Further evidence for charge transfer complexes in brown carbon aerosols from excitation-emission matrix fluorescence spectroscopy.

The light-absorbing fraction of organic molecules in ambient aerosols, known as "brown carbon," is an important yet poorly characterized component. Despite the fact that brown carbon could alter the radiative forcing of aerosols significantly, identification of specific chromophores has remained challenging. We recently demonstrated that charge transfer (CT) complexes formed in organic molecules could be responsible for a large fraction of absorption observed in water-extracted ambient particulate matter.1 In the present study, we use excitation-emission matrix fluorescence spectroscopy to further corroborate the importance of CT complexes in defining aerosol optical properties. Monotonically increasing and decreasing quantum yields, decreasing Stokes shifts, and red-shifting emission maxima are observed from ambient particulate matter collected in Athens, Georgia, strongly suggesting that a superposition of independent chromophores is not sufficient to explain brown carbon absorption and fluorescence. Instead, we show that a model in which such chromophores are energetically coupled to a dense manifold of CT complexes is consistent with all of the observations. Further, we suggest that a significant fraction of the observed fluorescence originates from CT complexes and that their contribution to brown carbon absorption is likely greater than we reported previously.