Air Chemistry and Dispersion

Abstract

Photochemical oxidants are called secondary pollutants. They are formed in polluted air only under certain meteorological conditions, during hours and days of transport, in a complex sequence of photochemically induced reactions from a multicomponent mixture of anthropogenic precursors. Ozone, the most easily measured photochemical pollutant, is also formed in the stratosphere and mixed down into the troposphere, where it gives rise to a natural background concentration. “Natural” ozone molecules are chemically and physically identical with “anthropogenic” ozone molecules which prevail in photochemical smog. These considerations cause certain difficulties in the development of suitable strategies for the reduction of oxidant pollution:

• 1. The measures to be taken depend upon the scale on which they should be most effective (local ozone reduction versus large-scale reductions), and upon the type of effect required (elimination of very high peak concentrations of short duration; reduction of long-term averages; reduction of a specific photochemical oxidant, e. g., peroxyacetyl nitrate).

• 2. Justification of measures imposed upon the emitters of specific precursors is extremely difficult, and presupposes long-term field measurements of oxidants, precursors, and meteorological parameters.

• 3. Beneficial effects which are expected from measures taken against specific precursors cannot, at present, be quantitatively predicted. It is equally difficult, after such measures have been taken, to obtain irrefutable proof of the effect, owing to the year-to-year variability of meteorological parameters (Schurath 1979b).