[6] Hence the topic was of high interest with many open questions: How many soot particles are formed per mass unit of burned fuel and how large are these soot particles? How large is the conversion fraction ε of fuel sulfur to sulfuric acid? How many CIs are emitted and how important are CIs for particle formation? How many volatile particles are formed per mass of burned fuel? What is the impact of fuel sulfur on contrail formation? How important is the FSC for volatile aerosol formation, soot activation, and contrail ice crystal formation?

[2] A series of experiments (SULFUR 1–7, abbreviated as S1–S7) was performed in the years from 1994 to 1999 in order to determine the particle and contrail formation properties of aircraft exhaust plumes for different fuel sulfur content (FSC) and atmospheric conditions. This paper describes the series of experiments and summarizes the results obtained. In particular, the paper discusses the evolution of our understanding of particle formation and contrails as obtained during the course of these and related experiments.

[12] Table 4 lists the parameters during the flight measurements reported. The aircraft were operated mainly in the upper troposphere, with or without contrails or at threshold conditions where contrails were just forming or disappearing. Some of the chased fast aircraft were operated at reduced power settings to let the slower Falcon follow at close distance.

[11] The experiments cover a wide range of FSC values. Aviation fuels are produced with FSC values from near 1 μg/g to an upper limit of 3000 μg/g. The median FSC value of fuels provided for airliners is near 400 μg/g [ IPCC , 1999 ]. In cases S1, S6, and S7 the FSC was varied by using different fuel deliveries. In cases S2–S5, up to 60 kg of dibuthylsulfide (C 8 H 18 S) containing a 22% mass fraction of sulfur were added to one of the fuel tanks to increase the FSC relative to the fuel in other tanks to the desired level. The melting‐, boiling‐, and flame‐point temperatures and the density of the additive (−80°C, 182°C, 62°C, 840 kg m −3 ) are sufficiently similar to those of standard Jet‐A1 kerosene fuel (−50°C, 164 to 255°C, 52°C, 800 kg m −3 ) to allow for reasonable mixing, and the additive can be handled easily. Later, other experimenters used tetrahydrothiophene (C 4 H 8 S; −96°C, 120°C, 12°C, 1000 kg m −3 ) for this purpose [ Cofer et al. , 1998 ; Miake‐Lye et al. , 1998 ] which shows larger differences to kerosene. The FSC was analyzed from fuel samples with standard laboratory methods or inferred from the amount of sulfur added to a reference fuel. The reproducibility of sulfur analysis from the same sample in various laboratories with 95% confidence interval is better than 10% [ Schröder et al. , 2000b ]. Larger differences (maximum of 25%, S5) were found between FSC analysis of fuel samples and FSC values derived from the amount of sulfur added, possibly due to incomplete mixing and partial evaporation of the additive. For the two B747 aircraft and the DC10 during POLINAT the FSC is determined from SO 2 and CO 2 measurements in the plume with about 30% accuracy [ Arnold et al. , 1994 ; Schulte et al. , 1997 ; Schumann et al. , 2000a ].

[10] In most of the experiments the DLR Advanced Technology Testing Aircraft System (ATTAS) was used as the aircraft causing the exhaust, see Table 3 . The ATTAS is a midsized two‐engine jet aircraft of type VFW 614, with Rolls‐Royce SNECMA M45H Mk501 engines, see Figure 1c , built in 1971 [ Busen and Schumann , 1995 ]. In more recent experiments, also younger and larger jet aircraft were investigated; their engines have higher thrust, bypass ratio, and pressure ratio. Higher bypass engines offer higher overall propulsion efficiency causing less waste heat released into the plume gases [ Cumpsty , 1997 ], which impacts contrail formation [ Schumann , 1996a , 2000 ]. The fuel sulfur conversion fraction depends probably on combustion pressure [ Brown et al. , 1996b ], which increases with the pressure ratio.

[9] The observations have been performed with an increasingly refined set of instruments by various partners, see Table 2 , as described in detail in the individual papers listed. The instruments include innovative methods, such as four different mass‐spectrometer instruments of MPIK, and a system of instruments to measure the number, size and volatility of aerosols in the diameter range from 3 nm to 20 μm of DLR‐IPA and partners.

(a) Photo of the Airbus A340 at cruise during S7 measuring about 70 m behind the right turbofans. The nose boom of the Falcon used for turbulence measurements is visible at the lower edge of the photo; (b) the Falcon with particle measurement systems below the wing and aerosol inlet at top of the fuselage during S4; (c) the ATTAS at cruise during S2 burning low (high) fuel sulfur content on left (right) engine; (d) the A340 and the B707, wing by wing, one with, the other without contrails during S7; (e) sample inlet behind the ATTAS engines at ground during S3; (f) Falcon measuring in the B707 plume 70 m behind the engines during S7.

[8] The series of SULFUR experiments, see Table 1 , was initiated by Deutsches Zentrum für Luft‐ und Raumfahrt, Institut für Physik der Atmosphäre (DLR, IPA), and performed in close cooperation with partners, in particular with Max‐Planck‐Institut für Kernphysik (MPIK), Heidelberg, to investigate particle formation from various aircraft, burning fuels with different FSCs during the same flight. The experiments S1–S7 included 10 flights with measurements in the young exhaust plume of various aircraft at separation distances varying from 25 m to about 5 km (plume ages 0.15–30 s, see Figure 1a , for example), and measurements at ground during S3 to S6. The measurements inside and outside of aircraft plumes have been performed on board the Falcon aircraft of DLR, see Figure 1b . Ground experiments were performed to measure particles and particle precursors close to jet engine exit.

3. Results