Jonathan P Reid

University of Bristol, Bristol, England, United Kingdom

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Publications (87)436.67 Total impact

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    ABSTRACT: Secondary organic material (SOM) constitutes a large mass fraction of atmospheric aerosol particles. Understanding its impact on climate and air quality relies on accurate models of interactions with water vapour. Recent research shows that SOM can be highly viscous and can even behave mechanically like a solid, leading to suggestions that particles exist out of equilibrium with water vapour in the atmosphere. In order to quantify any kinetic limitation we need to know water diffusion coefficients for SOM, but this quantity has, until now, only been estimated and has not yet been measured. We have directly measured water diffusion coefficients in the water soluble fraction of α-pinene SOM between 240 and 280 K. Here we show that, although this material can behave mechanically like a solid, at 280 K water diffusion is not kinetically limited on timescales of 1 s for atmospheric-sized particles. However, diffusion slows as temperature decreases. We use our measured data to constrain a Vignes-type parameterisation, which we extend to lower temperatures to show that SOM can take hours to equilibrate with water vapour under very cold conditions. Our modelling for 100 nm particles predicts that under mid to upper tropospheric conditions radial inhomogeneities in water content produce a low viscosity surface region and more solid interior, with implications for heterogeneous chemistry and ice nucleation.
    Chemical Science 06/2015; DOI:10.1039/C5SC00685F · 8.60 Impact Factor
  • Thomas C. Preston, Jonathan P. Reid
    Journal of the Optical Society of America A 06/2015; 32(6):1053. DOI:10.1364/JOSAA.32.001053 · 1.45 Impact Factor
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    ABSTRACT: A new experiment is presented for the measurement of single aerosol particle extinction efficiencies, Qext, combining cavity ring-down spectroscopy (CRDS, λ = 405 nm) with a Bessel beam trap (λ = 532 nm) in tandem with phase function (PF) measurements. This approach allows direct measurements of the changing optical cross-sections of individual aerosol particles over indefinite time-frames facilitating some of the most comprehensive measurements of the optical properties of aerosol particles so far made. Using volatile 1,2,6-hexanetriol droplets, Qext is measured over a continuous radius range with the measured Qext envelope well described by fitted cavity standing wave (CSW) Mie simulations. These fits allow the refractive index at 405 nm to be determined. Measurements are also presented of Qext variation with RH for two hygroscopic aqueous inorganic systems ((NH4)2SO4 and NaNO3). For the PF and the CSW Mie simulations, the refractive index, nλ, is parameterised in terms of the particle radius. The radius and refractive index at 532 nm are determined from PFs, while the refractive index at 405 nm is determined by comparison of the measured Qext to CSW Mie simulations. The refractive indices determined at the shorter wavelength are larger than at the longer wavelength consistent with the expected dispersion behaviour. The measured values at 405 nm are compared to estimates from volume mixing and molar refraction mixing rules, with the latter giving superior agreement. In addition, the first single-particle Qext measurements for accumulation mode aerosol are presented for droplets with radii as small as ~300 nm.
    Physical Chemistry Chemical Physics 05/2015; 17(24). DOI:10.1039/C5CP00252D · 4.20 Impact Factor
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    Chemical Reviews 05/2015; DOI:10.1021/cr5005502 · 45.66 Impact Factor
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    ABSTRACT: We present a comprehensive evaluation of the variabilities and uncertainties present in determining the kinetics of water transport in ultraviscous aerosol droplets, alongside new measurements of the water transport timescale in sucrose aerosol. Measurements are performed on individual droplets captured using aerosol optical tweezers and the change in particle size during water evaporation or condensation is inferred from shifts in the wavelength of the whispering gallery mode peaks at which spontaneous Raman scattering is enhanced. The characteristic relaxation timescale (τ) for condensation or evaporation of water from viscous droplets following a change in gas phase relative humidity can be described by the Kohlrausch-Williams-Watts function. To adequately characterise the water transport kinetics and determine τ, sufficient time must be allowed for the particle to progress towards the final state. However, instabilities in the environmental conditions can prevent an accurate characterisation of the kinetics over such long time frames. Comparison with established thermodynamic and diffusional water transport models suggests the determination of τ is insensitive to the choice of thermodynamic treatment. We report excellent agreement between experimental and simulated evaporation timescales, and investigate the scaling of τ with droplet radius. A clear increase in τ is observed for condensation with increase in drying (wait) time. This trend is qualitatively supported by model simulations.
