Massive positive and negative chemiions in the exhaust of an aircraft jet engine at ground-level: mass distribution measurements and implications for aerosol formation

Abstract

Mass distributions of positive and negative chemiions (CI) were measured in the exhaust plume of an aircraft jet engine at the ground at exhaust gas ages around 0.1 s using an ion mass spectrometer with a mass range of 8500 amu (large ion mass spectrometer, LIOMAS). Most of the CI had mass numbers m<2000, but very massive CI with m up to at least 8500 were also observed. About 1% of the negative CI and 0.2% of the positive CI had m>8500. An increase of the fuel sulfur content from 2 to 66 mg/kg did not change the ion mass distributions. This indicates that most of the observed CI did not contain sulfur-bearing molecules, but probably contained low volatility organic compounds (LVOC). An LVOC-emission index of 22.5 mg LVOC/kg fuel is inferred from our data.

Introduction

Ions formed in the combustion process of jet engines by radical radical reactions, so-called chemiions (CI), are potentially important as they may play a role in the formation of aerosol particles. Upon mutual recombination of a positive and a negative ion, a stable quasi-neutral aerosol particle may be formed (cf. Arnold, 1980) which spontaneously grows by condensation of a supersaturated gas. These aerosol particles may survive long enough to act as potential cloud condensation nuclei (CNN) (Yu and Turco, 1999).

CI in jet engine exhaust and their potential role in aerosol formation were originally discussed by Frenzel and Arnold (1994). Mass spectrometric CI measurements made by these authors in jet fuel combustion revealed that the most abundant negative CI with mass numbers m⩽700 were cluster ions containing sulfuric acid. Additional negative CI composition measurements made by the MPIK (Max-Planck Institut für Kernphysik, MPI for nuclear physics) group at the ground (Arnold et al., 1998; Kiendler et al., 2000; Kiendler and Arnold, 2001) strengthened the above findings, but additionally also detected organic negative CI.

First quantitative model calculations of CI-induced aerosol formation in the wake of a jet aircraft were reported by Yu and Turco (1997) suggesting that larger volatile aerosols in jet engine exhaust are formed preferably via CI. Further model calculations (Yu and Turco, 1998; Kärcher et al., 1998; Yu et al., 1999; Turco et al., 1998) support this hypothesis. Importantly, Yu and Turco (1999) recently suggested that aircraft-produced CI may even lead to CCN.

Recently, CI were detected for the first time in the wake of a jet aircraft in flight at plume ages t_p around 1 s by the MPIK-Heidelberg group (Arnold et al., 1999; Wohlfrom et al., 2000) and were found to have m (mass number) of at least up to 8500. These measurements indicated that for low fuel sulfur contents (FSC=2 mg/kg) massive CI are also present and are probably composed of hydrocarbon molecules. For FSC=118 mg/kg significant additional CI growth was found at least for negative CI which most likely was induced by H₂SO₄/H₂O clustering to CI (Wohlfrom et al., 2000). Also recently, the first total CI concentration measurements in jet engine exhaust at the ground were reported (Arnold et al., 2000). The initial total CI concentrations were found to be of the order of 10⁸–10⁹ cm^–3 and to decrease rapidly with increasing plume age t_p. As proposed by the authors this decrease is mostly due to ion–ion recombination. Support for this view was, recently, provided by model calculations of Sorokin and Mirabel (2001).

The present paper reports on the first measurements of the mass distributions of very massive positive and negative CI in the very young exhaust plume of an aircraft engine at the ground using an ion mass spectrometer with a very large mass range of up to 8500 amu and a low mass resolution. The instrument is named large ion mass spectrometer (LIOMAS). The LIOMAS measurements were accompanied by ion composition measurements using an ion trap mass spectrometer (PIT-MAS; with a high mass resolution), which will be presented in separate papers (Kiendler and Arnold (2001), Kiendler and Arnold (2002)).