Infrared Optical Constants of Organic Aerosols: Organic Acids and Model Humic-Like Substances (Hulis)

Aerosols are important atmospheric constituents as they impact the Earth's energy balance and climate. An analysis of the impact of aerosols depends on the detailed knowledge of aerosol optical properties. However, there is a lack of refractive index data for atmospherically relevant organic compounds in the infrared (IR) region which complicates the quantitative estimation of the aerosol influence on the radiative balance. In this study we investigate the optical properties of atmospherically relevant carboxylic acids and HUmic-LIke Substances (HULIS) proxies. Aerosol size distributions are measured simultaneously with Fourier transform infrared (FTIR) extinction spectra to calculate the complex refractive index. Scanning electron microscopy (SEM) images are also collected to investigate particle shape. Analysis of SEM images shows evidence for agglomeration at higher concentrations in some of the samples. The experimentally measured IR resonances do not appear to be highly sensitive to agglomeration effects. However, there is an increase in the scattering efficiency at shorter wavelengths as the result of larger overall particle size of the agglomerates. Refractive indices are retrieved from the IR extinction spectra of organic acids and HULIS proxies. Mie simulation results confirm the quality of the retrieved optical constants. The optical constants determined for the acids are in agreement with the published data for fire smoke plumes.