    Physical Chemistry Chemical Physics 03/2015; 17(15). DOI:10.1039/C4CP05383D · 4.20 Impact Factor
  • Rory M Power, Daniel R Burnham, Jonathan P Reid
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    ABSTRACT: Optical tweezers have found widespread application in biological and colloidal physics for the measurement of pN forces over nanometer to micrometer length scales. Similar aerosol-phase measurements of interparticle force have not been reported in spite of the potential to better resolve particle coagulation kinetics. Various refractive index mismatches in the beam path as well as the need to explicitly account for gravity and inertial particle motion provide a number of challenges that must be overcome to make such measurements tractable. In this regard, we demonstrate schemes by which the particle position and trap stiffness may be unambiguously measured using bright-field microscopy with resolution comparable with analogous condensed-phase measurements. Moreover, some of the challenges of working with highly dynamic aqueous particles are introduced and exploited to observe size-dependent phenomena in aerosol optical tweezers. Notably, when combined with cavity-enhanced Raman spectroscopy, this provides a unique opportunity to explore trapping forces over a continuum of particle size and refractive index. It is expected that the methods developed will provide a basis for the measurement of pairwise interaction forces in aerosol optical tweezers while providing a probe of fundamental airborne particle trapping dynamics.
    Applied Optics 12/2014; 53(36):8522-34. DOI:10.1364/AO.53.008522 · 1.78 Impact Factor
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    ABSTRACT: We report measurements of the vapour pressure of water over the super-cooled temperature range 248 to 273 K derived from evaporation kinetics measurements of single water droplets. Accurate measurements of the relative humidity of the surrounding gas phase are derived from comparative and sequential measurements of the evaporation kinetics of droplets containing sodium chloride. The temperature dependence of the vapour pressure of super-cooled water is shown to conform closely to the parameterisation provided by Murphy and Koop (2005) once the uncertainties in experimental and thermophysical parameters are accounted for by ensuring an accurate representation of evaporation rates at temperatures above 273 K. Further, from a sensitivity analysis of all of the data over the full temperature range from 248 to 293 K, we can conclude that the evaporation coefficient of water, and thus the mass accommodation coefficient, is greater than, or equal to, 0.5.
    09/2014; 119(18). DOI:10.1002/2014JD022093
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    ABSTRACT: The time-dependent growth and shrinkage of aqueous aerosol particles trapped in an electrodynamic balance exposed to changes in relative humidity (RH) depend on the translational diffusion coefficient of water (DH2O). Resonances in the Mie scattering patterns of the illuminated micrometre-sized droplets are used to follow the compositional evolution through stepwise changes in RH. Under conditions where the diffusion of water molecules becomes sufficiently slow, e.g. in the highly viscous or even glassy regime, the concentration and temperature dependent values of DH2O can be determined iteratively by comparing the observed shifts in the Mie resonant wavelengths with predicted shifts from a diffusion model of a multi-layered sphere. It is shown that condensation and evaporation of water vapour from or to highly viscous or glassy droplets follow different kinetic regimes, a result that is consistent with previous studies of adsorption and desorption on glassy surfaces.
