Search

The complex interplay of processes that govern the size, composition, phase and morphology of aerosol particles in the atmosphere is challenging to understand and model. Measurements on single aerosol particles (2 to 100 μm in diameter) held in electrodynamic, optical and acoustic traps or deposited on a surface can allow the individual processes to be studied in isolation under controlled laboratory conditions. In particular, measurements can now be made of particle size with unprecedented accuracy (sub-nanometre) and over a wide range of timescales (spanning from milliseconds to many days). The physical state of a particle can be unambiguously identified and its composition and phase can be resolved with a high degree of spatial resolution. In this review, we describe the advances made in our understanding of aerosol properties and processes from measurements made of phase behaviour, hygroscopic growth, morphology, vapour pressure and the kinetics of water transport for single particles. We also show that studies of the oxidative aging of single particles, although limited in number, can allow the interplay of these properties to be investigated. We conclude by considering the contributions that single particle measurements can continue to make to our understanding of the properties and processes occurring in atmospheric aerosol.

Bessel beams were used to create a counter-propagating optical trap for capturing and manipulating aerosol particles. Aerosol droplets were characterized through measurement of the elastic scattered light at three wavelengths; the trapping wavelength of 532 nm was used in conjunction with two probe beams at 405 nm and 633 nm to reduce the uncertainty in estimating droplet radii of 1 μm or less. Control of the aerosol size distribution sampled by the counter-propagating trap was demonstrated by varying the trapping beam core diameters and intensities. Smaller droplet sizes were preferentially selected with a 1.7 μm core diameter compared to cores of 2.7 μm and 4.5 μm. Further, an increase in core intensity was shown to broaden the range in droplet sizes that were optically trapped. The possibility of using such an approach to isolate and analyze the properties of single accumulation mode aerosol particles is discussed.

The cavity enhanced Raman scattering spectrum recorded from an aerosol droplet provides a unique fingerprint of droplet radius and refractive index, assuming that the droplet is homogeneous in composition. Aerosol optical tweezers are used in this study to capture a single droplet and a Raman fingerprint is recorded using the trapping laser as the source for the Raman excitation. We report here the retrieval of the real part of the refractive index with an uncertainty of ± 0.0012 (better than ± 0.11%), simultaneously measuring the size of the micrometre sized liquid droplet with a precision of better than 1 nm (± 0.05% error). In addition, the equilibrium size of the droplet is shown to depend on the laser irradiance due to optical absorption, which elevates the droplet temperature above that of the ambient gas phase. Modulation of the illuminating laser power leads to a modulation in droplet size as the temperature elevation is altered. By measuring induced size changes of1 nm, we show that the imaginary part of the refractive index can be retrieved even when less than 10 × 10(-9) with an accuracy of better than ± 0.5 × 10(-9). The combination of these measurements allows the complex refractive index of a droplet to be retrieved with high accuracy, with the possibility of making extremely sensitive optical absorption measurements on aerosol samples and the testing of frequently used mixing rules for treating aerosol optical properties. More generally, this method provides an extremely sensitive approach for measuring refractive indices, particularly under solute supersaturation conditions that cannot be accessed by simple bulk-phase measurements.

We have developed and studied methods to characterize the time-varying composition of liquid microdroplets, under controlled changes to environmental conditions, using Raman tweezers. This work has focussed on measurements of inorganic salts, such as nitrate and sulfate anions, which comprise a major fraction of the dissolved solutes in atmospheric aerosols. The experimental Raman intensities for the anions of inorganic salts in optically tweezed droplets were found to be in good agreement with theoretical estimates of photon scattering. The detection limit for sodium nitrate salt in a single particle was found to be ~4 pg. The mass of an inorganic salt in the droplet can be estimated from the Raman intensity of the anion bands using a calibration curve which is independent of droplet volume. The volume of the droplet, and concentration of solute, can be found directly from the spacing of morphology dependent resonances in the Raman band of water, or indirectly from the integrated-intensity of the Raman band for the solvent. The later strategy eliminates the uncertainty in the collection efficiency of Raman-scattered light related to varying particle sizes.

We present evidence that aerosol droplets, approximately 1-2microm in diameter, can be optically bound over a 4mm distance within a volume formed by the overlap of the central cores and rings of two counterpropagating Bessel beams. The sizes of the individual polydisperse aerosol particles can be estimated from the angular variation of the elastic light scattering. Scattered light from the two orthogonally polarized trapping beams and from a Gaussian probe beam of different wavelength can be used to provide independent estimations of size. The coalescence of two droplets was observed and characterized.

