Profiles of ambient semi- and intermediate- volatility organic compounds (S/IVOCs) in winter of urban Chengdu.

Semi-volatile/intermediate-volatility organic compounds (S/IVOCs) are considered to be missing precursors of secondary organic aerosol (SOA). In this study, ambient S/IVOCs were collected in the winter of Chengdu and were analyzed by a thermal desorption-comprehensive two-dimensional gas chromatography-quadrupole mass spectrometer (TD-GC×GC-qMS). Molecular characteristics and volatility-polarity distributions of the S/IVOCs were specified. A total of 351 compounds were quantified with S/IVOCs concentration of 2.63 ± 1.25 μg m-3 in gas phase and 1.83 ± 1.15 μg m-3 in particle phase. The volatility-polarity distributions are generally similar during polluted days and non-polluted days. Most of the gas-phase compounds fell in the high volatility and intermediate polarity bins, with IVOCs and SVOCs accounting for 16.9 % and 0.3 %, respectively, while IVOCs and SVOCs took up 73.0 % and 9.6 % of the total particle-phase concentration. Among gaseous S/IVOCs, acids and ketones were the main compounds, and benzoic acid was the most abundant species (0.86 ± 0.47 μg m-3), followed by 1,2-diphenyl-ethanone (0.20 ± 0.17 μg m-3), 2-hydroxy-1-phenyl-ethanone (0.20 ± 0.17 μg m-3) and C12-C15 alkenes. Alcohols and esters were the most abundant compounds in particle phase (62.8 %, including volatile chemical product compounds, VCPs), with tracers like Texanol detected (0.11 ± 0.07 μg m-3). Partial least squares-discriminant analysis (PLS-DA) indicated that combustion sources, VCPs and other industrial productions were closely related to pollution levels. The SOA yield method was used to estimate contributions of gas-phase S/IVOCs oxidation to SOA. Despite of their lower concentrations (2.63 ± 1.25 μg m-3 in gas phase, 17.2 %), S/IVOCs could contributed 27.4 % of the total estimated gas-phase SOA production. Emissions with more reactive S/IVOCs during non-polluted days may result in relatively higher SOA contribution fraction of S/IVOCs. Our result provided a deep insight into the molecular composition of ambient organics and elucidated the importance of S/IVOCs to SOA formation in the ambient air of Chengdu.