    Physical Chemistry Chemical Physics 07/2014; 16(31). DOI:10.1039/c4cp01939c · 4.20 Impact Factor
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    Rory M Power, Jonathan P Reid
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    ABSTRACT: The use of optical trapping techniques to manipulate probe particles for performing micro-rheological measurements on a surrounding fluid is well-established. Here, we review recent advances made in the use of optical trapping to probe the rheological properties of trapped particles themselves. In particular, we review observations of the continuous transition from liquid to solid-like viscosity of sub-picolitre supersaturated solution aerosol droplets using optical trapping techniques. Direct measurements of the viscosity of the particle bulk are derived from the damped oscillations in shape following coalescence of two particles, a consequence of the interplay between viscous and surface forces and the capillary driven relaxation of the approximately spheroidal composite particle. Holographic optical tweezers provide a facile method for the manipulation of arrays of particles allowing coalescence to be controllably induced between two micron-sized aerosol particles. The optical forces, while sufficiently strong to confine the composite particle, are several orders of magnitude weaker than the capillary forces driving relaxation. Light, elastically back-scattered by the particle, is recorded with sub-100 ns resolution allowing measurements of fast relaxation (low viscosity) dynamics, while the brightfield image can be used to monitor the shape relaxation extending to times in excess of 1000 s. For the slowest relaxation dynamics studied (particles with the highest viscosity) the presence and line shape of whispering gallery modes in the cavity enhanced Raman spectrum can be used to infer the relaxation time while serving the dual purpose of allowing the droplet size and refractive index to be measured with accuracies of ±0.025% and ±0.1%, respectively. The time constant for the damped relaxation can be used to infer the bulk viscosity, spanning from the dilute solution limit to a value approaching that of a glass, typically considered to be >1012 Pa s, whilst the frequencies of the normal modes of the oscillations of the particle can be used to infer surface properties. We will review the use of optical tweezers for studying the viscosity of aerosol particles and discuss the potential use of this micro-rheological tool for probing the fundamental concepts of phase, thermodynamic equilibrium and metastability.
    Reports on Progress in Physics 07/2014; 77(7):074601. DOI:10.1088/0034-4885/77/7/074601 · 15.63 Impact Factor
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    ABSTRACT: The equilibrium position of an aerosol droplet trapped in a counter-propagating Bessel beam and gas flow is studied both experimentally and theoretically. This provides an aerosol analogue to the separation of particles of differing size and refractive index in counter-propagating laser beam and liquid flow, referred to as optical chromatography. Using the model system of a pure glycerol droplet it is found that, as evaporation takes place and the size of the droplet decreases, the size-dependent equilibrium position does not change in a simple monotonic fashion. Instead, the position of the droplet is greatly affected by the excitation of whispering gallery modes. This leads to sharp peaks in the equilibrium position curve, not unlike those that occur in single particle spectroscopy. The conditions necessary to excite whispering gallery modes are thoroughly investigated.
    Journal of optics 01/2014; 16(2). DOI:10.1088/2040-8978/16/2/025702 · 2.01 Impact Factor
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    ABSTRACT: The size of aerosol particles prior to, and during, inhalation influences the site of deposition within the lung. As such, a detailed understanding of the hygroscopic growth of an aerosol during inhalation is necessary to accurately model the deposited dose. In the first part of this study, it is demonstrated that the aerosol produced by a nebulizer, depending on the airflows rates, may experience a (predictable) wide range of relative humidity prior to inhalation and undergo dramatic changes in both size and solute concentration. A series of sensitive single aerosol analysis techniques are then used to make measurements of the relative humidity dependent thermodynamic equilibrium properties of aerosol generated from four common nebulizer formulations. Measurements are also reported of the kinetics of mass transport during the evaporation or condensation of water from the aerosol. Combined, these measurements allow accurate prediction of the temporal response of the aerosol size prior to and during inhalation. Specifically, we compare aerosol composed of pure saline (150mM sodium chloride solution in ultrapure water) with two commercially available nebulizer products containing relatively low compound doses: Breath(®), consisting of a simple salbutamol sulfate solution (5mg/2.5mL; 1.7mM) in saline, and Flixotide(®) Nebules, consisting of a more complex stabilized fluticasone propionate suspension (0.25mg/mL; 0.5mM in saline. A mimic of the commercial product Tobi(©) (60mg/mL tobramycin and 2.25mg/mL NaCl, pH 5.5-6.5) is also studied, which was prepared in house. In all cases, the presence of the pharmaceutical was shown to have a profound effect on the magnitude, and in some cases the rate, of the mass flux of water to and from the aerosol as compared to saline. These findings provide physical chemical evidence supporting observations from human inhalation studies, and suggest that using the growth dynamics of a pure saline aerosol in a lung inhalation model to represent nebulizer formulations may not be representative of the actual behavior of the aerosolized drug solutions.
