Seasonal and diurnal characteristics of carbonyls in the urban atmosphere of Changsha, a mountainous city in south-central China

Seasonal and diurnal variations of carbonyl compounds were investigated in the ambient air of a mountainous city in China, from September 2014 to July 2015. The most abundant carbonyl compounds are formaldehyde, acetaldehyde and acetone, propionaldehyde and methacryladehyde (MACR), which were all measured in most samples. The average concentrations of formaldehyde, acetaldehyde, acetone, propionaldehyde and MACR in the atmosphere in Changsha were broken down into each season: 6.57, 3.29, 3.66, 0.67 and 0.54 μg/m³ respectively during Spring, 14.09, 8.28, 9.02, 1.28 and 0.6 μg/m³, respectively during Summer, 9.24, 5.48, 8.62, 0.73 and 0.62 μg/m³, respectively during Autumn, and 5.88, 4.84, 7.84, 0.87 and 0.26 μg/m³ respectively during Winter. And majority of the species had higher concentration during noon, showing photochemical oxidation and human activities played an important role in diurnal variation. The highest average C1/C2 (formaldehyde/acetaldehyde) ratio was observed in summer (2.10) compared to those (1.33–2.03) in other seasons, implying the photochemical activities had a positive effect on increasing the ratio of C1/C2. In this study, the monthly concentration of formaldehyde produced from isoprene accounts for 4.8%–39.1% of formaldehyde in ambient air. Strong correlation among some carbonyl compounds means that they came from the same sources. Photochemical reaction was the main source of carbonyl compounds in summer and vehicular exhaust (gasoline and diesel engines) in winter. Changsha is not a completely urbanized city and it is rich in vegetation of broadleaf evergreen shrubs. Both atmospheric photochemical reactions and anthropogenic sources, including vehicular exhaust and industrial processes, dominate the levels of carbonyls. The ILTCR and HQ values of formaldehyde and acetaldehyde are 1.23E-04 and 1.34E-05, 2.80E-01 and 1.86E-01, respectively.