Formation of Polycyclic Aromatic Hydrocarbon Oxidation Products in α-Pinene Secondary Organic Aerosol Particles Formed through Ozonolysis

Abstract

Accurate long-range atmospheric transport (LRAT) modeling of polycyclic aromatic hydrocarbons (PAHs) and PAH oxidation products (PAH-OPs) in secondary organic aerosol (SOA) particles relies on the known chemical composition of the particles. Four PAHs, phenanthrene (PHE), dibenzothiophene (DBT), pyrene (PYR), and benz(a)anthracene (BaA), were studied individually to identify and quantify PAH-OPs produced and incorporated into SOA particles formed by ozonolysis of α-pinene in the presence of PAH vapor. SOA particles were characterized using real-time in situ instrumentation, and collected on quartz fiber filters for offline analysis of PAHs and PAH-OPs. PAH-OPs were measured in all PAH experiments at equal or greater concentrations than the individual PAHs they were produced from. The total mass of PAH and PAH-OPs, relative to the total SOA mass, varied for different experiments on individual parent PAHs: PHE and 6 quantified PHE-OPs (3.0%), DBT and dibenzothiophene sulfone (4.9%), PYR and 3 quantified PYR-OPs (3.1%), and BaA and benz(a)anthracene-7,12-dione (0.26%). Further exposure of PAH-SOA to ozone generally increased the concentration ratio of PAH-OPs to PAH, suggesting longer atmospheric lifetimes for PAH-OPs, relative to PAHs. These data indicate that PAH-OPs are formed during SOA particle formation and growth.

Figure 1.

Experimental setup: SOA was produced by injecting 400 ppb α-pinene, 600–800 ppb ozone, and 100 ppm cyclohexane (to act as OH radical scrubber) into a ~100 L Teflon reaction chamber filled with zero air equilibrated with PAH vapor from solids. Particle growth was monitored using SMPS. When particle growth stopped (~ 1 hour), the fresh filter (A) was collected by directing a flow of the sample through charcoal denuders, to remove gas phase organic molecules, and onto a QFF in an inline filter trap, after real-time in-situ particle characterization was performed using single particle mass spectrometer (miniSPLAT). After fresh SOA collection was complete, the sample flow was directed into an oxidation flow tube reactor (PAM), for ozone exposure samples (B), allowed to mix, and again analyzed using miniSPLAT and collected using the inline filter trap.

Figure 2.

A) Mean wt% (± 1 standard error) of compounds measured in collected freshly formed and ozone-exposed PHE-SOA, measured using GC/MS. * indicates a statistically convincing evidence of a difference (p-value < 0.10) in the measured concentration (wt%) from fresh to ozone-exposed. ‡ indicates a statistically significant difference in measured concentration of PHE-OP than the PHE in ozone exposed PHE-SOA. B) Mobility size distributions of pure α-P SOA particles (blue) and PHE-SOA particles formed by ozonolysis of α-pinene in the presence of gas-phase PHE (red). C) Evaporation kinetics of α-P SOA (blue), PHE-SOA (red), and PHE-SOA exposed to additional ozone (green) measured as the volume fraction of particles remaining over time in minutes, measured with single particle mass spectrometer (miniSPLAT).

Figure 3.

HR-ToF-AMS data for the four PAHs tested in this study. Each plot shows the relative abundance of the peaks plotted in log scale as a function of the m/z measured. Each plot has α-pinene SOA (blue) with the PAH tested; A = phenanthrene, B = dibenzothiophene, C = pyrene, D = benz(a)anthracene. Each plot has an arrow pointing to the experimental PAH peak, and m/z peaks which may be associated with PAH-OPs based on previous works.