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 (naph-SOA) in the absence and presence of NO x . Highperformance 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 Ångstrom 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 m 2 g −1 and AAE 300−450nm ∼ 8.87 (low-NO x ) to MAC OM 405nm ∼ 0.19 m 2 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.

Optical Properties of Secondary Organic Aerosol Produced by Photooxidation of Naphthalene under NOx Condition

“… 78 , 79 It is positively correlated with the number of atoms in a molecule. As the NOx/VOC increases, the mean polarizability of naph -SOA increases and the density remains constant, while the MW decreases, as indicated by the significant decay of large molecules observed by Siemens et al 55 Therefore, an increase of the real part of RI is expected with the increase of the NOx/VOC ratio. At higher NOx/naphthalene ratios, the increased prevalence of NOx promotes RO 2 to react more with NOx to form nitro-organic compounds than reacting with another RO 2 to produce larger molecules (e.g., ROOR).…”

“…A higher nitro-organic compound content under higher NOx/naphthalene ratios was observed by HR-ToF-AMS ( Figures S3 and S4 ) and HPLC-HRMS ( Figure S7 ). Siemens et al 55 have analyzed the double-bond equivalent value (DBE) of the assigned species. They suggest that the high DBE values of the CHON compounds retain the aromaticity and are likely nitroaromatics.…”

Concurrent photochemical whitening and darkening of ambient brown carbon

“…Due to the high NOx emission, photooxidation of traffic VOCs may have largely involved nitrogen chemistry. Previous studies found the NOx-involved SOA could produce considerable chromophores (Lin et al, 2015;Siemens et al, 2022), such as the traffic VOCs may produce SOA in a time scale of hours, containing nitro-aromatics (Wang et al, 2019b;Keyte et al, 2016). The daytime formation of organic nitrate may follow the gas-phase photooxidation mechanism, in which the excess NO could add to the peroxy radical to produce organic nitrate (Liebmann et al, 2019).…”

Unexpected molecular diversity of brown carbon formed by Maillard-like reactions in aqueous aerosols

“…11 In recent years, the sources, and optical and chemical characteristics of secondary BrC have been widely studied in eld and laboratory studies. [12][13][14][15][16] In particular, BrC produced by Maillard-like reactions between carbonyls and reduced nitrogen species (e.g., organic amines and ammonium) in clouds and aqueous aerosols has attracted increasing attention, 9,10,17,18 because the related precursors are abundant in the atmosphere and the related reactions can be greatly accelerated in atmospheric aerosols. The light-absorbing capacities of BrC formed by reactions between a series of carbonyl compounds and ammonium sulfate (AS) and/or amines were evaluated, and it is conrmed that amines are considerably more effective than AS for the production of BrC on a per mole basis.…”

“…11 In recent years, the sources, and optical and chemical characteristics of secondary BrC have been widely studied in field and laboratory studies. 12–16 In particular, BrC produced by Maillard-like reactions between carbonyls and reduced nitrogen species ( e.g. , organic amines and ammonium) in clouds and aqueous aerosols has attracted increasing attention, 9,10,17,18 because the related precursors are abundant in the atmosphere and the related reactions can be greatly accelerated in atmospheric aerosols.…”