Reinhold Busen

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 investigated in the exhaust plumes of a twin-engine jet aircraft. Different fuels were used with sulfur mass fractions of 170 and 5500 ppm in the fuel, one lower than average, the other above the specification limit of standard Jet-Al fuel. During various phases of the same flight, the two engines burnt either high- or low-sulfur fuel or different fuels in the two engines. Besides visual, photographic, and video observations from close distance, in situ measurements were made within the plumes at plume ages of 20 to 30 s, at altitudes between 9 and 9.5 km, and temperatures between −49 and −55°C, when the visible contrail was about 2 km long. The data include particle number densities for particles larger than 7 nm, 18 nm, 120 nm, and 1 μm in diameter, together with wind, temperature and humidity measurements. The observations 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 about 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 caused a larger optical thickness of the contrail shortly after onset, with slightly brown-colored contrail when the Sun was behind the observer, and more contrast when viewed against the Sun. The high-sulfur contrail 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 sub visible 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 little larger than the estimated number of soot particles emitted. The high-sulfur plume shows more particles than the low-sulfur plume. The differences are about 25% for particles above 7 nm and about 50% above 18 nm. The results indicate that part of the fuel sulfur is converted to sulfuric acid which nucleates with water vapor heterogeneously on soot or nucleates acid droplets homogeneously which then coagulate partly with soot. During descent through the level of contrail onset, the high-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 high sulfur contents, aircraft generate an invisible aerosol trail which enhances the background level of condensation nuclei, in particular in regions with dense air traffic at northern latitudes and near the tropopause.

Ultrafine particle size distributions measured in aircraft exhaust plumes

Fast-response measurements of particle size distributions were made for the first time in the near-field plume of a Boeing 737–300 aircraft burning fuel with fuel sulfur (S) contents (FSCs) of 56 and 2.6 ppmm, as well as in fresh and dissipating contrails from the same aircraft, using nine particle counters operating in parallel. Nonsoot particles were present in high concentrations, with number maxima at diameters ≤3nm. From these and ancillary measurements we determined the apparent emission index, EI*, or amount produced per kilogram of fuel burned, for particle nuipber, surface, and volume, and the value of η*, the apparent fraction of fuel S found in the particulate phase in the plume assuming the particles were composed of sulfuric acid and water. All of these parameters were functions of the age of the plume since emission, FSC, and presence or absence of contrail. The measurements support the use of values of η* of <10% in numerical models of the effects of the current aircraft fleet on the atmosphere, suggest that non-S species become important contributors to particulate mass at very low FSCs, and place significant constraints on numerical models of plume microphysical processes.

Influence of fuel sulfur on the composition of aircraft exhaust plumes: The experiments SULFUR 1–7

[1] The series of SULFUR experiments was performed to determine the aerosol particle and contrail formation properties of aircraft exhaust plumes for different fuel sulfur contents (FSC, from 2 to 5500 mg/g), flight conditions, and aircraft (ATTAS, A310, A340, B707, B747, B737, DC8, DC10). This paper describes the experiments and summarizes the results obtained, including new results from SULFUR 7. The conversion fraction e of fuel sulfur to sulfuric acid is measured in the range 0.34 to 4.5% for an older (Mk501) and 3.3 ± 1.8% for a modern engine (CFM56-3B1). For low FSC, e is considerably smaller than what is implied by the volume of volatile particles in the exhaust. For FSC � 100 mg/g and e as measured, sulfuric acid is the most important precursor of volatile aerosols formed in aircraft exhaust plumes of modern engines. The aerosol measured in the plumes of various aircraft and models suggests e to vary between 0.5 and 10% depending on the engine and its state of operation. The number of particles emitted from various subsonic aircraft engines or formed in the exhaust plume per unit mass of burned fuel varies from 2 � 10 14 to 3 � 10 15 kg � 1 for nonvolatile particles (mainly black carbon or soot) and is of order 2 � 10 17 kg � 1 for volatile particles >1.5 nm at plume ages of a few seconds. Chemiions (CIs) formed in kerosene combustion are found to be quite abundant and massive. CIs contain sulfur-bearing molecules and organic matter. The concentration of CIs at engine exit is nearly 10 9 cm � 3 . Positive and negative CIs are found with masses partially exceeding 8500 atomic mass units. The measured number of volatile particles cannot be explained with binary homogeneous nucleation theory but is strongly related to the number of CIs. The number of ice particles in young contrails is close to the number of soot particles at low FSC and increases with increasing FSC. Changes in soot particles and FSC have little impact on the threshold temperature for contrail formation (less than 0.4 K). INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); KEYWORDS: chemiion, sulfur, soot, contrail, aircraft, emission