    International Journal of Pharmaceutics 01/2014; 463(1). DOI:10.1016/j.ijpharm.2013.12.048 · 3.79 Impact Factor
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    ABSTRACT: A single horizontally-propagating zeroth order Bessel laser beam with a counter-propagating gas flow was used to confine single fine-mode aerosol particles over extended periods of time, during which process measurements were performed. Particle sizes were measured by the analysis of the angular variation of light scattered at 532 nm by a particle in the Bessel beam, using either a probe beam at 405 nm or 633 nm. The vapour pressures of glycerol and 1,2,6-hexanetriol particles were determined to be 7.5 ± 2.6 mPa and 0.20 ± 0.02 mPa respectively. The lower volatility of hexanetriol allowed better definition of the trapping environment relative humidity profile over the measurement time period, thus higher precision measurements were obtained compared to those for glycerol. The size evolution of a hexanetriol particle, as well as its refractive index at wavelengths 532 nm and 405 nm, were determined by modelling its position along the Bessel beam propagation length while collecting phase functions with the 405 nm probe beam. Measurements of the hygroscopic growth of sodium chloride and ammonium sulfate have been performed on particles as small as 350 nm in radius, with growth curves well described by widely used equilibrium state models. These are the smallest particles for which single-particle hygroscopicity has been measured and represent the first measurements of hygroscopicity on fine mode and near-accumulation mode aerosols, the size regimes bearing the most atmospheric relevance in terms of loading, light extinction and scattering. Finally, the technique is contrasted with other single particle and ensemble methods, and limitations are assessed.
    Physical Chemistry Chemical Physics 12/2013; 16(5). DOI:10.1039/c3cp54368d · 4.20 Impact Factor
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    ABSTRACT: Evaporation studies of single aqueous sucrose aerosol particles as a function of relative humidity (RH) are presented for coarse and fine mode particles down into the submicron size range (600 nm < r < 3.0 μm). These sucrose particles serve as a proxy for biogenic secondary organic aerosols that have been shown to exist, under ambient conditions, in an ultraviscous glassy state, which can affect the kinetics of water mass transport within the bulk phase and hinder particle response to changes in the gas phase water content. A counter-propagating Bessel beams (CPBBs) optical trapping setup is employed to monitor the real-time change in the particle radius with RH decreasing from 75% to 5%. The slow-down of the size change upon each RH step and the deviation from the theoretical equilibrium hygroscopic growth curve indicate the onset of glassy behavior in the RH range of 10-40%. Size-dependent effects were not observed within the uncertainty of the measurements. The influence of the drying time below the glass transition RH on the timescale of subsequent water condensation and re-equilibration for sucrose particles is explored by optical tweezers measurements of micron-sized particles (3 μm < r < 6 μm). The timescale for water condensation and re-equilibration is shown to increase with increasing drying time, i.e. the time over which a viscous particle is dried below 5% RH. These studies demonstrate the importance of the history of the particle conditioning on subsequent water condensation and re-equilibration dynamics of ultraviscous and glassy aerosol particles.
    Physical Chemistry Chemical Physics 12/2013; 16(21). DOI:10.1039/c3cp54233e · 4.20 Impact Factor
  • Thomas C. Preston, Jonathan P. Reid
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    ABSTRACT: We present an algorithm that can be used to simultaneously determine the radius and the refractive index (with dispersion) of a spherical, homogeneous particle. This is accomplished by fitting characteristic resonances calculated using Mie scattering coefficients to the measured whispering gallery mode resonances. The advantage of this algorithm over those that have been presented previously is that a large portion of the search can be reduced to two dimensions (a search that includes radius and refractive index with dispersion will always be at least three dimensions). Using this algorithm, we analyze two large sets of cavity-enhanced Raman spectra from optically trapped aerosol particles. The speed of the algorithm allows for best fits to be found in real time. Precision is found to be limited by the resolution of the spectrograph.
    Journal of the Optical Society of America B 08/2013; 30(8). DOI:10.1364/JOSAB.30.002113 · 1.81 Impact Factor
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    ABSTRACT: Uncertainties in quantifying the kinetics of evaporation and condensation of water from atmospheric aerosol are a significant contributor to the uncertainty in predicting cloud droplet number and the indirect effect of aerosols on climate. The influence of aerosol particle surface composition, particularly the impact of surface active organic films, on the condensation and evaporation coefficients remains ambiguous. Here, we report measurements of the influence of organic films on the evaporation and condensation of water from aerosol particles. Significant reductions in the evaporation coefficient are shown to result when condensed films are formed by monolayers of long-chain alcohols [CnH(2n+1)OH], with the value decreasing from 2.4 × 10(-3) to 1.7 × 10(-5) as n increases from 12 to 17. Temperature-dependent measurements confirm that a condensed film of long-range order must be formed to suppress the evaporation coefficient below 0.05. The condensation of water on a droplet coated in a condensed film is shown to be fast, with strong coherence of the long-chain alcohol molecules leading to islanding as the water droplet grows, opening up broad areas of uncoated surface on which water can condense rapidly. We conclude that multicomponent composition of organic films on the surface of atmospheric aerosol particles is likely to preclude the formation of condensed films and that the kinetics of water condensation during the activation of aerosol to form cloud droplets is likely to remain rapid.