This year, as the laser celebrates its 50th anniversary, a field that was made possible through laser technology reaches an important milestone as well. Over the past 40 years, optical manipulation research has deepened our understanding of physics and biology, and it has yielded the optical-tweezer technique that is used across all the sciences.

We demonstrate that it is possible to lock the radius of an optically trapped salt-water microdroplet to the n=1 whispering gallery resonances (WGRs) at the trapping laser wavelength. The optical properties of the droplet are determined using stimulated Raman scattering. The droplet is in thermodynamic equilibrium with the surrounding vapour and the proposed locking mechanism consists of a balance between bulk heating and WGR heating. Raman measurements allow the size parameter (nka) of the droplet to be determined with a precision of ~10−5 and the resonance linewidth to be estimated.

The time-dependent evolution in the equilibrium size of an optically trapped aqueous sodium chloride droplet (2 microm radius) within an environment of varying relative humidity (RH) is shown to depend on both the depression in vapour pressure due to the presence of the solute and the elevation in temperature due to optical absorption. In particular, the level of optical absorption is highly dependent on the size of the droplet relative to the wavelength of the absorbed light. Thus, as the droplet size tunes into a Mie resonance at the trapping laser wavelength, the increased level of optical absorption leads to an elevation in droplet temperature. This increase in resonant heating can balance a continual increase in RH, leading to only marginal growth in droplet size and change in solute concentration. Once the RH is sufficiently high that the resonance condition can be surpassed, the droplet cools instantaneously and the solute concentration again dominates in determining the vapour pressure, with a rapid increase in size and a decrease in solute concentration returning the droplet to equilibrium with the gas phase RH. Thus, a growing droplet is observed to pass through periods of apparent size stability followed by instantaneous growth, consistent with the variation in absorption efficiency with droplet size. This provides a clear example of the coupling between the optical and physical properties of an aerosol and their influence on the equilibrium state.

Micron and sub-micron sized aerosol particles are captured, manipulated and characterised in a Bessel beam optical trap. Bright field microscopy and elastic light scattering measurements are used in combination to interrogate trapped particles and explore the optical landscape of the trap. We conclude that the Bessel trap has a number of advantages over optical tweezers in terms of characterisation of accumulation mode particles, manipulation of particles over macroscopic length scales and effective control of the gas phase. As such, the Bessel trap is a valuable addition to the aerosol optical toolkit.

The evaporation dynamics of stationary water droplets held within an electrodynamic trap are investigated in a nitrogen flow of variable velocity. In particular, the influence of the nitrogen gas flow on the temperature of the evaporating water droplets is studied. By applying a contact free measurement technique, based on spontaneous Raman scattering, time averaged and time resolved measurements of temperature in the droplet volume are compared. This technique determines the temperature from an intensity ratio in the OH stretching band of the Stokes-Raman scattering after calibration. The measured trends in temperature over the first 5 s of evaporation are found to be in agreement with theoretical calculations of the heat and mass transfer rates.

We demonstrate the ability to direct the flow of aerosol droplets through a trapping cell using a tailored optical landscape generated by spatial light modulation. Using an optical barrier, droplets held in an optical trap can be effectively isolated from other droplets within the aerosol. To illustrate the effective isolation we compare the influence of different optical landscapes on the flow of free aerosol around a trapped droplet. We also present spectroscopic evidence of the optical barrier effect and apply the technique to permit controlled loading of different aerosol particles into neighbouring optical traps. This method will enable comparative measurements of aerosol properties to be made and facilitate the study of aerosol chemistry in sub-picolitre droplets. It also facilitates the use of an isolated droplet of known composition as a sensitive probe of the gas phase conditions in an aerosol ensemble.

The application of optical tweezers (a single-beam gradient force optical trap) to the manipulation and characterisation of aerosol particles is discussed in this tutorial review. Optical tweezers allow not only the indefinite control over a single droplet, but control over arrays of particles. Typical particle sizes span the 1-10 microm diameter range. When coupled with spectroscopic techniques for probing evolving particle size (with nanometre accuracy), composition, phase and mixing state, detailed investigations of the thermodynamic properties of aerosol, the kinetics of particle transformation, and the nature of interparticle forces and coagulation can be undertaken.

