Secondary Organic Aerosol Mass Yields and Composition from NO3 Oxidation of α-pinene and Δ-carene: Effect of RO2 Radical Fate
DOUGLAS DAY, Juliane Fry, Hyun-Gu Kang, Jordan Krechmer, Benjamin Ayres, Natalie Keehan, Samantha Thompson, Weiwei Hu, Pedro Campuzano-Jost, Jason Schroder, Harald Stark, Marla DeVault, Paul Ziemann, Kyle Zarzana, Robert Wild, William Dubè, Steven S. Brown, Jose-Luis Jimenez,
CIRES, University of Colorado, Boulder Abstract Number: 103
Working Group: Aerosol Chemistry
AbstractDark chamber experiments were conducted to study SOA formed from α-pinene and Δ-carene oxidation by NO
3 under different peroxy radical (RO
2) fate regimes: RO
2 + NO
3, RO
2 + RO
2, and RO
2 + HO
2. SOA mass yields from α-pinene oxidation were <1-25% and strongly dependent on OA mass up to ~100 μg m
-3. Yields from Δ-carene were consistently higher, ranging ~10-45% with some dependence on OA for <25 μg m
-3. Explicit kinetic modeling including vapor wall-losses was conducted to enable comparisons across precursors and RO
2 fate regimes, and to determine atmospherically-relevant yields. SOA yields were similar across nominal RO
2 fate regimes, and thus the Volatility Basis Sets constructed were independent of the chemical regime. Elemental O/C ratios were ~0.4-0.6 (including nitrate groups) and nitrate:organic mass ratios ~0.16 for both monoterpenes in all regimes. A previously reported empirical relationship for estimating particle density using elemental ratios for non-nitrate containing OA, was shown to be applicable to organic nitrate-rich SOA after adaptation.
Observations suggest that Δ-carene more readily forms low-volatility gas-phase highly oxygenated molecules (HOMs) than α-pinene, which primarily forms volatile and semi-volatile species. The similar Δ-carene SOA yields across regimes, high O/C ratios, and presence of HOMs, suggest that unimolecular and multi-step processes such as alkoxy radical isomerization and decomposition may play a role in SOA formation from Δ-carene. The scarcity of peroxide functional groups (14% of C10 groups carried a peroxide group in one RO
2+RO
2 test experiment) appears to rule out a major role for auto-oxidation and organic peroxide formation. The substantially lower SOA yields for α-pinene suggest such pathways are less available for this precursor. The marked and regime-robust difference in SOA yield from two monoterpenes suggests that in order to accurately model SOA production in forested regions, the chemical mechanism must feature some distinction among different monoterpenes.