Formation of Ozone
Ozone is a radiative active compound; it absorbs long-wave infrared (LWIR) radiation emitted from the Earth's surface and so contributes to the greenhouse effect. But ozone near the tropopause has a much larger influence on the radiative balance compared to ozone at surface level. Ozone absorbs infrared radiation and re-emits this radiation at an energy level equivalent to about 18° Celsius. This means that the impact of ozone at surface temperature is not very important. It is a much more effective greenhouse gas at the tropopause where temperatures of –60 to –80°C are encountered.
In the Intergovernmental Panel on Climate Change (IPCC) 2000 report, the forcing (change) in the radiative balance is estimated to be about 0.3 watt/m2 (range: 0.1 to 0.6) due to the increase of tropospheric ozone near the tropopause and a forcing of –0.1 (range: -0.05 to -0.2) watt/m2 due to the decrease of stratospheric ozone. The total forcing from all greenhouse gases together is about 2.5 watt/m2 (see Figure 1).
There has been much discussion concerning the origin of the elevated ozone concentrations near the tropopause (see Figure 2). The process of ozone formation in the higher troposphere is the same as in the lower troposphere, contrary to the situation in the stratosphere, (see Figure 3). Hydrocarbons and nitrogen oxides (NOx) are involved in reactions producing ozone and other oxidants. The formation of ozone by NOx molecules is stopped by the conversion of nitrogen oxides to nitric acid.
A popular hypothesis was that aircraft emissions were responsible for increased nitrogen oxide concentrations near the tropopause, leading in turn to increased ozone concentrations. Most of the emissions from airplanes are discharged at this altitude and vertical mixing was seen as a slow process. Hence, the contribution of surface sources to the nitrogen oxide (NOx) concentrations at 10 to 15 km altitude would be quite low. Due to this slow exchange, long lifetimes of nitrogen oxides—up to one month—were assumed.
The other hypothesis was that these elevated ozone concentrations were a product of ozone exchange between the stratosphere and the troposphere and that the tropopause layer is not the absolute separation between troposphere and stratosphere, as is sometimes assumed.
The fact that commercial jet aircraft do indeed fly just above or below the tropopause supports the first hypothesis. The height of the tropopause varies from about 10 km at the poles to 17 km over the equator. Especially in the busy transatlantic route between the U.S. and Europe, airplanes fly at altitudes between 10 and 12 km, just above or below the tropopause. A large part of the aircraft emissions (currently estimated to be 2.6 Mton NOx (a few percent of total human-made NOx emissions) is indeed emitted near the tropopause in this North-Atlantic flight corridor. Rough estimates indicate that an extra contribution of 20 to 50 ppt (parts per trillion, indicating a mixing ratio of 1 NOx on 1012 air molecules) would be added by aircraft emissions to the local background of about 200 ppt.
It should be noted that such an increase in nitrogen dioxide concentrations cannot be linearly extrapolated to increases in ozone concentrations. Other factors such as hydrocarbon concentrations and, even more important, lifetime of nitrogen oxide molecules near the tropopause, must be taken into account.
Role of Aerosols and Mixing by Cold Fronts
Measurement campaigns, mainly by U.S. and European research groups, have made it clear that nitrogen oxide (NOx) lifetime is limited near the tropopause because the conversion to nitric acid is much faster than indicated by initial assessments. In the daytime, NOx is converted by reaction with OH-radicals to nitric acid. At night, this conversion proceeds via the formation of N2O5 (see Figure 3).
Nitrogen oxides (NOx) are converted at night to nitric acid by means of a reaction between N2O5 and water vapor. The reaction of N2O5 with water is very slow if no aerosol surface is available, but quite fast on aerosol surfaces. Measurements have shown that sufficient concentrations of aerosols are present in the upper troposphere to make this reaction an important sink for reactive nitrogen species, and that the chemical lifetime of NOx is limited to about four days, not months as was previously thought.
Another important factor is the mixing of the troposphere and lower stratosphere by deep cold fronts. According to meteorological observations, an annual average of 340 cyclonic cold fronts pass through the North-Atlantic Ocean, with an average frequency of one every 3–4 days for a given location. During such passages, complete vertical mixing takes place, often as far as the lower stratospheric layers. Cold air displaces warm air. As the cold air has a higher density and is heavier (according to the general gas law), the warm air is pushed up steeply and large portions of the troposphere in the North-Atlantic, and even the lower stratosphere, are mixed.
Aircraft emissions are rapidly diluted by this process and large amounts of NOx, emitted at the Earth's surface, are transported upwards. It is evident that this frequent vertical mixing process determines the physical residence time of aircraft emissions in the upper troposphere. Rapid vertical transport from the lower air layers to the upper troposphere has been proven in past studies of vertical distribution of unstable short-lived hydrocarbons. A general overview of a steep cold front is given in Figure 4.
The combination of reduced chemical residence time, due to faster conversion to nitric acid as well as the faster mixing by fronts, has led to the conclusion that the impact of NOx emissions by airplanes is probably less important than suggested by earlier evaluations.
The mixing between the troposphere and stratosphere will lead to a larger contribution of stratospheric ozone and to the amount of ozone in the troposphere near the tropopause. Increased vertical transport will also lead to a larger role of surface NOx emissions in ozone formation near the tropopause.