Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
observations show that aircraft exhaust does not contain large numbers of ice nuclei active
at temperatures above -35°C (Rogers et al., 1998). Soot and metals were found to be
significant, but not dominant, components of ice nuclei in contrails (Chen et al., 1998;
Petzold et al., 1998; Twohy and Gandrud, 1998).
Aircraft measurements in and near clouds have indicated the presence of light-absorbing
material contained inside ice crystals. The distribution pattern and the amount of measured
absorbers suggest that the material is related to aircraft soot (Ström and Ohlsson, 1998)
(Figure 3-17). For the same abundance of aerosol particles, clouds perturbed by absorbing
material contained 1.6 to 2.8 times more ice crystals than unperturbed portions of clouds.
These observations suggest that aircraft-produced particles enhance cloud ice particle
concentrations, although they have not revealed the physical mechanism involved.
Specifically, exhaust soot particles may have been involved in ice crystal formation within
the cirrus or formed contrail ice particles within the exhaust plume before being incorporated
into the cloud. These observations are roughly consistent with calculations of a cirrus cloud
forming in a region of recent exhaust trails (Jensen and Toon, 1997).
Cirrus clouds may also be perturbed by enhanced sulfate aerosol. Small sulfate particles (e.
g., 10-nm radius) are unlikely to be ice nuclei or cloud condensation nuclei. Larger particles
would be more efficient. Such large particles may originate as ambient sulfate particles
enlarged by growth from sulfur gases emitted by aircraft or by processing of liquid exhaust
particles in short-lived contrails (see Section 3.2). Because ice nucleation processes tend to
be self-limiting (Jensen and Toon, 1994), cirrus cloud properties would change only slightly
if the number of sulfate particles increased while the size distribution and composition of
ambient sulfate particles remained constant. On the other hand, large sulfate particles-such
as those induced by the Mt. Pinatubo volcanic cloud-could lead to a significant increase in
the number of ice crystals, optical depth, and radiative forcing of cirrus clouds (Jensen and
Toon, 1992). Cloud modifications would require a large increase in the number of larger
sulfate particles; such modifications would occur primarily in cirrus that were very cold (T < -
50°C) and weakly forced by slow updrafts. Lidar observations of a cirrus cloud embedded in
aerosol from the Mt. Pinatubo eruption show significantly higher than normal ice crystal
concentrations (Sassen, 1992; Sassen et al., 1995). Satellite data analyzed after the Mt.
Pinatubo eruption suggest that the volcanic aerosol reduced the occurrence of cirrus clouds
with high extinction coefficients, increased the occurrence of clouds with low extinction
coefficients, and increased extinction in optically thick clouds (Minnis et al., 1993; Wang et
al., 1995). Hence, some available models and data suggest that aircraft exhaust could play
a significant role in modifying the properties of clouds. However, the magnitude and nature
of these modifications are not well understood.
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Figure 3-18: Change in cirrus occurrence frequency between 1987-
1991 and 1982-1986 (in %) computed from surface observations over
the North Atlantic Ocean averaged over grid boxes with fuel
consumption above 8 km greater than indicated on the x-axis (open
circles).
Figure 3-19: Observed monthly variation of contrail occurrence
frequency (solid squares) and seasonal trends of cirrus cover (open
circles) for (a) the United States of America (%/yr) and (b) Europe (%/