Photochemical formation of large molecular weight oligomers from low-molecular weight keto-carbonyls in haze aerosol

Abstract

The optical properties of the atmospheric aerosol play a fundamental role in the Earth's radiative balance. Since ~60% of the aerosol mass consists of complex natural and anthropogenic organic matter that absorbs in the ultraviolet and, particularly, in the visible regions of the solar spectrum, it is important to establish the chemical identity of the organic chromophores and the mechanism by which they contribute to aerosol color. Here we report studies on the chemical composition versus chromism of photolyzed (λ > 305 nm) solutions of PA, under a variety of experimental conditions that include substrate concentration, temperature and the presence of relevant spectator solutes, such ammonium sulfate. We use high resolution mass and ^1H, ^(13)C NMR spectrometries to track chemical speciation in photolyzed solutions as they go through thermochromic and photobleaching cycles. The chemical identity of the components of these mixtures does not change in these cycles, in which photobleached solutions gradually recover their yellow color in the dark with non-conventional kinetics typical of aggregation processes, we infer that visible absorptions involve the intermolecular coupling of carbonyl chromophores in supramolecular assemblies.