Fourier Transform Infrared Spectroscopy for Identification and Quantification of Organic Functional Groups in Aqueous Phase Secondary Organic Aerosol

Particles in the atmosphere influence visibility, climate, and human health. Secondary organic aerosols (SOA) formed from chemical reactions in the atmosphere constitute a portion of total organic particle mass. Most research on SOA has focused on gas phase reactions; however, reactions taking place in cloud and fog drops may be significant. One group of water-soluble compounds that participate in these reactions is phenols. Phenols, emitted from biomass burning, react in the aqueous phase to form low-volatility SOA products. The products formed from these reactions are currently poorly characterized. In order to characterize laboratory-generated samples, we are developing an attenuated total reflectance-Fourier transform infrared spectroscopic (ATR-FTIR) technique to identify and quantify organic functional groups in SOA. Aqueous SOA is made in the laboratory by illuminating solutions of phenolic compounds with an oxidant. The illuminated solution is then blown to dryness in order to determine the mass of SOA produced. The dry SOA is reconstituted in water and drops of this solution are placed onto a single-reflection ATR accessory. In order to identify and quantify functional groups in the complex SOA samples, it is necessary to calibrate with compounds and mixtures of compounds containing bond types similar to those found in the laboratory-generated SOA. Initially, focus has been placed on multiple peaks located in the region between 1800 cm-1 and 800 cm-1, including peaks for C=O and C-O. We distinguish between characteristic absorbances to begin determining the organic functional group composition of the SOA samples. This ATR-FTIR technique complements information from mass spectrometry measurements and allows us to quantify organic mass for non-volatile SOA products.