Reactions of Criegee Intermediates with Alcohols at Air–Aqueous Interfaces

The fate of Criegee intermediates (CIs) from the gas-phase ozonolysis of unsaturated organic compounds in the troposphere is largely controlled by their reactions with water vapor. We recently found that against all expectations carboxylic acids compete at millimolar concentrations with water for CIs at the air–liquid interface of aqueous organic media. This outcome is consistent with both the low water concentration in the outermost interfacial layers and the enrichment of the competing acids therein. Here we show, via online electrospray mass spectrometric detection, that CIs generated in situ in the fast ozonolysis of sesquiterpenes (C₁₅H₂₄) on the surface of water:acetonitrile microjets react with <i>n</i> ≥ 4 linear alcohols C_<i>n</i>H_2<i>n</i>+1OH to produce high molecular weight C_15+<i>n</i> ethers in one step. The OH group of 1-octanol proved to be ∼25 times less reactive than that of <i>n</i>-octanoic toward CIs at the same bulk molar concentration, revealing that the reactivity of hydroxylic species depends on both acidities and interfacial affinities. CI interfacial reactions with surface-active hydroxylic species, by bypassing water, represent shortcuts to molecular complexity in atmospheric aerosols

Confocal Fluorescence Microscopy of the Morphology and Composition of Interstitial Fluids in Freezing Electrolyte Solutions

Ice rheology, the integrity of polar ice core records, and ice−atmosphere interactions are among the phenomena controlled by the morphology and composition of interstitial fluids threading polycrystalline ice. Herein, we investigate how ionic impurities affect such features via time-resolved confocal fluorescence microscopy of freezing electrolyte solutions doped with a pH probe. We find that the 10 μM probe accumulates into 12 μm thick glassy channels in frozen water, but it is incorporated into randomly distributed <1 μm diameter inclusions in freezing 1 mM NaCl. We infer that morphology is largely determined by the dynamic instabilities generated upon advancing ice by the rejected solute, rather than by thermodynamics. The protracted alkalinization of the fluid inclusions reveals that the excess negative charge generated by the preferential incorporation of Cl⁻ over Na⁺ in ice is neutralized by the seepage of the OH⁻ slowly produced via H₂O → H⁺ + OH⁻ thermal dissociation

Thermochromism of Model Organic Aerosol Matter

Laboratory experiments show that the optical absorptivity of model organic matter is not an intrinsic property, but a strong function of relative humidity, temperature, and insolation. Suites of representative polyfunctional C_<i>x</i>H_<i>y</i>O_<i>z</i> oligomers in water develop intense visible absorptions upon addition of inert electrolytes. The resulting mixtures reach mass absorption cross sections σ(532 nm) ∼ 0.1 m²/gC in a few hours, absorb up to 9 times more solar radiation than the starting material, can be half-bleached by noon sunlight in ∼1 h, and can be repeatedly recycled without carbon loss. Visible absorptions red-shift and evolve increasingly faster in subsequent thermal aging cycles. Thermochromism and its strong direct dependences on ionic strength and temperature are ascribed to the dehydration of >CH−C(OH)< groups into >CC< unsaturations by a polar E1 mechanism, and bleaching to photoinduced retrohydration. These transformations are deemed to underlie the daily cycles of aerosol absorption observed in the field, and may introduce a key feedback in the earth’s radiative balance

Fenton Oxidation of Gaseous Isoprene on Aqueous Surfaces

We report that gaseous isoprene ISO­(g) is oxidized into soluble species on the surface of aqueous acidic FeCl₂ solutions simultaneously exposed to H₂O₂(g). In our experiments, ISO­(g) and/or H₂O₂(g) streams intersect aqueous pH ∼ 2 FeCl₂ microjets for ∼10 μs. The products formed in these reactive encounters are identified in situ via online electrospray ionization mass spectrometry. We found that the (ISO)_<i>n</i>H⁺ oligomers generated from ISO­(g) on the surface of pH < 4 water are oxidized into myriad products whose combined yields exceed 5%. MS² analysis reveals that the positive ions derived from the protonation of neutral products split H₂O and O neutrals, whereas the less abundant negative carboxylate ion products undergo CO, H₂O, and CO₂ losses. Significantly, all products are fully quenched by ·OH scavenger <i>tert</i>-butyl alcohol. These results are consistent with an oxidation process initiated by the addition of ·OH from (Fe²⁺(aq) + H₂O₂(g)) to (ISO)_<i>n</i>H⁺, followed by fast reactions involving dissolved H₂O₂, HO₂·, and O₂ that lead to polyols; carbonyls; and, to a lesser extent, carboxylic acids. Our experiments demonstrate that gas-phase olefins are oxidized upon colliding on the surface of Fe-containing acidic aqueous media under typical tropospheric conditions