The recent SASS and European assessment reports indicate that progress has been made in some of the above areas. However, important details still remain poorly understood. This section addresses several issues related to aerosols and contrails that require more investigation. Among them are the emission, formation, and ambient distribution of sulfate and carbon aerosols, the formation of contrails and their role in cirrus-cloud development, and the heterogeneous chemistry occurring on all of these surfaces.

Friedl (1997) notes a major difficulty in quantitatively understanding how sulfur compounds in aircraft fuel are converted to sulfate in the aircraft engine. The sulfur content of jet-aircraft fuels varies with each fuel's specifications, but it is commonly around 200 ppm by weight or lower. As the fuel is burned in the engine, the chemically bound sulfur compounds occurring in the hydrocarbon fuel matrix form SO₂, SO₃, and H₂SO₄ and its hydrates.

An experimental determination of aerosols in aircraft wakes was made in 1995, when an ER-2 aircraft was able to sample the exhaust plume of a Concorde aircraft (Fahey et al., 1995a). A huge number of aerosol particles was found in the plume with peak values ranging up to 15,000 particles/cm³ (the background concentration was approximately 6–18 particles/cm³). Heated at 192°C, a large fraction of these submicron particles was volatilized, and their composition was consistent with that of sulfuric acid. While the Concorde engine design is quite different than that of today's subsonic fleet, these studies are still relevant to SASS, as they confirmed that a large number of aerosols can be generated in the aircraft wake by the simultaneous condensation of sulfuric acid and water vapors (heteromolecular nucleation), even if the local atmosphere is under saturated with water vapor. Similar conclusions were reached by Schlager et al. (1997) when they sampled flight corridors in the troposphere and in the stratosphere, showing the similarity between supersonic and subsonic plumes with regard to aerosol formation.

Several theoretical studies have been performed to predict aerosol production in aircraft wakes, including those of Miake-Lye et al. (1993, 1994), Zhao and Turco (1995), Kärcher (1995), Danilin et al. (1997), Yu and Turco (1997), and Taleb et al. (1997). The mechanism of aerosol production involves the formation of sulfuric acid in the jet regime, followed by its condensation with water vapor (heteromolecular nucleation). The rate of formation of the aerosols is generally calculated according to the "classical" theory of (binary) nucleation. When this theory is applied to aircraft wakes, nucleation rates as high as 10¹²/cm³ are predicted, depending on the sulfur content of the fuel. However, it has been shown that the steady-state assumption used in the classical theory is not valid (Taleb et al., 1997), and that a certain delay is needed to reach the steady state, the net effect being a reduction of the number density of the newly formed particles.