Unraveling the Light‐Absorbing Properties of Brown Carbon at a Molecular Level

Abstract Brown carbon (BrC) exhibits a highly complex chemical composition with diverse light‐absorbing properties, which complicates our understanding of its climate impacts. This study examined the impact of molecular characteristics (including molecular mass, unsaturation, oxidation state, and polarity) and heteroatoms on the light‐absorbing properties (absorptivity and wavelength dependence) of BrC from a molecular perspective, based on the ultraviolet‐visible spectra of over 40,000 light‐absorbing substances in aerosol from different sources and ambience. Our findings reveal that the light‐absorptivity of BrC molecules increases with decreasing polarity and O/C ratio, while it rises with higher molecular mass and unsaturation. We developed predictive models for molecular absorptivity based on its double bond equivalent and O/C ratio. In addition, we observed an inverse correlation between absorptivity and wavelength dependence at the molecular level, as determined through mathematical analysis. This molecular‐level understanding provides valuable insights into BrC absorbing mechanisms, facilitating more accurate characterization in atmospheric models.