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1. Temperature, humidity, and ionisation effect of iodine oxoacid nucleation

   https://doi.org/10.1039/d4ea00013g  

   Rörup, Birte; He, Xu-Cheng; Shen, Jiali; Baalbaki, Rima; Dada, Lubna; Sipilä, Mikko; Kirkby, Jasper; Kulmala, Markku; Amorim, Antonio; Baccarini, Andrea; et al (May 2024, Environmental Science: Atmospheres)

   Iodine oxoacids are recognised for their significant contribution to the formation of new particles in marine and polar atmospheres. Nevertheless, to incorporate the iodine oxoacid nucleation mechanism into global simulations, it is essential to comprehend how this mechanism varies under various atmospheric conditions. In this study, we combined measurements from the CLOUD (Cosmic Leaving OUtdoor Droplets) chamber at CERN and simulations with a kinetic model to investigate the impact of temperature, ionisation, and humidity on iodine oxoacid nucleation. Our findings reveal that ion-induced particle formation rates remain largely unaffected by changes in temperature. However, neutral particle formation rates experience a significant increase when the temperature drops from +10 ^oC to −10 ^oC. Running the kinetic model with varying ionisation rates demonstrates that the particle formation rate only increases with a higher ionisation rate when the iodic acid concentration exceeds 1.5 × 10⁷ cm^sup>−3, a concentration rarely reached in pristine marine atmospheres. Consequently, our simulations suggest that, despite higher ionisation rates, the charged cluster nucleation pathway of iodic acid is unlikely to be enhanced in the upper troposphere by higher ionisation rates. Instead, the neutral nucleation channel is likely to be the dominant channel in that region. Notably, the iodine oxoacid nucleation mechanism remains unaffected by changes in relative humidity from 2% to 80%. However, under unrealistically dry conditions (below 0.008% RH at +10 ^oC), iodine oxides (I₂O₄ and I₂O₅) significantly enhance formation rates. Therefore, we conclude that iodine oxoacid nucleation is the dominant nucleation mechanism for iodine nucleation in the marine and polar boundary layer atmosphere. 

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2. Ozone depletion due to dust release of iodine in the free troposphere

   https://doi.org/10.1126/sciadv.abj6544  

   Koenig, Theodore K.; Volkamer, Rainer; Apel, Eric C.; Bresch, James F.; Cuevas, Carlos A.; Dix, Barbara; Eloranta, Edwin W.; Fernandez, Rafael P.; Hall, Samuel R.; Hornbrook, Rebecca S.; et al (December 2021, Science Advances)

   Iodine is an atmospheric trace element emitted from oceans that efficiently destroys ozone (O 3 ). Low O 3 in airborne dust layers is frequently observed but poorly understood. We show that dust is a source of gas-phase iodine, indicated by aircraft observations of iodine monoxide (IO) radicals inside lofted dust layers from the Atacama and Sechura Deserts that are up to a factor of 10 enhanced over background. Gas-phase iodine photochemistry, commensurate with observed IO, is needed to explain the low O 3 inside these dust layers (below 15 ppbv; up to 75% depleted). The added dust iodine can explain decreases in O 3 of 8% regionally and affects surface air quality. Our data suggest that iodate reduction to form volatile iodine species is a missing process in the geochemical iodine cycle and presents an unrecognized aeolian source of iodine. Atmospheric iodine has tripled since 1950 and affects ozone layer recovery and particle formation. 

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3. Isoprene Epoxydiol-Derived Sulfated and Non-Sulfated Oligomers Suppress Particulate Mass Loss during Oxidative Aging of Secondary Organic Aerosol

   ARMSTRONG, N. CAZIMIR; Chen, Yuzh; Cui, Tianqu; Zhang, Yue; Zhang, Zhenfa; Turpin, Barbara; Chan, Man Nin; Gold, Avram; Ault, Andrew; Surratt, Jason (October 2022, AAAR 40th Annual Conference)

