Online headspace monitoring of volatile organic compounds using proton transfer reaction-mass spectrometry: Application to the multiphase atmospheric fate of 2,4-hexadienedial.

Chemical processes in clouds have been suggested to contribute significantly to the mass of organic aerosol particles in the atmosphere. Experimental and theoretical evidence suggest that organic mass production in clouds can be substantial and depends on the concentration of organic precursor compounds available in the gas phase. The present study aims at studying the aqueous phase reactivity of one of these overlooked precursors, i.e. 2,4-hexadienedial, an important and toxic intermediate in the atmospheric oxidation of aromatic species. Cautious synthesis and purification of 2,4-hexadienedial was performed. Its effective Henry's law constant was measured using a new simple and fast method based on online flow-injection analysis. The reactivity of 2,4-hexadienedial in the aqueous phase relevant to atmospheric conditions was studied, including hydrate formation, photolysis, ∙OH- and SO4∙--oxidation as well as reaction with NH3. The results revealed a low hydration constant compared to other dicarbonyls (Khyd1 = 7 × 10-2) and no dihydrate formation, indicating in an intermediate solubility (KH = 1.0 × 104 M atm-1) and high absorption cross sections (σ278nm > 10-16 cm2 molecule-1). Compared to its gas phase photolysis, its aqueous phase photolysis showed low quantum yields (Φ290-380nm = 0.9 %), and a significant red shift of the absorbance maximum, leading to a fast aqueous photolysis kinetics (Jaq,atm = 8.7 × 10-5 s-1) under atmospheric solar radiation, but no triplet state formation was detected. Radical oxidation experiments revealed extremely rapid oxidation kinetics (k∙OH = 1.10 × 1010 M-1 s-1 and kSO4∙- = 1.4 × 109 M-1 s-1) driven by fast addition of the radicals to the unsaturated bonds. In contrast, the reaction with aqueous NH3 (kNH3 = 2.6 × 10-3 M-1 s-1) was found slower than glyoxal and 2-butenedial, likely due to the hyperconjugation of 2,4-hexadienedial. Using these new data complemented with assumed aqueous phase kinetics (for NO3, 3C* and 1O2 reactions) and previous gas-phase kinetic ones, the multiphase atmospheric fate of 2,4-hexadienedial was established under atmospheric conditions reported from previous field measurements and models. The results revealed a short day lifetime (∼1 h) and a long night lifetime (>12 h). It was shown that daytime atmospheric chemistry of 2,4-hexadienedial can be influenced by aqueous-phase reactivity during cloud events, up to ∼50 % under thick cloud conditions (Liquid Water Content >2000 g/m3), indicating that even a compound of intermediate solubility can be strongly affected by condensed-phase reactivity. Besides its fast aqueous phase reactivity towards ∙OH and photolysis, its daytime condensed-phase reactivity may be driven by reactions with dissolved triplet states (3C*), up to 35 %, highlighting the need to study further the kinetics, the nature and concentrations of dissolved 3C* under various atmospheric conditions. In addition, the molecular properties and atmospheric behavior of 2,4-hexadienedial were found different from those of glyoxal and 2-butenedial, highlighting the need for detailed atmospheric reactivity studies of polyfunctional compounds, in particular unsaturated compounds.

A fast and efficient method for the analysis of α-dicarbonyl compounds in aqueous solutions: Development and application.

Among the highly oxygenated species formed in situ in the atmosphere, α-dicarbonyl compounds are the most reactive species, thus contributing to the formation of secondary organic aerosols that affect both air quality and climate. They are ubiquitous in the atmosphere and are easily transferred to the atmospheric aqueous phase due to their high solubility. In addition, α-dicarbonyl compounds are toxic compounds found in food in biochemistry studies as they can be produced endogenously through various pathways and exogenously through the Maillard reaction. In this work, we take advantage of the high reactivity of α-dicarbonyl compounds in alkaline solutions (intramolecular Cannizzaro reaction) to develop an analytical method based on high performance ion chromatography. This fast and efficient method is suitable for glyoxal, methylglyoxal and phenylglyoxal which are detected as glycolate, lactate and mandelate anions respectively, with 100% conversion at pH > 12 and room temperature for exposure times to hydroxide ranging from 5 min to 4 h. Diacetyl is detected as 2,4-dihydroxy-2,4-dimethyl-5-oxohexanoate due to a base-catalysed aldol reaction that occurs before the Cannizzaro reaction. The analytical method is successfully applied to monitor glyoxal consumption during aqueous phase HO∙-oxidation, an atmospherically relevant reaction using concentrations that can be observed in fog and cloud water. The method also reveals potential analytical artifacts that can occur in the use of ion chromatography for α-hydroxy carboxylates measurements in complex matrices due to α-dicarbonyl conversion during the analysis time. An estimation of the artifact is given for each of the studied α-hydroxy carboxylates. Other polyfunctional and pH-sensitive compounds that are potentially present in environmental samples (such as nitrooxycarbonyls) can also be converted into α-hydroxy carboxylates and/or nitrite ions within the HPIC run. This shows the need for complementary analytical measurements when complex matrices are studied.