HALO

AERO-CLOUD​

Aircraft aerosol effects on high and low clouds

Mission Status: in planning

Persons in Charge

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Project description

Short summary: AERO-CLOUD will investigate aerosol effects on high and low clouds and on climate with a focus on aircraft emissions

Scientific rationale: Today, air traffic contributes about 5% to the global anthropogenic radiative forcing, mainly originating from contrail cirrus, aircraft CO 2 emissions and other minor components. This estimate does not include indirect aerosol effects on clouds, as neither the indirect effects of soot and volatile emissions on high altitude cirrus clouds nor effects of fuel sulfur emissions on low liquid clouds are adequately understood – hence preventing their accurate quantification. Indirect aerosol effects on high clouds include soot aerosol emissions modifying existing cirrus or fostering additional cirrus cloud formation. This effect could possibly be enhanced by pre-activation of soot due to processing in contrails. In addition, volatile particles formed and processed in the aircraft exhaust plume may be transported to the (subtropical) lower troposphere, where they might affect low clouds.

Fig.1: Aviation-induced radiative forcing from different components, adapted from Grewe et al., Aerospace, 2017

Both effects could have a potentially high impact on the aviation’s climate impact but are poorly confined by experimental evidence and hence render the major open science questions in aviation climate research. Some model estimates might even tip the originally positive sign of aircraft’s climate impact into a negative one when the aerosol effects are taken into account (Figure 1).While some model studies reveal a very large cooling due to soot-induced cirrus modifications, such effects could not be identified in other studies. Large model uncertainties still result from the representation of aerosols, cirrus clouds, and dynamical forcing. Existing model quantifications of the potentially large cooling effect of aerosol-driven changes of low clouds suffer from uncertain representations of aerosol processing in aircraft plumes, aerosol long-range transport, and aerosol-induced liquid cloud formation. New insights from observations are urgently required to constrain model simulations towards more robust quantifications of the
aerosol climate effects.

Objectives
Experimental evidence for sulfur effects on low clouds is missing, as week-long transport times of the particulate and trace gas aircraft engine emissions to the lower troposphere prevent the exact attribution of measured low-level cloud data to a specific aircraft source high up. However, the transport scenarios strongly depend on the processing and the aerosol size distribution in the aged aircraft exhaust, which is measurable by instruments on HALO.
Very few experimental studies investigate changes in natural cirrus clouds by aircraft soot emissions into clear air outside of contrails. Ström and Ohlson (1998) find enhanced particle number densities in perturbed cirrus which contain higher levels of absorbing material. However, a direct source attribution is missing. Recently, Urbanek et al. (2018) find enhanced particle linear depolarization ratios in cirrus which were possibly stronger affected by aircraft emissions. Contrasting observations and aerosol characterizations upstream of cirrus regions and within the cirrus in regions strongly or minor polluted by aircraft emissions could help to answer questions on indirect aviation aerosol effects on high clouds.

The mission will address the following topics:
– Processing of aerosol in aircraft exhaust
– (Aircraft) aerosol effects on low clouds
– (Aircraft) aerosol effects in high clouds

Partners

  • LR, Institute for Atmospheric Physics
  • University of Mainz, Institute for Atmospheric Physics
  • Max-Planck-Institute for Chemistry
  • Karlsruhe Institute of Technology (KIT)
  • Institute of Meteorology and Climate Research, Karlsruhe
  • Research Center Jülich, Institute for Energy and Climate Research
  • University of Leipzig, Leipzig Institute for Meteorology
  • Leibniz Institute for Tropospheric Research
  • Ludwig-Maximilians-University
  • Goethe University, Institute for Atmospheric and Environmental Sciences

Scientific instruments and payload configuration

  • List of scientific instruments for the mission:

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Cabin and exterior configuration of HALO for the mission

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HALO flights for this mission

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More information

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Press releases, media etc

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