The Post-Fuego Stratospheric Aerosol: Lidar Measurements, with Radiative and Thermal Implications

Fifteen lidar observations of the stratospheric aerosol were made between Feb. and Nov. 1975. All observations revealed the greatly increased particulate backscattering that followed the eruption of the Fuego volcano in Oct. 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. The observations began in mid-Feb., at about the time of maximum northern mid-latitude influence of the volcanic injection. From late Feb. on, vertically integrated particulate backscattering 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 the average 1975 profile was 1.7, and the vertically integrated particulate backscattering was 3.6 x 10 super(-) super(4) sr super(-) super(1) (both at lambda = 694 nm). The mean mid-visible particulate optical thickness, derived from measured backscattering 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 by 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 IR 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.