The concentrations of two carbonyl compounds (formaldehyde and acetaldehyde) were monitored at the PolyU station. In total 31 valid samples were collected. Motor vehicle exhaust is the major source of carbonyl compounds in urban areas, which are major contributors of air pollution generated by photochemical reactions. Formaldehyde is most abundant species among all atmospheric aldehydes, followed by acetaldehyde. The measured concentrations of carbonyls during the sampling period are shown in Figures 4.1e & f and a summary of statistical data is shown in Table 4.1b. In this study, concentrations of formaldehyde were found to be higher than those of acetaldehyde. The mean acetaldehyde levels ranged from 0.16 to 6.75ugm-3 and formaldehyde from 0.82 to 11.34ugm-3. The average concentrations of acetaldehyde and formaldehyde were found to be 2.11 ugm-3 and 4.65 ugm-3 respectively (Figure 4.1g).

Monthly Variation of Carbonyls

The monthly variation of carbonyls is shown in Figure 4.1h. Both acetaldehyde and formaldehyde follow similar trends, peaking in May and reaching their lowest levels between October and December. This indicates that they came from the same emission source. There is a strong correlation between formaldehyde and acetaldehyde results. The formaldehyde to acetaldehyde ratio was relatively stable (about 2) except in wintertime.

Seasonal Variation of Carbonyls at PolyU Station

The seasonal variation of carbonyls is shown in Figure 4.1i. The concentrations of acetaldehyde and formaldehyde in summer were higher than in winter. Atmospheric photo-oxidation is an important secondary source of aldehydes. As such, it is believed that high amount of aldehydes were produced photochemically in summer, while direct vehicular emissions were their major source in winter.