well-mixed anthropogenic carbon dioxide concentration addition. Aviation CO 2 will contribute
to the entire CO 2 burden of the atmosphere like
all other emissions into the atmosphere close to
the surface of the Earth. For the second major
emission, H 2 O, the point where it is emitted is
decisive: At the surface this addition is negligible
in comparison to the natural fluxes of water, but
not so at about 10 km height at a water vapor
mixing ratio ranging from about 100 ppmv to as
low as 5 ppmv. Flying by 1 km higher or lower in
heights around the tropopause (the interface
between the troposphere and the stratosphere;
typically at 10 km height in mid-latitudes), however, can often avoid a long-lasting linear contrail (for more see section “Aviation Induced
Cloudiness”).
At present the global consumption of aviation
fuel per year is equivalent to an emission of about
0.2 Gigatonnes of Carbon per year (GtC/a),
which is about 2% of the entire anthropogenic
carbon emissions, when emissions from land use
change (deforestation being the main contribution to it) are included. If only counting the
emissions originating from the burning of fossil
fuels and cement production, the percentage of
aviation emissions grows to about 2.4. Figure 4
presents an estimate of the time series of global
CO 2 emissions by aviation since 1990 and
extrapolation attempts by different groups as
compiled by Lee et al. [25]. Besides an overall
growth of the emissions, it is evident that different scenarios of future economic development
and technological progress lead to very different
emission paths.
It is also interesting to compare the different
modes of transport with respect to emissions per
passenger and per tonne transported. Here again
results from the recently finished major European
Union’s research project QUANTIFY [30], to
which the author has contributed, are presented:
Aviation is the strongest contributor to radiative
forcing by CO 2 emissions per passenger kilometer
in comparison to road traffic, shipping, and rail.
As clearly visible from Fig. 6, this is not the case
for freight transport, where road traffic contributes
similarly to radiative forcing per tonne-kilometer
(Fig. 5).
Aviation-Induced Cloudiness
There exist two types of impact on clouds caused
by particle emissions from traffic. First, the particle emissions can cause the formation of clouds
that would not form naturally. The most prominent example is jet exhaust condensation trails
(contrails). This cloud circumvents the lack of
so-called freezing nuclei in the rather clean and
cold upper troposphere. The large emission of
water vapor (1 kg/l of kerosene burned) in the
hot exhaust plume together with the additional
particles, also within the exhaust plume, leads to
cloud droplet formation, which rapidly freeze by
mixing with the ambient air. If the flight corridor
of an aircraft is supersaturated with respect to an
ice surface, the contrail formed will survive and
expand depositing all the supersaturated water
vapor onto the contrail ice particles.
A second type of anthropogenic cirrus clouds,
which is much more difficult to observe, are aerodynamic contrails involving a sequence of effects: The
emitted particles first get mixed with the background
aerosol particles and later – when the mix of anthropogenic and natural aerosol particles becomes
involved into cloud formation processes – the particles emitted by air traffic will affect cloud formation,
cloud evolution, cloud properties, and eventually
precipitation. Also most so-called ship tracks belong
to this category. A variant of this second aviationinduced cloud type, the hypothetical soot cirrus, is of
interest here. It describes an ice cloud that is strongly
affected by soot particles emitted by aircraft. To
estimate the effect of aerosol particles emitted by
air traffic (often soot) on ice clouds is difficult,
because of the presence of various pathways to ice
formation. In clean unpolluted air, ice clouds form
via homogeneous nucleation, while in polluted air,
like in air traffic corridors, heterogeneous nucleation
might be dominant. Because there exist always very
many other aerosol particles, which become solution
droplets at higher relative humidity over a water
surface (about 100/cm
3
), an ice cloud forming
homogeneously has a high number concentration
of ice crystals. In contrast, heterogeneous ice
cloud formation is a very selective process: Only
about every millionth particle is an appropriate ice
forming nucleus (IN). Thus, ice clouds forming
Aviation and Atmosphere
335
to the entire CO 2 burden of the atmosphere like
all other emissions into the atmosphere close to
the surface of the Earth. For the second major
emission, H 2 O, the point where it is emitted is
decisive: At the surface this addition is negligible
in comparison to the natural fluxes of water, but
not so at about 10 km height at a water vapor
mixing ratio ranging from about 100 ppmv to as
low as 5 ppmv. Flying by 1 km higher or lower in
heights around the tropopause (the interface
between the troposphere and the stratosphere;
typically at 10 km height in mid-latitudes), however, can often avoid a long-lasting linear contrail (for more see section “Aviation Induced
Cloudiness”).
