Ambient concentrations of ethanol and methyl tert-butyl ether in Porto Alegre, Brazil, March 1996 April 1997

While air quality and other perceived benefits of oxygenated fuel programs are currently the topic of much debate, there are very few reports of ambient concentrations of oxygenated fuels (e.g., ethanol) and fuel additives (e.g., ethanol, methyl tert-butyl ether, hereafter called MTBE) in urban air. Ambient concentrations of MTBE and ethanol have been measured by GC-FID and CC-MS analysis of samples collected in electropolished canisters at a downtown location in Porto Alegre, Brazil, where 17% of the vehicles run on ethanol, 74% run on a mixture of 85% gasoline and 15% MTBE, and 9% use diesel fuel. During the ca. 1-year period of March 20, 1996-April 16, 1997, ambient levels of MTBE ranged from 0.2 to 17.1 ppbv(average = 6.6 +/- 4.3 ppbv); those of ethanol ranged from 0.4 to 68.2 ppbv (average = 12.1 +/- 13.3 ppbv). Ambient levels of ethanol and MTBE are compared to those of carbon I monoxide (for which vehicle exhaust accounts for ca. 99% of total emissions in the city of Porto Alegre) and of acetylene. Linear regression of the ambient concentration data (44 samples) yielded near-zero intercepts and slopes of 11.40 +/- 0.45 for acetylene (ppbv) vs CO (ppmv), 1.73 +/- 0.20 for MTBE (ppbv) vs CO (ppmv), 0.153 +/- 0.016 for MTBE (ppbv) vs acetylene (ppbv), and 4.64 +/- 0.78 for ethanol (ppbv) vs CO (ppmv). These slopes together with an estimated vehicle exhaust emission rate for CO in mid-1996 are used to estimate vehicle emission rates of 2338 +/- 393 t/year for ethanol and 1668 +/- 193 t/year for MTBE.