|
EXECUTIVE SUMMARY
Aircraft emit a number of chemically active species that can alter the concentration
of atmospheric ozone. The species with the greatest potential impact are nitric
oxide (NO) and nitrogen dioxide (NO2) (collectively termed NOx), sulfur oxides,
water, and soot.
Ozone concentrations in the upper troposphere and lowermost stratosphere are
expected to increase in response to NOx increases and decrease in response to
sulfur and water increases. At higher altitudes, increases in NOx lead to decreases
in ozone.
Soot surfaces destroy ozone and possibly convert nitric acid to NOx. However,
because atmospheric soot reactions are highly unlikely to be catalytic and because
ambient soot concentrations are low, the effect on ambient ozone is expected
to be negligible.
Soot surfaces destroy ozone and possibly convert nitric acid to NOx. However,
because atmospheric soot reactions are highly unlikely to be catalytic and because
ambient soot concentrations are low, the effect on ambient ozone is expected
to be negligible.
Aircraft emissions are calculated to have increased NOx at cruise altitudes
in northern mid-latitudes by approximately 20%. The uncertainty in this calculation
is primarily related to uncertainties in the NOx chemical lifetime and in the
relative magnitude of the aircraft source compared to lightning, rapid vertical
convection of surface NOx, and other sources of upper tropospheric NOx. The
calculated increase is substantially smaller than the observed variability in
NOx.
NOx emissions from current aircraft are calculated to have increased ozone
by about 6% in the region 30-60°N latitude and 9-13 km altitude. Calculated
total ozone column changes in this latitude range are approximately 0.4%. Calculated
effects are substantially smaller outside this region. Some of the uncertainty
in these calculations is captured by the range of model results. However, the
models are notably deficient in coupling representations of stratospheric and
tropospheric chemistry and in describing exhaust plume processes, HOx sources,
and non-methane chemistry in the upper troposphere. In addition, there is high
uncertainty associated with the model description of vertical and horizontal
transport in the upper troposphere/ lower stratosphere.
The effect of current aircraft particle and particle precursor emissions (i.e.,
soot, sulfur, and water) in the stratosphere on ozone is estimated to be smaller
than, and of opposite sign to, the NOx effect. Model representations of aerosol
microphysics and chemistry are, however, largely incomplete.
Aircraft-related increases in NOx in the upper troposphere are calculated to
increase the concentration of hydroxyl (OH) radicals by a few percent throughout
the Northern Hemisphere. The OH change results in a corresponding decrease in
the concentration of methane (CH4). Uncertainties in the global budget of CH4
and the factors that control OH preclude testing this calculation with atmospheric
observations. Because the chemical processes that lead to the reduction of CH4
are the same processes that increase ozone, calculated CH4 and ozone effects
are correlated.
|