Unambiguous identification and measurement of sulfuric acid cluster chemiions in aircraft jet engine exhaust

Measurements of negative chemiions (CI) emitted by a jet engine at the ground were made with an ion trap mass spectrometer. The new instrument offered a high-mass resolution, which led to a first unambiguous identification of negative CI formed by a jet engine. The observed ions are HSO4−(H2SO4)a clusters proved by an isotope study. From the mass spectra an efficiency for fuel sulfur conversion to SVI of 2%±0.8 could be inferred. In addition thermodynamic properties of the observed cluster ions were inferred from measured ion abundance ratios. An effective free energy ΔGa−1,a0=−14 kcal/mol was calculated (for a=3) and an enthalpy of ΔHa−1,a0=−24 (for a=3) kcal/mol was estimated. This indicates a low stability of HSO4−(H2SO4)a (a 3) cluster ions against thermal detachment of H2SO4 at the high temperatures of our experiment. However the low temperatures at cruise altitudes around 10–12 km lead to high H2SO4/H2O supersaturation and therefore a rapid growth of HSO4−(H2SO4)a cluster ions seems to be possible which is not hindered by thermal H2SO4 detachment.