Prof. Dr. Sjaak Slanina held a position at the Netherlands Energy Research Foundation as head of the environmental research department and as senior research scientist in atmospheric chemistry and air pollution. Dr. Slanina passed away 28 March 2009. More information can be found at http://register.slanina.com
- Ozone Depletion Reduces Ocean Carbon Uptake
- Beyond Carbon Dioxide: Growing Importance Of Hydrofluorocarbons (HFCs) in Climate Warming
- Ozone Layer Faces Bumpy Return to Health
- Chemicals That Eased One Woe Worsen Another
- Geoengineering To Mitigate Global Warming May Cause Other Environmental Harm
- Ozone protection feels the heat
We would like to dedicate this weeks Environment in Focus to the memory of Sjaak Slanina. We appreciate his years of work on atmospheric science and all of his contributions to the Encylopedia of Earth.
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.