Ambient concentrations, sources, emission rates, and photochemical reactivity of C2-C10 hydrocarbons in Porto Alegre, Brazil

The combination of vehicle fuels used in Porto Alegre, Brazil, is unique in the world (17%, 9%, and 74% of the vehicles run on ethanol, diesel, and a mixture of 85% gasoline and 15% MTBE, respectively). As a result, air pollution control strategies for Porto Alegre, where vehicles account for ca. 99% and ca. 86% of total city wide CO and hydrocarbons emissions, respectively, must make use of air quality data specific to the Porto Alegre area. In this study, ambient concentrations of 66 C-2 - C-10 hydrocarbons have been measured for ca. 1 year (March 1996-April 1997) at downtown Porto Alegre locations. On the average, the 10 most abundant hydrocarbons on a mass concentration basis were, in decreasing order, as follows: acetylene, ethylene, propane, toluene, isopentane, (mfp)xylene, propene, n-butane, benzene, and n-pentane. Scatterplots of ambient concentrations showed a high degree of correlation among hydrocarbon classes (e.g., alkenes, isobutene vs 1-pentene), between hydrocarbons (alkanes, alkenes, and aromatics) and acetylene, and between hydrocarbons and carbon monoxide. Mid-1946 hydrocarbon emission rates (e.g., 3274 +/- 828 and 1418 +/- 369 t per year for acetylene and isopentane, respectively) are estimated from regression analysis of ambient hydrocarbon vs ambient CO. Reactivity rankings are presented, one involving reaction with OH (using hydrocarbon-OH reaction rate constants) and the other involving the production of ozone (using maximum incremental reactivity coefficients). Production of ozone is dominated by the following alkenes and aromatics (in order of decreasing importance), ethylene, (m+p)-xylene (calculated for m-xylene), propene, 1,2,4-trimethylbenzene, toluene, trans-2-pentene, 2-methyl-2-butene, isobutene, p-ethyltoluene, and o-xylene. Reaction with OH is dominated by these nine alkenes (in order of decreasing importance): ethylene, 2-methyl-2-butene, propene, isobutene, trans-2-pentene, 2-methyl-1-butene, cyclopentene, trans-2-butene, and 1,3-butadiene, If carbon monoxide, ethanol, and MTBE are included along with the 66 hydrocarbons, the re activity rankings indicate a major role for CO (ranked second after ethylene for ozone formation and ranked first for reaction with OH), a modest role for ethanol (ranked 16 for ozone formation and 18 for reaction with OH) and a minor role for MTBE (ranked 30 for ozone formation and 28 for reaction with OH).