Analytical Chemistry Seminar: Doug Day, and Swaleha Inamdar, ANYL researchers

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

Doug Day,
(ANYL Research Scientist, Jimenez group)

"Dark chamber experiments were conducted to study the SOA formed from the oxidation of α-pinene and Δ-carene under different peroxy radical (RO₂) fate regimes: RO₂ + NO₃, RO₂ + RO₂, and RO₂ + HO₂. SOA mass yields from α-pinene oxidation were <1 to ∼25% and strongly dependent on available OA mass up to ∼100 μg m–3. The strong yield dependence of α-pinene oxidation is driven by absorptive partitioning to OA and not by available surface area for condensation. Yields from Δ-carene + NO₃ were consistently higher, ranging from ∼10–50% with some dependence on OA for <25 μg 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 ∼0.4–0.6 and nitrate/organic mass ratios of ∼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 Δ-carene more readily forms low-volatility gas-phase highly oxygenated molecules (HOMs) than α-pinene, which primarily forms volatile and semivolatile species, when reacted with NO₃, regardless of RO2 regime. The similar Δ-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 Δ-carene + NO₃. The scarcity of peroxide functional groups (on average, 14% of C10 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 α-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."

and

A study of iodine chemistry in the Indian and Southern Ocean marine boundary layer

Swaleha Inamdar,
(ANYL Professional Research Assistant, Volkamer Group)

"Reactive atmospheric iodine significantly affects the tropospheric chemistry by ozone (O₃) depletion and participation in various heterogeneous reaction cycles mainly involving O₃, causing changes to the atmospheric oxidation capacity. There is growing experimental evidence that halogen chemistry plays a key role as part of tropospheric photochemistry. Much of the proposed halogen chemistry in literature is propagated through the reactions of a series of halogen atom and radicals. Although, some measurements of iodine oxide (IO) are reported around the globe, the remote open ocean remains undersampled. Recently, first reported observations of IO in the Indian and Southern Ocean marine boundary layer (MBL) suggest that atmospheric IO may not be well correlated with the inorganic fluxes. Further, biogenic fluxes play a significant role in active iodine chemistry in this region. This contradicts with the studies reporting that inorganic iodine emissions are responsible for 75% of the reactive iodine in the tropical Atlantic MBL. Thus, to improve our understanding of iodine chemistry in this region, field observations in the remote open ocean of the Southern Ocean are carried out in this study via several ship-based expeditions in this region. To identify the geographical emissions of reactive iodine precursors, this study compiles observations from the Indian and Southern Ocean MBL for a comprehensive region-specific parameterization for estimating the inorganic iodine fluxes via sea surface iodide concentrations. This study addresses whether the existing parameterization tools for inorganic iodine fluxes are sufficient to explain the detected IO in the remote open ocean MBL."