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This study is based on fine-mode aerosol samples collected in the\udupper troposphere (UT) and the lowermost stratosphere (LMS) of the Northern\udHemisphere extratropics during monthly intercontinental flights at\ud8.8–12 km altitude of the IAGOS-CARIBIC platform in the time period\ud1999–2014. The samples were analyzed for a large number of chemical elements\udusing the accelerator-based methods PIXE (particle-induced X-ray emission)\udand PESA (particle elastic scattering analysis). Here the particulate sulfur\udconcentrations, obtained by PIXE analysis, are investigated. In addition, the\udsatellite-borne lidar aboard CALIPSO is used to study the stratospheric\udaerosol load. A steep gradient in particulate sulfur concentration extends\udseveral kilometers into the LMS, as a result of increasing dilution towards\udthe tropopause of stratospheric, particulate sulfur-rich air. The stratospheric air is diluted with tropospheric air, forming the extratropical transition layer (ExTL). Observed\udconcentrations are related to the distance to the dynamical tropopause. A\udlinear regression methodology handled seasonal variation and impact from\udvolcanism. This was used to convert each data point into stand-alone estimates\udof a concentration profile and column concentration of particulate sulfur in\uda 3 km altitude band above the tropopause. We find distinct responses to\udvolcanic eruptions, and that this layer in the LMS has a significant\udcontribution to the stratospheric aerosol optical depth and thus to its\udradiative forcing. Further, the origin of UT particulate sulfur shows\udstrong seasonal variation. We find that tropospheric sources dominate during\udthe fall as a result of downward transport of the Asian tropopause aerosol\udlayer (ATAL) formed in the Asian monsoon, whereas transport down from the\udJunge layer is the main source of UT particulate sulfur in the first half of\udthe year. In this latter part of the year, the stratosphere is the clearly\uddominating source of particulate sulfur in the UT during times of volcanic\udinfluence and under background conditions

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