Coalescence Sampling and Analysis of Aerosols using Aerosol Optical Tweezers.

We present a first exploratory study to assess the use of aerosol optical tweezers as an instrument for sampling and detecting accumulation- and coarse-mode aerosol. A subpicoliter aqueous aerosol droplet is captured in the optical trap and used as a sampling volume, accreting mass from a free-flowing aerosol generated by a medical nebulizer or atomizer. Real-time measurements of the initial stability in size, refractive index, and composition of the sampling droplet inferred from Raman spectroscopy confirm that these quantities can be measured with high accuracy and low noise. Typical standard deviations in size and refractive index of the sampling droplet over a period of 200 s are <±2 nm and <±0.0005, respectively, equivalent to <±0.04% in both measured quantities. A standard deviation of <±1% over a 200 s period is achieved in the spontaneous Raman intensity measurement. When sampling coarse-mode aerosol, mass changes of <10 pg can be detected by the sampling droplet as discrete coalescence events. With accumulation-mode aerosol, we show that fluxes as low as 0.068 pg s-1 can be detected over a 50 s period, equivalent to ∼3 pg of sampled material.

Short-wave infrared excited spatially offset Raman spectroscopy (SORS) for through-barrier detection.

Spatially offset Raman spectroscopy (SORS) using 1064 nm excitation is demonstrated here to detect chemicals through a physical barrier such as a container. This excitation wavelength overcomes the issue of fluorescence from the target chemical, whilst retaining the benefits of the SORS technique for through-barrier detection. These advantages have a wide range of applications in both civilian and military environments.

Characterization of aerosols containing Zn, Pb, and Cl from an industrial region of Mexico City.

Recent ice core measurements show lead concentrations increasing since 1970, suggesting new nonautomobile-related sources of Pb are becoming important worldwide (1). Developing a full understanding of the major sources of Pb and other metals is critical to controlling these emissions. During the March, 2006 MILAGRO campaign, single particle measurements in Mexico City revealed the frequent appearance of particles internally mixed with Zn, Pb, Cl, and P. Pb concentrations were as high as 1.14 microg/m3 in PM10 and 0.76 microg/m3 in PM2.5. Real time measurements were used to select time periods of interest to perform offline analysis to obtain detailed aerosol speciation. Many Zn-rich particles had needle-like structures and were found to be composed of ZnO and/or Zn(NO3)2 x 6H2O. The internally mixed Pb-Zn-Cl particles represented as much as 73% of the fine mode particles (by number) in the morning hours between 2-5 am. The Pb-Zn-Cl particles were primarily in the submicrometer size range and typically mixed with elemental carbon suggesting a combustion source. The unique single particle chemical associations measured in this study closely match signatures indicative of waste incineration. Our findings also show these industrial emissions play an important role in heterogeneous processing of NO(y) species. Primary emissions of metal and sodium chloride particles emitted by the same source underwent heterogeneous transformations into nitrate particles as soon as photochemical production of nitric acid began each day at approximately 7 am.

Probing the evaporation of ternary ethanol-methanol-water droplets by cavity enhanced Raman scattering.

Cavity enhanced Raman scattering is used to characterise the evolving composition of ternary aerosol droplets containing methanol, ethanol and water during evaporation into a dry nitrogen atmosphere. Measurements made using non-linear stimulated Raman scattering from these ternary alcohol-water droplets allow the in situ determination of the concentration of the two alcohol components with high accuracy. The overlapping spontaneous Raman bands of the two alcohol components, arising from C-H stretching vibrational modes, are spectrally-resolved in stimulated Raman scattering measurements. We also demonstrate that the evaporation measurements are consistent with a quasi-steady state evaporation model, which can be used to interpret the evaporation dynamics occurring at a range of pressures at a particular evaporation time.

Oxygenated interface on biomass burn tar balls determined by single particle scanning transmission X-ray microscopy.

Carbonaceous particles originating from biomass burning can account for a large fraction of organic aerosols in a local environment. Presently, their composition, physical and chemical properties, as well as their environmental effects are largely unknown. Tar balls, a distinct type of highly spherical carbonaceous biomass burn particles, have been observed in a number of field campaigns. The Yosemite Aerosol Characterization Study that took place in summer 2002 occurred during an active fire season in the western United States; tar balls collected during this field campaign are described in this article. Scanning transmission X-ray microscopy and near-edge X-ray absorption fine structure spectroscopy are used to determine the shape, structure, and size-dependent chemical composition of approximately 150 individual spherical particles ranging in size from 0.15 to 1.2 mum. The elemental composition of tar balls is approximately 55% atomic carbon and approximately 45% atomic oxygen. Oxygen is present primarily as carboxylic carbonyls and oxygen-substituted alkyl (O-alkyl-C) functional groups, followed by moderate amounts of ketonic carbonyls. The observed chemical composition, density, and carbon functional groups are distinctly different from soot or black carbon and more closely resemble high molecular weight polymeric humic-like substances, which could account for their reported optical properties. A detailed examination of the carboxylic carbonyl and O-alkyl-C functional groups as a function of particle size reveals a thin oxygenated interface layer. The high oxygen content, as well as the presence of water-soluble carboxylic carbonyl groups, could account for the reported hygroscopic properties of tar balls. The presence of the oxygenated layer is attributed to atmospheric processing of biomass burn particles.

