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

• Published in: Environmental science & technology Vol. 56; no. 6; pp. 3340 - 3353
• Main Authors: Siemens, Kyla, Morales, Ana, He, Quanfu, Li, Chunlin, Hettiyadura, Anusha P. S, Rudich, Yinon, Laskin, Alexander
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 15.03.2022
• Subjects: Absorption; Absorptivity; Aerosols - chemistry; Aging; Air Pollutants - analysis; Anthropogenic factors; Anthropogenic Impacts on the Atmosphere; Aromatic compounds; Biodegradation; Biomass; Carbon; Carbon - chemistry; Chemical composition; Chromophores; Electromagnetic absorption; Emissions; High performance liquid chromatography; Ionization; Liquid chromatography; Naphthalene; Naphthalenes; Nitrogen oxides; Optical properties; Photodiodes; Photooxidation; Smoldering; high-resolution mass spectrometry (HRMS); optical properties; NO x -involved photooxidation of naphthalene; anthropogenic secondary organic aerosol (SOA); molecular characterization; atmospheric aging; brown carbon (BrC); high-performance liquid chromatography (HPLC); NOx-involved photooxidation of naphthalene
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/acs.est.1c03135

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 MACOM 405nm ∼ 0.12 m2 g–1 and AAE300–450nm ∼ 8.87 (low-NO x ) to MACOM 405nm ∼ 0.19 m2 g–1 and AAE300–450nm ∼ 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.