Molecular Analysis of Secondary Brown Carbon Produced from the Photooxidation of Naphthalene

We investigate the chemical composition of organic light-absorbing components, also known as brown carbon (BrC) chromophores, formed in a proxy of anthropogenic secondary organic aerosol generated from the photooxidation of naphthalene (naph-SOA) in the absence and presence of NO_x. High-performance liquid chromatography equipped with a photodiode array detector and electrospray ionization high-resolution mass spectrometer is employed to characterize naph-SOA and its BrC components. We provide molecular-level insights into the chemical composition and optical properties of individual naph-SOA components and investigate their BrC relevance. This work reveals the formation of strongly absorbing nitro-aromatic chromophores under high-NO_x conditions and describes their degradation during atmospheric aging. NO_x addition enhanced the light absorption of naph-SOA while reducing wavelength-dependence, as seen by the mass absorption coefficient (MAC) and absorption Ångström exponent (AAE). Optical parameters of naph-SOA generated under low- and high-NO_x conditions showed a range of values from MAC_{OM 405nm} ∼ 0.12 m² g^-1 and AAE_300-450nm ∼ 8.87 (low-NO_x) to MAC_{OM 405nm} ∼ 0.19 m² g^-1 and AAE_300-450nm ∼ 7.59 (high-NO_x), consistent with "very weak" and "weak" BrC optical classes, respectively. The weak-BrC class is commonly attributed to biomass smoldering emissions, which appear to have optical properties comparable with the naph-SOA. Molecular chromophores contributing to naphthalene BrC absorption were identified with substantial nitro-aromatics, indicating that these species may be used as source-specific markers of BrC related to the anthropogenic emissions.