Seventeen carbonyls in urban ambient air were quantified in summer (June 2009) and winter (January 2010) in an urban site located in Xi'an, China. Formaldehyde, acetaldehyde and acetone were the three most abundant carbonyls in the atmosphere with the concentrations of 6.54 +/- 2.38 ppbv, 2.08 +/- 1.07 ppbv and 2.74 +/- 1.14 ppbv in summer (from 14th to 24th June, 2009), respectively. In winter, the concentrations were 4.46 +/- 1.74 ppbv, 6.52 +/- 3.88 ppbv and 3.87 +/- 2.33 ppbv respectively from 4th January, 2010 to 10th January, 2010. Most carbonyls had higher concentrations in winter than in summer. And majority of the species had higher concentrations in daytime than in nighttime, indicating photochemical oxidation/human activities played an important role in diurnal variation. Formaldehyde/acetaldehyde ratios (F/A) in summer (2.14) was much higher than that in winter (0.47), showing significant effect of photochemical oxidation in the urban air during summer. Acetaldehyde/propionaldehyde (A/P) average ratio was 12.2 in wintertime, implying anthropogenic emission was the major source of carbonyls in Xi'an. In addition, the ratio of acetone to methylglyoxal (A/M) is used to determine the impact of photochemistry in the atmosphere. The average acetone/methylglyoxal ratio (10.3 +/- 2.3) in summer was lower than that in winter (21.3 +/- 5.1) in Xi'an. Strong correlations among some carbonyls imply that they came from the same pollution sources. Formaldehyde and acetaldehyde play a very important role in photochemical smog formation. Methylglyoxal and glyoxal also have significant contribution to ozone formation potential.