We demonstrate that temperature gradients can be investigated in evaporating volatile water/alcohol droplets by characterising the volume averaged temperature by laser induced fluorescence and the near-surface temperature by cavity enhanced Raman scattering. In the former technique, the fluorescence spectrum from Rhodamine B can be used to determine the droplet temperature with an accuracy of +/-1 K. The latter technique uses the band width of the OH stretching Raman band to determine the temperature change within the near-surface volume in which whispering gallery modes propagate with an accuracy of +/-4 K. We demonstrate that the temperatures measured with varying evaporation time, buffer gas pressure, droplet size and composition are consistent with the predictions from a quasi-steady theoretical treatment of the evaporation rate and can be used to investigate temperature gradients within evaporating droplets.

Optical tweezers are used to control aerosol droplets, 4–14 μm in diameter, over time frames of hours at trapping powers of less than 10 mW. When coupled with cavity enhanced Raman scattering (CERS), the evolution of the size of a single droplet can be examined with nanometre accuracy. Trapping efficiencies for water and decane droplets are reported and the possible impact of droplet heating is discussed. We demonstrate that the unique combination of optical tweezing and CERS can enable the fundamental factors governing the coagulation of two liquid droplets to be studied.

Micron-sized liquid droplets exhibit unique optical properties, including lower threshold energies for non-linear optical processes than are observed in the bulk liquid phase. Whispering gallery modes (WGMs) and cavity quantum electrodynamics lead to cavity enhancement of fluorescence and Raman scattering at wavelengths commensurate with WGMs. The principles of cavity enhanced droplet spectroscopy will be reviewed, along with the major developments in the field over two decades. The prospect of applying cavity enhanced techniques in the simultaneous determination of droplet size and composition is discussed. A systematic experimental examination of the cavity enhanced Raman spectroscopy of water droplets is presented. In particular, the influences of laser power and illumination geometry, ionic strength and the presence of a second Raman active scatterer, nitrate, on the shape of the water Raman band are explored. Prospects for applying cavity enhanced techniques to studies of aerosol dynamics are also discussed.

Cavity enhanced Raman scattering (CERS) fingerprints are obtained on a single particle basis, providing a spectroscopic signature of the water droplet sizes generated by a vibrating orifice aerosol generator (VOAG). By sampling the aerosol train generated by the VOAG over a period of seconds, the size distribution of water droplets can be accumulated. The objective of this paper is to validate the size distribution measurements made and to quantify the accuracy of the sizes measured, along with providing an examination of the dependence of the CERS fingerprint on illumination geometry and laser pulse energy. The size distribution measurements are verified by elastic light scattering data, confirming both the sizes measured by CERS and the evolving droplet sizes produced by the VOAG during an operating time of 5 min. The experimental limitations imposed on the accuracy of the sizes measured by CERS are discussed and estimated. The estimated uncertainty in the droplet radius increases from 4 nm for a 10 μm radius droplet to 22 nm for a 50 μm radius droplet.

Investigation of sub-micron aerosol particles can provide insight into cloud forming properties. Bessel beam optical trapping techniques are used to confine and characterise droplets using elastic scattered light and cavity ring down techniques.

Introductioncontinuous positive airway pressure (CPAP) and high-flow nasal oxygen (HFNO) provide enhanced oxygen delivery and respiratory support for patients with severe COVID-19. CPAP and HFNO are currently designated as aerosol-generating procedures despite limited high-quality experimental data. We aimed to characterise aerosol emission from HFNO and CPAP and compare with breathing, speaking and coughing.Materials and methodsHealthy volunteers were recruited to breathe, speak and cough in ultra-clean, laminar flow theatres followed by using CPAP and HFNO. Aerosol emission was measured using two discrete methodologies, simultaneously. Hospitalised patients with COVID-19 had cough recorded using the same methodology on the infectious diseases ward.ResultsIn healthy volunteers (n=25 subjects; 531 measures), CPAP (with exhalation port filter) produced less aerosol than breathing, speaking and coughing (even with large >50 L/min face mask leaks). Coughing was associated with the highest aerosol emissions of any recorded activity. HFNO was associated with aerosol emission, however, this was from the machine. Generated particles were small (ConclusionsIn healthy volunteers, standard non-humidified CPAP is associated with less aerosol emission than breathing, speaking or coughing. Aerosol emission from the respiratory tract does not appear to be increased by HFNO. Although direct comparisons are complex, cough appears to be the main aerosol-generating risk out of all measured activities.