Predicting Real Refractive Index of Organic Aerosols from Elemental Composition
YAOWEI LI, Bin Bai, John Dykema, Nara Shin, Andrew Lambe, Qi Chen, Mikinori Kuwata, Nga Lee Ng, Frank Keutsch, Pengfei Liu,
Harvard University Abstract Number: 357
Working Group: Aerosol Physics
AbstractAtmospheric aerosol particles have a significant impact on the Earth's climate by interacting with radiation and water. Despite their large contribution to sub-micrometer aerosol particle mass, our understanding of the optical properties of atmospheric organic aerosols (OA) is still limited. One of the challenges is that the refractive index, i.e., the intrinsic optical constant of OA material, is poorly constrained. The lack of knowledge on the refractive index of OA can cause large uncertainties in estimating their optical properties and radiative effects on climate.
Here we present a semiempirical model that predicts the real refractive index
n of OA material from its widely measured oxygen-to-carbon (O:C) and hydrogen-to-carbon (H:C) elemental ratios. The model was developed based on the theoretical framework of Lorenz-Lorentz equation and trained with
n-values measured at 589 nm (
n589nm) of 160 pure compounds. The predictions can be expanded to predict n-values in a wide spectrum between 300 and 1200 nm. The model was validated with newly measured and literature datasets of
n-values for various laboratory secondary organic aerosol (SOA) materials. The uncertainties of n589nm predictions for all SOA samples are within ±5%. The model suggests that the
n589nm values of OA, consisting mainly of C, H, and O, may vary within a relatively small range for typical O:C and H:C values observed in the atmosphere. Our results support a simplified representation of using a constant
n value for ambient OA consisting mainly of C, H, and O in large-scale Earth system models. However, our parameterization can be also readily implemented in process-based aerosol models with OA elemental ratios represented.
This abstract has supplementary material available. Click PDF icon to download.