Sources of increase in lowermost stratospheric sulphurous and carbonaceous aerosol background concentrations during 1999-2008 derived from CARIBIC flights

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.