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

Abstract

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 (<i>naph</i>-SOA) in the absence and presence of NO_<i>x</i>. High-performance liquid chromatography equipped with a photodiode array detector and electrospray ionization high-resolution mass spectrometer is employed to characterize <i>naph</i>-SOA and its BrC components. We provide molecular-level insights into the chemical composition and optical properties of individual <i>naph</i>-SOA components and investigate their BrC relevance. This work reveals the formation of strongly absorbing nitro-aromatic chromophores under high-NO_<i>x</i> conditions and describes their degradation during atmospheric aging. NO_<i>x</i> addition enhanced the light absorption of <i>naph</i>-SOA while reducing wavelength-dependence, as seen by the mass absorption coefficient (MAC) and absorption Ångström exponent (AAE). Optical parameters of <i>naph</i>-SOA generated under low- and high-NO_<i>x</i> conditions showed a range of values from MAC_{OM 405nm} ∼ 0.12 m² g^-1 and AAE_300-450nm ∼ 8.87 (low-NO_<i>x</i>) to MAC_{OM 405nm} ∼ 0.19 m² g^-1 and AAE_300-450nm ∼ 7.59 (high-NO_<i>x</i>), 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 <i>naph</i>-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.