E.J. Davis

PHYSICAL AND CHEMICAL (RAMAN) CHARACTERIZATION OF BIOAEROSOLS—POLLEN

The chemical characterization of pollen is explored using fluorescence spectroscopy and two Raman spectroscopic techniques. The on-line Raman technique involves trapping single pollen particles in an electrodynamic balance coupled to a Raman spectrometer. The off-line method requires the pollen samples to be deposited on a substrate and analyzed using a Raman microprobe. It is shown that fluorescence spectra of various grass and tree pollens do not show sufficient differences to provide an adequate method for characterizing them. Furthermore, the strong fluorescence masks any underlying Raman spectra. Although photochemical bleaching can reduce the fluorescence signal, it is shown that use of a near-IR laser provides Raman signals that can be used to characterize the pollen particles. In the on-line system, the particle density was determined from electrodynamic springpoint measurements and the geometric pollen size measured with a video microscope.

THE RAYLEIGH LIMIT OF CHARGE REVISITED : LIGHT SCATTERING FROM EXPLODING DROPLETS

Abstract Droplet fission associated with charged droplets has been re-examined to explore the issue of droplet fission prior to reaching the classical Rayleigh limit. Elastic scattering from an evaporating electrodynamically levitated droplet has been used to determine the coulombic charge and droplet size at the onset of droplet explosion. Since precise determination of the charge requires knowledge of the electric field in the levitator, computations of the a.c. and d.c. electric fields were performed as part of the study. The light-scattering data consist of angular scattering measurements and morphological resonance spectra made prior to and subsequent to droplet explosion. Water droplets containing sodium dodecyl sulfate (SDS) surfactant and 1-dodecanol droplets were used in the experiments. The aqueous solution droplets were 4–15 μm in diameter when they exploded, and the 1-dodecanol droplets were typically 10–20 μm in diameter. Both types of droplets fissioned at approximately 90% of the theoretical Rayleigh limit. In an attempt to determine if the external electric field led to this prior explosion, a second a.c. field (30–100 kHz) was superposed on the normal d.c. field and 60 Hz a.c. field used for electrodynamic trapping of the organic droplets. The external field was found to have no effect on the stability characteristics of the droplet.

ELECTRODYNAMIC TRAPPING AND MANIPULATION OF PARTICLE CLOUDS

Apparatus and techniques were developed to electrodynamically trap and manipulate groups of microparticles. The equipment consists of a vibrating orifice aerosol generator, an inductive particle charger, a plenum chamber, and a double-ring electrodynamic balance. Salt particles (NaNO3) of controllable and measurable mass and charge were produced and introduced into the balance in nitrogen at flow rates up to 25 cm3/min. Ordered arrays of any number of particles up to 26 were assembled and manipulated. Methods for compressing the arrays are presented, and controlled ejection of single particles from a trapped array is demonstrated. Particles of opposite polarity were successfully levitated and kept apart, and aggregation of these particles was then induced by changing the electric field. Raman spectra were recorded for multiple salt particles, each having a diameter of 3.5 μm, by aligning them in a laser beam. The enhanced Raman signal is compared with that from a single particle isolated from the array. F...

The characterization of fine particles originating from an uncharged aerosol: Size dependence and detection limits for Raman analysis

A new experimental method for chemical in situ analysis of ambient aerosol particles is presented. Aerosol particles from the atmosphere with diameters > 1 μm were charged and subsequently captured in an electrodynamic balance. Raman scattering from the particles was excited with an argon ion laser. Raman spectra were taken with a CCD detector through a spectrograph and used to identify chemical substances in the particles. Test particles of sodium sulfate and diethyl sebacate (DES) were employed to determine the detection limit of the method and the size dependence of Raman scattering. The detection limit for sodium sulfate was 0.27 pg, corresponding to a particle diameter of 580 nm. The size-averaged Raman scattering was found to be approximately proportional to volume for particles with diameters > 500 nm using excitation in the visible region.

Mathematical models of the uptake of ClONO2 and other gases by atmospheric aerosols

Abstract The reactions of chlorine nitrate and other gas-phase chlorine compounds with aqueous sulfuric acid aerosols under stratospheric and laboratory conditions are investigated. A mathematical model that accounts for three chemical reactions occurring within the droplet is developed and solved numerically. A relevant discussion concerning gas transport to an aerosol particle in the continuum, free-molecule, and transition regimes is presented. It is demonstrated that some recent experimental investigations of atmospherically important gas/aerosol chemical reactions have not adequately accounted for mass transport to the particle.

