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Long-chain alkanes are a type of important intermediate-volatile organic compounds (IVOCs) in the atmosphere, which contribute to a large proportion of secondary organic aerosol (SOA). However, the optical properties of SOA derived from long-chain alkanes remain poorly understood. Here, we investigate the refractive index (RI) of SOA derived from photo-oxidation of dodecane (C12), pentadecane (C15) and heptadecane (C17) under low-NO x and high-NO x conditions with the absence or presence of inorganic aerosol seeds. The RIs of these SOAs are found to be in the range of 1.33 to 1.57 at the wavelength of 532 nm. The results from mass spectroscopy indicate that both reaction mechanisms influenced by NO x level and gas-particle partitioning influenced by seeds have important impact on the chemical compositions of SOAs, which further influence the optical properties like RI. Finally, by comparing the RI values to other literature and model results, we suggest that various RIs of SOAs derived from long-chain alkanes should be applied in atmospheric and climate models. [ABSTRACT FROM AUTHOR]

With the development of the economy, anthropogenic emissions in the atmospheric environment increases, and air pollution has caused wide public concern. Vehicle exhaust is an important emission source in the atmosphere, and alkanes are the representative components in it. In this study, the optical properties of secondary organic aerosol (SOA) derived from several C 12 alkanes (2-methylundecane, hexylcyclohexane, and cyclododecane) in the absence of NO X were determined. Absorption (imaginary part of the refractive index (RI), k) at 532 nm was negligible for all the derived SOA, and the scattering (real part of RI, n) of the SOA at 532 nm followed the order of cyclododecane SOA < hexylcyclohexane SOA < 2-methylundecane SOA, at both room- (25 °C) and low- (5 °C) temperature. The chemical compositions of the SOA formed were analyzed with an electrospray ionization time-of-flight mass spectrometer (ESI-TOF-MS). The mass spectra showed that the oligomers were generated in the reactions. It was shown that the different reaction pathways (due to various alkane structures) leaded to the difference in SOA chemical composition, which changed the RI values. The low-temperature condition promoted the progress of the oligomerization reaction so that the final RI values also changed. This work suggested that when estimating the radiative forcing of SOA using regional or global models, the structure of the precursors and reaction conditions should be taken into account. Unlabelled Image • Optical properties of SOA derived from several C 12 alkanes at 25 °C and 5 °C were determined. • SOA RI followed the order: cyclododecane < hexylcyclohexane < 2-methylundecane. • Different alkane structures leaded to different SOA chemical composition, which changed the RI. • Low-temperature promoted the oligomerization reaction progress, final RI values changed. [ABSTRACT FROM AUTHOR]