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Volume 17, issue 18 | Copyright
Atmos. Chem. Phys., 17, 10937-10953, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 15 Sep 2017

Research article | 15 Sep 2017

Particulate sulfur in the upper troposphere and lowermost stratosphere – sources and climate forcing

Bengt G. Martinsson1, Johan Friberg1, Oscar S. Sandvik1, Markus Hermann2, Peter F. J. van Velthoven3, and Andreas Zahn4 Bengt G. Martinsson et al.
  • 1Division of Nuclear Physics, Lund University, Lund, Sweden
  • 2Leibniz Institute for Tropospheric Research, Leipzig, Germany
  • 3Royal Netherlands Meteorological Institute (KNMI), De Bilt, the Netherlands
  • 4Institute of Meteorology and Climate Research, Institute of Technology, Karlsruhe, Germany

Abstract. This study is based on fine-mode aerosol samples collected in the upper troposphere (UT) and the lowermost stratosphere (LMS) of the Northern Hemisphere extratropics during monthly intercontinental flights at 8.8–12km altitude of the IAGOS-CARIBIC platform in the time period 1999–2014. The samples were analyzed for a large number of chemical elements using the accelerator-based methods PIXE (particle-induced X-ray emission) and PESA (particle elastic scattering analysis). Here the particulate sulfur concentrations, obtained by PIXE analysis, are investigated. In addition, the satellite-borne lidar aboard CALIPSO is used to study the stratospheric aerosol load. A steep gradient in particulate sulfur concentration extends several kilometers into the LMS, as a result of increasing dilution towards the tropopause of stratospheric, particulate sulfur-rich air. The stratospheric air is diluted with tropospheric air, forming the extratropical transition layer (ExTL). Observed concentrations are related to the distance to the dynamical tropopause. A linear regression methodology handled seasonal variation and impact from volcanism. This was used to convert each data point into stand-alone estimates of a concentration profile and column concentration of particulate sulfur in a 3km altitude band above the tropopause. We find distinct responses to volcanic eruptions, and that this layer in the LMS has a significant contribution to the stratospheric aerosol optical depth and thus to its radiative forcing. Further, the origin of UT particulate sulfur shows strong seasonal variation. We find that tropospheric sources dominate during the fall as a result of downward transport of the Asian tropopause aerosol layer (ATAL) formed in the Asian monsoon, whereas transport down from the Junge layer is the main source of UT particulate sulfur in the first half of the year. In this latter part of the year, the stratosphere is the clearly dominating source of particulate sulfur in the UT during times of volcanic influence and under background conditions.

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We find that the aerosol of the lowermost stratosphere has a considerable climate forcing. The upper tropospheric (UT) particulate sulfur is strongly influenced by stratospheric sources the first half of the year, whereas tropospheric sources dominate in fall; 50 % of the UT particulate sulfur (S) was found to be stratospheric at background condition, and 70 % under moderate influence from volcanism. The Asian monsoon is found to be an important tropospheric source of S in the NH extratropical UT.
We find that the aerosol of the lowermost stratosphere has a considerable climate forcing. The...