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  • Author or Editor: J. F. Boatman x
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Y. J. Kim and J. F. Boatman

Abstract

A modified Mohnen slotted-rod collector was used to collect cloud-water samples in summer clouds over the northeastern United States. Cloud-droplet-size distributions were measured with a forward-scattering spectrometer probe (FSSP) mounted on the National Oceanic and Atmospheric Administration (NOAA) King Air research aircraft. Cloud-droplet-volume distributions and liquid water content were determined for each cloud-water sample through analyses of the FSSP data. The theoretical collection efficiency of the slotted-rod collector was calculated as a function of droplet size for flight conditions encountered during each cloud-water sampling. The mass ratio was then calculated for each cloud sample by ratioing the actual collected water mass to the maximum possible collectable water mass. Mass-ratio values higher than unity were obtained having an average of 1.40±0.27. This could be due to an underestimation of the liquid water content by the FSSP or to the collection of large hydrometeors by the slotted rod. The modified Mohnen slotted rod collected fast representative cloud-water samples in sufficient quantities for chemical analysis of the sampled cloud water.

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Y. J. Kim and J. F. Boatman

Abstract

The response of the Forward Scattering Spectrometer probe (FSSP) is affected by the optical properties of measured particles. The manufacturer's size calibration data are specifically applicable to nonabsorbing water droplets. Response functions of the FSSP probe are calculated for different complex refractive indices corresponding to different types of atmospheric aerosols under various relative humidity Conditions. Based on the results of these response calculations, new corrected size calibrations are determined for six relative humidity values (0%, 50%, 70%, 80%, 90% and 99%) and for three atmospheric aerosol types (Rural, Urban and Maritime). Sample calculations with these corrected size calibration data show that a significant underestimation of the aerosol size/volume distribution can result, especially for dry atmospheric aerosols, if the manufacturer's size calibration data are used.

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Young J. Kim and Joe F. Boatman

Abstract

Distortion of the size spectra measured with a forward mattering spectrometer probe (FSSP) under different transit time modes—“inhibit”, “normal”, and “delayed”—was evaluated using the theoretical analyses by Baumgardner and Spowart and the results of the response time and beam intensity profile measurements of the NOAA FSSP. The Baumgardner and Spowart work is extended to correct the FSSP atmospheric aerosol data collected under the “inhibit” or “delayed” mode. A correction algorithm is developed using the non-negative least squares (NNLS) method to reconstruct the original size distribution from a distorted one measured with an FSSP under the inhibit or delayed mode. A lognormal fit to the corrected size spectra was able to successfully recover from the original size distributions from the distorted artificial ones obtained from the theoretical simulation of the FSSP performance. When the actual test flight data for atmospheric aerosols measured with the NOAA FSSP under the inhibit and delayed modes were corrected using the NNLS correction scheme, the two corrected size spectra converged, implying the measurement of the same sample of particles.

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