Carboxylate Ion Availability at the Air–Water Interface

Amphiphilic organic compounds at the air–water interface play key roles in the nucleation, growth, and aging process of atmospheric aerosol. Surface-active species are expected to react preferentially with atmospheric oxidants, such as the OH radical, at the air–water interface via specific mechanisms. Establishing the relative availability of the different amphiphilic species to gas-phase oxidants at the air–water interface under atmospherically relevant conditions is, however, challenging. Here we report the interfacial availability of atmospherically relevant carboxylate ions R_<i>n</i>-COO^– (<i>n</i> = 1–7) and <i>n</i>-, cyclo-, aromatic-R₆-COO^– at the air–water interface via a novel application of mass spectrometry of aqueous microjets. The breakup mechanism of microjets lets us determine the relative interfacial affinities of carboxylate ions in equimolar solutions of the corresponding carboxylic acids in the 1 μM to 1 mM range under ambient conditions. We find that the interfacial affinity of R_<i>n</i>-COO^– increases exponentially with both chain-length and solvent-accessible surface area (SASA) except in the case of R₁-COO^–. The relative interfacial affinities for <i>n</i>-heptanoate (<i>n</i>-R₆-COO^–) > cyclohexane­carboxylate (<i>c</i>-R₆-COO^–) > benzoate (Ar-R₆-COO^–) are also determined. We attribute the smallest availability of Ar-R₆-COO^– at the air–water interface among the three carboxylate ions to a strong π–H bonding between the aromatic ring and water molecule. Molecular mechanisms on the availability of carboxylate ions at the air–water interface and the atmospheric implications are discussed

Kinetics Study of OH Uptake onto Deliquesced NaCl Particles by Combining Laser Photolysis and Laser-Induced Fluorescence

Despite the role of hydroxyl radical (OH) uptake onto sea-salt particles as a daytime chlorine source, affecting the chemical processes in the marine boundary layer, its uptake coefficient has not yet been confirmed by direct measurement methods. This study reports the application of a combination technique of laser flash photolysis generation and laser-induced fluorescence detection for the direct kinetic measurement of OH uptake onto deliquesced NaCl particles. The uptake coefficient was not constant and inversely depended on the initial OH concentration, indicating that the first uptake step is Langmuir-type adsorption. The resistance model, including surface processes, well reproduced the observed uptake coefficient. The model predicted an uptake coefficient for the atmospheric relevant OH concentration within the range from 0.77 to 0.95. Such values may lead to emissions of Cl₂ higher than those predicted in previous studies based on other values. Hence, the proposed value may provide more reliable estimations of ozone formation, oxidation of volatile organic compounds, secondary organic aerosol formation, and lifetime of methane and elemental mercury in the marine boundary layer

Characterization of Chromophoric Water-Soluble Organic Matter in Urban, Forest, and Marine Aerosols by HR-ToF-AMS Analysis and Excitation–Emission Matrix Spectroscopy

Chromophoric water-soluble organic matter in atmospheric aerosols potentially plays an important role in aqueous reactions and light absorption by organics. The fluorescence and chemical–structural characteristics of the chromophoric water-soluble organic matter in submicron aerosols collected in urban, forest, and marine environments (Nagoya, Kii Peninsula, and the tropical Eastern Pacific) were investigated using excitation–emission matrices (EEMs) and a high-resolution aerosol mass spectrometer. A total of three types of water-soluble chromophores, two with fluorescence characteristics similar to those of humiclike substances (HULIS-1 and HULIS-2) and one with fluorescence characteristics similar to those of protein compounds (PLOM), were identified in atmospheric aerosols by parallel factor analysis (PARAFAC) for EEMs. We found that the chromophore components of HULIS-1 and -2 were associated with highly and less-oxygenated structures, respectively, which may provide a clue to understanding the chemical formation or loss of organic chromophores in atmospheric aerosols. Whereas HULIS-1 was ubiquitous in water-soluble chromophores over different environments, HULIS-2 was abundant only in terrestrial aerosols, and PLOM was abundant in marine aerosols. These findings are useful for further studies regarding the classification and source identification of chromophores in atmospheric aerosols