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, the role of organic compounds is not yet completely understood due to their massive chemical complexity and numerous transformations in the atmosphere. Traditionally, the driving forces for secondary organic aerosol (SOA) growth are associated with homogeneous gas phase oxidation and the formation of condensable species. But models based on such mechanisms can not account for the SOA mass 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 SOA. By means of an aerosol flow tube reactor, we identified that traces of the aerosol-phase product of glyoxal chemistry, namely imidazole-2-carboxaldehyde (IC) act as a strong photo-sensitizer when irradiated in the near-UV 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 autophotocatalyses SOA growth.