Predicting Real Refractive Index of Organic Aerosols From Elemental Composition

Accurate estimates of aerosol refractive index (RI) are critical for modeling aerosol-radiation interaction, yet this information is limited for ambient organic aerosols, leading to large uncertainties in estimating aerosol radiative effects. We present a semi-empirical model that predicts the real RI n of organic aerosol material from its widely measured oxygen-to-carbon (O:C) and hydrogen-to-carbon (H:C) elemental ratios. The model was based on the theoretical framework of Lorenz-Lorentz equation and trained with n-values at 589 nm (n589nm ${n}_{589\mathrm{n}\mathrm{m}}$) of 160 pure compounds. The predictions can be expanded to predict n-values in a wide spectrum between 300 and 1,200 nm. The model was validated with newly measured and literature datasets of n-values for laboratory secondary organic aerosol (SOA) materials. Uncertainties of n589nm ${n}_{589\mathrm{n}\mathrm{m}}$ predictions for all SOA samples are within & PLUSMN; $\pm $5%. The model suggests that n589nm ${n}_{589\mathrm{n}\mathrm{m}}$-values of organic aerosols may vary within a relatively small range for typical O:C and H:C values observed in the atmosphere.