Formation of Ozone in Urban Air by Photochemical Oxidation of Hydrocarbons: Captive Air Experiments in Porto Alegre, RS

Abstract

Captive air experiments have been carried out at a downtown location in Porto Alegre, RS, where both methyl-tert-butyl ether (MTBE) and ethanol are being used as oxygenated fuels. In a captive air experiment, ambient air is introduced in a Teflon chamber and exposed to sunlight for several hours, thus allowing photochemical reactions to take place under "real-world" conditions. Parameters measured in six experiments, the first of their kind to be carried out in Brazil, included oxides 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 (PAN). Photochemical conversion of NO to NO2 and photochemical production of ozone (maximum concentrations 156-348 ppb) and PAN (maximum 13.3-29.5 ppb) were observed in all experiments, together with net (formation minus removal) photochemical production of formaldehyde and acetaldehyde. For each experiment, the contributions of each pollutant to reaction with the hydroxyl radical (OH) and to the production of ozone have been calculated. The results indicate that on the average the following 10 compounds are the most important with respect to the production of ozone (listed in order of decreasing importance): ethylene, carbon monoxide, acetaldehyde, (m + p)-xylene, formaldehyde, propene, 1,2,4-trimethylbenzene, toluene, ethanol and trans-2-pentene. MTBE makes only a small contribution to reaction with OH (ranked #27) and to the production of ozone (ranked #30), and plays only a minor photochemical role in the atmosphere of Porto Alegre.