Photolysis of pyruvic acid in ice: Possible relevance to CO and CO₂ ice core record anomalies

Abstract

The abnormal spikes detected in some CO and CO2 polar ice core records indicate persistent chemical activity in glacial ice. Since CO and CO2 spikes are correlated, and their amplitudes scale with reported CO/CO2 yields for the photolysis of dissolved natural organic matter, a common photochemical source is implicated. Given that sufficient actinic radiation is constantly generated throughout ice by cosmic muons (Colussi and Hoffmann, 2003), it remains to be shown that the photolyses of typical organic contaminants proceed by similar mechanisms in water and ice. Here we report that the photodecarboxylation of pyruvic acid (PA, an ubiquitous ice contaminant) indeed leads to the same products nearly as efficiently in both media. CO2 is promptly released from frozen PA/H2O films upon illumination and continues to evolve after photolysis. By analogy with our studies in water (Guzmán et al., 2006b), we infer that 3PA* reacts with PA in ice producing CH3C(O)C(O)O· and (CH3 (OH)C(O)OH) radicals. The barrierless decarboxylation, CH3C(O)C(O)O· → CH3C(O)· + CO2, accounts for prompt CO2 emissions down to ∼140 K. Bimolecular radical reactions subsequently ensue in fluid molecular environments, both in water and ice, leading to metastable intermediates that decarboxylate immediately in water, but protractedly in ice. The overall quantum yield of CO2 production in the λ ~313 nm photolysis of PA in ice at 250 K is ∼60% of that in water at 293 K. The in situ photolysis of natural organic matter is, therefore, a plausible explanation of CO and CO2 ice core record anomalies.

A review of air–ice chemical and physical interactions (AICI): liquids, quasi-liquids, and solids in snow

“…Surprisingly and in contrast with expectations from the phase diagram, the NMR investigations by Cho et al (2002), Guzmán et al (2006) and Robinson et al (2006) have revealed the existence of liquid NaCl and (NH 4 ) 2 SO 4 solutions well below the respective eutectic temperatures. Cho et al (2002) argued that kinetic limitations leading to supercooled solutions could not explain the observations, because the amount of liquid closely followed the temperature during warming periods toward the eutectic temperature.…”

Acetaldehyde in the Alaskan subarctic snowpack

“…This is illustrated by the fact that acid hydrolysis of 2,4-pentadione formed acetone and acetaldehyde (Noziere and Riemer, 2003). The photochemistry of HULIS contained in organic particles in the snow may also produce acetaldehyde (Guzman et al, 2007), which may remain trapped within the complex macromolecular structures and be released upon melting and analysis.…”

Organics in environmental ices: sources, chemistry, and impacts

“…A comparison of the organic content in both polar ice caps shows that the Greenland ice cores are enriched by the organic aerosol released in major boreal forest fires during the last two millennia (Gayley and Ram, 1985;Kawamura et al, 1995;Desideri et al, 1998;Savarino and Legrand, 1998). Hoffmann and coworkers found that the photodegradation of dissolved organic matter trapped in ice cores by ultraviolet Cerenkov radiation from cosmic muons can account for the anomalous CO and CO 2 levels, and the correlation between them (Colussi and Hoffmann, 2003;Guzman et al, 2007;Guzman, 2007).…”

“…Scheme A. Initiation steps of the pyruvic acid oligomerization mechanism, after Guzman et al (2006bGuzman et al ( , 2007 and Rincon et al (2009Rincon et al ( , 2010. See text for details.…”