Humankind is facing a changing environment possibly due to anthropogenic stress on the atmosphere. In this context, aerosols play a key role through their influence on radiative climate forcing, hydrological cycle, and their adverse effect on health. However, such understanding is not yet achieved due to the massive chemical complexity and numerous transformations of organic compounds in the environment. Traditionally, the driving forces for particle growth are associated with homogeneous gas phase oxidation by radicals that form condensable species. Models based on available parameterizations from laboratory studies do not account for aerosol particle growth observed in the atmosphere, suggesting that unknown processes might exist. The present study shows experimental evidence that particulate phase chemistry produces photo-sensitizers that lead to photo-induced formation and growth of secondary organic aerosol (SOA). By means of an aerosol flow tube reactor equipped with Scanning Mobility Particle Sizer (SMPS) having Kr-85 source aerosol neutralizer, Differential Mobility Analyser (DMA) and Condensation Particle Sizer (CPC), we identified that traces of the aerosol-phase product of glyoxal chemistry, namely imidazole-2-carboxaldehyde (IC) are strong photo-sensitizer when irradiated by near-UV irradiation and exposed to volatile organic compounds such as terpenes. This novel photo-sensitizer contributed to more than 30% of SOA growth in 19min irradiation time in the presence of terpenes in the system. This identification underlines that upon ageing, dicarbonyl compounds in organic aerosols produces photo-sensitizers which auto-photo-catalyses SOA growth.