Positive and negative ion measurements in jet aircraft engine exhaust: concentrations, sizes and implications for aerosol formation

Positive and negative ions have been measured by the ion mobility analyzer in the exhaust of a jet aircraft engine behind the combustor exit at the ground-level. The operational conditions of the combustor were varied covering two fuel flows (FFs) and three fuel sulfur contents (FSCs). About 50% of the observed ions have mass number m (amu, atomic mass units) larger than 100 and the most massive ions have m up to about 1500–3000 depending on the FF and FSC. Considering such large m, many of these ions must be relatively large molecular ions, as for example polycyclic aromatic hydrocarbons (PAH). Labile cluster ions are unlikely due to the high temperature (about 1000 K). Measured ion concentrations referred to standard conditions are np=4×107–1.7×108 cm−3 (positive ions) and nn=6×107–2.1×108 cm−3 (negative ions). The total ion emission index is Ei=(Ep+En)=1.2×1016–2.0×1016 ions kg−1 fuel burnt. Emission index Ei increases markedly with FF. For low FF, Ei increases markedly with higher FSC. Our Ei is the same order as was observed for large volatile particles in wakes of a jet aircraft in-flight formed by the ion-induced nucleation followed by the ion-assisted growth of freshly nucleated aerosols.