Estimate of diffusion parameters of aircraft exhaust plumes near the tropopause from nitric oxide and turbulence measurements

Horizontal and vertical plume scales and respective diffusivities for dispersion of exhaust plumes from airliners at cruising altitudes are determined from nitric oxide (NO) and turbulence data measured with the DLR Falcon research aircraft flying through the plumes. Ten plumes of known source aircraft were encountered about 5 to 100 min after emission at about 9.4 to 11.3 km altitude near the tropopause in the North Atlantic flight corridor at 8°W on three days in October 1993. The ambient atmosphere was stably stratified with bulk Richardson numbers greater than 10. The measured NO peaks had half widths of 500 to 2000 m with maximum concentrations up to 2.4 parts per billion by volume (ppbv), clearly exceeding the background values between 0.13 and 0.5 ppbv. For analysis the measured plumes are approximated by an analytical Gaussian plume model which accounts for anisotropic diffusion in the stably stratified atmosphere and for shear. Two methods are given to obtain diffusivity parameters from either the individual plume data or the set of all plume measurements. Using estimates of the emitted mass of NO per unit length, the vertical plume width is found to be 140 m on average. This width is related to mixing in the initial trailing vortex pair of the aircraft. The range of the plume data suggests vertical diffusivity values between 0 and 0.6 m2 s−1. The turbulence data exhibit strong anisotropic air motions with practically zero turbulent dissipation and weak vertical velocity fluctuations. This implies very small vertical diffusivities. The horizontal diffusivity is estimated as between 5 and 20 m2 s−1 from the increase of horizontal plume scales with time. For constant diffusivities, shear dominates the lateral dispersion after a time of about 1 hour even for the cases with only a weak mean shear of 0.002 s−1.

Water vapour measurements inside cirrus clouds in Northern and Southern hemispheres during INCA

Water vapour data inside cirrus clouds from in-situ measurements with an aircraft-borne frost-point hygrometer are analysed. These data have been obtained during two field campaigns, performed in the Southern and Northern hemisphere mid latitudes. There were many occurrences of ice supersaturation inside the investigated cirrus, with a higher frequency of occurrences in the Southern Hemisphere. The source of the differences in the humidity data from the two hemispheres is not clear, and it is speculated that these differences may be related to different levels of pollution. A distribution law for the relative humidity inside cirrus clouds is inferred.

Ultrafine aerosol particles in aircraft plumes : In situ observations

Measurements of ultrafine particles in the near field of the DLR research aircraft ATTAS using low (0.02 g/kg fuel) and high (2.7g/kg) fuel sulfur contents (FSCs) are presented. Soot emissions of ∼ 1015/kg show no significant dependence on FSC. Strong evidence is found that ∼ 1/3 of the soot particles must be involved in ice nucleation in contrails, in addition to freezing of newly formed volatile particles. In the absence of contrails, numbers of volatile particles with diameters D > 5 nm reach ∼ 1017/kg for high FSC, and still reach ∼ 1016/kg for low FSC. A clear contribution of H2SO4 to volatile particle growth is observed. If growth is exclusively linked to H2SO4, the S to H2SO4 conversion efficiency increases with decreasing FSC. Depletion of ultrafine particles is observed in contrails, very likely due to scavenging by contrail ice crystals.