    Proceedings of the National Academy of Sciences 05/2013; DOI:10.1073/pnas.1305277110 · 9.81 Impact Factor
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    ABSTRACT: A Bessel beam optical trap is combined with continuous wave cavity ringdown spectroscopy to measure the extinction cross section of individual aerosol particles. Particles, 1 μm in size, can be captured indefinitely and processes that transform size or refractive index studied. The measured light extinction induced by the particle is shown to depend on the position of the particle in the cavity, allowing accurate measurements of the mode structure of a high finesse optical cavity without significant perturbation. The variation in extinction efficiency of a sodium chloride droplet with relative humidity is shown to agree well with predictions from Mie scattering theory.
    Journal of Physical Chemistry Letters 05/2013; 4(10):1748–1752. DOI:10.1021/jz4008068 · 6.69 Impact Factor
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    ABSTRACT: The hygroscopic properties of an aerosol originating from a nebulizer solutioncan affect the extent of peripheral deposition within the respiratory tract, which in turns affects drug efficacy of drugs delivered to the lungs. Thus, the ability to tailor the degree and rate of hygroscopic growth of an aerosol produced by a nebulizer through modification of the formulation would serve to improve drug efficacy through targeted lung deposition. In this study, the kinetic and thermodynamic hygroscopic properties of sodium chloride aerosol mixed with commercially available Pluronic polymers, specifically F77 and F127, are reported using three complimentary single aerosol analysis techniques, specifically aerosol optical tweezers, a double ring electrodynamic balance and a concentric cylinder electrodynamic balance. The F77 polymer is shown to have a predictable effect on the hygroscopic properties of the aerosol: the ability of the droplet to uptake water from the airdepends on the solute weight percent of sodium chloride present in a linear dose dependant manner. Unlike the smaller F77, a non-linear relationship was observed for the larger molecular weight F127 polymer, with significant suppression of hygroscopic growth (>50% by mass) for solution aerosol containing even only 1wt % of the polymer and 99wt % sodium chloride. The suppression of growth is shown to be consistent with the formation of mixed phase aerosol particles containing hydrophilic inorganic rich domains and hydrophobic polymer rich domains that sequester some of the inorganic component, with the two phases responding to changes in relative humidity independently. This independence of coupling with the gas phase is apparent in both the equilibrium state and the kinetics of water evaporation/condensation. By starting with a saline nebulizer solution with a concentration of F127 ∼10(-2) mM, a 12% reduction in the radius of all aerosol produced at a relative humidity (RH) of 84% is possible. The difference in diameter is RHdependent, and may be much greater at higher humidities. These findings suggest that the addition of μM concentrations of larger Pluronic polymers to nebulizer formulations may greatly reduce the size of aerosols produced.
