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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.
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