Droplet evaporation and condensation in the near-continuum regime

The problem of quasi-steady state evaporation and condensation of aerosol droplets is re-examined to determine the effect of the molecular interaction model on the predicted mass transfer rates in the Knudsen regime. A new expression for the mass flux is obtained that contains explicitly the dependence of the rate process on the accommodation coefficient and on the molecular weight ratio of the vapor and gas molecules. The analysis, based on the solution of the Boltzmann equation by the method of Grad for Maxwellian molecules, is shown to yield results in the near-continuum regime (Kn<1) very close to a number of previous theoretical analyses based on hard sphere molecules and semi-theoretical correlations, including the Fuchs–Sutugin equation. These results indicate that the theoretical predictions are not sensitive to the molecular interaction model used, but depend strongly on the method of solution in the near-free-molecule regime where the method of Grad fails. As the continuum regime is approached, the solution becomes independent of the accommodation coefficient. Theoretical predictions agree with previously published evaporation data for isothermal evaporation of dibutyl phthalate (DBP) in air and dibutyl sebacate (DBS) in nitrogen using an accommodation coefficient of 1.0 for DBP and 0.9 for DBS.

A study of aerosol chemical reactions by optical resonance spectroscopy

Abstract A new spectroscopic technique has been developed for the study of chemical reactions between a reactive gas and an aerosol. A single microdroplet of 1-octadecene was suspended by means of electrical fields in a laser beam and the chemical reaction between bromine vapor and the droplet was monitored by recording the optical resonance spectrum of the scattered light and its phase function. Resonances are extremely sensitive to droplet size and refractive index, and hence they were used to determine the refractive index change that occurred as the reaction proceeded. Reaction rates and the solubility of the reactive vapor are shown to be measurable by this method.

Non-isothermal droplet evaporation and condensation in the near-continuum regime

The problem of non-isothermal quasi-steady state evaporation and condensation of aerosol spheres is examined to determine the rates of simultaneous heat and mass transport in the Knudsen (transition) regime. New expressions for the mass and heat 5uxes are obtained that show explicitly the dependence of the rate processes on the Knudsen number, the accommodation coe6cients for mass and energy transport and on the molecular weight ratio of the vapor and gas molecules. The analysis, based on the solution of the Boltzmann equation by the method of Grad for Maxwellian molecules, is shown to yield results in the continuum regime (Kn1) in reasonable agreement with classical methods based on continuum theory and with measured water droplet evaporation rates in dry air. Computations of heat and mass transport rates for ice sublimation for upper tropospheric and stratospheric conditions for sizes that correspond to the continuum and transition regimes show that the process is very nearly isothermal. Parametric studies explore the e<ects of temperature, humidity and accommodation coe6cients on the heat and mass transport processes. Although the method of Grad is known to fail in the free-molecule regime, the results agree with more rigorous theoretical solutions for isothermal processes in the near-continuum regime and with an earlier solution for hard sphere molecules in the near-continuum regime. It is shown that 5ux-matching or resistance models used for the transition regime do not show the correct dependence on the Knudsen number and other parameters. c

Observations of Non-Rayleigh Limit Explosions of Electrodynamically Levitated Microdroplets

ABSTRACT Explosions far below the Rayleigh limit of charge were observed while levitating liquid microparticles in a double-ring electrodynamic balance. Video tapes of these novel explosions showed that when fission occurred a stream of many small droplets was emitted from the parent drop for several seconds. It was estimated that hundreds of daughter droplets were emitted during the event. Prior to fission, the droplets had radii of ∼ 15 μm and surface charge densities as low as ∼ 3% of the Rayleigh limit of charge. It is demonstrated that the explosions are not consistent with a variety of charge-loss mechanisms that were explored, and it is suggested that droplet fission may be due to localized heating and/or chemical reaction.

AERODYNAMIC PARTICLE SIZE MEASUREMENT BY ELECTRODYNAMIC OSCILLATION TECHNIQUES

Abstract Four dynamical techniques for measuring the aerodynamic size of spherical and non-spherical aerosol particles are evaluated and compared. For spheres the classical method of angular light scattering is also included. One method based on particle dynamics, the so-called springpoint method, has been extensively used, but the other particle oscillation methods have been used rarely or not at all. All the dynamical methods involve imbalancing a particle in an electrodynamic balance (EDB) by changing the dc potential to produce particle oscillation. A linescan CCD camera and associated electronics were used to measure the amplitude of the oscillations, the offset of oscillation centers, and the phase lag relative to the ac drive. These measurements are compared with theoretical solutions of the equation of particle motion to establish the aerodynamic size of the particle. The stability characteristics of the particle are analyzed by solving the particle equation of motion using the method of continued fractions. The various techniques are compared for spheres, spheroids and crystalline or amorphous particles of irregular shape. All five methods are shown to be in good agreement for spheres (within 3.9%). For non-spherical germanium dioxide particles the three oscillation methods agree with the springpoint method within 3.4%.