The impact of aircraft exhaust emissions on the atmosphere ; scenario studies with a three dimensional global model

The objectives of this study were to study the effects of emissions of nitrogen oxides (NOx) and other exhaust species from aircraft using the three dimensional (3D) model MOGUNTIA, in particular with respect to changes in the upper troposphere.

The main questions were:

• What are the increases of NO_x concentrations due to aircraft emissions?
• What are the effects of the emissions with respect to changes in the concentrations of ozone (O₃) and related species?
• How large are plume and corridor effects?
• What are the effects of emissions of future air traffic?
• What are the consequences of aircraft emissions with respect to the in the Netherlands so-called environmental themes: the enhanced greenhouse effect, the depletion of the stratospheric ozone layer, acid deposition and surface pollution by ozone?

The changes due to aircraft emissions in the concentrations of NO_x, nitric acid (HNO₃), O₃ and the hydroxyl radical (OH) have been calculated with the global 3D chemical-transport model MOGUNTIA. The calculated relative increases in the background concentrations of NO_x, HNO₃, O₃ and OH at cruising heights (9-12 km) between 30-60 degrees N in 1990 due to aircraft emissions are for NO_x: 20-70%, HNO₃: 10-20%, O₃: 1-4.5% and OH: 5-15%, respectively.

These figures are consistent with other studies. However, it must be kept in mind that many uncertainties are left in tropospheric modelling. The chemical processes in an aircraft plume in the upper troposphere occurring before large-scale mixing were studied with an Aircraft Exhaust Plume Model. We assumed a gaussian plume growth. To take the sub-grid processes into account in the 3D model study, a parameterisation of the subgrid chemistry was developed with which the base aircraft emission fields can be processed. In the processed emission fields then, the NO_x emissions are differentiated into NO_x (30%) and NO_y (70%) species (HNO₃, pernitric acid (HO₂NO₂) and nitrogen pentoxide (N₂O₅). As part of the NO_x emissions are converted in a non-O₃ producing form, the calculated relative contribution of aircraft to the changes in the concentrations of NO_x, O₃ and OH are also smaller (10-60%) in these scenarios. No large changes occur in the global distribution.

The effects of future air traffic have been calculated for the years 2003 and 2015 using three different scenarios: European Renaissance, Global Shift and Balanced Growth. The scenarios differ for different regional economic growths. Effects are increasing regardless of the scenario used. Calculated relative increases at cruising heights between 30-60 degrees N are 50-370% for NOx and 1.5-11% for O₃, respectively.

The importance of effects due to aircraft emissions is also growing relative to the effects of other anthropogenic surface emissions. We quantified the effects of subsonic aircraft with respect to the so-called environmental themes: the enhanced greenhouse effect, the depletion of the stratospheric ozone layer, acid deposition and surface pollution by ozone. However, this is still very difficult to do due to the large uncertainties in the effects of many of the emitted species.

The current subsonic air traffic has little global influence on the greenhouse effect: 1-3% of the global radiative forcing since pre-industrial times. The model used is a global model with limited suitability for acidification, photochemical smog and ozone layer studies. Other studies showed very little influence of aviation on acid deposition, the depletion of the ozone layer and the amount of ozone at surface level. However, air traffic is growing faster than other means of travel and therefore the relative importance will grow as well. With respect to the so-called environmental themes the impact of aircraft emissions on the greenhouse effect seems to be the most important consequence of air traffic at this moment.