Formation of aqueous-phase α-hydroxyhydroperoxides (α-HHP): Potential atmospheric impacts
The focus of this work is on quantifying the degree of the aqueous-phase formation of alpha-hydroxyhydroperoxides (alpha-HHPs) via reversible nucleophilic addition of H2O2 to aldehydes. Formation of this class of highly oxygenated organic hydroperoxides represents a poorly characterized aqueous-phase processing pathway that may lead to enhanced SOA formation and aerosol toxicity. Specifically, the equilibrium constants of alpha-HHP formation have been determined using proton nuclear-magnetic-resonance (H-1 NMR) spectroscopy and proton-transfer-reaction mass spectrometry (PTR-MS). Significant alpha-HHP formation was observed from formaldehyde, acetaldehyde, propionaldehyde, glycolaldehyde, glyoxylic acid, and methylglyoxal, but not from methacrolein and ketones. Low temperatures enhanced the formation of alpha-HHPs but slowed their formation rates. High inorganic salt concentrations shifted the equilibria toward the hydrated form of the aldehydes and slightly suppressed alpha-HHP formation. Using the experimental equilibrium constants, we predict the equilibrium concentration of alpha-HHPs to be in the mu M level in cloud water, but it may also be present in the mM level in aerosol liquid water (ALW), where the concentrations of H2O2 and aldehydes can be high. Formation of alpha-HHPs in ALW may significantly affect the effective Henry's law constants of H2O2 and aldehydes but may not affect their gas-phase levels. The photochemistry and reactivity of this class of atmospheric species have not been studied.