Process-oriented analysis of aircraft soot-cirrus interactions constrains the climate impact of aviation

Abstract

Fully accounting for the climate impact of aviation requires a process-level understanding of the impact of aircraft soot particle emissions on the formation of ice clouds. Assessing this impact with the help of global climate models remains elusive and direct observations are lacking. Here we use a high-resolution cirrus column model to investigate how aircraft-emitted soot particles, released after ice crystals sublimate at the end of the lifetime of contrails and contrail cirrus, perturb the formation of cirrus. By allying cloud simulations with a measurement-based description of soot-induced ice formation, we find that only a small fraction (<1%) of the soot particles succeeds in forming cloud ice alongside homogeneous freezing of liquid aerosol droplets. Thus, soot-perturbed and homogeneously-formed cirrus fundamentally do not differ in optical depth. Our results imply that climate model estimates of global radiative forcing from interactions between aircraft soot and large-scale cirrus may be overestimates. The improved scientific understanding reported here provides a process-based underpinning for improved climate model parametrizations and targeted field observations.

An Observational Constraint on Aviation‐Induced Cirrus From the COVID‐19‐Induced Flight Disruption

“…We have exclusively looked at the changes in cirrus extent; ERF also depends on the cirrus' optical thickness (Burkhardt & Kärcher, 2011 ; Kärcher, 2018 ) and on what surface the cirrus overlays (Yang et al., 2010 ). Optical thickness is in turn a function of ice crystal properties (shape, size, and number) and of ambient conditions (e.g., temperature, humidity, wind shear) (Bock & Burkhardt, 2016a ; Kärcher et al., 2009 , 2021 ; Yang et al., 2010 ). The net effect of these properties may not be independent of cirrus coverage.…”

“…Because aircraft emission plumes may be temporarily supersaturated with respect to ice (Minnis et al., 2004 ), contrails can form in conditions where natural cirrus cannot and therefore have the potential to substantially impact regional high‐level cloudiness (Burkhardt & Kärcher, 2011 ; Sassen, 1997 ). High‐level cloud is also affected by the aerosols emitted by aircraft, which can serve as ice nuclei; the ice nucleation efficiency of black carbon, in particular, remains uncertain (Karcher et al., 2007 ; Kärcher et al., 2021 ; Voigt et al., 2021 ). Finally, the formation of contrail cirrus can compete with natural cirrus for water vapor, reducing the formation of the latter (Burkhardt & Kärcher, 2011 ).…”

Aerosol-cloud interactions: The representation of heterogeneous ice activation in cloud models

“…Figure 4 shows size-resolved and size-integrated cumulative and differential AFs of aircraft soot particles processed in contrails (Kärcher et al, 2021). Size-resolved cumulative AFs decrease strongly with mobility diameter from 400 nm to 100 nm https://doi.org/10.5194/acp-2021-511 Preprint.…”

Aerosol–cloud interactions: the representation of heterogeneous ice activation in cloud models

“…Heterogeneous freezing curves become even broader when a mixture of different INP types is investigated. Yet, when one and the same droplet is repeatedly probed in freezingthawing cycles during refreeze experiments, freezing occurs in a temperature range that is similarly narrow to that for homogeneous freezing (Kaufmann et al, 2017).…”