Chemical and isotopic composition of secondary organic aerosol generated by <i>α</i>-pinene ozonolysis
Abstract. Secondary organic aerosol (SOA) plays a central role in air pollution and climate. However, an exact description of the sources and mechanisms leading to SOA is elusive despite decades of research. Stable isotope analysis may help to constrain atmospheric SOA budgets but the isotope effects associated with the underlying processes have to be determined in order to do so. In this paper, SOA formation from ozonolysis of α-pinene – an important precursor and perhaps the best-known model system used in laboratory studies – was investigated using stable carbon isotope analysis, position-specific isotope analysis (PSIA), and high-resolution chemical analysis based on a thermal-desorption proton-transfer-reaction mass-spectrometer (PTR-MS). SOA was formed in a constant-flow chamber under dark, dry and low-NOx conditions, with OH scavengers in the absence of seed particles. Product SOA was collected on doubly-stacked quartz filters (front and back filters). During analysis, the filters were heated stepwise over the range of 100–400 °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 13C analysis. In addition, the total carbon isotopic composition of selected samples was measured. For the first time PSIA has been performed for α-pinene. More than 400 ions in the mass range from 39–800 Da were detected and quantified using the PTR-MS. The largest mass fraction desorbed from the filters at 150 °C. The measured O:C ratio of front filter material increased from 0.18 to 0.25 as the desorption temperature was raised from 100 to 250 °C. The rising trend is consistent with the fact that functionalization decreases the volatility of chemical species. At temperatures above 250 °C the O:C ratio of thermally desorbed material, presumably from oligomeric precursors, was constant. The observation of a number of components across the full range of desorption temperatures suggests that they are generated by thermal decomposition of oligomers. SOA on front filters was enriched in 13C by 0.2–2.9 ‰ relative to the initial α-pinene, at all desorption temperatures. The total carbon isotopic composition was similar to the enrichment of the major fraction desorbing at 150 °C. Gas-phase compounds desorbing from the back filters showed much lower concentrations but were depleted in 13C by 0.7 ‰ compared to the initial α-pinene and by 1.9 ‰ compared to the corresponding front filter. PSIA showed that the isotope enrichment at individual carbon positions in α-pinene ranged from −6.9 to +10.5 ‰ relative to the bulk composition. However, there was not a clear mechanistic connection between those values and the observed isotopic enrichment of bulk SOA. Instead, fragmentation reactions favouring the loss of small, isotopically light products to the gas phase are consistent with the observations. In monoterpene ozonolysis, functionalization is known to follow fragmentation, however it is the fragmentation step that seems to govern the isotope budget. The isotope effect associated with oligomerization is small. The suggested isotope effects are important for the interpretation of isotopic compositions of ambient aerosol.
