Abstract
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 8.8–12 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.
Highlights
The global mean surface temperature has increased considerably in the last 2 years (NOAA, 2016), which followed on a 15-year period with slow temperature evolution
The resulting groups of data from each season were modeled by forced linear regression. These model results are used in a second regression step to model seasonal influences from transport, which in turn are used to obtain the response of the lowermost stratosphere (LMS) and upper troposphere (UT) particulate sulfur concentrations to changes induced mainly by volcanic eruptions
Particulate sulfur in the upper troposphere (UT) and the lowermost stratosphere (LMS) obtained from the IAGOS-CARIBIC platform was investigated at northern midlatitudes in the time period 1999–2014, which covwww.atmos-chem-phys.net/17/10937/2017/
Summary
The global mean surface temperature has increased considerably in the last 2 years (NOAA, 2016), which followed on a 15-year period with slow temperature evolution. CMIP5 (Coupled Model Intercomparison Project) models predict stronger-than-observed temperature increases in this period (Fyfe et al, 2013, 2016) Reasons for these differences were sought, and the Interdecadal Pacific Oscillation connected with increased subduction and upwelling (England et al, 2014; Meehl and Teng, 2014), variations in volcanic aerosol (Solomon et al, 2011; Santer et al, 2014) and solar (Myhre et al, 2013) forcings were identified as main causes of the discrepancies. Air from the tropical troposphere containing aerosol precursor gases is lifted into the tropical stratosphere in the Brewer–Dobson circulation. Carbonyl sulfide (OCS) is the most abundant sulfur-containing gas in the atmosphere
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