Complexities in Modeling Organic Aerosol Light Absorption

Aerosol particles dynamically evolve in the atmosphere
by physicochemical
interactions with sunlight, trace chemical species, and water. Current
modeling approaches fix properties such as aerosol refractive index,
introducing spatial and temporal errors in the radiative impacts.
Further progress requires a process-level description of the refractive
indices as the particles age and experience physicochemical transformations.
We present two multivariate modeling approaches of light absorption
by brown carbon (BrC). The initial approach was to extend the modeling
framework of the refractive index at 589 nm (nD), but that result was insufficient. We developed a second
multivariate model using aromatic rings and functional groups to predict
the imaginary part of the complex refractive index. This second model
agreed better with measured spectral absorption peaks, showing promise
for a simplified treatment of BrC optics. In addition to absorption,
organic functionalities also alter the water affinity of the molecules,
leading to a hygroscopic uptake and increased light absorption, which
we show through measurements and modeling.