Near‐field measurements on contrail properties from fuels with different sulfur content

Microphysical properties of jet exhaust aerosol and contrails were studied in the near field of the emitting aircraft for different fuel sulfur contents. Measurements were performed behind two different aircraft (ATTAS test aircraft of type VFW 614 and Airbus A310-300) using fuels with sulfur contents of 6 ppm and 2700 ppm, respectively. At closest approach (plume age ‹ 1 s), the total number concentrations exceeded the measuring range of the condensation particle counter, i.e., N › 10 5 cm -3 . The concentration of the dry accumulation mode aerosol, i.e., predominantly soot particles, was not affected by the fuel sulfur content. At a plume age of 10 s, an increase in total number concentration (D p › 0.01 µm) by a factor of 3.5 in the high sulfur case compared to the low sulfur case was observed. The ultrafine condensation nuclei fraction (0.007 µm ‹ D p ‹ 0.018 µm) contributed at maximum 70% to the total aerosol in the plume while this fraction was much less outside the plume. The high fuel sulfur content also caused an increase in the typical number concentrations of contrail particles by about one third with respect to low sulfur fuel, while the effective diameter of the size distribution was lowered at a fuel sulfur independent ice water content. The major differences in accumulation mode aerosol and microphysical contrail properties between the used aircraft were an increased number concentration of both the accumulation mode aerosol and the contrail particles in the Airbus A310-300 plume relative to the ATTAS plume. Part of the difference in contrail particles may be caused by different ambient conditions, but the major differences are assumed to be caused by different engine and wake properties.

Physicochemistry of aircraft‐generated liquid aerosols, soot, and ice particles: 2. Comparison with observations and sensitivity studies

Results from a coupled microphysical-chemical-dynamical trajectory box model have been compared to recent in situ observations of particles generated in the wake of aircraft. Sulfur emissions mainly cause the formation of ultrafine volatile particles in young aircraft plumes (mean number radius ∼5 nm). Volatile particle numbers range between 1016 and 1017 per kg fuel burnt for average to high fuel sulfur levels, exceeding typical soot emission indices by a factor of 10–100. Model results come into closer agreement with observations when chemi-ions from fuel combustion are included in the aerosol dynamics. Ice particles (mean number radius 1 μm.) crystals. Contrails with larger crystals would also form without soot and sulfur emissions. The lifecycle of cirrus clouds can be modified by exhaust aerosols.

Anatomy of cirrus clouds: Results from the Emerald airborne campaigns

The Emerald airborne measurement campaigns have provided a view of the anatomy of cirrus clouds in both the tropics and mid-latitudes. These experiments have involved two aircraft that combine remote sensing and in-situ measurements. Results are presented here from two separate flights: one in frontal cirrus above Adelaide, Australia, the other in the cirrus outflow from convection above Darwin. Recorded images of ice crystals are shown in relation to the cloud structure measured simultaneously by an airborne lidar. In mid-latitude frontal cirrus, columnar and irregular ice crystals were observed throughout the cloud while rosettes were found only at the top. The cirrus outflow from a tropical thunderstorm extended for hundreds of kilometres between the heights of 12.2 and 15.8 km. This was composed mainly of hexagonal plates, columns, and large crystal aggregates that originated from within the main core region of the convection. A small number of bullet rosettes were found at the top of the outflow cirrus and this is interpreted as an indication of in-situ crystal formation. It was found that the largest aggregates fell to the lower regions of the outflow cirrus cloud while the single crystals and small aggregates remained at the top.

First direct sulfuric acid detection in the exhaust plume of a jet aircraft in flight

Sulfuric acid (SA) was for the first time directly detected in the exhaust plume of a jet aircraft in flight. The measurements were made by a novel aircraft-based VACA (Volatile Aerosol Component Analyzer) instrument of MPI-K Heidelberg while the research aircraft Falcon was chasing another research aircraft ATTAS. The VACA measures the total SA in the gas and in volatile submicron aerosol particles. During the chase the engines of the ATTAS alternatively burned sulfur-poor and sulfur-rich fuel. In the sulfur-rich plume very marked enhancements of total SA were observed of up to 1300 pptv which were closely correlated with ΔCO2 and ΔT and were far above the local ambient atmospheric background-level of typically 15-50 pptv. Our observations indicate a lower limit for the efficiency ɛ for fuel-sulfur conversion to SA of 0.34 %.