Nabro volcano aerosol in the stratosphere over Georgia, South Caucasus from ground-based spectrometry of twilight sky brightness

Ground-based spectral measurements of twilight sky brightness were carried out between October 2009 and August 2011 in Georgia, South Caucasus. The algorithm which allowed to retrieve the lower stratospheric and upper tropospheric aerosol extinction profiles was developed. The Monte-Carlo technique was used to correctly represent multiple scattering in a spherical atmosphere. The estimated stratospheric aerosol optical depths at a wavelength of 780 nm were: 3.0 × 10−3 ± 1 × 10 −3 (31 August 2009–15 January 2011) and 1.1 × 10−2 ± 3 × 10−3 (18 July 2011–03 August 2011, 10 observations). The first optical depth value corresponds to the background stratospheric aerosol level, the last one to the volcanically disturbed one after the Nabro eruption in June 2011. Reconsideration of measurements acquired soon after the Pinatubo eruption in 1991 allowed to model the phenomenon of the "second purple light", a twilight sky brightness enhancement at large solar zenith angles (97–102°). Monte-Carlo modeling reveals that the second purple light is caused by multiple scattering in the stratospheric aerosol layer. The modeling also shows that, assuming a hypothetical mesospheric aerosol layer with optical extinction comparable to typical noctilucent cloud values, a measurable twilight sky brightness increase at wavelength 440 nm follows at solar zenith angles 98–99&deg.