Chemical and isotopic composition of secondary organic aerosol generated by α-pinene ozonolysis

Secondary organic aerosol (SOA) plays a central role in air pollution and climate. However, the description of the sources and mechanisms leading to SOA is elusive despite decades of research. While stable isotope analysis is increasingly used to constrain sources of ambient aerosol, in many cases it is difficult to apply because neither the isotopic composition of aerosol precursors nor the fractionation of aerosol forming processes is well characterised. In this paper, SOA formation from ozonolysis of alpha-pinene an important precursor and perhaps the best-known model system used in laboratory studies-was investigated using position-dependent and average determinations of C-13 in alpha-pinene and advanced analysis of reaction products using thermal-desorption proton-transfer-reaction mass spectrometry (PTR-MS). The total carbon (TC) isotopic composition delta C-13 of the initial alpha-pinene was measured, and the delta C-13 of the specific carbon atom sites was determined using position specific isotope analysis (PSIA). The PSIA analysis showed variations at individual positions from -6.9 to +10.5 % relative to the bulk composition. SOA was formed from alpha-pinene and ozone in a constant-flow chamber under dark, dry, and low-NOx, conditions, with OH scavengers and in the absence of seed particles. The excess of ozone and long residence time in the flow chamber ensured that virtually all alpha-pinene had reacted. Product SOA was collected on two sequential quartz filters. The filters were analysed offline by heating them stepwise from 100 to 400 degrees C to desorb organic compounds that were (i) detected using PTR-MS for chemical analysis and to determine the o : C ratio, and (ii) converted to CO2 for C-13 analysis. More than 400 ions in the mass range 39-800 Da were detected from the desorbed material and quantified using a PTR-MS. The largest amount desorbed at 150 degrees C. The 0 : C ratio of material from the front filter increased from 0.18 to 0.25 as the desorption temperature was raised from 100 to 250 degrees C. At temperatures above 250 degrees C, the o : C ratio of thermally desorbed material, presumably from oligomeric precursors, was constant. The observation of a number of components that occurred across the full range of desorption temperatures suggests that they are generated by thermal decomposition of oligomers. The isotopic composition of SOA was more or less independent of desorption temperature above 100 degrees C. TC analysis showed that SOA was enriched in C-13 by 0.6-1.2%c relative to the initial cy-pinene. According to mass balance, gas phase products will be depleted relative to the initia alpha-pinene. Accordingly, organic material on the back filters, which contain adsorbed gas-phase compounds, is depleted in C-13 in TC by 0.7 % relative to the initial alpha-pinene, and by 1.3 % compared to SOA collected on the front filter. The observed difference in C-13 between the gas and particle phases may arise from isotope-dependent changes in the branching ratios in the alpha-pinene + O-3 reaction. Alternatively, some gas phase products involve carbon atoms from highly enriched and depleted sites, as shown in the PSIA analysis, giving a non-kinetic origin to the observed fractionations. In either case, the present study suggests that the site-specific distribution of C-13 in the source material itself governs the abundance of C-13 in SOA.