F. Schröder

On the Transition of Contrails into Cirrus Clouds

In situ observations of the microphysical properties of upper-tropospheric contrails and cirrus clouds have been performed during more than 15 airborne missions over central Europe. Experimental and technical aspects concerning in situ characterization of ice clouds with the help of optical and nonoptical detection methods (preferably FSSP-300 and Hallet-type replicator) are addressed. The development of contrails into cirrus clouds on the timescale of 1 h is discussed in terms of a representative set of number densities, and size distributions and surface area distributions of aerosols and cloud elements, with special emphasis on small ice crystals (diameter ,20 mm). Contrails are dominated by high concentrations (.100 cm23) of nearly spherical ice crystals with mean diameters in the range 1‐10 mm. Young cirrus clouds, which mostly contain small regularly shaped ice crystals in the range 10‐20-mm diameter and typical concentrations 2‐5 cm23, have been observed. Measurement results are compared to simple parcel model calculations to identify parameters relevant for the contrail‐cirrus transition. Observations and model estimates suggest that contrail growth is only weakly, if at all, affected by preexisting cirrus clouds.

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.

Elemental composition and morphology of ice-crystal residual particles in cirrus clouds and contrails

Aircraft sampling of residual particles from evaporated ice crystals was performed using a Counterflow Virtual Impactor. Samples of crystals taken in both contrails and cirrus clouds were compared with interstitial aerosols found in natural cirrus. The samples were analyzed with a scanning electron microscope which was equipped with a windowless energy-dispersive X-ray detector (SEM/EDX). In the contrail and cirrus cases black carbon (BC) particles dominated the residual size spectra for particles smaller than 1 μm. The coarse residual particle mode (D p ≥ 1.5 μm) in contrails consisted almost completely of mechanically generated metallic particles which contributed only about 1% to residual particle number but approximately 50% to residual particle volume. Observed particle number concentrations and BC mass concentration of the residual particles were 0.2 cm -3 and 16 ng m -3 inside the contrail and 0.02 cm -3 and < 2 ng m -3 inside the cirrus. The fraction of BC particles (0.1 μm < D p < 0.8 μm) in the interstitial aerosol samples increased with altitude from < 70% at 8 km to 95% at 11 km near the air-traffic corridors with number concentrations of ≃ 0.1 cm -3 .

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.

Ultrafine aerosol particles in aircraft plumes: Analysis of growth mechanisms

An analysis of in situ measurements of ultrafine volatile aerosols in the plume near field of the DLR aircraft ATTAS using low (0.02 g/kg) and high (2.7g/kg) fuel sulfur contents (FSCs) is presented. The observed growth of nanoparticles (diameter 5–10 nm) is reproduced in detail by a microphysical simulation with chemi-ion emissions of 2.6×1017/kg fuel. Volatile aerosol dynamics is controlled by sulfuric acid (H2SO4) for high FSC, consistent with a S to H2SO4 conversion of 1.8%. The very high conversion for low FSC (55%) prescribed in the model to match the observations contradicts direct in situ measurements of H2SO4 and suggests that species other than H2SO4, likely exhaust hydrocarbons, control particle growth in such cases.

In Situ Observations of the Microphysical Properties of Young Cirrus Clouds

Abstract In situ measurements made in cold (−35° to −60°C) cirrus clouds over southern Germany in March 1994 are presented. The clouds appeared to be in an early stage of their life cycle and their properties in many ways resemble those reported for ice fogs. Crystal concentrations were high (median 2.5 cm−3, STP) and sizes small with a diameter of mean mass of typically 16 μm. The cloud on 18 March presents an interesting case for modeling studies of cirrus formation. On that particular day, the bulk properties of the cloud appeared to be connected to wave structures in the vertical wind field consistent with the Brunt–Vaisala frequency, which gave a corresponding wavelength of 40–50 km. Furthermore, analyses of potential temperature and vertical wind suggested that the vertical displacement producing these clouds was less than 100 m. Size distribution measurements of interstitial particles and crystal residues (particles remaining after evaporation of the crystals) show that small aerosol particles (dia...

In situ measurements of the scattering phase function of stratocumulus, contrails and cirrus

Original measurements were obtained in stratocumulus, contrails and cirrus clouds by using a new optical airborne probe, the ‘Polar Nephelometer’, which is the first airborne instrument to make direct in situ measurements of the scattering phase function of cloud particles over a broad range of sizes (from a few micrometers to about 500 µm diameter). Preliminary measurements show that in stratocumulus water droplet cloud, the measured phase function fits very well with the phase function derived from direct PMS probes measurements. This definitively confirms the reliability of the Polar Nephelometer for airborne measurements. In contrails and natural cirrus, measured scattering phase function indicates major differences with those used in cloud models which assume ice spheres or simple geometric shape of ice particles. These results highlight new potential insights on both modelling of climate processes and methodologies for cloud remote sensing from satellite measurements.

Carbonaceous aerosol in jet engine exhaust: emission characteristics and implications for heterogeneous chemical reactions

Characteristic parameters of black carbon aerosol (BC) emitted from jet engine were measured during ground tests and in-flight behind the same aircraft. Size distribution features were a primary BC mode at a modal diameter D+0.045 lm, and a BC agglomeration mode at D(0.2 lm. The total BC number concentration at the engine exit was 2.9]107 cm~3 with good agreement between model results and in-flight measured number concentrations of non-volatile particles with D*0.014 lm. A comparison between total number concentration of BC particles and the non-volatile fraction of the total aerosol at the exit plane suggests that the non-volatile fraction of jet engine exhaust aerosol consists almost completely of BC. In-flight BC mass emission indices ranged from 0.11 to 0.15 g BC (kg fuel)~1. The measured in-flight particle emission value was 1.75

Aerosol states in the free troposphere at northern midlatitudes

0.15]1015 kg~1 with corresponding ground test values of 1.0—8.7]1014 kg~1. Both size distribution properties and mass emission indices can be scaled from ground test to in-flight conditions. Implications for atmospheric BC loading, BC and cirrus interaction and the potential of BC for perturbation of atmospheric chemistry are briefly outlined. ( 1999 Elsevier Science Ltd. All rights reserved.

On the mechanisms controlling the formation and properties of volatile particles in aircraft wakes

accumulation state, and a cloud-processed state. This interpretation implies that aerosol sources (nucleation from the gas phase) are closely tied to aerosol sinks (cloud scavenging of aerosol particles). Processes that could modify the proposed aerosol life cycle are discussed. Number size and surface area distributions typical for the various aerosol states are examined. The dry aerosol surface area ranges from 1 to 20 mm 2 cm � 3 in the free troposphere and from 2 to 13 mm 2 cm � 3 in the tropopause region. Parameterizations of the aerosol size distributions and total surface area concentrations are provided to facilitate the use of the data in atmospheric models. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry