Time resolved infrared spectroscopic analysis of aerosol formed by photo-oxidation of 1,3,5-trimethylbenzene and α-pinene

Secondary organic aerosol generated from the photo-oxidation of 1,3,5- trimethylbenzene and alpha-pinene in a smog chamber was investigated. Fourier Transform Infrared Spectroscopy (FT-IR) was used to monitor the time dependent change of five different functional groups in the aerosol (carboxylic acids, alcohols, organonitrates, ketones/ aldehydes carbonyls, and aliphatic carbon) sampled with an impactor on zinc- selenide discs. Based on model compounds for oxidation products of 1,3,5- trimethylbenzene and alpha- pinene, calibration factors for the different functional groups were calculated, and relative molar fractions of the functional groups were estimated from the analysis of the FT-IR spectra of the smog chamber samples. We show chemical evolutions of secondary organic aerosol on a time scale of up to 20 h. Time series with up to eight measurements per experiment show a strong increase in the relative amounts of carboxylic acid groups and a moderate increase of alcohol and carbonyl groups, whereas the relative amounts of organonitrates and the aliphatic carbon decrease. These findings support the assumption that the chemical composition of the aerosol continues to change for a long time after the particle formation has considerably slowed down. According to these observed changes with time, average, sum formulas of the molecules in the secondary organic aerosol are suggested.