    International Journal of Pharmaceutics 01/2013; DOI:10.1016/j.ijpharm.2012.12.039 · 3.79 Impact Factor
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    Rachael E H Miles, Jonathan P Reid, Ilona Riipinen
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    ABSTRACT: We compare and contrast measurements of the mass accommodation coefficient of water on a water surface made using ensemble and single particle techniques under conditions of supersaturation and subsaturation, respectively. In particular, we consider measurements made using an expansion chamber, a continuous flow streamwise thermal gradient cloud condensation nuclei chamber, the Leipzig Aerosol Cloud Interaction Simulator, aerosol optical tweezers, and electrodynamic balances. Although this assessment is not intended to be comprehensive, these five techniques are complementary in their approach and give values that span the range from near 0.1 to 1.0 for the mass accommodation coefficient. We use the same semianalytical treatment to assess the sensitivities of the measurements made by the various techniques to thermophysical quantities (diffusion constants, thermal conductivities, saturation pressure of water, latent heat, and solution density) and experimental parameters (saturation value and temperature). This represents the first effort to assess and compare measurements made by different techniques to attempt to reduce the uncertainty in the value of the mass accommodation coefficient. Broadly, we show that the measurements are consistent within the uncertainties inherent to the thermophysical and experimental parameters and that the value of the mass accommodation coefficient should be considered to be larger than 0.5. Accurate control and measurement of the saturation ratio is shown to be critical for a successful investigation of the surface transport kinetics during condensation/evaporation. This invariably requires accurate knowledge of the partial pressure of water, the system temperature, the droplet curvature and the saturation pressure of water. Further, the importance of including and quantifying the transport of heat in interpreting droplet measurements is highlighted; the particular issues associated with interpreting measurements of condensation/evaporation rates with varying pressure are discussed, measurements that are important for resolving the relative importance of gas diffusional transport and surface kinetics.
    The Journal of Physical Chemistry A 10/2012; 116(44). DOI:10.1021/jp3083858 · 2.78 Impact Factor
  • Rory Power, David L. Bones, Jonathan P. Reid
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    ABSTRACT: Holographic aerosol optical tweezers can be used to trap arrays of aerosol particles allowing detailed studies of particle properties and processes at the single particle level. Recent observations have suggested that secondary organic aerosol may exist as ultra-viscous liquids or glassy states at low relative humidity, potentially a significant factor in influencing their role in the atmosphere and their activation to form cloud droplets. A decrease in relative humidity surrounding a particle leads to an increased concentration of solute in the droplet as the droplet returns to equilibrium and, thus, an increase in the bulk viscosity. We demonstrate that the timescales for condensation and evaporation processes correlate with particle viscosity, showing significant inhibition in mass transfer kinetics using ternary sucrose/sodium chloride/water droplets as a proxy to atmospheric multi-component aerosol. We go on to study the fundamental process of aerosol coagulation in aerosol particle arrays, observing the relaxation of non-spherical composite particles formed on coalescence. We demonstrate the use of bright-field imaging and elastic light scattering to make measurements of the timescale for the process of binary coalescence contrasting the rheological properties of aqueous sucrose and sodium chloride aerosol over a range of relative humidities.
    Proceedings of SPIE - The International Society for Optical Engineering 10/2012; DOI:10.1117/12.931655 · 0.20 Impact Factor
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    ABSTRACT: Studies of the oxidative aging of single mixed component aerosol particles formed from oleic acid (OL) and sodium chloride over a range of relative humidities (RH) and ozone concentrations by aerosol optical tweezers are reported. The rate of loss of OL and changes in the organic phase volume are directly measured, comparing particles with effloresced and deliquesced inorganic seeds. The kinetics of the OL loss are analyzed and the value of the reactive uptake coefficient of ozone by OL is compared to previous studies. The reaction of OL is accompanied by a decrease in the particle volume, consistent with the evaporation of semivolatile products over a time scale of tens of thousands of seconds. Measurements of the change in the organic phase volume allow the branching ratio to involatile components to be estimated; between 50 and 85% of the initial organic volume remains involatile, depending on ozone concentration. The refractive index (RI) of the organic phase increases during and after evaporation of volatile products, consistent with aging followed by a slow restructuring in particle morphology. The hygroscopicity of the particle and kinetics of the response of the organic phase to changes in RH are investigated. Both size and RI of unoxidized and oxidized particles respond promptly to RH changes with values of the RI consistent with linear mixing rules. Such studies of the simultaneous changes in composition and size of mixed component aerosol provide valuable data for benchmarking kinetic models of heterogeneous atmospheric aging.
    Journal of Geophysical Research Atmospheres 10/2012; 117(D20):20204-. DOI:10.1029/2012JD018163 · 3.44 Impact Factor

Publication Stats

1k Citations
436.67 Total Impact Points


  • 2005–2014
    • University of Bristol
      • School of Chemistry
      Bristol, England, United Kingdom
  • 2012
    • Stockholm University
      • Department of Applied Environmental Science
      Tukholma, Stockholm, Sweden
  • 2002–2004
    • University of Birmingham
      • School of Chemistry
      Birmingham, England, United Kingdom