FORMATION OF OZONE IN URBAN AIR BY PHOTOCHEMICAL OXIDATION OF HYDROCARBONS - CAPTIVE AIR EXPERIMENTS IN PORTO-ALEGRE, RS

Captive air experiments have been carried out at a downtown location i n Porto Alegre, RS, where both methyl-tert-butyl ether (MTBE) and etha nol are being used as oxygenated fuels. In a captive air experiment, a mbient air is introduced in a Teflon chamber and exposed to sunlight f or several hours, thus allowing photochemical reactions to take place under ''real-world'' conditions. Parameters measured in six experiment s, the first of their kind to be carried out in Brazil, included oxide s of nitrogen (NO and NO2), carbon monoxide, carbon dioxide, methane, ca. 75 non-methane hydrocarbons, formaldehyde, acetaldehyde, ethanol, MTBE, and the photochemical oxidants ozone and peroxyacetyl nitrate (P AN). Photochemical conversion of NO to NO2 and photochemical productio n of ozone (maximum concentrations 156-348 ppb) and PAN (maximum 13.3- 29.5 ppb) were observed in all experiments, together with net (formati on minus removal) photochemical production of formaldehyde and acetald ehyde. For each experiment, the contributions of each pollutant to rea ction with the hydroxyl radical (OH) and to the production of ozone ha ve been calculated. The results indicate that on the average the follo wing 10 compounds are the most important with respect to the productio n of ozone (listed in order of decreasing importance): ethylene, carbo n monoxide, acetaldehyde, (m + p)-xylene, formaldehyde, propene, 1,2,4 -trimethylbenzene, toluene, ethanol and trans-2-pentene. MTBE makes on ly a small contribution to reaction with OH (ranked #27) and to the pr oduction of ozone (ranked #30), and plays only a minor photochemical r ole in the atmosphere of Porto Alegre.