Ambient levels of gas phase pollutants in Porto Alegre, Brazil

Abstract

Air samples have been collected using electropolished canisters in downtown Porto Alegre, Brazil, where ethanol is used as a vehicle fuel and methyl-tert-butyl ether (MTBE) is used as a vehicle fuel additive. The 150 volatile organic compounds (VOC) identified by GC-FID and GC-MS included 46 alkanes, 30 alkenes, 22 aromatics, 17 carbonyls, 3 alcohols, 8 bicyclic aromatics, 11 halogenated hydrocarbons and 13 other compounds. The most abundant VOC on a mass concentration basis (after CO 2, CH 4 and CO) included acetylene, MTBE, ethanol, the alkanes propane, n-butane, n-pentane, isopentane, n-hexane, 2-methylpentane and indane, the alkenes ethylene and propene, and the aromatics benzene, toluene, ethylbenzene and ( m+ p) xylene. During the ca. one-year period studied, 20 March, 1996–16 April, 1997, ambient concentrations of VOC correlated well with those of carbon monoxide, for which vehicle exhaust emissions account for ca. 99% of total emissions in Porto Alegre. Two VOC photochemical reactivity rankings are presented: one involves reaction with OH (product of VOC concentration and VOC–OH reaction rate constant) and the other involves production of ozone (product of VOC concentration and VOC maximum incremental reactivity coefficient). Reaction with OH is dominated by CO followed by 2-methyl-2-butene and by several other alkenes. Ozone production is dominated by ethylene and CO (about equal contribution) followed by several alkenes, alkylbenzenes and aldehydes. The two fuel oxygenates, ethanol and MTBE, play only a minor role as photochemical precursors (reaction with OH and production of ozone) in the atmosphere of Porto Alegre.