We measured Raman spectra (RS) of ambient aerosols semi-continuously, to determine the compositions of individual aerosol particles at 15-20 minute time-resolution. The automated aerosol-Raman spectrometer (ARS) we use is Battelle’s (Columbus, OH) REBS. Many RS exhibit a broad luminescence, which can vary from one replicate to another because of photobleaching, thermal changes in crystal structure, or other phenomena. Materials consistent with the measured RS include DG carbon, organic carbon, and minerals such as calcite and quartz. We illustrate differences in the compositions consistent with the RS observed at a semi-urban site in Maryland and a rural site in New Mexico.
Laboratory measurements demonstrating the effects of ozone on aerosolized B. thuringiensis, as revealed by
fluorescence spectra, are reported. The fluorescence emission peak around 330 nm (excited at 263nm) decreases
somewhat in intensity and becomes slightly blue-shifted. Further, the fluorescence emission around 400 nm-550 nm is
less affected by the ozone than is the 330 nm (tryptophan) peak.
We report the development of a Single-Particle Fluorescence Spectrometer (SPFS) system capable of measuring two UV-laser excited fluorescence spectra from a single particle on-the-fly. The two dispersed fluorescence spectra are obtained from excitation by two lasers at different wavelengths (263 nm and 351 nm). The SPFS samples single particles with sizes primarily in the 1-10 &mgr;m range. The fluorescence spectra are recorded from 280 nm to 600 nm (in 20 channels) for 263 nm excitation and from 370 nm to 700 nm (in 22 channels) for 351 nm excitation. The elastic scattering (channel 4 and 9) is also recorded for sizing each particle. A time stamp for single particles is marked with a variable time interval resolution from 10 ms to 10 minutes. The SPFS employs a virtual-impactor concentrator to concentrate respirable-sized particles with a resulting (size-dependent) effective flow rate of around 100 liters/min. The SPFS can measure single-particle spectra at a maximum rate of 90,000/sec, although the highest rates we have experienced for the ambient are only several hundred/sec. When the SPFS is combined with an aerodynamic deflector (puffer) to sort particles according to their fluorescence spectral characteristics, the SPFS/puffer system can selectively deflect and collect an enriched sample of targeted particles (at rates limited by the puffer) of 1200 particles/sec, for further examination. In laboratory tests, aerosol particles with similar UV-LIF spectra (e.g. B. subtilis and E.coli) are puffed into the reservoir of a micro-fluidic cell, where fluorescent-labeled antibodies bind to them and were classified by their labeled fluorescence. Measurements of the background ambient aerosol with the SPFS system made at sites with different regional climate (Connecticut, Maryland, and New Mexico) were clustered (unstructured hierarchical analysis) into 8-10 groups, with over 90% of all the fluorescent particles contained within these clusters (threshold dot product=0.9). However, the percentage of aerosols in each profile differed by sampling location. The unique features and performance of the SPFS/puffer system compared to other fluorescence-based bio-aerosol sensors will be discussed, with emphasis on reduction of false alarm rates.
Real-time and in-situ detection and discrimination of aerosol particles, especially bio-aerosols, continues to be an important challenge. The technique labeled TAOS (Two-dimensional Angular Optical Scattering) characterizes particles based upon the angular distribution of elastically scattered light. The detected angular distribution of light, labeled the TAOS pattern, depends upon the particle’s shape, size, surface features, and its complex refractive index. Thus, the absorptive properties of a particle affect the TAOS pattern. Furthermore, we expect to use this change in the TAOS pattern, which occurs when the particle absorption band includes the input wavelength, to characterize the strength of the absorption. Thus, by illuminating a particle in the mid-infrared wavelength range, high frequency vibrational modes that are unique to the aerosol can be reached and quantified.
Spherical aerosol particles (in the diameter range of 50-60 micrometers) were generated via a droplet generator and illuminated by an Interband Cascade (IC) laser designed to emit in the 3-5 micrometers wavelength range. The TAOS pattern of the elastically scattered light was detected with an InSb-focal-plane-array infrared camera.
