Photochemical smog or increased tropospheric ozone

The IPCC Fourth Assessment Report (IPCC, 2007a) shows that the radiative forcing of climate by tropospheric ozone has reached the same level as accumulated methane with about 0.3 Wm-2. Ozone in the troposphere is formed by a totally different chemical reaction chain than in the stratosphere where solar radiation at wavelengths < 0.24 ^m dissociates oxygen molecules into two oxygen atoms, which can combine with other oxygen molecules to form ozone (O3). In the troposphere so-called precursor gases, namely nitrogen oxides (NOx = NO + NO2) and hydrocarbons are needed above a certain concentration level of NOx to form ozone. The key reaction determining the ozone formation rate is the dissociation of nitrogen dioxide (NO2) into nitrogen monoxide (NO) and an oxygen atom (O) by ultraviolet radiation at wavelengths 1 < 0.4 ^m, which can penetrate the atmosphere down to the surface. As large quantities of hydrocarbons are also emitted by vegetation the key pollution gas in this context is nitrogen dioxide originating from traffic, power plants, heating of buildings by oil and gas and industrial processes but also from vegetation fires, especially deforestation by slash and burn practices. Tropospheric ozone is therefore a continent wide air pollution problem. Because ozone formation rate reaches a maximum at a certain NOx to hydrocarbon ratio, it is difficult to forecast effects of clean air acts as reduction of one component, e.g. NO2, might cause higher ozone levels far away from the emission source.

Photochemical smog, as it is often called, has become a nearly global phenomenon and its abatement is much more difficult than anticipated. The involvement of so many environmental factors makes forecasting of effects of air pollution control a real challenge. Whenever authorities report near surface ozone concentrations above the alarm level (180 ^g m-3 O3 in ambient air in several European countries) this has also a climate change importance.

Many more chemical reactions among trace gases as well as between trace gases and aerosol particles are relevant for climate but are not discussed here. However, they do not have the same importance as the changes of ozone. While we reduced it where we need it as a UV radiation shield (in the stratosphere) we augmented it by more than a factor of 2 in the 20th century in the troposphere where it is an air pollution component.

The overall climate effect of the changed ozone profile is difficult to assess because we have to include halogenated hydrocarbons, very potent greenhouse gases, into the discussion, too. In addition, even no change in the total ozone column content as a result of depletion above and augmentation below the tropopause can have a strong climate effect, as we redistribute the solar radiation absorption thus heating rates in the vertical and also the vertical profile of terrestrial radiation flux density is changed, leading to changed cooling rates.

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