Optical property variations from a precursor (isoprene) to its atmospheric oxidation products

Isoprene is a crucial precursor of secondary organic aerosols (SOAs) that reacts with radicals or oxidants such as OH, O3, and NO3. The reaction mechanisms under various conditions have been elucidated through years of research. In this study, the UV–vis light absorptive ability of oxydates for isoprene was calculated at the PBE1PBE/6-311g (d) level using Gaussian 09. The results indicated that reactions of isoprene with OH and O3 would produce less absorptive organics; the intensity of the absorption peaks and the maximum absorption wavelength decreased relative to those of the precursor. The oxydates formed from isoprene through an NO3-initiated reaction had different light absorption properties. The optical properties of the oxydates exhibited a bathochromic shift and hyperchromic effect relative to those of the precursor. NO3 mainly originated from vehicular traffic, indicating that anthropogenic activity heavily affected the formation of oxydates and light absorption. The light absorption properties and irradiation effect of SOAs caused by isoprene heavily depended on the reaction route. Transformation of functional groups was the dominated factor in photobleaching and photoenhancement of oxidative products. This study provides a detailed mechanism that explains the changes in optical absorption properties during isoprene oxidation.