Jet Engine Exhaust Chemiion Measurements: Implications for Gaseous SO3 and H2SO4

Abstract

We have made mass spectrometric measurements of negative chemiions (CI) in the exhaust of a jet engine on the ground. The measurements took place at plume ages between 6.6 and 19 ms at low- and high-fuel sulfur content (FSC). Total negative CI-number densities reached up to 1.4 · 107 cm-3 corresponding to an emission index for negative CI of 3×1015 CI per kg fuel. The most abundant negative CI species were found to be HSO-4H2SO4, HSO-4SO3, HSO-4HNO3, and NO-3(HNO3)m. Probably HSO-4-containing ions are formed from NO-3-containing ions by reactions with SO3 and gaseous sulfuric acid (GSA). Hence our experiments indicate the presence of SO3 and GSA. Building on this ion reaction scheme the S(VI) number density (S(VI)=SO3+H2SO4) was inferred from the CI-composition measurements. For low FSC one obtains (S(VI))=6.4×1011 cm-3 which corresponds to an efficiency for fuel sulfur conversion to gaseous S(VI) of =0.012. Our findings have important implications for bimolecular H2SO4–H2O-nucleation in jet aircraft exhaust plumes at cruise altitudes. New aerosol particles may form via homogeneous and/or CI-induced nucleation while heterogeneous nucleation on soot particles may activate soot particles to become water vapour condensation nuclei. Our findings imply that nucleation and condensational growth are more efficient than predicted by most previous models which assumed smaller e.