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Stratospheric aerosol parameters for the Fuego volcanic incursion Louis Elterman Air Force Cambridge Research Laboratories, Bedford, Massachusetts 01730. Received 10 January 1975. In Guatemala, the Highlands begin a short distance west of Guatemala City and covers the western part of the coun- try. It contains ridges, valleys, and volcanic regions, all of these features resting on a vast plateau 1.52 km above sea level. In these Highlands, the Fuego Volcano (14° 29′ N, 90° 52′ W), which rises sharply to a height of 3.84 km above m.s.l., has been a threatening landmark for several years. Its eruption in 1971 was followed by a sustained emission of hissing steam that continued until February 1973, when it erupted again. The Smithsonian Institution 1 reported re- sumption of intense activity on 13 October 1974 and espe- cially on 18 October. Voluminous ash emissions were de- posited as far north as the Mexican border, 150 km distant. The stratospheric effects, due to the volcanic dust infusion, were first noted by Meinel and Meinel 2 while on a flight over Mexico, 9 November. Since the Meinels' initial ob- servations, characteristic colorful skies have been reported throughout the U. S., Puerto Rico, England, and France. Volz 3 estimates the volcanic dust moved westward from its source to Hawaii, turned north, and entered the westerlies. In Hawaii, laser measurements of the event were first re- ported by Fegley and Ellis 4 from 29 October through 26 December 1974. Although they were unaware of the erup- tion, they noted the presence of a unique layer, especially on 5 November when the aerosol backscatter at a 19.5-km altitude was greater than a corresponding Rayleigh re- sponse by a factor of 20. In Virginia on 26 November laser measurements were carried out during a 3-h period by McCormick and Fuller. 5 They found layers with maxima at 16.0 km and 20.25 km having backscatter values an order of magnitude larger than their earlier measurements. This Letter reports searchlight probing results of volcan- ic dust over New Mexico during the nights 11 and 12 No- vember when a total of twelve profiles were acquired, most of them consecutively. Since the aerosol attenuation coef- ficient approximates proportionality to the aerosol concen- tration, the profiles provide information concerning the aerosol concentration as it varies with altitude. Figure 1 presents the profile for 11 November 1974 at 22:17 MST, acquired over a period of 25 min. It was selected because quantitatively it resembles the average profile for that night. In these profiles, we note that our data reduction methods rule out any smoothing of field data, even of indi- vidual points, at stratospheric altitudes between 11.7 km and 26.3 km. Between 26.3 km and about 31 km, the field data were smoothed to obviate occurrence of isolated in- stances of negative aerosol attenuation coefficients. Be- tween about 31 km and 41.1 km (our highest measure- ment), it was determined that a first-order least-square was the most suitable fit. As is our current practice, all data are normalized to the least-square value at 41.1 km with the assumption that the existence of an aerosol constituent at that altitude results in small error. Figure 2 is the turbidity chart for the same profile shown in Fig. 1. Several layers are prominent, and these features, including their aerosol optical thickness, are described in Table I. During the night, over several hours of measure- ment, the major layer retained remarkably well its altitude of maximum turbidity. The other layers were less stable. The following night (12 November), the major layer was found at a somewhat lower altitude, approximately 15 km, but similar in appearance. A single point such as a turbidity maximum cannot de- scribe quantitatively the aerosol content. Accordingly, the aerosol optical thickness was computed for the stratospher- ic layer, or layers, usually observed to be within about the 12-26-km altitude region. Table II assembles these pa- rameters and other pertinent information for all the pro- files acquired during both nights. These parameters represent an early phase of the volcan- ic dust infusion showing a stratospheric dust increase of about 50% compared with recent searchlight probing. The behavior of this atmospheric condition will be reported with greater detail in a future publication. The author wishes to acknowledge the fine participation with field operations provided by John D. Essex and Mi- Fig. 1. Vertical profile of aerosol attentuation coefficients β p (h) for 11 November 1974 at 22:17 MST over New Mexico; λ = 0.55 μm. The stratospheric region characterizes the Fuego volcanic dust incursion. The eruption (Guatemala, 14°29'N, 90°52′W) oc- curred 13-18 October 1974. Fig. 2. Turbidity profile corresponding to Fig. 1. Turbidity: ratio of aerosol to Rayleigh attenuation coefficients, β p (h)/β r (h) for λ = 0.55 μm. The features are described in Tables I and II. 1262 APPLIED OPTICS / Vol. 14, No. 6 / June 1975
