Stable carbon isotope composition of secondary organic aerosol fromβ-pinene oxidation

A chamber study was carried out to investigate the stable carbon isotopic composition (delta C-13) of secondary organic aerosol (SOA) formed from ozonolysis of beta-pinene. beta-Pinene (600 ppb) with a known delta C-13 value (-30.1%) and 500 ppb ozone were injected into the chamber in the absence of light and the resulting SOA was collected on preheated quartz fiber filters. Furthermore, delta C-13 values of the gas-phase beta-pinene and one of its oxidation products, nopinone, were measured using a gas chromatograph coupled to an isotope ratio mass spectrometer (GC-IRMS). beta-Pinene was progressively enriched with the heavy carbon isotope due to the kinetic isotope effect (KIE). The KIE of the reaction of beta-pinene with ozone was measured to be 1.0026 ((O3)epsilon 2.6 +/- 1.5 parts per thousand). The delta C-13 value of total secondary organic aerosol was very similar to that of its precursor (average = -29.6 +/- 0.2 parts per thousand) independent of experiment time. Nopinone, one of the major oxidation products of beta-pinene, was found in both the gas and aerosol phases. The gas-phase nopinone was heavier than the initial beta-pinene by 1.3 parts per thousand but lighter than the corresponding aerosol-phase nopinone. On average, the gas-phase nopinone was lighter by 2.3 parts per thousand than the corresponding aerosol- phase nopinone. The second product found in the SOA was detected as acetone, but it desorbed from the filter at a higher temperature than nopinone, which indicates that it is a pyrolysis product. The acetone showed a much lower delta C-13 (-36.6%) compared to the initial beta-pinene delta C-13.