   Acid-driven multiphase chemistry of isoprene epoxydiols (IEPOX) with inorganic sulfate aerosols contributes substantially to formation of secondary organic aerosol (SOA), which constitutes a large mass fraction of atmospheric fine particulate matter (PM2.5). However, atmospheric chemical sinks of freshly generated IEPOX-SOA particles remain unclear. We examined the role of heterogeneous oxidation of freshly-generated IEPOX-SOA particles by gas-phase hydroxyl radical (•OH) under dark conditions as one potential atmospheric sink. After 4 h of gas-phase •OH exposure (~3x108 molecules cm-3), chemical changes in smog chamber-generated IEPOX-SOA particles were assessed by hydrophilic interaction liquid chromatography coupled with electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (HILIC/ESI-HR-QTOFMS). Comparison of molecular-level compositional changes in IEPOX-SOA particles during aging with or without •OH revealed that decomposition of oligomers by heterogeneous •OH oxidation acts as a sink for •OH and maintains a reservoir of low-volatility compounds including monomeric sulfate esters and oligomer fragments. We propose tentative structures and formation mechanisms for previously uncharacterized SOA constituents in PM2.5. Our results suggest that this •OH-driven renewal of low-volatility products may extend atmospheric lifetimes of IEPOX-SOA particles by slowing production of low-molecular weight, high-volatility organic fragments, and likely contributes to large quantities of 2-methyltetrols and methyltetrol sulfates reported in PM2.5. 

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4. Quantitative constraints on autoxidation and dimer formation from direct probing of monoterpene-derived peroxy radical chemistry

   https://doi.org/10.1073/pnas.1812147115  

   Zhao, Yue; Thornton, Joel A.; Pye, Havala O. (November 2018, Proceedings of the National Academy of Sciences)

   Organic peroxy radicals (RO 2 ) are key intermediates in the atmospheric degradation of organic matter and fuel combustion, but to date, few direct studies of specific RO 2 in complex reaction systems exist, leading to large gaps in our understanding of their fate. We show, using direct, speciated measurements of a suite of RO 2 and gas-phase dimers from O 3 -initiated oxidation of α-pinene, that ∼150 gaseous dimers (C 16–20 H 24–34 O 4–13 ) are primarily formed through RO 2 cross-reactions, with a typical rate constant of 0.75–2 × 10 −12 cm 3 molecule −1 s −1 and a lower-limit dimer formation branching ratio of 4%. These findings imply a gaseous dimer yield that varies strongly with nitric oxide (NO) concentrations, of at least 0.2–2.5% by mole (0.5–6.6% by mass) for conditions typical of forested regions with low to moderate anthropogenic influence (i.e., ≤50-parts per trillion NO). Given their very low volatility, the gaseous C 16–20 dimers provide a potentially important organic medium for initial particle formation, and alone can explain 5–60% of α-pinene secondary organic aerosol mass yields measured at atmospherically relevant particle mass loadings. The responses of RO 2 , dimers, and highly oxygenated multifunctional compounds (HOM) to reacted α-pinene concentration and NO imply that an average ∼20% of primary α-pinene RO 2 from OH reaction and 10% from ozonolysis autoxidize at 3–10 s −1 and ≥1 s −1 , respectively, confirming both oxidation pathways produce HOM efficiently, even at higher NO concentrations typical of urban areas. Thus, gas-phase dimer formation and RO 2 autoxidation are ubiquitous sources of low-volatility organic compounds capable of driving atmospheric particle formation and growth. 

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5. Synergistic HNO3–H2SO4–NH3 upper tropospheric particle formation

   https://doi.org/10.1038/s41586-022-04605-4  

   Wang, Mingyi; Xiao, Mao; Bertozzi, Barbara; Marie, Guillaume; Rörup, Birte; Schulze, Benjamin; Bardakov, Roman; He, Xu-Cheng; Shen, Jiali; Scholz, Wiebke; et al (May 2022, Nature)

   Abstract New particle formation in the upper free troposphere is a major global source of cloud condensation nuclei (CCN) 1–4 . However, the precursor vapours that drive the process are not well understood. With experiments performed under upper tropospheric conditions in the CERN CLOUD chamber, we show that nitric acid, sulfuric acid and ammonia form particles synergistically, at rates that are orders of magnitude faster than those from any two of the three components. The importance of this mechanism depends on the availability of ammonia, which was previously thought to be efficiently scavenged by cloud droplets during convection. However, surprisingly high concentrations of ammonia and ammonium nitrate have recently been observed in the upper troposphere over the Asian monsoon region 5,6 . Once particles have formed, co-condensation of ammonia and abundant nitric acid alone is sufficient to drive rapid growth to CCN sizes with only trace sulfate. Moreover, our measurements show that these CCN are also highly efficient ice nucleating particles—comparable to desert dust. Our model simulations confirm that ammonia is efficiently convected aloft during the Asian monsoon, driving rapid, multi-acid HNO 3 –H 2 SO 4 –NH 3 nucleation in the upper troposphere and producing ice nucleating particles that spread across the mid-latitude Northern Hemisphere. 

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