Speciated ambient carbonyls in Rio de Janeiro, Brazil.

Carbonyls in urban air continue to receive scientific and regulatory attention as toxic air contaminants and for their important role in photochemical smog. However, few data are available for speciated carbonyls in urban air. Ambient concentrations of up to 61 carbonyls have been measured in Rio de Janeiro, Brazil. The most abundant carbonyls were formaldehyde and acetaldehyde (study-averaged concentrations of 10.8 +/- 4.1 and 10.4 +/- 4.6 microg m(-3), respectively, in samples of 3-h duration collected from May to November 2000 at a downtown location during the morning vehicle commute) followed by acetone, 2-butanone, and benzaldehyde. Ambient concentrations of other carbonyls (except acetophenone) correlated well with those of acetaldehyde and of formaldehyde. This study examines the ambient acetaldehyde/ambient formaldehyde concentration ratio in Brazilian cities since the mid-1980s in the context of changes in Brazil's reliance on ethanol as a vehicle fuel. This ratio has begun to decrease in recent years due to fleet turnover and is likely to decrease further as older cars fueled with ethanol are replaced by lower-emitting models that run on a gasoline-ethanol blend. The carbonyls measured are ranked with respect to ozone formation potential (using MIR coefficients) and reaction with OH (using carbonyl-OH reaction rate constants). Ozone formation is dominated by formaldehyde (43% of total) followed by acetaldehyde (32%) and methylglyoxal (8%); other carbonyls each contributed < or = 4% of total. For reaction with OH, acetaldehyde ranks first closely followed by formaldehyde.