Secondary Organic Aerosol Mass Yields from NO3 Oxidation of Α-Pinene and Δ-Carene: Effect of RO2 Radical Fate

Dark chamber experiments were conducted to study the SOA formed from the oxidation of alpha-pinene and Delta-carene under different peroxy radical (RO2) fate regimes: RO2 + NO3, RO2 + RO2, and RO2 + HO2. SOA mass yields from alpha-pinene oxidation were <1 to similar to 25% and strongly dependent on available OA mass up to similar to 100 mu g m(-3). The strong yield dependence of alpha-pinene oxidation is driven by absorptive partitioning to OA and not by available surface area for condensation. Yields from Delta-carene + NO3 were consistently higher, ranging from similar to 10-50% with some dependence on OA for <25 mu g m(-3). Explicit kinetic modeling including vapor wall losses was conducted to enable comparisons across VOC precursors and RO2 fate regimes and to determine atmospherically relevant yields. Furthermore, SOA yields were similar for each monoterpene across the nominal RO2 + NO3, RO2 + RO2, or RO2 + HO2 regimes; thus, the volatility basis sets (VBS) constructed were independent of the chemical regime. Elemental O/C ratios of similar to 0.4-0.6 and nitrate/organic mass ratios of similar to 0.15 were observed in the particle phase for both monoterpenes in all regimes, using aerosol mass spectrometer (AMS) measurements. An empirical relationship for estimating particle density using AMS-derived elemental ratios, previously reported in the literature for non-nitrate containing OA, was successfully adapted to organic nitrate-rich SOA. Observations from an NO3- chemical ionization mass spectrometer (NO3-CIMS) suggest that Delta-carene more readily forms low-volatility gas-phase highly oxygenated molecules (HOMs) than alpha-pinene, which primarily forms volatile and semivolatile species, when reacted with NO3, regardless of RO2 regime. The similar Delta-carene SOA yields across regimes, high O/C ratios, and presence of HOMs, suggest that unimolecular and multistep processes such as alkoxy radical isomerization and decomposition may play a role in the formation of SOA from Delta-carene + NO3. The scarcity of peroxide functional groups (on average, 14% of C-10 groups carried a peroxide functional group in one test experiment in the RO2 + RO2 regime) appears to rule out a major role for autoxidation and organic peroxide (ROOH, ROOR) formation. The consistently substantially lower SOA yields observed for alpha-pinene + NO3 suggest such pathways are less available for this precursor. The marked and robust regime-independent difference in SOA yield from two different precursor monoterpenes suggests that in order to accurately model SOA production in forested regions the chemical mechanism must feature some distinction among different monoterpenes.