Molecular composition of secondary brown carbon from styrene at low-to-high oxidation degrees

Brown carbon (BrC) and its light absorption properties, linked to molecular chromophores, influence Earth's radiative balance. However, the chemical complex formation of secondary BrC has obscured the relationship between molecular composition and optical characteristics. In this study, we investigated the secondary organic aerosol (SOA) optical properties and BrC chromophores formed from the oxidation of styrene, a typical compound from various anthropogenic emissions. The photochemical age spanned low to high oxidation degrees by deploying a smog chamber and an oxidation flow reactor (OFR). Light absorption increased with rising NO_x concentrations under low oxidation degrees in the smog chamber experiments, followed by a decline with increasing photochemical ages at high oxidation degrees in the OFR experiments, likely attributed to the transition from functionalization to fragmentation reactions. Employing a high-resolution quadrupole time-of-flight mass spectrometer with a diode array detector, we characterized the BrC chromophores at a molecular level. We show that the dominant BrC chromophores in styrene SOA are nitrophenol compounds (e.g., C₆H₅NO₅, C₆H₅NO_4, and C₆H₅NO₃). These nitrophenol compounds are proposed to primarily originate from the OH oxidation of benzaldehyde, an intermediate product in the oxidation of styrene. Overall, this study presents a comprehensive characterization of the light absorption of styrene SOA under varying oxidation degrees and provides insights into the contribution of styrene-derived BrC and its implications for radiative forcing.