U. Schumann

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.

The Initial Composition of Jet Condensation Trails

Abstract Physicochemical processes that generate and transform aerosols in jet aircraft plumes are discussed on the basis of theoretical models and recent observations of young contrails in the upper troposphere. The initial evolution of optical depth and ice water content under threshold contrail formation conditions is studied. Constrained by the measurements, a lower bound is deduced for the number density of ice crystals initially present in contrails. This bound serves as a visibility criterion for young contrails. An analysis of the primary contrail particles (aqueous solution droplets nucleated in situ, emitted insoluble combustion aerosols, and entrained background aerosols) reveals that only soot must he involved as ice forming nuclei if the visibility criterion is to be fulfilled. Possible activation pathways of the soot aerosols are investigated, including an analysis of their wetting behavior and droplet scavenging and heterogeneous nucleation properties. To support these investigations, resul...

In Situ Observations of Air Traffic Emission Signatures in the North Atlantic Flight Corridor

Focussed aircraft measurements have been carried out over the eastern North Atlantic to search for signals of air traffic emissions in the flight corridor region. Observations include NO, NO2, HNO3, SO2, O3, H2O, total condensation nuclei (CN), and meteorological parameters. A flight pattern with constant-altitude north-south legs across the major North Atlantic air traffic tracks was flown. Signatures of air traffic emissions were clearly detected for NOx, SO2, and CN with peak concentrations of 2 ppbv, 0.25 ppbv, and 500 cm−3, respectively, exceeding background values by factors of 30 (NOx), 5 (SO2), and 3 (CN). The observed NOx, SO2, and CN peaks were attributed to aircraft plumes based on radar observations of the source air traffic and wind measurements. Major aircraft exhaust signatures are due to relatively fresh emissions, i.e., superpositions of 2 to 5 plumes with ages of about 15 min to 3 hs. The observed plume peak concentrations of NOx compare fairly well with concentrations computed with a Gaussian plume model using horizontal and vertical diffusivities as obtained by recent large-eddy simulations, measured vertical wind shear, and the corridor air traffic information. For the major emission signatures a mean CN/NOx abundance ratio of 300 cm−3ppbv−1 was measured corresponding to an emission index (EI) of about 1016 particles per 1 kg fuel burnt. This is higher than the expected soot particle EI of modern wide-bodied aircraft. For the most prominent plumes no increase of HNO3 concentrations exceeding variations of background values was observed. This indicates that only a small fraction of the emitted NOx is oxidized in the plumes within a timescale of about 3 hs for the conditions of the measurements.

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.

Operational prediction of ash concentrations in the distal volcanic cloud from the 2010 Eyjafjallajökull eruption

[1] During the 2010 eruption of Eyjafjallajokull, improvements were made to the modeling procedure at the Met Office, UK, enabling peak ash concentrations within the volcanic cloud to be estimated. In this paper we describe the ash concentration forecasting method, its rationale and how it evolved over time in response to new information and user requirements. The change from solely forecasting regions of ash to also estimating peak ash concentrations required consideration of volcanic ash emission rates, the fraction of ash surviving near-source fall-out, and the relationship between predicted mean and local peak ash concentrations unresolved by the model. To validate the modeling procedure, predicted peak ash concentrations are compared against observations obtained by ground-based and research aircraft instrumentation. This comparison between modeled and observed peak concentrations highlights the many sources of error and the uncertainties involved. Despite the challenges of predicting ash concentrations, the ash forecasting method employed here is found to give useful guidance on likely ash concentrations. Predicted peak ash concentrations lie within about one and a half orders of magnitude of the observed peak concentrations. A significant improvement in the agreement between modeled and observed values is seen if a buffer zone, accounting for positional errors in the predicted ash cloud, is used. Sensitivity of the predicted ash concentrations to the source properties (e.g., the plume height and the vertical distribution of ash at the source) is assessed and in some cases, seemingly minor uncertainties in the source specification have a large effect on predicted ash concentrations.

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.

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.

NO x emission indices of subsonic long‐range jet aircraft at cruise altitude: In situ measurements and predictions

In the course of the Commissions of the European Communities project “Pollution From Aircraft Emissions in the North Atlantic Flight Corridor (POLINAT)”, in situ measurements of NO, NOx , and CO2 volume mixing ratios in the near-field exhaust plumes of seven subsonic long-range jet aircraft have been carried out by using the research aircraft Falcon of the Deutsche Forschungsanstalt fur Luft- und Raumfahrt (DLR). For three additional aircraft, only NO and CO2 were measured. Plume ages of 50 s to 150 s have been covered, with maximum observed exhaust gas enhancements of 319 parts per billion by volume and 51 parts per million by volume for Δ[NOx] and Δ[CO2], respectively, in relation to ambient values. Aircraft cruising altitudes and Mach numbers ranged from 9.1 to 11.3 km and from 0.77 to 0.85, respectively. These measurements are used to derive NOx emission indices for seven of the individual aircraft/engine combinations. The NOx emission indices derived range from 12.3 g/kg to 30.4 g/kg. They are compared with predicted emission index values, calculated for the same aircraft engine and the actual conditions by using two newly developed fuel flow correlation methods. The calculated emission indices were mostly within or close to the error limits of the measured values. On average, the predictions from both methods were 12% lower than the measured values, with an observed maximum deviation of 25%. The ratio γ = [NO2]/[NOx] found during the present measurements ranged from 0.06 to 0.11 for five daytime cases and was around 0.22 for two nighttime cases. By use of a simple box model of the plume chemistry and dilution these data were used to estimate the initial value γ0 present at the engine exit plane. We found γ0 values between 0 and 0.15. These were applied to estimate the corresponding NO2 for the three cases in which only NO was measured.

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 %.