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6.3.3. Radiative Forcing for O3
Ozone is a potent greenhouse gas whose concentration is highly variable and
controlled by atmospheric chemistry and dynamics. Aircraft emissions of NOx
accelerate local photochemical production of O3 in the
troposphere; modeling studies suggest that these emissions are responsible today
for average O3 enhancements of 2-5 ppbv in the middle
troposphere at northern mid-latitudes, where most aircraft fly (see Chapters
2 and 4). This ozone increase will generally be proportional
to the amount of NOx emitted (Grewe et al., 1999), but
evolving atmospheric composition, including increases in surface sources of
combustion-related NOx, will affect the aircraft impact.
Four subsonic ozone perturbations based on detailed 3-D patterns of NOx
emissions were chosen (along with their control atmospheres) for the calculation
of RFs: NASA-1992, NASA-2015, FESGa (tech 1), and FESGe (tech 1) at 2050. Results
for NASA-1992 were scaled by 1.15 to give NASA-1992*, and those for NASA-2015
by 1.05 to give NASA-2015*. Table 6-1 describes
some of the basic properties of these aircraft scenarios, including total NOx
emissions. Chapter 4 supplied the seasonal pattern of
O3 perturbations for these scenarios based on model calculations
and reported a factor of 2 uncertainty in this best value. The modeled RFs from
these O3 perturbations agree quite well, and the stratospheric
temperature adjustment does not greatly affect the result (as was confirmed
by two independent model calculations). Given the predominantly tropospheric
perturbation, the uncertainty in modeling RF is small, and the uncertainty in
the final result may be a factor of only 3. The ozone RFs are +0.023 W m-2
for NASA-1992*, +0.040 W m-2 for NASA-2015*, and +0.060
W m-2 for FESGa (tech1) 2050 (see also Table
6-1).
The development of atmospheric chemistry models in the past 2 years has allowed
a consensus to build such that aircraft ozone perturbations can be calculated
with a likely (2/3 probability) range of about a factor of 2 (higher/lower,
see Chapter 4). Our estimate of the resulting RF for this
predominantly tropospheric perturbation does not significantly enhance that
interval.
The NASA-1992, NASA-2015, and FESGa (tech 1) at 2050 scenarios produce global
mean column ozone increases (predominantly tropospheric) of 0.5, 1.1, and 1.7
Dobson Units (DU), respectively. Our estimate of the increase in tropospheric
ozone associated with all anthropogenic changes (IS92a including aircraft plus
surface emissions of NOx, CO, and hydrocarbons) is about 3 DU from 1992 to 2015
and 7 DU from 1992 to 2050. These results represent an advance in our understanding
since the Second Assessment Report (IPCC, 1996), when future ozone changes were
scaled only to CH4 increases and did not include the
effects of doubling NOx emissions from 1990 to 2050.
Two HSCT cases with detailed 3-D emission scenarios-one with 500 aircraft and
the other with 1,000 aircraft-were used to calculate RF from stratospheric O3
and H2O perturbations (see Table
6-1). Most of the ozone change occurs above the tropopause; thus, there
is poorer agreement among RF models and a greater difference in RF values after
stratospheric temperatures adjust. The ozone perturbation calculated for 500
supersonic aircraft with the 2015 background atmosphere is substantially different
in nature from that calculated for 1,000 aircraft with the 2050 atmosphere,
in part because of specified changes in chlorine and methane contents of the
stratosphere (see Chapter 4). Nevertheless, the best RF
values are about the same: -0.01 W m-2. The uncertainty
range in these values is large and changes sign (-0.04 to +0.01 W m-2),
reflecting not only the range in O3 perturbations given
by Chapter 4 but also the large uncertainty in deriving
RF for stratospheric perturbations. This ozone-related RF for the HSCT fleet
is based only on the stratospheric ozone perturbation calculated by the models
in Chapter 4; the tropospheric changes are discarded,
and a correction for the displacement of about 11% of the subsonic traffic is
included, as shown in Table 6-1 (scenario Fa1H).
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