Spectroscopy of growing and evaporating water droplets: exploring the variation in equilibrium droplet size with relative humidity.

We demonstrate that the thermodynamic properties of a single liquid aerosol droplet can be explored through the combination of a single-beam gradient force optical trap with Raman spectroscopy. A single aqueous droplet, 2-6 microm in radius, can be trapped in air indefinitely and the response of the particle to variations in relative humidity investigated. The Raman spectrum provides a unique fingerprint of droplet composition, temperature, and size. Spontaneous Raman scattering is shown to be consistent with that from a bulk phase sample, with the shape of the OH stretching band dependent on the concentration of sodium chloride in the aqueous phase and on the polarization of the scattered light. Stimulated Raman scattering at wavelengths commensurate with whispering gallery modes is demonstrated to provide a method for determining the size of the trapped droplet with nanometer precision and with a time resolution of 1 s. The polarization dependence of the stimulated scatter is consistent with the dependence observed for the spontaneous scatter from the droplet. By characterizing the spontaneous and stimulated Raman scattering from the droplet, we demonstrate that it is possible to measure the equilibrium size and composition of an aqueous droplet with variation in relative humidity. For this benchmark study we investigate the variation in equilibrium size with relative humidity for a simple binary sodium chloride/aqueous aerosol, a typical representative inorganic/aqueous aerosol that has been studied extensively in the literature. The measured equilibrium sizes are shown to be in excellent agreement with the predictions of Köhler theory. We suggest that this approach could provide an important new strategy for characterizing the thermodynamic properties and kinetics of transformation of aerosol particles.

Measuring temperature gradients in evaporating multicomponent alcohol/water droplets.

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.

A comparative study of the mass and heat transfer dynamics of evaporating ethanol/water, methanol/water, and 1-propanol/water aerosol droplets.

The mass and heat transfer dynamics of evaporating multicomponent alcohol/water droplets have been probed experimentally by examining changes in the near surface droplet composition and average droplet temperature using cavity-enhanced Raman scattering (CERS) and laser-induced fluorescence (LIF). The CERS technique provides a sensitive measure of the concentration of the volatile alcohol component in the outer shell of the droplet, due to the exponential relationship between CERS intensity and species concentration. Such volatile droplets, which are probed on a millisecond time scale, evaporate nonisothermally, resulting in both temperature and concentration gradients, as confirmed by comparisons between experimental measurements and quasi-steady state model calculations. An excellent agreement between the experimental evaporation trends and quasi-steady state model predictions is observed. An unexpectedly slow evaporation rate is observed for the evaporation of 1-propanol from a multicomponent droplet when compared to the model; possible explanations for this observation are discussed. In addition, the propagation depth of the CERS signal, and, therefore, the region of the droplet from which compositional measurements are made, can be estimated. Such measurements, when considered in conjunction with quasi-steady state theory, can allow droplet temperature gradients to be measured and vapor pressures and activity coefficients of components within the droplet to be determined.

Evaporation of ethanol/water droplets: examining the temporal evolution of droplet size, composition and temperature.

The evolving size, composition, and temperature of evaporating ethanol/water aerosol droplets 25-57 microm in radius are probed by cavity enhanced Raman scattering (CERS) and laser induced fluorescence. This represents the first study in which the evolving composition of volatile droplets has been probed with spatial selectivity on the millisecond time scale, providing a new strategy for exploring mass and heat transfer in aerosols. The Raman scattering intensity is shown to depend exponentially on species concentration due to the stimulated nature of the CERS technique, providing a sensitive measure of the concentration of the volatile ethanol component. The accuracy with which we can determine droplet size, composition, and temperature is discussed. We demonstrate that the CERS measurements of evolving size and composition of droplets falling in a train can be used to characterize, and thus avoid, droplet coagulation. By varying the surrounding gas pressure (7-77 kPa), we investigate the dependence of the rate of evaporation on the rate of gas diffusion, and behavior consistent with gas diffusion-limited evaporation is observed. We suggest that such measurements can allow the determination of the vapor pressures of components within the droplet and can allow the determination of activity coefficients of volatile species.