Measurements of atmospheric carboxylic acids and carbonyl compounds in SaoPaulo city, Brazil

Winter atmospheric measurements of gaseous lower carbonyl and carboxylic ac ids were carried out simultaneously (in 1999) at two distinct urban sites l ocated in the city of Sao Paulo, Brazil. The greater metropolitan area of S ao Paulo is the largest industrialized region of Latin, America and has a h ighly polluted atmosphere. It has an unconventional mix of vehicle types in that a variety of gasoline blends, including oxygenated ones, are used. Mi xing ratios of formic and acetic acids ranged, respectively, from 0.6 to 19 .4 and from 0.1 to 10.6 ppbv in one of the sites studied and from 1.4 to 18 .4 and from 0.4 to 6.7 ppbv in the other site. High values of formic to ace tic ratios were found, especially in the latter site (average = 4.3), sugge sting that photochemical production was the predominant source of the formi c and acetic acid during the afternoon. Differing from the acids, levels of carbonyls were similar at both sites. Higher average mixing ratios of acet aldehyde and formaldehyde were found in the morning (18.9 and 17.2 ppbv) an d gradually decreased from midday (9.5 and 11.8 ppbv) to evening (7.2 and 1 0.2 ppbv). In the morning, vehicular direct emission seemed to be the main primary source of formaldehyde and acetaldehyde, whereas at midday and even ing these compounds appeared to be mainly formed by photochemistry. Seconda ry photochemical production of organic acids and aldehydes (rather than pri mary emissions from vehicles) was shown to be more important in Sao Paulo's atmosphere from midday to evening, particularly on days with strong solar radiation.