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Due to their small size, measurements of the complex composition of atmospheric aerosol particles and their surfaces are analytically challenging. This is particularly true for microspectroscopic methods, where it can be difficult to optically identify individual particles smaller than the diffraction limit of visible light (∼350 nm) and measure their vibrational modes. Recently, surface enhanced Raman spectroscopy (SERS) has been applied to the study of aerosol particles, allowing for detection and characterization of previously undistinguishable vibrational modes. However, atmospheric particles analyzed via SERS have primarily been >1 μm to date, much larger than the diameter of the most abundant atmospheric aerosols (∼100 nm). To push SERS towards more relevant particle sizes, a simplified approach involving Ag foil substrates was developed. Both ambient particles and several laboratory-generated model aerosol systems (polystyrene latex spheres (PSLs), ammonium sulfate, and sodium nitrate) were investigated to determine SERS enhancements. SERS spectra of monodisperse, model aerosols between 400–800 nm were compared with non-SERS enhanced spectra, yielding average enhancement factors of 10 2 for both inorganic and organic vibrational modes. Additionally, SERS-enabled detection of 150 nm size-selected ambient particles represent the smallest individual aerosol particles analyzed by Raman microspectroscopy to date, and the first time atmospheric particles have been measured at sizes approaching the atmospheric number size distribution mode. SERS-enabled detection and identification of vibrational modes in smaller, more atmospherically-relevant particles has the potential to improve understanding of aerosol composition and surface properties, as well as their impact on heterogeneous and multiphase reactions involving aerosol surfaces.
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Constraints on the Role of Laplace Pressure in Multiphase Reactions and Viscosity of Organic Aerosols
Abstract Aerosol chemistry has broad relevance for climate and global public health. The role of interfacial phenomena in condensed‐phase aerosol reactions remains poorly understood. In this work, liquid drop formalisms are coupled with high‐pressure transition state theory to formulate an expression for predicting the size‐dependence of aerosol reaction rates and viscosity. Insights from high‐pressure synthesis studies suggest that accretion and cyclization reactions are accelerated in 3–10‐nm particles smaller than 10 nm. Reactions of peroxide, epoxide, furanoid, aldol, and carbonyl functional groups are accelerated by up to tenfold. Effective rate enhancements are ranked as: cycloadditions >> aldol reactions > epoxide reactions > Baeyer‐Villiger oxidation >> imidazole formation (which is inhibited). Some reactions are enabled by the elevated pressure in particles. Viscosity increases for organic liquids but decreases for viscous or solid particles. Results suggest that internal pressure is an important consideration in studies of the physics and chemical evolution of nanoparticles.
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- Award ID(s):
- 1624696
- PAR ID:
- 10445935
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 49
- Issue:
- 12
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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