Light absorption by brown carbon due to individual components and their complexes

Brown carbon aerosol is a complex mixture of light-absorbing organic compounds suspended in the air. Brown carbon has complex effects on the radiative balance on Earth, as it may contribute to warming the planet through absorption of light and to cooling through scattering. In addition to these interactions with radiation, it has interactions with water, as well, influencing cloud formation and properties. The direct effect of brown carbon through interactions with radiation is not adequately understood, so there is significant uncertainty in the magnitude and direction of this effect on radiative forcing. One source of this uncertainty is the complexity of the mixture, which is composed of hundreds and thousands of functionalized organic compounds, including methoxyphenols and quinones. With only few exceptions, these compounds are not available commercially for use as standards, so typically identifications are tentative, detections are non-quantitative, and mass and molar absorption coefficients are unknown. Here, we first demonstrate the utility of time-dependent density functional theory as a computationally affordable method to predict the molar absorption coefficients of a homologous series of cinnamaldehydes, including coniferaldehyde and sinapaldehyde, which are two of the most abundant methoxyphenols generated from biomass burning. The predicted UV-vis absorption spectra show good agreement with experimental results. We next posed the following question: even if we could know the mass and molar absorption coefficients of every organic component in brown carbon, would we know the total absorption of their mixture – or do non-additive intermolecular interactions play a role? To address this question, we next demonstrate that the light absorption of mixtures of guaiacol and benzoquinone in dilute bulk solutions, upon liquid-liquid partitioning, and upon gasliquid partitioning in thin disks is greater than is expected from the sum of their individual molar absorptivities. Kinetic, thermodynamic, and composition analyses support the conclusion that this enhanced absorption comes from non-covalent complexes rather than covalent products. Finally, we demonstrate that similarly enhanced light absorption due to intermolecular interactions occurs for a series of other liquid methoxyphenols, in addition to guaiacol, in both the previous setup with thin disks as well as a new setup with acoustically levitated microdroplets. Together, these studies of the fundamental origins of the light absorption of brown carbon can help improve our understanding of how biomass burning impacts Earth’s climate.