Manuel Barret

Thermodynamics of the formaldehyde-water and formaldehyde-ice systems for atmospheric applications.

Formaldehyde (HCHO) is a species involved in numerous key atmospheric chemistry processes that can significantly impact the oxidative capacity of the atmosphere. Since gaseous HCHO is soluble in water, the water droplets of clouds and the ice crystals of snow exchange HCHO with the gas phase and the partitioning of HCHO between the air, water, and ice phases must be known to understand its chemistry. This study proposes thermodynamic formulations for the partitioning of HCHO between the gas phase and the ice and liquid water phases. A reanalysis of existing data on the vapor-liquid equilibrium has shown the inadequacy of the Henrys law formulation, and we instead propose the following equation to predict the mole fraction of HCHO in liquid water at equilibrium, X(HCHO,liq), as a function of the partial pressure P(HCHO) (Pa) and temperature T (K): X(HCHO,liq) = 1.700 × 10(-15) e((8014/T))(P(HCHO))(1.105). Given the paucity of data on the gas-ice equilibrium, the solubility of HCHO and the diffusion coefficient (D(HCHO)) in ice were measured by exposing large single ice crystals to low P(HCHO). Our recommended value for D(HCHO) over the temperature range 243-266 K is D(HCHO) = 6 × 10(-12) cm(2) s(-1). The solubility of HCHO in ice follows the relationship X(HCHO,ice) = 9.898 × 10(-13) e((4072/T))(P(HCHO))(0.803). Extrapolation of these data yields the P(HCHO) versus 1/T phase diagram for the H(2)O-HCHO system. The comparison of our results to existing data on the partitioning of HCHO between the snow and the atmosphere in the high arctic highlights the interplay between thermodynamic equilibrium and kinetics processes in natural systems.

Formaldehyde in the Alaskan Arctic snowpack: Partitioning and physical processes involved in air‐snow exchanges

[1] The snowpack is a photochemically active medium which produces numerous key reactive species involved in the atmospheric chemistry of polar regions. Formaldehyde (HCHO) is one such reactive species produced in the snow, and which can be released to the atmospheric boundary layer. Based on atmospheric and snow measurements, this study investigates the physical processes involved in the HCHO air‐snow exchanges observed during the OASIS 2009 field campaign at Barrow, Alaska. HCHO concentration changes in a fresh diamond dust layer are quantitatively explained by the equilibration of a solid solution of HCHO in ice, through solid‐state diffusion of HCHO within snow crystals. Because diffusion of HCHO in ice is slow, the size of snow crystals is a major variable in the kinetics of exchange and the knowledge of the snow specific surface area is therefore crucial. Air‐snow exchanges of HCHO can thus be explained without having to consider processes taking place in the quasi‐liquid layer present at the surface of ice crystals. A flux of HCHO to the atmosphere was observed simultaneously with an increase of HCHO concentration in snow, indicating photochemical production in surface snow. This study also suggests that the difference in bromine chemistry between Alert (Canadian Arctic) and Barrow leads to different snow composition and post‐deposition evolutions. The highly active bromine chemistry at Barrow probably leads to low HCHO concentrations at the altitude where diamond dust formed. Precipitated diamond dust was subsequently undersaturated with respect to thermodynamic equilibrium, which contrasts to what was observed elsewhere in previous studies.

The specific surface area and chemical composition of diamond dust near Barrow, Alaska

2006, when conditions were similar. The SSA of DD ranges from 79.9 to 223 m 2 kg � 1 . The calculated ice surface area in the atmosphere reaches 11000 (� 70%) mm 2 cm � 3 , much higher than the aerosol surface area. However, the impact of DD on the downwelling and upwelling light fluxes in the UV and visible is negligible. The composition of DD is markedly different from that of snow on the surface. Its concentrations in mineral ions are much lower, and its overall composition is acidic. Its concentrations in aldehydes, DOC, H2O2 and mercury are much higher than in surface snows. Our interpretation is that DOC from the oceanic surface microlayer, coming from open leads in the ice off of Barrow, is taken up by DD. Active chemistry in the atmosphere takes place on DD crystal surfaces, explaining its high concentrations in aldehydes and mercury. After deposition, active photochemistry modifies DD composition, as seen from the modifications in its absorption spectra and aldehyde and H2O2 content. This probably leads to the emissions of reactive species to the atmosphere.

Sensitive determination of glyoxal, methylglyoxal and hydroxyacetaldehyde in environmental water samples by using dansylacetamidooxyamine derivatization and liquid chromatography/fluorescence

In this study we improved the dansylacetamidooxyamine (DNSAOA)-LC-fluorescence method for the determination of aqueous-phase glyoxal (GL), methylglyoxal (MG) and hydroxyacetaldehyde (HA). As derivatization of dicarbonyls can potentially lead to complex mixtures, a thorough study of the reaction patterns of GL and MG with DNSAOA was carried out. Derivatization of GL and MG was shown to follow the kinetics of successive reactions, yielding predominantly doubly derivatized compounds. We verified that the bis-DNSAOA structure of these adducts exerted only minor influence on their fluorescence properties. Contrary to observations made with formaldehyde, derivatization of GL, MG and, to a lesser extent of HA, was shown to be faster in acidic (H(2)SO(4)) medium with a maximum of efficiency for acid concentrations of ca. 2.5 mM. Concomitant separation of GL, MG, HA and of single carbonyls was achieved within 20 min by using C(18) chromatography and a gradient of CH(3)CN in water. Detection limits of 0.27, 0.17 and 0.12 nM were determined for GL, MG and HA, respectively. Consequently, low sample volumes are sufficient and, unlike numerous published methods, neither preconcentration nor large injection volumes are necessary to monitor trace-level samples. The method shows relative measurement uncertainties better than ±15% at the 95% level of confidence and good dynamic ranges (R(2)>0.99) from 0.01 to 1.5 μM for all carbonyls. GL, MG and HA were identified for the first time in polar snow samples, but also in saline frost flowers for which unexpected levels of 0.1-0.6 μM were measured. Concentrations in the 0.02-2.3 μM range were also measured in cloud water. In most samples, a predominance of HA over GL and MG was observed.