Airborne ITCIMS measurements of SO2, HCl and HNO3 in an aged stratospheric plume of volcanic origin

Kurzfassung

The climate impacts of volcanic gases and particles emitted into the stratosphere are extensive both in time and space. Sulphate particles, formed in the stratosphere through oxidation and nucleation of primary sulphur emissions, provide additional surface for halogen activation and trigger ozone depletion. The timescale of conversion of volcanic SO2 into particulate sulphate in the stratosphere is important for these activation processes. Other primary emitted gases such as HCl have longer lifetimes once they reach the stratosphere and may serve as a volcanic fingerprint. Secondary HCl produced through reaction of sulphuric acid with halite particles emitted during volcanic eruptions may contribute to the chlorine budget of the stratosphere. Airborne ITCIMS (Ion Trap Chemical Ionization Mass Spectrometry) measurements of SO2, HCl and HNO3 were carried out during the CONCERT (CONtrail and Cirrus ExpeRimenT) campaign in October 2008 and detected a broad SO2 enhancement in the lower stratosphere. The plume originated from the Mt. Kasatochi eruption (Aleutian Island) in August 2008 (Jurkat et al., 2010). The high-latitude volcanic eruption injected about 1.4 Mt of SO2 into the stratosphere. Simultaneous measurements of particulate sulphate with an aerosol mass spectrometer (Schmale et al., 2010) onboard the research aircraft Falcon (DLR) provide the unique opportunity to derive the e-folding lifetime of SO2 in the stratosphere. The ratio of particulate sulphate to total sulphur inside the plume was 0.76 0.07 corresponding to an e-folding time of 60 days in the stratosphere. Additional trace gas measurements of NOy, O3 and CO allow for a characterization of the air mass encountered and for analysis of heterogeneous reactions that took place on these sulphate particles. Correlation analysis show an enhancement of 19% of the ratios HCl/O3 and HNO3/NOy inside the SO2 – plume as compared to outside plume conditions. The enhanced ratio HCl/O3 can be explained by direct emission of HCl into the stratosphere. A consistent estimate of the e-folding lifetime of SO2 in the northern latitude lower stratosphere with an inferred dilution ratio of HCl was obtained. HNO3 was enhanced up to 50 % in the plume when compared to background conditions and former HNO3 measurements in the UT/LS. Heterogeneous reactions that trigger HNO3 production will be discussed. The data set provides new and extensive insights into trace gas fields of the UT/LS and show how they are affected by sulphate particles of volcanic origin.