Ambient levels of carbonyls in Mexico City

Measurements of ambient carbonyls were carried out at one sampling site at the University of Mexico campus, Mexico City in dry season (November and De cember 1993, March 1994, and February and March 1996), and rainy season (Ju ne and September 1995, May, June, July, and September 1996). In general, fo rmaldehyde had higher average hourly concentrations than those of the other sampled carbonyls, however, in May 1996, acetaldehyde concentrations were above 65 ppbv; much higher than the obtained for all the period of study. I n June 1996, these levels decreased to 20 ppbv, even higher than the observ ed values in all the period of study, except February 1996 with 22.5 ppbv. It was not found a satisfactory explanation for the acetaldehyde levels obs erved in February,May and at the beginning of June 1996. Average hourly con centrations of acetone from 4 to 28 ppbv for all the period study were slig htly higher or equal to formaldehyde and acetaldehyde concentrations at som e days. Propionaldehyde (1 to 9 ppbv) and butyraldehyde (1 to 4 ppbv) level s were small but significant. In general, a decrease in carbonyl concentrat ions during the evening was observed. Comparison of carbonyl concentrations between March 1994 and 1995, June 1995 and June 1996 and September 1995 an d 1996, indicated a significant difference (p less than or equal to 0.05) f or acetaldehyde, acetone and propionaldehyde. On the other hand, formaldehy de concentration (14.9 ppbv) was higher in rainy season and acetaldehyde, a cetone and butyraldehyde were higher in dry season (14.1, 12.4, 2.5 y 2.3 p pbv, respectively).