Rebecca J. Hopkins

Control and characterisation of a single aerosol droplet in a single-beam gradient-force optical trap

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.

Correlations between optical, chemical and physical properties of biomass burn aerosols

Correlations between Optical, Chemical and Physical Properties of Biomass Burn Aerosols R. J. Hopkins, 1 K. Lewis, 2 Y. Desyaterik, 3 Z. Wang, 1,4 A. V. Tivanski, 1 W. P. Arnott, 2 A. Laskin, 3 and M. K. Gilles 1,* Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA. Department of Physics, University of Nevada, Reno, Nevada, USA. William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National College of Engineering, University of California, Berkeley, California, USA. Laboratory, Richland, Washington, USA. Abstract Aerosols generated from burning different plant fuels were characterized to determine relationships between chemical, optical and physical properties. Single scattering albedo (ω) and Angstrom absorption coefficients (α ap ) were measured using a photoacoustic technique combined with a reciprocal nephelometer. Carbon-to-oxygen atomic ratios, sp 2 hybridization, elemental composition and morphology of individual particles were measured using scanning transmission X-ray microscopy coupled with near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) and scanning electron microscopy with energy dispersion of X-rays (SEM/EDX). Particles were grouped into three categories based on sp 2 hybridization and chemical composition. Measured ω (0.4 – 1.0 at 405 nm) and α ap (1.0 - 3.5) values displayed a fuel dependence. The category with sp 2 hybridization >80% had values of ω ( 0.8) and α ap (1.0 to 3.5) values, indicating increased absorption spectral selectivity.

Internal structure, hygroscopic and reactive properties of mixed sodium methanesulfonate-sodium chloride particles

Internal structures, hygroscopic properties and heterogeneous reactivity of mixed CH(3)SO(3)Na/NaCl particles were investigated using a combination of computer modeling and experimental approaches. Surfactant properties of CH(3)SO(3)(-) ions and their surface accumulation in wet, deliquesced particles were assessed using molecular dynamics (MD) simulations and surface tension measurements. Internal structures of dry CH(3)SO(3)Na/NaCl particles were investigated using scanning electron microscopy (SEM) assisted with X-ray microanalysis mapping, and time-of-flight secondary ion mass spectrometry (TOF-SIMS). The combination of these techniques shows that dry CH(3)SO(3)Na/NaCl particles are composed of a NaCl core surrounded by a CH(3)SO(3)Na shell. Hygroscopic growth, deliquescence and efflorescence phase transitions of mixed CH(3)SO(3)Na/NaCl particles were determined and compared to those of pure NaCl particles. These results indicate that particles undergo a two step deliquescence transition: first at ∼69% relative humidity (RH) the CH(3)SO(3)Na shell takes up water, and then at ∼75% RH the NaCl core deliquesces. Reactive uptake coefficients for the particle-HNO(3) heterogeneous reaction were determined at different CH(3)SO(3)Na/NaCl mixing ratios and RH. The net reaction probability decreased notably with increasing CH(3)SO(3)Na and at lower RH.

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.

Spatially offset Raman spectroscopy (SORS) for through-barrier proximal chemical and explosive detection

The capability to detect toxic chemicals and explosive materials through a wide range of container types has a variety of applications, including liquid screening at airport entrance points. Conventional Raman spectroscopy is commonly used for chemical detection, but can result in an intense spectral response due to scattering and/or fluorescence from the container when used for through-barrier applications. Such a response can reduce the effectiveness of the technique for analysis of the container contents by swamping the Raman signature of the target material. By producing two spectra containing different contributions from the container and the contents, spatially offset Raman spectroscopy (SORS) allows a spectrum of the contents to be obtained, even through fluorescing containers. This innovative technique could provide a through-barrier detection capability for a wider range of containers than conventional Raman spectroscopy, including containers made from coloured glass and opaque plastic. In this paper, the use of SORS for through-barrier detection is introduced, and its ability to detect a range of analytes through a range of container materials evaluated. The potential advantages of using a longer excitation wavelength (e.g. 1064 nm) to reduce sample fluorescence are also explored, focussing on target analytes mixed with fluorescent materials.

Applications of spatially offset Raman spectroscopy to defense and security

Spatially offset Raman spectroscopy (SORS) allows for sub-surface and through barrier detection and has applications in drug analysis, cancer detection, forensic science, as well as defense and security. This paper reviews previous efforts in SORS and other through barrier Raman techniques and presents a discussion on current research in defense and security applications.

Optical manipulation and characterisation of aerosol droplets

Aerosol droplets are trapped and manipulated with a single-beam gradient-force optical trap for timescales of hours. By coupling the optical trap with cavity enhanced Raman scattering, the size of the trapped droplet can be determined with nanometre accuracy and high time resolution. This allows the evolution in droplet size and composition to be monitored during the growth or evaporation of a single trapped droplet, providing a method for characterising the factors that govern aerosol droplet size. The simultaneous trapping of two or more aerosol droplets in parallel optical traps can permit studies of aerosol coagulation.