Furans are predominant heterocyclic volatile organic compounds in the atmosphere from both primary and secondary sources, such as direct emissions from wildfires and atmospheric oxidation of dienes. The formation of secondary organic aerosols (SOAs) from the oxidation of furans has been reported. Previous research has shown that furan SOA generated from nighttime oxidation contributes to brown carbon (BrC) formation; however, how nighttime oxidant levels [represented by nitrate radical (NO₃) levels] and pre-existing particles influence the SOA chemical composition and BrC optical properties is not well constrained. In this study, we conducted chamber experiments to systematically investigate the role of these two environmental factors in furan-derived secondary BrC formation during the nighttime. Our results suggest that the bulk compositions of SOA measured as ion fragment families by an aerosol mass spectrometer are unaffected by changes in NO₃ levels but can be influenced by the presence of pre-existing ammonium sulfate particles. Based on the mass absorption coefficient profiles of SOA produced under different experimental conditions, BrC light absorption was enhanced by higher NO₃ levels and reduced by the presence of pre-existing ammonium sulfate seed particles, suggesting that NO₃-initiated oxidation of furan can promote the formation of light-absorbing products, while pre-existing particles may facilitate the partitioning of nonabsorbing organics in the aerosol phase. Furthermore, molecular-level compositional analysis reveals a similar pattern of chromophores under various studied environmental conditions, in which highly oxygenated monomers (e.g., C₄H₄O₆ and C₄H₃NO₇), dimers, and oligomers can all contribute to BrC chromophores. Taken together, the NO₃ levels and pre-existing particles can influence secondary BrC formation by altering SOA compositions, which is critical for assessing BrC optical properties in a complex environment.