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Atmospheric acidity is increasingly determined by carbon dioxide and organic acids.sup.1-3. Among the latter, formic acid facilitates the nucleation of cloud droplets.sup.4 and contributes to the acidity of clouds and rainwater.sup.1,5. At present, chemistry-climate models greatly underestimate the atmospheric burden of formic acid, because key processes related to its sources and sinks remain poorly understood.sup.2,6-9. Here we present atmospheric chamber experiments that show that formaldehyde is efficiently converted to gaseous formic acid via a multiphase pathway that involves its hydrated form, methanediol. In warm cloud droplets, methanediol undergoes fast outgassing but slow dehydration. Using a chemistry-climate model, we estimate that the gas-phase oxidation of methanediol produces up to four times more formic acid than all other known chemical sources combined. Our findings reconcile model predictions and measurements of formic acid abundance. The additional formic acid burden increases atmospheric acidity by reducing the pH of clouds and rainwater by up to 0.3. The diol mechanism presented here probably applies to other aldehydes and may help to explain the high atmospheric levels of other organic acids that affect aerosol growth and cloud evolution. The oxidation of hydrated formaldehyde from cloud droplets is the dominant source of atmospheric formic acid, increasing atmospheric acidity by reducing cloud and rainwater pH., Author(s): B. Franco [sup.1] [sup.2] , T. Blumenstock [sup.3] , C. Cho [sup.1] , L. Clarisse [sup.2] , C. Clerbaux [sup.2] [sup.4] , P.-F. Coheur [sup.2] , M. De Mazière [...]

Recent studies have revealed that siderophores, biogenic chelating agents that strongly complex Fe(III) and other hard metals, may function in concert with low-molecular-mass organic acids (LMMOAs) to facilitate Fe (hydr) oxide dissolution via synergistic reactions. The siderophore desferrioxamine B (DFOB) may also participate in a number of chemical reactions at Mn (hydr)oxide surfaces. The goal of this study was to determine the rates and products of 6-Mn[O.sub.2], a layer type Mn(IV) oxide, dissolution as promoted by LMMOA-DFOB mixtures. As with Fe (hydr)oxides, the rate of DFOB-promoted dissolution of [delta]-Mn[O.sub.2] is strongly influenced by the presence of the LMMOAs citrate and oxalate. In the presence of DFOB, citrate increases the dissolution rate relative to the sum of dissolution rates from corresponding single-ligand systems by promoting both Mn(III)HDFOB+ complex formation and Mn(II) production in a pH-dependent mechanism. In contrast, oxalate--DFOB mixtures produce predominantly Mn(II), with rates enhanced up to threefold from the sum of dissolution rates in single-ligand systems at acidic pH values. We investigated possible mechanisms to describe this synergistic dissolution processes by building on observations of single-ligand systems. These results suggest that biological exudation of LMMOAs in conjunction with siderophores may allow the selection of dissolution products, including regulation of the formation of potentially reactive aqueous Mn(III) complexes. doi: 10.2136/sssaj2009.0465

Field and laboratory studies were conducted to examine how the availability of electron acceptors affects the distribution and biodegradability of low molecular weight organic acids in a shallow, gasoline-contaminated aquifer. Field results showed that low molecular weight organic acids were associated with the degradation of hydrocarbons in anoxic zones of the aquifer. Changes in electron acceptor availability over time resulted in changes in the composition and concentration of the organic acid pool. Laboratory results confirmed that the fate of low molecular weight organic acids in groundwater depends on the nature of the acids and electron acceptor availability.

A study was conducted to investigate the killing of Escherichia coli 0157:H7 strains in the presence or absence of selected organic acids, using glyconic acid as a noninhibitory low-pH buffer. It was observed that acetic, malic, and L-Lactic acids can have protective effects on the survival of E. coli, when compared to pH.

The contribution of ligands during weathering on Earth is discussed. The effects of the ligands generally followed trends in cation-ligand, stability constants, but aromatic ligands are less effective in element mobilization than aliphatic ligands.

Calcareous soils are frequently typified by a low availability of plant nutrients due to poor solubility of these elements at high pH. Calcicole plants have recently been shown to release organic acids in response to the nutrient deficient conditions prevailing in these soils. It has been speculated, however, that the efficiency of this nutrient mobilization mechanism may be significantly reduced by microbial degradation of the organic acids. In conventional methods, root exudate degradation is typically determined by the addition of [sup.14]C-radiolabeled substrates to soil and subsequent tracking of their fate with time by trapping evolved [sup.14][CO.sub.2] in a strong alkali trap. However, in calcareous soils, [sup.14][CO.sub.2] and [H.sub.2][sup.14][CO.sub.3] produced by microbial decomposition may become trapped as [Ca([H[sup.14][CO.sub.3]).sub.2]. The aim of this study was to develop and validate an experimental procedure for the accurate quantification of [sup.14]C-labeled substrate degradation rates in calcareous soils. Conventional methods for determining [sup.14]C-labeled substrate decomposition rates in calcareous soils are inaccurate due to incomplete recovery of [sup.14][CO.sub.2]. Up to 49% of the [sup.14][CO.sub.2] produced during microbial degradation of [sup.14]C-labeled organic acids (malate, oxalate, citrate) was trapped as carbonate in this calcareous soil (pH 7.58). For an acid soil (pH 4.32) no detectable amount of [sup.14][CO.sub.2] was trapped. We describe a simple, accurate, and reliable method, which includes a postincubation HCI addition, for the accurate determination of [sup.14][CO.sub.2]-evolution and substrate degradation in calcareous soils.