Seasonal and diurnal variations of carbonyl compounds were investigated in roadside (MK), urban (TW), and suburban (UST) environments in Hong Kong. Thirteen carbonyls (C1-C6) were identified and quantified in the samples. The average total quantified carbonyl concentrations decreased in the order of roadside (12.16 +/- 3.52 mu g m(-3)), urban (10.45 +/- 4.82 mu g m(-3)), and suburban (5.14 +/- 3.35 mu g m(-3)). Formaldehyde was the most abundant species, accounting for similar to 52%, similar to 52%, and similar to 46% of total measured carbonyls at MK, TW, and UST, respectively. At MK, the summer/winter ratios of most carbonyls were below one. The diurnal variations were associated with traffic flows, with high concentrations in daytime but low in nighttime. Good correlations were found between most carbonyls and carbon monoxide (CO), showing primary vehicular emission was the major source at the roadside location. In contrast, the summer/winter ratios of most carbonyls were larger than one at the urban site of TW and their carbonyls correlated well with secondary pollutant of ozone (O-3), indicating photochemical reactions contributed significantly in the formation of carbonyls, especially in summer. Distinct diurnal variations of carbonyls were observed at TW on a hazy winter day of 28 Jan 2012. Most carbonyl concentrations reached the first peak at noontime and had the second peak in the evening. The first peak was ascribed to be the products of photochemical reactions as maximum concentrations of nitrogen monoxide (NO) and O-3 occurred just before and after the carbonyl peak, respectively. The correlations among the carbonyl species were strong in summer but fair in winter at UST. Natural sources (e.g. biogenic) were usually the major source at the suburban, as far as its surrounding environment is concerned.