Studies of Volcanic Influence on Aerosols, Clouds and Climate

Sammanfattning: This thesis focuses on the influence of volcanism on the compositions of the aerosols in the upper troposphere (UT) and lowermost stratosphere (LMS), and their direct and indirect impact on climate. Aerosol data were obtained by aircraft-borne sampling, using the CARIBIC (Civil Aircraft for the Regular Investigation of the Atmosphere Based on an Instrument Container) platform, and laboratory-based ion beam analysis of aerosol samples at the Lund Ion Beam Analysis Facility (LIBAF). Aerosol composition data were compared to particle size distributions obtained from onboard optical particle counter (OPC) measurements, demonstrating good agreement between the two analysis systems. The impact on climate was investigated using satellite observations of aerosol and optical properties of cirrus clouds. These were provided by the CALIOP and MODIS instruments onboard the NASA satellites CALIPSO, Terra and Aqua. The aerosol load in the LMS has varied considerably since 2000, mainly due to volcanic injections of particles and particleforming gases. Tropical volcanoes affect the LMS for up to two years after eruption, through transport within the Brewer- Dobson circulation. In contrast, extra-tropical volcanoes inject aerosols directly into the LMS, which subside to the UT within months. The eruption of Kasatochi in August 2008 increased the aerosol load in the northern hemisphere LMS by a factor of ~10. Apart from sulfate and ash, both fresh and aged volcanic aerosols contain surprisingly large amounts of carbonaceous aerosols, and the value of the oxygen:carbon ratio (O/C) of ~2 indicates an organic origin. Entrainment of the organic aerosol present in the tropospheric background within volcanic jets and plumes was suggested to be the cause. Using CALIOP data, it was shown that the stratospheric aerosol at altitudes below 15 km constitutes a significant part of the volcanic forcing. During the period from 2008 to the middle of 2012, volcanic forcing in the LMS constituted 30% of that in the rest of the stratosphere. In addition, volcanism was found to have a significant influence on aerosol concentrations in the UT of the northern hemisphere. Comparison with cirrus reflectance (CR) data obtained using the MODIS instrument revealed a strong anti-correlation between the CR and particulate sulfur mass concentration, suggesting that the volcanic aerosol affected midlatitude cirrus clouds. In 2011, the CR was 8% lower than in 2001. Since cirrus clouds warm the Earth, this decrease is associated with regional cooling. The results of these studies show that previous estimates of the impact of volcanism on climate have been underestimated. The investigations of the direct and indirect radiative effects of volcanism on the UT and LMS presented here provide new information on the effect of volcanism on the Earth’s climate. This will allow more realistic estimates of the impact of volcanism on climate variability, and improve climate models providing more realistic projections of future global temperatures.