Light Absorption by Cinnamaldehyde Constituents of Biomass Burning Organic Aerosol Modeled Using Time-Dependent Density Functional Theory

Abstract

Organic aerosol emitted from biomass burning absorbs visible radiation. However, the impact of this light absorption on the overall climate effects of atmospheric aerosol is not well known, partly due to variability in particle composition and absorptivity. Cinnamaldehydes, which consist of an aromatic ring with an unsaturated aldehyde substituent, are an important class of chromophores in light-absorbing organic aerosol, or brown carbon. Here, light absorption by a homologous series of three cinnamaldehydes─coumaraldehyde, coniferaldehyde, and sinapaldehyde─is modeled with time-dependent density functional theory (TD-DFT) calculations, in the gas and aqueous phases. Based on a survey of hydration and acid dissociation equilibria, the neutral aldehyde is expected to be the predominant form of each species in the atmospheric aqueous phase. These species have complicated conformational landscapes compared to many other brown carbon constituents, like rigid polycyclic aromatic hydrocarbons. For coumaraldehyde, coniferaldehyde, and sinapaldehyde, a total of 8, 26, and 18 conformers were located, respectively. For each species, most of the total population is accounted for by the four most-populated conformers. The relative contributions of the conformers to the total light absorption of the respective species are dictated more by differences in the relative free energies than by differences in the molar absorption coefficients. As the functionalization increases, the absorption is red-shifted. The peaks predicted in water agree well with experimental spectra of coniferaldehyde and sinapaldehyde. No conformers have vertical transitions in the visible spectral range, so absorption above 380 nm is due to the shoulders of transitions of major conformers at ultraviolet wavelengths. These results demonstrate the importance of exploring potential energy landscapes, determining conformer stability and absorptivity, to predict the light absorption of chromophores in brown carbon.