[Formation and transport of ozone and other photochemical oxidants]

Photochemical oxidants (O3, PAN, HCHO) are the typical pollutants in photochemical Smog. They can be formed as the result of the sunlight induced oxidation of precursor pollutants emitted into the atmosphere (NOx, CxHy). Other important precursor substances and reactive intermediates are compounds of the organic hydroxyl group (-O-H, for instance the methoxy radical CH3O), hydroxyl radical .OH, hydroperoxy radical HO2, and singlet oxygen O. For the production of photochemical oxidants in smog, especially ozone, NOx primarily act as catalysator. But CxHy serve as "combustibles". The quantities of the several formed photooxidants are mainly dependent on intensity and duration of daily sunshine, temperature, oxidant concentrations at the beginning of a build-up period, and on emission rates and concentrations of primary pollutants (NO, CxHy). Maximum mixing ratios of secondary pollutants often occur at certain distances downwind of primary pollutant sources, dependent on meteorological conditions (wind velocity, rel. humidity, etc.) and on rate constants for oxidant formation cycles. During the period of July 5 to 11, 1984, for instance, 6 kilometers SW from Zürich we found maximum ground-level mixing ratios of about 150 ppbv ozone, 4 ppbv PAN and 9 ppbv formaldehyde, that is 5 (ozone) to 10 (PAN) times above the supposed oxidant concentrations of unpolluted continental air. The increase in ozone in the late morning and early afternoon is accompanied by an increase in PAN and HCHO and by a decrease in NOx. This shows that in this case HCHO originates from photochemical reactions rather than being emitted directly to the atmosphere from vehicles.(ABSTRACT TRUNCATED AT 250 WORDS)