These two accounts provide direct evidence of a persistent stratospheric aerosol veil that spanned at least 2600 km into both Northern and Southern Hemispheres and establish that the source was a tropical volcano. The second, made by physician José Hipólito Unanue in Lima, Peru, describes sunset after-glows (akin to well-documented examples known to be caused by stratospheric volcanic aerosols) from mid-December 1808 to February 1809. The first, Francisco José de Caldas, describes a stratospheric aerosol haze, a "transparent cloud that ob-structs the sun's brilliance", that was visible over the city of Bogotá, Colombia, from 11 December 1808 to at least mid-February 1809. The ob-servations were made by two highly respected Latin Ameri-can scientists. Here we report on two me-teorological observations dating from the end of 1808 that describe phenomena we attribute to volcanic-induced atmo-spheric effects caused by the Unknown eruption. However, no eyewitness accounts of the event, and therefore its location, or the atmospheric optical effects associated with its aerosols have been docu-mented from historical records. The Unknown eruption of 1808/1809 was the sec-ond most explosive SO 2 -rich volcanic eruption in the last two centuries, eclipsed only by the cataclysmic VEI 7 Tamb-ora eruption in April 1815.
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