IN-SITU OBSERVATIONS OF PARTICLES IN JET AIRCRAFT EXHAUSTS AND CONTRAILS FOR DIFFERENT SULFUR-CONTAINING FUELS

The impact of sulfur oxides on particle formation and contrails is inv estigated in the exhaust plumes of a twin-engine jet aircraft. Differe nt fuels were used with sulfur mass fractions of 170 and 5500 ppm in t he fuel, one lower than average, the other above the specification lim it of standard Jet-Al fuel. During various phases of the same flight, the two engines burnt either high- or low-sulfur fuel or different fue ls in the two engines. Besides visual, photographic, and video observa tions from close distance, in situ measurements were made within the p lumes at plume ages of 20 to 30 s, at altitudes between 9 and 9.5 km, and temperatures between -49 and -55 degrees C, when the visible contr ail was about 2 km long. The data include particle number densities fo r particles larger than 7 nm, 18 nm, 120 nm, and 1 mu m in diameter, t ogether with wind, temperature and humidity measurements. The observat ions show visible and measurable differences between contrails caused by the different sulfur levels. At ambient temperatures 5 K below the threshold temperature for contrail onset, the plume became visible abo ut 10 m after the engine exit for high sulfur content, but 15 m after the engine exit for low sulfur content. The higher sulfur emission cau sed a larger optical thickness of the contrail shortly after onset, wi th slightly brown-colored contrail when the Sun was behind the observe r, and more contrast when viewed against the Sun. The high-sulfur cont rail grew more quickly but also evaporated earlier than the low-sulfur contrail. At plume ages of about 20 s, each engine plume was diluted to an effective diameter of 20 m. The plumes contained many subvisible particles. Peak number densities were 30,000 cm(-3) for particles of diameter above 7 nm and 15,000 cm(-3) above 18 nm. The latter is a lit tle larger than the estimated number of soot particles emitted. The hi gh-sulfur plume shows more particles than the low-sulfur plume. The di fferences are about 25% for particles above 7 nm and about 50% above I s nm. The results indicate that part of the fuel sulfur is converted t o sulfuric acid which nucleates with water vapor heterogeneously on so ot or nucleates acid droplets homogeneously which then coagulate partl y with soot. During descent through the level of contrail onset, the h igh-sulfur contrail remained visible at slightly lower altitude (25 to 50 m) or higher temperature (0.2 to 0.4 K). At least for average to h igh sulfur contents, aircraft generate an invisible aerosol trail whic h enhances the background level of condensation nuclei, in particular in regions with dense air traffic at northern latitudes and near the t ropopause.