Abstract
We present a broad assessment on the studies of optically-trapped single airborne aerosol particles, particularly chemical aerosol particles, using laser technologies. To date, extensive works have been conducted on ensembles of aerosols as well as on their analogous bulk samples, and a decent general description of airborne particles has been drawn and accepted. However, substantial discrepancies between observed and expected aerosols behavior have been reported. To fill this gap, single-particle investigation has proved to be a unique intersection leading to a clear representation of microproperties and size-dependent comportment affecting the overall aerosol behavior, under various environmental conditions. In order to achieve this objective, optical-trapping technologies allow holding and manipulating a single aerosol particle, while offering significant advantages such as contactless handling, free from sample collection and preparation, prevention of contamination, versatility to any type of aerosol, and flexibility to accommodation of various analytical systems. We review spectroscopic methods that are based on the light-particle interaction, including elastic light scattering, light absorption (cavity ring-down and photoacoustic spectroscopies), inelastic light scattering and emission (Raman, laser-induced breakdown, and laser-induced fluorescence spectroscopies), and digital holography. Laser technologies offer several benefits such as high speed, high selectivity, high accuracy, and the ability to perform in real-time, in situ. This review, in particular, discusses each method, highlights the advantages and limitations, early breakthroughs, and recent progresses that have contributed to a better understanding of single particles and particle ensembles in general.
Highlights
Besides the main gaseous components (N2, O2, Ar, CO2, etc.), the Earth’s atmosphere includes a significant amount of fine particles floating in the air, known as atmospheric aerosols
In single-particle cavity ringdown spectroscopy (SP-Cavity Ringdown Spectroscopy (CRDS)), where the size is trivially determined from elastic scattering, a particle trapped inside the cavity is allowed to interact with a probe beam at each of its multiple trips between highly reflective surfaces, and the attenuation time or ringdown time (RDT) is measured
These morphology-dependent resonances (MDR) peaks provide a good indicator on the phase distribution, since they are quenched by uneven distribution in refractive index [178], yet they still exist in a phase separation resulting in core-shell morphology, and Gorkowski et al [172] proposed an algorithm to fit unlabeled whispering gallery modes (WGMs) in a multicomponent droplet
Summary
Besides the main gaseous components (N2 , O2 , Ar, CO2 , etc.), the Earth’s atmosphere includes a significant amount of fine particles (solid or liquid droplets) floating in the air, known as atmospheric aerosols. It has been shown that atmospheric aerosols can consist of multiple phases, and each individual phase might be homogeneous or heterogeneous [39,40,41] Those microproperties possess the ability to influence aerosol’s behavior in a particular environment by altering size and phase, evaporation rates, adsorption, absorption, surface enhanced chemistry, etc., which leads to the necessity for scrutinizing the behavior of a single particle with respect to its inherent composition [39,42,43,44]. With no claim of being exhaustive, this article compiles valuable recent information scattered across the literature and portrays a general picture that serves as a point of reference to a wide scientific community, especially for scientists and researchers who are new to the field or just jumpstarting their careers
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