Particulate sulfur in the upper troposphere and lowermost stratosphere – sources and climate forcing
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 8.8–12 km altitude of the IAGOS-CARIBIC platform in the time period 1999–2014. The samp...
Main Authors: | , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2017-09-01
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Series: | Atmospheric Chemistry and Physics |
Online Access: | https://www.atmos-chem-phys.net/17/10937/2017/acp-17-10937-2017.pdf |
Summary: | 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
8.8–12 km 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 3 km 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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ISSN: | 1680-7316 1680-7324 |