The goal of the project is to conduct a systematic investigation of the role of nitrogen-containing organic compounds (NOC) in the light-absorption by aged secondary organic aerosols (SOA). Using the state-of-the-art molecular-level characterization capabilities of high-resolution mass spectrometry, the project will be able to address the following important questions: 1) What is the chemical composition and molecular identity of the light absorbing NOC in lab-generated and field SOA? 2) What is the impact of individual light-absorbing NOC molecules (chromophores) on the optical properties of SOA? 3) What types of the NOC chromophores contribute the most to the absorption of visible radiation by SOA? 4) How does atmospheric aging affect NOC composition and subsequently the optical properties of SOA?