The microphysical pathway to contrail formation

Abstract

A conceptual framework to predict microphysical and optical properties of contrail particles within a wingspan behind the source aircraft is developed. Results from two decades of contrail observations and numerical simulations are reviewed forming the basis of theoretical model development. The model utilizes cloud theory applied to the dynamics and thermodynamics of jet aircraft exhaust plumes in upper tropospheric conditions. Droplet nuclei include soot particles emitted from aircraft engines and atmospheric particles entrained into the plume. These precursor particles activate into copious homogeneously freezing water droplets as the plume relative humidity rises beyond liquid water saturation. A unimodal size spectrum of ice particles develops wherein ice particles grow to micrometer mean sizes. Contrail particle formation is analyzed over a wide range of soot emissions relating to conventional jet fuels as well as to alternative aviation fuels producing much less soot and volatile particle emissions. For current aviation fuels and propulsion technology, the number of contrail ice particles scales roughly in proportion to the number of emitted soot particles that act as water condensation nuclei despite their poor hygroscopicity. Close to the contrail formation threshold, only few plume particles can be water activated and freeze. Implications for effects of alternative fuels on contrails, an arena for future scientific exploration, are outlined.

Variability in Contrail Ice Nucleation and Its Dependence on Soot Number Emissions

“…Plume aerosol particles have varying sizes and properties with larger and more hygroscopic particles activating into droplets first (Köhler, ) depleting supersaturation ( s w ). The basic assumption of the parameterization of Kärcher et al () is that all droplets in the rapidly cooling plume form at the same time, ( t o ), at the “activation relaxation time.” The parameterization determines the droplet number concentration ( n(t o ): = n o ) which is needed to balance a further increase of s w by condensation loss. This implies two approximations: The first one is that quenching of s w is assumed to occur instantly since the remaining smaller and less hydrophilic particles cannot be activated any more once s w is decreasing.…”

“…Dry core radii r act,l of each particle type l that can activate into water droplets at certain plume supersaturation s w are obtained from Kappa‐Köhler theory (Petters & Kreidenweis, ) by where r K ~1 nm is the fixed Kelvin radius and κ l denotes the hygroscopicity parameter characterizing the water uptake and activation behavior of the particle type l depending on its chemical composition. Please note that we use the parameterization from Kärcher et al () but we calculate the dry core radius for activation according to Petters and Kreidenweis (). This means that we keep the factor 4/27 under the third root consistent with prescribing the Kelvin radius.…”

“…Typical values of κ derived from laboratory measurements range between 0.5 and 1.4 for hydrophilic salts, between 0.01 and 0.5 for slightly to very hygroscopic organic species, and are zero for purely hydrophobic compounds (Petters & Kreidenweis, ). Following Kärcher et al (), we assume fully soluble upper tropospheric background particles with an average κ a of 0.5 and soot particles with a soluble volume fraction of 1% and an effective κ s of 0.005. Equation indicates that the smaller and less hygroscopic particles need higher plume supersaturations to be activated.…”

Mitigating the Climate Impact from Aviation: Achievements and Results of the DLR WeCare Project

“…Note that changes in particulate emissions, such as soot, were not taken into account. A reduction in the soot number densities from lean combustion would result in changes of the properties of the contrails (lower ice particle densities, lower shortwave optical thickness, and enhanced sedimentation of ice particles), which would lead to a much larger decrease of the climate impact by contrails [136][137][138]. A detailed description of the study can be found in [135].…”

Contrail Modeling and Simulation

“…One remaining unknown pointed out in the latter work is the role played by ultrafine droplets consisting of organics. The problem of nucleation of ice particles in contrails was recently revisited by Kärcher et al (2015).…”