Jerome Kasparian, Riad Bourayou, Veronique Boutou, Catherine Favre, Guillaume Mejean, Didier Mondelain, Andre Mysyrowicz, Miguel Rodriguez, Estelle Salmon, Roland Sauerbrey, Holger Wille, Jean-Pierre Wolf, Ludger Woeste, Jin Yu, L. Klingbeil, K. Rethmeier, W. Kalkner, A. Hartzes, H. Lehman, J. Eisloeffel, Bringfried Stecklum, J. Winkler, Uwe Laux, S. Hoenger, Yong-Le Pan, Richard Chang, Steve Hill
The propagation of ultrashort, ultra-intense laser pulses gives rise to strongly nonlinear processes. In particular, filamentation is observed, yielding an ionized, conducting plasma channel where white-light supercontinuum due to self-phase modulation occurs. This supercontinuum, extending from the UV to the IR, is a suitable "white laser" source for atmospheric remote sensing, and especially Lidar (Light Detection and Ranging). Recent significant results in this regard are presented, as well as lightning control using ultrashort laser pulses. The application of ultrashort-pulse lidar to aerosol monitoring is also discussed.
Real-time methods which is reagentless and could detect and partially characterize bioaerosols are of current interest. We present a technique for real-time measurement of UV-excited fluorescence spectra and two-dimensional angular optical scattering (TAOS) from individual flowing biological aerosol particles. The fluorescence spectra have been observed from more than 20 samples including Bacillus subtilis, Escherichia coli, Erwinia herbicola, allergens, dust, and smoke. The S/N and resolution of the spectra are sufficient for observing small lineshape differences among the same type of bioaerosol prepared under different conditions. The additional information from TAOS regarding particle size, shape, and granularity has the potential of aiding in distinguishing bacterial aerosols from other aerosols, such as diesel and cigarette smoke.
The detection and characterization of micro-particles, particularly airborne biological particles, is currently of great interest. We present a novel technique for recording the 2D angular scattering pattern from a single airborne microparticle. Angular scattering measurements were performed in both the near-forward and near-backward regions for a variety of particles including for ethanol droplets, single polystyrene latex spheres, psl clusters, and clusters of Bacillus subtilis spores, all of various sizes. Because the angular scattering pattern is sensitive to size, shape and refractive index, the angular feature associate with clusters may be used to better characterize such airborne micro-particles. A watershed image processing routine has also been implemented. Through this routine, the number of intensity patches per solid angles is found to increase with cluster diameter.
Our group has been developing a system for single-particle fluorescence detection of aerosolized agents. This paper describes the most recent steps in the evolution of this system. The effects of fluorophore concentrations, droplet size, and excitation power have also been investigated with microdroplets containing tryptophan in water to determine the effects of these parameters on our previous results. The vibrating orifice droplet generator was chosen for this study base don its ability to generate particles of well- known and reproducible size. The power levels required to reach saturation and photodegradation were determined. In addition, the collection of fluorescence emission was optimized through the use of a UV achromatic photographic lens. This arrangement permitted collection of images of the droplet stream. Finally, the use of a dual-beam, conditional firing scheme facilitated the collection of improved signal- to-noise single-shot spectra from individual biological particles.
Fluorescence or Raman emission can be used in characterizing particles inside or near a surface, e.g., a biological cell or spore on a filter, or a contaminant particle on a silicon wafer. Here we model the emission from a sphere on a surface. The internal fields in a sphere on a surface are known for plane-wave excitation. These fields induce dipole moments in molecules in the sphere. These oscillating dipoles are the sources of the incident radiation at the shifted frequency. The Green function for emission is found by using the reciprocity theorem for Green functions along with the internal fields generated by a plane wave at the shifted frequency. Reciprocity provides a simple method for obtaining the far fields for systems for which the near/internal fields are known for plane-wave excitation.
