Bernd Kärcher

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...

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 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.

Physically based parameterization of cirrus cloud formation for use in global atmospheric models

Motivated by the need to study the climatic impact of aerosol-related cirrus cloud changes, a physically based parameterization scheme of ice initiation and initial growth of ice crystals in young cirrus clouds has been developed. The scheme tracks the number density and size of nucleated ice crystals as a function of vertical wind speed, temperature, ice saturation ratio, aerosol number size distributions, and preexisting cloud ice, allowing for competition between heterogeneous ice nuclei and liquid aerosol particles during freezing. Predictions of the parameterization are compared with numerical parcel simulations of ice nucleation and growth from atmospheric aerosols, with a special focus on explaining the indirect effects of ice nuclei on the properties of young cirrus clouds. The uncertainties of the parameterization are discussed and its implementation in a general circulation model is briefly outlined. This new scheme establishes a flexible framework for a comprehensive assessment of indirect aerosol effects on and properties of cirrus clouds in global climate, chemistry transport, and weather forecast models.

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

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.

Role of aircraft soot emissions in contrail formation

The susceptibility of microphysical properties of young contrails to changes in aircraft soot emissions is studied with a microphysical plume model. Liquid plume and ambient particles compete with exhaust soot particles for the formation of contrail ice particles, assuming that soot particles are activated into water droplets prior to homogeneous freezing. Soot controls ice formation in contrails for high number emission indices including the range of current global fleet values. A fivefold reduction of soot emissions from average levels of 5 x 10 14 - 10 15 (kgfuel) -1 approximately halves the initial contrail visible optical depth. Further soot reduction reverses this trend at temperatures well below the formation threshold temperature unless emissions of sulfur and organics are cut substantially. Whether and to which degree reductions in soot emissions help mitigate the contrail climate impact depends on subsequent aircraft wake vortex processing of contrails and their development into contrail cirrus.

Nitric Acid Uptake on Subtropical Cirrus Cloud Particles

The redistribution of HNO 3 via uptake and sedimentation by cirrus cloud particles is considered an important term in the upper tropospheric budget of reactive nitrogen. Numerous cirrus cloud encounters by the NASA WB-57F high-altitude research aircraft during the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE) were accompanied by the observation of condensed-phase HNO 3 with the NOAA chemical ionization mass spectrometer. The instrument measures HNO 3 with two independent channels of detection connected to separate forward and downward facing inlets that allow a determination of the amount of HNO 3 condensed on ice particles. Subtropical cirrus clouds, as indicated by the presence of ice particles, were observed coincident with condensed-phase HNO 3 at temperatures of 197-224 K and pressures of 122-224 hPa. Maximum levels of condensed-phase HNO 3 approached the gas-phase equivalent of 0.8 ppbv. Ice particle surface coverages as high as 1.4 x 10 14 molecules cm -2 were observed. A dissociative Langmuir adsorption model, when using an empirically derived HNO 3 adsorption enthalpy of -11.0 kcal mol -1 , electively describes the observed molecular coverages to within a factor of 5. The percentage of total HNO 3 in the condensed phase ranged from near zero to 100% in the observed cirrus clouds. With volume-weighted mean particle diameters up to 700 μm and particle fall velocities up to 10 m s -1 , some observed clouds have significant potential to redistribute HNO 3 in the upper troposphere.

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.