Volcanic Aerosols in the Upper Troposphere and Stratosphere – Aircraft- and Satellite-Based Observations

Abstract: Volcanic eruptions are the largest source of variability in the stratospheric particle concentration as they inject SO2 that is subsequently converted into sulphate particles. No major perturbation of the stratospheric aerosol load has been observed since the large eruption of Pinatubo in 1991, but several moderate eruptions after 2000 have caused deviations from stratospheric background conditions. Atmospheric particles are of concern with regard to climate because they scatter and absorb radiation. This thesis presents results from observations of the aerosol in the upper troposphere and stratosphere during the period 1999-2013. Measurements of aerosol particles and trace gases were performed with the CARIBIC flying observatory, where instrumentation is installed on a Lufthansa passenger aircraft during monthly intercontinental flights at altitudes of 9-12 km. The elemental composition of particles in the diameter range 0.08-2 µm was obtained from sampling and subsequent ion beam analysis (PIXE and PESA). Size distributions of particles with diameters of 0.12-1 µm were measured during flight by an optical particle counter. Lidar observations from the satellite CALIPSO were used to reveal the vertical distribution and global effect of three volcanic eruptions from 2008 to 2011. These observations were used to determine stratospheric aerosol optical depth and radiative forcing, in order to estimate the climate impact received from the lowermost stratosphere (LMS). It was found that the LMS particle concentrations were on average higher in the years 2005-2008 than 1999-2002. While the latter period is considered to represent background conditions, the increase in 2005-2008 was attributed to three tropical eruptions with subsequent transport of the aerosol into the LMS by the shallow and deep branches of the Brewer–Dobson circulation. In 2008-2012, volcanism had considerable impact during due to both extra-tropical and tropical eruptions. Three main components were identified in volcanic particles sampled one week to more than 18 weeks after the eruptions: sulphate, ash and a carbonaceous fraction. The residence time of SO2 in the stratosphere was estimated to be 45±22 days. The stratosphere below 15 km altitude was found to contribute 55% to the total stratospheric aerosol optical depth and 42% to the total aerosol radiative forcing from the Kasatochi (2008), Sarychev (2009) and Nabro (2011) eruptions. Thus, currently used estimates of the impact of volcanic eruptions on climate based on data obtained above 15 km or the 380 K isentrope should be updated to include the effect of volcanism on the LMS. Furthermore, higher concentrations of sulphate particles were observed in the upper troposphere during volcanically influenced periods, with a positive trend in 2001-2011. The opposite trend was observed in cirrus cloud reflectance in the same period. This suggests that the volcanic aerosol has a significant indirect effect on climate, which requires further investigation.