At present the global consumption of aviation
fuel per year is equivalent to an emission of about
0.2 Gigatonnes of Carbon per year (GtC/a),
which is about 2% of the entire anthropogenic
carbon emissions, when emissions from land use
change (deforestation being the main contribution to it) are included. If only counting the
emissions originating from the burning of fossil
fuels and cement production, the percentage of
aviation emissions grows to about 2.4. Figure 4
presents an estimate of the time series of global
CO 2 emissions by aviation since 1990 and
extrapolation attempts by different groups as
compiled by Lee et al. [25]. Besides an overall
growth of the emissions, it is evident that different scenarios of future economic development
and technological progress lead to very different
emission paths.
It is also interesting to compare the different
modes of transport with respect to emissions per
passenger and per tonne transported. Here again
results from the recently finished major European
Union’s research project QUANTIFY [30], to
which the author has contributed, are presented:
Aviation is the strongest contributor to radiative
forcing by CO 2 emissions per passenger kilometer
in comparison to road traffic, shipping, and rail.
As clearly visible from Fig. 6, this is not the case
for freight transport, where road traffic contributes
similarly to radiative forcing per tonne-kilometer
(Fig. 5).
Aviation-Induced Cloudiness
There exist two types of impact on clouds caused
by particle emissions from traffic. First, the particle emissions can cause the formation of clouds
that would not form naturally. The most prominent example is jet exhaust condensation trails
(contrails). This cloud circumvents the lack of
so-called freezing nuclei in the rather clean and
cold upper troposphere. The large emission of
water vapor (1 kg/l of kerosene burned) in the
hot exhaust plume together with the additional
particles, also within the exhaust plume, leads to
cloud droplet formation, which rapidly freeze by
mixing with the ambient air. If the flight corridor
of an aircraft is supersaturated with respect to an
ice surface, the contrail formed will survive and
expand depositing all the supersaturated water
vapor onto the contrail ice particles.
A second type of anthropogenic cirrus clouds,
which is much more difficult to observe, are aerodynamic contrails involving a sequence of effects: The
emitted particles first get mixed with the background
aerosol particles and later – when the mix of anthropogenic and natural aerosol particles becomes
involved into cloud formation processes – the particles emitted by air traffic will affect cloud formation,
cloud evolution, cloud properties, and eventually
precipitation. Also most so-called ship tracks belong
to this category. A variant of this second aviationinduced cloud type, the hypothetical soot cirrus, is of
interest here. It describes an ice cloud that is strongly
affected by soot particles emitted by aircraft. To
estimate the effect of aerosol particles emitted by
air traffic (often soot) on ice clouds is difficult,
because of the presence of various pathways to ice
formation. In clean unpolluted air, ice clouds form
via homogeneous nucleation, while in polluted air,
like in air traffic corridors, heterogeneous nucleation
might be dominant. Because there exist always very
many other aerosol particles, which become solution
droplets at higher relative humidity over a water
surface (about 100/cm
3
), an ice cloud forming
homogeneously has a high number concentration
of ice crystals. In contrast, heterogeneous ice
cloud formation is a very selective process: Only
about every millionth particle is an appropriate ice
forming nucleus (IN). Thus, ice clouds forming
Aviation and Atmosphere
335