Chemical characteristics of atmospheric carbonyls over the South China Sea: Influence of continental outflow

For the first time, atmospheric carbonyls were identified in an island in the South China Sea during biomass burning episodes to investigate their chemical characteristics and possible sources. PM2.5 and its chemical species, including water-soluble ions, water-soluble organic compounds, organic carbon and elemental carbon, were also identified. The levels of atmospheric carbonyls in Weizhou Island were significantly higher than those in other remote areas and most polluted cities in China during the sampling period. The sum of formaldehyde (27.23 ± 12.25 μg/m3), acetaldehyde (10.67 ± 4.46 μg/m3) and isovaleraldehyde (2.35 ± 1.21 μg/m3) accounted for ∼84% of the total 16 carbonyls. Different from other studies, isovaleraldehyde ranked the third highest carbonyl in this study, while it was extremely low in other rural areas, and it was believed to be a photochemical production from industrial pollutants. Backward trajectory analysis showed that transport from Southeast Asia could be possible sources of isovaleraldehyde. Contrary to many previous studies, diurnal variation of carbonyls at this site showed that the concentrations of carbonyls were high in the early morning and evening (82.00 μg/m3 and 43.18 μg/m3, respectively) but low in the morning and afternoon (47.99 μg/m3 and 32.16 μg/m3, respectively), indicating that transport and intense photolysis might be responsible for the diurnal variation of carbonyls in Weizhou Island. The ozone formation potential and propene-equivalent concentration were 345 μg/m3 and 20 μg/m3, respectively, suggesting that carbonyls had a significant contribution to ozone formation in Weizhou Island. Diagnostic ratios and correlation analysis together with backward trajectory analysis showed that anthropogenic activities and biomass burning emissions from Southeast Asia and northern Gulf of China were two main sources of carbonyls at the sampling site.