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The alpha-hydroxyalkyl-hydroperoxides [R-(H)C(-OH)(-OOH), alpha-HH] produced in the ozonolysis of unsaturated organic compounds may contribute to secondary organic aerosol (SOA) aging. alpha-HHs' inherent instability, however, hampers their detection and a positive assessment of their actual role. Here we report, for the first time, the rates and products of the decomposition of the alpha-HHs generated in the ozonolysis of atmospherically important monoterpenes alpha-pinene (alpha-P), d-limonene (d-L), gamma-terpinene (gamma-Tn), and alpha-terpineol (alpha-Tp) in water/acetonitrile (W/AN) mixtures. We detect alpha-HHs and multifunctional decomposition products as chloride adducts by online electrospray ionization mass spectrometry. Experiments involving D2O and (H2O)-O-18, instead of (H2O)-O-16, and an OH-radical scavenger show that alpha-HHs decompose into gem-diols + H2O2 rather than free radicals. alpha-HHs decay mono- or biexponentially depending on molecular structure and solvent composition. e-Fold times, tau(1)(/e), in water-rich solvent mixtures range from tau(1)(/e), = 15-45 min for monoterpene-derived alpha-HHs to tau(1)(/e) > 10(3) min for the alpha-Tp-derived alpha-HH. All tau(1)(/e)'s dramatically increase in <20% (v/v) water. Decay rates of the alpha-Tp-derived alpha-HH in pure water increase at lower pH (2.3 <= pH <= 3.3). The hydroperoxides detected in day-old SOA samples may reflect their increased stability in water-poor media and/or the slow decomposition of alpha-HHs from functionalized terpenes. 

Recently, Gallo et al. ( Chem. Sci., 2019, 10, 2566) investigated whether the previously reported oligomerization of isoprene vapor on the surface of pH < 4 water in an electrospray ionization (ESI) mass spectrometer ( J. Phys. Chem. A, 2012, 116, 6027 and Phys. Chem. Chem. Phys., 2018, 20, 15400) would also proceed in liquid isoprene–acidic water emulsions. Gallo et al. hypothesized that emulsified liquid isoprene would oligomerize on the surface of acidic water because, after all, isoprene, liquid or vapor, is always a hydrophobe. In their emulsion experiments, isoprene oligomers were to be detected by ex situ proton magnetic resonance ( 1 H-NMR) spectrometry. 

One of the research priorities in atmospheric chemistry is to advance our understanding of heterogeneous reactions and their effect on the composition of the troposphere. Chemistry on aqueous surfaces is particularly important because of their ubiquity and expanse. They range from the surfaces of oceans (360 million km2), cloud and aerosol drops (estimated at ~10 trillion km2) to the fluid lining the human lung (~150 m2). Typically, ambient air contains reactive gases that may affect human health, influence climate and participate in biogeochemical cycles. Despite their importance, atmospheric reactions between gases and solutes on aqueous surfaces are not well understood and, as a result, generally overlooked. New, surface-specific techniques are required that detect and identify the intermediates and products of such reactions as they happen on liquids. This is a tall order because genuine interfacial reactions are faster than mass diffusion into bulk liquids, and may produce novel species in low concentrations. Herein, we review evidence that validates online pneumatic ionization mass spectrometry of liquid microjets exposed to reactive gases as a technique that meets such requirements. Next, we call attention to results obtained by this approach on reactions of gas-phase ozone, nitrogen dioxide and hydroxyl radicals with various solutes on aqueous surfaces. The overarching conclusion is that the outermost layers of aqueous solutions are unique media, where most equilibria shift and reactions usually proceed along new pathways, and generally faster than in bulk water. That the rates and mechanisms of reactions at air-aqueous interfaces may be different from those in bulk water opens new conceptual frameworks and lines of research, and adds a missing dimension to atmospheric chemistry.