Filters

2014 - 20152

More filters

2019 - 201912020 - 20221
Reset filters

Settings

Author: Friberg, Johan × Subject: volcanic aerosol ×

Search Results

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Influence of volcanic eruptions on midlatitude upper tropospheric aerosol and consequences for cirrus clouds
    (Malden, Mass. : American Geophysical Union, 2015) Friberg, Johan; Martinsson, Bengt G.; Sporre, Moa K.; Andersson, Sandra M.; Brenninkmeijer, Carl A.M.; Hermann, Markus; van Velthoven, Peter F.J.; Zahn, Andreas
    The influence of downwelling stratospheric sulfurous aerosol on the UT (upper troposphere) aerosol concentrations and on cirrus clouds is investigated using CARIBIC (Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrument Container observations) (between 1999–2002 and 2005–2013) and the cirrus reflectance product from Moderate Resolution Imaging Spectroradiometer (MODIS). The initial period, 1999–2002, was volcanically quiescent after which the sulfurous aerosol in the LMS (lowermost stratosphere) (SLMS) became enhanced by several volcanic eruptions starting 2005. From 2005 to 2008 and in 2013, volcanic aerosol from several tropical eruptions increased SLMS. Due to consequent subsidence, the sulfur loading of the upper troposphere (SUT) was increased by a factor of 2.5 compared to background levels. Comparison of SLMS and SUT during the seasons March–July and August–November shows a close coupling of the UT and LMS. Finally, the relationship between SLMS and the cirrus cloud reflectance (CR) retrieved from MODIS spectrometer (on board the satellites Terra and Aqua) is studied. SLMS and CR show a strong anticorrelation, with a factor of 3.5 increase in SLMS and decrease of CR by 8 ± 2% over the period 2001–2011. We propose that the increase of SLMS due to volcanism has caused the coinciding cirrus CR decrease, which would be associated with a negative radiative forcing in the Northern Hemisphere midlatitudes.
  • Thumbnail Image
    Item
    Sources of increase in lowermost stratospheric sulphurous and carbonaceous aerosol background concentrations during 1999–2008 derived from CARIBIC flights
    (Milton Park : Taylor & Francis, 2014) Friberg, Johan; Martinsson, Bengt G.; Andersson, Sandra M.; Brenninkmeijer, Carl A.M.; Hermann, Markus; Van Velthoven, Peter F.J.; Zahn, Andreas
    This study focuses on sulphurous and carbonaceous aerosol, the major constituents of particulate matter in the lowermost stratosphere (LMS), based on in situ measurements from 1999 to 2008. Aerosol particles in the size range of 0.082 mm were collected monthly during intercontinental flights with the CARIBIC passenger aircraft, presenting the first long-term study on carbonaceous aerosol in the LMS. Elemental concentrations were derived via subsequent laboratory-based ion beam analysis. The stoichiometry indicates that the sulphurous fraction is sulphate, while an O/C ratio of 0.2 indicates that the carbonaceous aerosol is organic. The concentration of the carbonaceous component corresponded on average to approximately 25% of that of the sulphurous, and could not be explained by forest fires or biomass burning, since the average mass ratio of Fe to K was 16 times higher than typical ratios in effluents from biomass burning. The data reveal increasing concentrations of particulate sulphur and carbon with a doubling of particulate sulphur from 1999 to 2008 in the northern hemisphere LMS. Periods of elevated concentrations of particulate sulphur in the LMS are linked to downward transport of aerosol from higher altitudes, using ozone as a tracer for stratospheric air. Tropical volcanic eruptions penetrating the tropical tropopause are identified as the likely cause of the particulate sulphur and carbon increase in the LMS, where entrainment of lower tropospheric air into volcanic jets and plumes could be the cause of the carbon increase.