Surfaces of Atmospheric Droplet Models Probed with Synchrotron XPS on a Liquid Microjet

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS:The atmosphere is a key part of the earth system comprising myriad chemical species in all basic forms of matter.Ubiquitous nano-and microscopic aerosol particles and cloud droplets suspended in the air play crucial roles in earth's climate and the formation of air pollution. Surfaces are a prominent part of aerosols and droplets, due to the high surface area to bulk volume ratios, but very little is known about their specific properties. Many atmospheric compounds are surface-active, leading to enhanced surface concentrations in aqueous solutions. Their distribution between the surface and bulk may determine heterogeneous chemistry and many other properties of aerosol and cloud droplets, but has not been directly observed.We used X-ray photoelectron spectroscopy (XPS) to obtain direct molecular-level information on the surface composition and structure of aqueous solutions of surface-active organics as model systems for atmospheric aerosol and cloud droplets. XPS is a vacuum-based technique enabled for volatile aqueous organic samples by the application of a high-speed liquid microjet. In combination with brilliant synchrotron X-rays, the chemical specificity of XPS allows distinction between elements in different chemical states and positions within molecular structures. We used core-level C 1s and N 1s signals to identify the alkyl and hydrophilic groups of atmospheric carboxylic acids, alkyl-amines, and their conjugate acids and bases. From this, we infer changes in the orientation of surface-adsorbed species and quantify their relative abundances in the surface. XPS-derived surface enrichments of the organics follow trends expected from their surface activities and we observed a preferential orientation at the surface with the hydrophobic alkyl chains pointing increasingly outward from the solution at higher concentrations. This provides a first direct experimental observation of well-established concepts of surface adsorption and confirms the soundness of the method. We mapped relative abundances of conjugate acid−base pairs in the aqueous solution surfaces from the respective intensities of distinctive XPS signals. For each pair, the protonation equilibrium was significantly shifted toward the neutral form in the surface, compared to the bulk solution, across the full pH range. This represents an apparent shift of the pK a in the surface, which may be toward either higher or lower pH, depending on whether the acid or base form of the pair is the neutral species. The surface shifts are broadly consistent with the relative differences in surface enrichment of the individual acid and base conjugates in binary aqueous solutions, with additional contributions from nonideal interactions in the surface. In aqueous mixtures of surface-active carboxylate anions with ammonium salts at near-neutral pH, we found that the conjugate carboxylic acids were further strongly enhanced. This occurs as the coadsorption of weakly basic carboxylate anions and weakly acidic ammoniu...

Perspective on Theoretical and Experimental Advances in Atmospheric Photochemistry

“…Intriguing questions about photochemistry in aerosol particles are attracting growing attention; for example, what are the consequences of high surface-area-to-volume ratios, surface composition and solute enrichment, electric fields at the surfaces of water droplets, supersaturated concentrations of solutes, photocatalytic molecular aggregates, changes in pH, and the nanofocusing of light to give photochemical hotspots in droplets? ,,,− Might there be polaritonic effects in sub-micrometer diameter aerosol particles, which can act as high quality resonators for specific wavelengths of light? The single-particle spectroscopy methods mentioned above and new tools such as X-ray microscopy and X-ray absorption and photoelectron spectroscopies (XAS and XPS) ,, are beginning to provide answers.…”

Ice Interfaces: Vapor, Liquid, and Solutions

“…Additional layers of water thus have no receiving valence for attachment; hence, the surface water layer is hydrophobic and water dewets the surface despite being covered with a monolayer of water, one of the experimental challenges presented by ice. The dewetting phenomena may be relevant to the nucleation of ice in the environment since atmospheric ice typically forms on particulate matter including pollen, dust, soot, and salts that result from ammonia neutralization of acidic gases common in the atmosphere. − The mismatch between the first solvation layer and ice likely impacts both growth kinetics and the fate of the nascent nuclei. Interestingly, a graphene overlayer appears to create a template for crystallization due to the planar structure of graphene preventing wrinkle defects; crystallization then propagates layer-by-layer in a linear fashion.…”