Stable carbon isotope composition of secondary organic aerosol from <i>β</i>‐pinene oxidation

Abstract

A chamber study was carried out to investigate the stable carbon isotopic composition ( δ 13 C) of secondary organic aerosol (SOA) formed from ozonolysis of β ‐pinene. β ‐Pinene (600 ppb) with a known δ 13 C 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, δ 13 C values of the gas‐phase β ‐pinene and one of its oxidation products, nopinone, were measured using a gas chromatograph coupled to an isotope ratio mass spectrometer (GC‐IRMS). β ‐Pinene was progressively enriched with the heavy carbon isotope due to the kinetic isotope effect (KIE). The KIE of the reaction of β ‐pinene with ozone was measured to be 1.0026 2.6 ± 1.5‰). The δ 13 C value of total secondary organic aerosol was very similar to that of its precursor (average = −29.6 ± 0.2‰) independent of experiment time. Nopinone, one of the major oxidation products of β ‐pinene, was found in both the gas and aerosol phases. The gas‐phase nopinone was heavier than the initial β ‐pinene by 1.3‰ but lighter than the corresponding aerosol‐phase nopinone. On average, the gas‐phase nopinone was lighter by 2.3‰ 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 δ 13 C (−36.6‰) compared to the initial β ‐pinene δ 13 C.