An exploration of aqueous oxalic acid production in the coastal marine atmosphere

Abstract

Oxalic acid is the most abundant dicarboxylic acid found in the troposphere, yet there is still no scientific consensus concerning its origins or formation process. Recent studies have suggested mechanisms for its formation in cloud water from gaseous precursors. Comparison of the characteristics of oxalic acid and nss sulfate, a chemical with a known incloud formation pathway, provides some support for an aqueous formation mechanism for oxalic acid. Analysis of the filters collected from the CIRPAS Twin Otter aircraft during CARMA I, a field campaign designed to study the marine stratocumulus off the coast of Monterey, CA, by a five stage Micro-Orifice Impactor (MOI) revealed a peak in the concentration distribution at a diameter of 0.26-0.44 mm, similar to the size distribution found for nss sulfate and corresponding to the droplet mode in the aerosol size distribution. An air-equivalent average of 2.0370.47 mg m À3 (standard error) of sulfate was observed in the collected marine cloud water, in excess to below-cloud concentrations by 1.16 mg m À3 on average. This suggests in-cloud production similar in concentration to previous field campaigns in coastal marine atmospheres. Oxalate was observed in the clouds at air-equivalent concentrations of 0.2170.04 mg m À3 , in excess to below-cloud concentrations by 0.14 mg m À3 and suggesting an in-cloud production as well. The tentative identification in cloud water of one of the intermediate species in the aqueous oxalate production mechanism lends further support to an in-cloud oxalate source. r

Observations of Sharp Oxalate Reductions in Stratocumulus Clouds at Variable Altitudes: Organic Acid and Metal Measurements During the 2011 E-PEACE Campaign

“…Between one to six cloudwater samples of approximately 10−30 min duration were collected each flight, which typically lasted ∼4−4.5 h. Samples were obtained with a modified Mohnen slotted-rod cloudwater collector. 26,45,52 The collection efficiency of the collector increases with decreasing aircraft speed and has little correlation with drop diameter up to a mass mean diameter of approximately 35 μm; 53 the range of drop sizes encountered during E-PEACE are expected to have been sampled by the collector with variable size-dependent collection efficiencies. The drop-size dependent sampling efficiency and possible large drop shatter might bias the concentrations of metals, organics, and other solutes since their concentrations change with drop size (e.g., refs 54−57).…”

“…INTERACTIONS IN STUDY REGION Oxalic acid can form metal complexes in the marine boundary layer due to the ubiquity of metals in directly emitted particle types (e.g., sea salt, dust, fly ash), the ubiquity of oxalate in aerosol particles in the study region, 26,36,69 and the favorable aqueous-phase medium provided by cloud drops to promote efficient complexation between metal and oxalate ions. 83 While we only considered complexation of iron by oxalate, other organic ligands (acids, humic like substances) might also form iron complexes.…”

“…84 The E-PEACE measurements, together with box model simulations, can explain the significant overestimate of predicted oxalate levels versus in-cloud measurements in the same study region almost a decade ago. 26 ■ ASSOCIATED CONTENT * S Supporting Information Two additional figures are provided to summarize documented organic acid chemical mechanisms and more results related to Section 3.3. Details related to the model used for Section 3.3 are provided.…”

“…34−37 Measured oxalate concentrations in marine boundary layer clouds have been shown to be an order of magnitude less than predictions. 26 Oxalate losses include the direct oxidation by OH and the photolysis of Fe(III) oxalato complexes: The latter process converts Fe(III) to a more soluble form, Fe(II). 38 Surface measurements in the Arctic indicate that enhanced sunlight and Fe from dust degrade oxalate.…”

Aerosol formation and heterogeneous chemistry in the atmosphere

“…Furthermore, a number of field studies have shown that the high amount of non volatile organic acids (such as oxalic, malonic, malic, succinic, glutaric acids) found in atmospheric waters and aerosols originate from aqueous phase processes Yao et al 2003;Crahan et al 2004;Yu et al 2005;Sorooshian et al 2007). These observations were in good agreement with the fact that, in the aqueous phase, the atmospheric reactivity is different from the gas phase one, leading to the formation of less volatile products.…”