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 NOx. 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-NOx conditions and describes their degradation during atmospheric aging. NOx addition enhanced the light absorption of naph-SOA while reducing wavelength-dependence, as seen by the mass absorption coefficient (MAC) and absorption Angstrom exponent (AAE). Optical parameters of naph-SOA generated under low- and high-NOx conditions showed a range of values from MAC(OM 405nm) similar to 0.12 m(2) g(-1) and AAE(300 similar to 450nm) (low-NOx) to MAC(OM 405nm) similar to 0.19 m(2) g(-1 )and AAE(300 similar to 450nm )similar to 7.59 (high-NOx), 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.