Indirect Impacts from Aviation:
Nitrogen Oxides, Ozone and Methane

Certain chemicals emitted by airplanes, though not direct GHGs themselves, can act to modify, produce, or destroy GHGs. Nitrogen oxides (NOx) and their impacts on methane and ozone concentration are of primary concern.

Nitrogen Oxides and Their Impact on Ozone and Methane

Nitrogen oxides are produced in aircraft engines under high temperature and high pressure conditions by the reaction between oxygen and atmospheric nitrogen. Nitrogen oxides catalyze the following chemical reactions which lead to both warming and cooling effects:

1. NOx (and hydroxides (-OH) produced from water vapor and volatile organic emissions) catalyze production of the short-lived greenhouse gas ozone (03). NOx released during air travel increases ozone in the upper troposphere but destroys ozone in the lower stratosphere.

The process of 03 formation is similar to ground level smog formation from transportation and industrial emissions. As with ground level smog, ozone formation depends on the concentration of NOx because NOx catalyzes not only the formation of ozone but also its destruction. This means, paradoxically, that lower ambient NOx concentrations can lead to a greater production of 03 than higher NOx concentrations. The 03 effects of a particular flight therefore depend on the existing atmospheric conditions.

Because of increased UV radiation at high altitude (above approximately 9 km) ozone is formed more effectively there than at ground level and leads to a larger radiative forcing (Berntsen et al., 2005). Ozone formation in the upper troposphere and the lower stratosphere is particularly sensitive to NOx. Furthermore, ozone occurring in the subtropics and tropics has greater radiative forcing than ozone emitted at higher latitudes. Thus the 03 effects of a particular flight depend on where changes occur geographically.

To summarize, the climate response of ozone formation is a function of UV radiation, water vapor concentration, temperature, and input of NOx, as well as input of volatile organic compounds (VOCs), all of which have ambient concentrations and effects that differ with 1) the physical and chemical background levels, 2) altitude and latitude, and 3) climate sensitivity.

2. NOx emissions also lead to indirect destruction of methane (CH4) through the creation of ozone. Methane is a strong GHG with an average lifetime of approximately 12 years. Creation of ozone results in hydroxyl radicals (-OH) that break down CH4 into CO2 and water, which are weaker GHGs than methane. NOx-induced ozone production that reduces atmospheric methane therefore results in a small net cooling. The effects of NOx on methane last a little more than a decade.

3. Lastly, NOx emissions and the resulting reduction in methane in turn lead to a longer-term decrease in O3, and therefore a small cooling effect over the same lifetime as the methane reaction. Ozone itself has an average lifetime on the order of weeks.

Summary

NOx emissions lead to an initial increase in ozone (net warming) followed by a longer-term decrease in methane (net cooling) and later a decrease of ozone (net cooling). However, the small decrease in ozone does not outweigh the larger initial increase in ozone. Therefore, overall changes in ozone concentration incur a warming effect whereas decreased methane has a cooling effect. But it would be incorrect to assume that the two effects cancel each other out, since they occur on different time scales and have different geographical distributions.

While the scientific understanding and modeling of NOx effects have substantially improved over the last few years, there is still uncertainty regarding the exact extent to which NOx emissions from air travel affect climate change. In general, uncertainties for emissions that have indirect impacts on GHGs are higher than for direct GHG emissions, as the climate response may be dependent on geographic location and time of emission.

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Nitrogen oxides: Strictly speaking, NO2 is also a (weak) GHG. Yet because NO2 tends to oxidize rapidly, it has a more important role in its impacts on ozone and methane than as a GHG.

higher NOx concentrations: Lower NOx concentrations do not always lead to a greater production of 03 than higher NOx concentrations. It depends on the proportion of NOx and volatile hydrocarbons and also on temperature, humidity, and UV light.

above approximately 9 km: Aircraft usually travel at or above 9km (29,500 feet) for distances of more than 500 km (IPCC, 1999).