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Laser-induced breakdown spectroscopy of liquid aerosols and droplet salt concentration measurements
Abstract
Results are presented of the application of laser-induced breakdown spectroscopy to droplet salt concentration measurements of seawater aerosol droplets. An excimer laser, operating with krypton fluoride gas $(\lambda$ = 248 nm), caused laser-induced breakdown of aerosol droplets to form a plasma. Emission lines consisting of the sodium doublet $(\lambda$ = 589 nm) and three Balmer-series hydrogen lines, $H\sb\alpha$ $(\lambda$ = 656 nm), $H\sb\beta$ $(\lambda$ = 486 nm) and $H\sb\gamma$ $(\lambda$ = 434 nm) were monitored with an optical multi-channel analyzer to characterize the plasma and provide quantitative measurements of the sodium concentration. Studies of temporally-resolved emission measurements from the plasma determined the optimum times for gating of the detector to be between 2 and 4 $\mu$s after initial pulse incidence. Spatially-resolved measurements of the plasma were studied by positioning a monodisperse stream of droplets at various locations relative to the probe volume of the system. Spatially-resolved emission intensity ratios of sodium to hydrogen, along the axis of the laser pulse, were found to be greatest in the shadow regions of the plasma and decreased significantly in the illuminated portions of the plasma. Translation of the droplet stream, along the axis of the detector relative to the laser beam, also exhibited large variations in the emission intensity ratios. The electron temperature of the plasma was estimated to range between 5,000 and 20,000 K, but usually 10,000-15,000 K, based upon an assumed Boltzmann distribution of the occupation of the excited states of hydrogen associated with the Balmer-series lines. Calibration curves are presented relating the Na to $H\sb\alpha$ intensity ratio as a function of Na concentration, ranging from 100 to 10,000 ppm. Limits of detection for Na by the current method were estimated to be approximately 165 ppm for monodisperse sprays and 925 ppm for one case involving a polydisperse spray. Droplet diameter was found to strongly influence the observed emission intensity ratio. Studies to determine the influence of moisture content of the atmosphere on the LIBS measurements were inconclusive.
Subject Area
Mechanical engineering|Atmosphere|Fluid dynamics|Gases
Recommended Citation
Poulain, Dana Earl, "Laser-induced breakdown spectroscopy of liquid aerosols and droplet salt concentration measurements" (1993). ETD collection for University of Nebraska-Lincoln. AAI9333981.
https://digitalcommons.unl.edu/dissertations/AAI9333981