AMBIENT LEVELS OF GAS-PHASE POLLUTANTS IN PORTO-ALEGRE, BRAZIL

Air samples have been collected using electropolished canisters in dow ntown Porto Alegre, Brazil, where ethanol is used as a vehicle fuel an d methyl-tert-butyl ether (MTBE) is used as a vehicle fuel additive. T he 150 volatile organic compounds (VOC) identified by GC-FID and CC-MS included 46 alkanes, 30 alkenes, 22 aromatics, 17 carbonyls, 3 alcoho ls, 8 bicyclic aromatics, 11 halogenated hydrocarbons and 13 other com pounds. The most abundant VOC on a mass concentration basis (after:CO2 , CH4 and CO) included acetylene, MTBE, ethanol, the alkanes propane, M-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 studi ed, 20 March, 1996-16 April, 1997, ambient concentrations of VOC corre lated well with those of carbon monoxide, for which vehicle exhaust em issions account for ca. 99% of total emissions in Porto Alegre. Two VO C photochemical reactivity rankings are presented: one involves reacti on with OH (product of VOC concentration and VOC-OH reaction rate cons tant) and the other involves production of ozone (product of VOC conce ntration and VOC maximum incremental reactivity coefficient). Reaction with OH is dominated by CO followed by 2-methyl-2-butene and by sever al other alkenes. Ozone production is dominated by ethylene and CO (ab out equal contribution) followed by several alkenes, alkylbenzenes and aldehydes. The two fuel oxygenates, ethanol and MTBE, play only a min or role as photochemical precursors (reaction with OH and production o f ozone) in the atmosphere of Porto Alegre. (C) 1998 Elsevier Science Ltd. All rights reserved.