This paper presents results supporting the existence of a natural source of oxalic acid over the oceans. Oxalate was detected in "clean-sector" marine aerosol samples at Mace Head (Ireland) (53 degrees 20'N, 9 degrees 54'W) during 2006, and at Amsterdam Island (37 degrees 48'S, 77 degrees 34'E) from 2003 to 2007, in concentrations ranging from 2.7 to 39 ng m(-3) and from 0.31 to 17 ng m(-3), respectively. The oxalate concentration showed a clear seasonal trend at both sites, with maxima in spring-summer and minima in fall-winter, being consistent with other marine biogenic aerosol components (e.g., methanesulfonic acid, non-sea-salt sulfate, and aliphatic amines). The observed oxalate was distributed along the whole aerosol size spectrum, with both a submicrometer and a supermicrometer mode, unlike the dominant submicrometer mode encountered in many polluted environments. Given its mass size distribution, the results suggest that over remote oceanic regions oxalate is produced through a combination of different formation processes. It is proposed that the cloud-mediated oxidation of gaseous glyoxal, recently detected over remote oceanic regions, may be an important source of submicrometer oxalate in the marine boundary layer. Supporting this hypothesis, satellite-retrieved glyoxal column concentrations over the two sampling sites exhibited the same seasonal concentration trend of oxalate. Furthermore, chemical box model simulations showed that the observed submicrometer oxalate concentrations were consistent with the in-cloud oxidation of typical marine air glyoxal mixing ratios, as retrieved by satellite measurements, at both sites.
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