Fifteen lidar observations of the stratosphere aerosol were made between February and November 1975. All observations revealed the greatly increased particulate backscattering that followed the eruption of the volcano Fuego in October 1974. Vertical structure consisted initially of multiple layers, which later merged to form a single, broader peak. Nearly all of the increased scattering was confined to altitudes below 20 km. Hence, aerosol layer centroids in 1975 were typically several kilometers below their altitude prior to the eruption.

Our observations began in mid-February, at about the time of maximum northern midlatitude influence of the volcanic injection. From late February on, both vertically integrated particulate back-scattering and the peak ratio of particulate to gaseous backscattering displayed approximately exponential declines, with mean 1/e lifetimes of eight and eleven months, respectively. These relatively short residence times are a combined consequence of the low altitude of the volcanic particles and their larger mean size as compared to the preinjection, or unperturbed, aerosol. The peak scattering ratio of our average 1975 profile was 1.7, and the vertically integrated particulate backscattering was 3.6 × 10−4 sr−1 (both at λ = 694 nm). The mean midvisible particulate optical thickness, derived from measured back-scattering and realistic optical models, was about 0.03, approximately six times the mean value in the year before the Fuego eruption, but not as large as values observed for some years after the 1963 Agung eruption.

Radiative and thermal consequences of the measured post-Fuego layer were computed using several recently published models. The models yield a 1975 mean layer albedo of about 1% or less; they predict a temperature increase of several kelvins at the altitude of the layer, caused by the infrared absorption bands of the sulfuric acid particles. At the surface, the models predict a temperature decrease of considerably less than 1 K, partly because of the small optical thickness of the volcanic layer, and partly because of its short residence time relative to the earth-ocean thermal response time.

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