We report the operation of an aerosol analyzer capable of measuring the fluorescence spectra of single micrometer-size bioaerosol particles. Aerosol particles in an air stream initially transverse a cw 488-nm 'trigger' laser beam where their elastic scattering and total fluorescence is measured with photomultipliers. When the elastic scattering and fluorescence signals meet certain criteria, a UV (266 nm) 'probe' laser is triggered and it illuminates the selected particles. The UV laser-excited spectra of particles are measured with the instrument's image-intensified CCD detector, gaged with signals from the trigger laser. We demonstrate the ability of the instrument to capture the fluorescence of single airborne biological particles. The results suggest that it may be possible to differentiate among biological particles based on their single particle fluorescence spectra.
Linear and nonlinear time-dependent backscattering calculations for dielectric spheres are presented. The time-dependent backscattered linear fields depend upon: (1) the linewidth of the pulse and the linewidth of the resonance of the sphere, and (2) the frequency separation between the center frequency of the pulse and the resonance frequency of the sphere. When the refractive index or absorption of the sphere is intensity-dependent, then the backscattering also depends upon the intensity of the laser pulse. When the droplets have a large intensity- dependent index of refraction or a large saturable absorption, and are illuminated near a resonance with a high-intensity laser pulse, hysteresis may be seen in the back-scattered intensity.
Light scattering by radially inhomogeneous fuel droplets has been calculated using both geometrical optics (GO) and the exact separation of variables (SV) solutions. The refractive index profiles of the fuel droplets were those calculated by Kneer et al. The GO and SV solutions agree very well in the forward direction (for scattering angles between 30 and 60 degrees), and less well in the backward direction (for scattering angles between 140 and 170 degrees). Both amplitudes and phases of the scattered light are compared. The agreement in the backward direction is much better for 40 micrometers diameter droplets than for 20 micrometers diameter droplets.
We have recently modeled third-order sum-frequency generation (TSFG) in droplets. The basic approach is similar to the model developed by Cooney and Gross for coherent anti- Stokes Raman scattering (CARS) from droplets. In this model, three generating waves interact to generate a third-order nonlinear polarization, which then radiates inside the sphere as described by the model of H. Chew et al. The intensity of the output waves at the sum frequency is proportional to the spatial overlap (amplitude and phase) of the nonlinear polarization with the output resonance of the droplet cavity mode, and to the integral of the products of the frequency dependence of the nonlinear polarization and the output resonance mode. Here we review our approach to modeling TSFG in droplets, discuss second-order sum frequency generation (SSFG) and CARS in droplets, stressing the similarities and differences among TSFG, SSFG, and CARS in droplets, and discuss the possible application of these mixing processes for fuel droplet characterization. We not that TSFG and SSFG from droplets are too weak to be useful for fuel droplet characterization, but that CARS is readily detectable from droplets and may be useful for determining the concentrations of chemical species in fuel droplets.
Several nonlinear optical processes concurrently occur in single micrometer-sized droplets that are irradiated by the second-harmonic output of a single-mode Q-switched Nd:YAG laser. The authors review the following observations of stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) in droplets: (1) the image of the droplets that are emitting SBS and SRS; (2) the long delay and decay time of the SRS pulse; (3) the correlated temporal profiles of SBS and SRS pulses; (4) the high-resolution spectra of SBS; (5) the frequency splitting of a degenerate spherical normal mode in the SRS spectra; and (6) the fine structures in the angular distribution of SRS.
Computed results for some linear optical problems relevant to nonlinear optics in droplets are presented and discussed. (1) The electric energy density distributions inside homogeneous spheres (water droplets) illuminated with plane waves are computed using Lorentz-Mie theory and geometrical optics ray tracing. (2) The electric energy densities inside spheres illuminated by Gaussian beams are computed using an angular spectrum of plane waves approach, a technique applicable to scattering of Gaussian beams by axisymmetric objects. (3) The Q's and resonance locations of spherically symmetric, radially-inhomogeneous spheres are computed numerically. (4) The Q's and resonance locations of perturbed, homogeneous, droplets are computed using the T-matrix method. (5) The Q's and resonance locations of inhomogeneous, spherically-asymmetric droplets are computed using the T-matrix method.
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