Field Characterization Of Phosphorus Smokes Using Spectral Transmittance And Real-Time Concentration Measurements

AbstractAccurate field characterization of aerosol number density, extinction cross-section, mass extinction, and aerosol mass concentration is of basic importance in the development of improved smoke screens and electro-optical weapons systems. During a recent field test involving smoke screen tests, data were obtained from 1) a spectral transmissometer which measured transmittance once per second for 200 wavelengths between 2.5 and 14 micrometres, 2) optical particle size analyzers, 3) an electric cascade impactor, 4) a quartz crystal microbalance, and 5) mechanical mass samplers. The aerosol parameters of interest (number density, extinction cross-section, etc.) were computed through combinations of independently measured parameters in order to determine data consistency and the spread in values which might be encountered from such measurements. Results obtained from some of these instruments are described which show how the mass extinction coefficient can be expected to vary with time. It is shown that this time dependence is a major reason for wide variability in field measurements of the extinction coefficient.IntroductionThe U. S. Army has conducted a number of large scale field tests designed to evaluate the capabilities of electro-optical weapons systems in an obscured environment. The sources of obscuration have ranged from dust raised by artillary explosions to phosphorus generated smokes to specially developed obscurants designed to defeat systems using infrared wave­ lengths. In such tests, the successful characterization of the optical properties of these obscurants for wavelengths ranging from visible to the far IR is of fundamental importance if improved obscurants, effective countermeasures, and effective predictions of electro- optical system performance are to be developed.Transmittance through obscurant clouds is one parameter indicative of obscurant effective­ ness. If the aerosol mass concentration along a line of sight and transmission path length are measured and the mean extinction cross-section per mean particle mass (hereafter called the extinction coefficient, a) is known, then the transmittance can be predicted using the Beer-Bougher transmission law. The extinction coefficient thus is a basic parameter necessary for characterizing and evaluating the optical attenuation properties of an obscurant.Serious discrepancies have been found to exist between laboratory and field measurements of the mass extinction coefficient of hygroscopic smokes.1 This is clearly shown in figure 1 which plots a number of different determinations of the extinction coefficient for phos­ phorus smokes at a wavelength of 3.4 micrometres. These were measured by numerous agencies using different techniques and methodologies. Most of the data for relative humidities above 65% were obtained during the High Humidity Hygroscopic Smoke (H^s) test conducted during July of 1979- Figure 1 indicates that the data differences can be traced to how the measurements were made in the field tests. When extinction coefficient is computed using measured particle size distributions and Mie scattering theory results reasonably consistent with laboratory measurements (Chemical Systems Laboratory, CSL) are obtained. However, when these results are compared to those obtained using long base line transmittance and chemical impinger data (Dugway Proving Ground, DPG) significant differences are evident. During Smoke Week III, a large field test held at Eglin Air Force Base during August 1980, measurements were made with the intent of generating data to help resolve the differences. These data were provided by a localized set of instruments called the "instrumentation cluster" placed on a concrete pad near the test grid center. Among the parameters measured by these instruments were: 1) particle size distribution, 2) particle number density, 3) aerosol mass concentration, and 4) transmittance over a 5m path for over 200 wavelengths between 2.5 and 14 ym. In a number of test cases, there was sufficient redundance in the instrumentation and in the measurements to determine data consistency and range of variation in the measurement. A description instrumentation cluster, its arrangement, hardware, and possible data correlations available with the measurements are discussed elsewhere.2 The purpose of this paper is to describe how the use of time resolved aerosol mass concentration and 5m path transmittance measurements have been used to determine the probable reason for the discrepancies between past laboratory and field determinations of the extinction coefficient. First, the results of the time dependent mass concentration