Riva et al, 2021), the distribution of electric field (Hao et al, 2022), and the amplification of light (Arroyo et al, 2022). Moreover, a stable pH gradient was observed by Wei et al (2018) in a 20 μm droplet, and this trend was recently confirmed as R p -dependent by Gong et al (2023) among the droplets within 2.9-134.6 μm. The decrease of pH toward interface may cause the distribution of different S (IV) species in aerosol, which requires further consideration in this model.…”

How the Acidity of Water Droplets and Films Is Controlled by the Air–Water Interface

“…The pH increase in small samples is a general result that applies to all neat water systems with a large surface-to-volume ratio. Some experiments − have recently used Raman microscopy techniques to determine the acidity of water droplets. However, they employed a pH probe and measured the acid (AH) and conjugate base (A – ) abundances.…”

“…First, the relative influence of the interface is expected to be enhanced in these smaller systems, leading to a dependence of the acidity on their size. Second, in the case of microdroplets, acidity has been suggested to be nonuniform , and to change between surface and core due to their different solvation conditions.…”

“…Since traditional pH probes for bulk solutions, such as reference electrodes, are obviously not suitable for very small droplets, experiments typically estimate the pH indirectly using spectroscopic methods to measure the ratio between the acidic and basic forms of an indicator molecule. Recent pioneering experiments have used Raman microscopy to map the acidity within microdroplets and to investigate whether pH gradients exist within aerosols. − However, these studies have yielded conflicting results, and a consensus on this key question is still lacking. Furthermore, it is important to immediately emphasize an important limitation of indicator-based spectroscopic approaches: they cannot provide an unambiguous measure of pH because the acid–base equilibrium also depends on other factors, including the indicator p K a which can significantly change from its bulk value due to partial desolvation of the acid and conjugate base at the interface. , …”

Spiers Memorial Lecture: Water at interfaces

“…A number of studies indicate that microdroplets exhibit unusual acid/base catalysis, with droplets sustaining stable long-range ion and pH gradients [95][96][97][98][99] and with some evidence for superacid and basic interfaces. 100 In contrast, other studies 101 observe a homogeneous pH within the droplets, with some evidence that pH is a function of droplet size and is either lower [102][103][104] or higher 96 than or the same 101,105 as that of the corresponding bulk solution.…”