Demonstration of Quantitative Mid-Infrared Hydrocarbon Aerosol Absorption Spectroscopy Using a Dual-Cell Photoacoustic System.

We report the development and demonstration of a simplified and robust photoacoustic (PA) technique by integrating an External Cavity Interband Cascade Laser (EC-ICL) with a dual PA cell configuration for the quantitative characterization of hydrocarbon aerosols in the mid-infrared range of 3.30 to 3.55 μm. The EC-ICL source was selected for its stable optical output, compact design, and low power consumption. By flowing a known gas through one PA cell, a wavelength-dependent transfer function (TF) is established and subsequently used to retrieve absolute absorption coefficients from the analyte aerosol measured in the second PA cell. As a proof of concept, propane was employed as the reference gas and diethyl-hexyl-sebacate (DEHS) aerosols as the analyte. Validation against bulk ellipsometry demonstrates a mean deviation of approximately 2.1% between the photoacoustically derived absorption coefficients and Rayleigh-theory predictions, establishing the quantitative accuracy of the transfer-function methodology. The proposed approach eliminates the need for external power normalization and frequent calibration procedures, providing a practical framework for quantitative mid-infrared aerosol absorption spectroscopy with applications in atmospheric aerosol characterization, source apportionment, and industrial emission monitoring.