Measuring a 10,000-fold enhancement of singlet molecular oxygen (O-1*(2)) concentration on illuminated ice relative to the corresponding liquid solution

作者：Bower Jonathan P; Anastasio Cort^*

来源：Atmospheric Environment, 2013, 75: 188-195.

DOI：10.1016/j.atmosenv.2013.04.054

• Snow
• Photochemistry
• Freeze-concentration
• Liquid-like regions
• Oxidants

Much attention has focused on the highly reactive hydroxyl radical in the oxidation of trace organic compounds on snow and ice (and subsequent release of volatile organics to the atmospheric boundary layer) but other oxidants are likely also important in this processing. Here we examine the ice chemistry of singlet molecular oxygen (O-1*(2)), which can be significant in atmospheric water drops but has not been examined in ice or snow. To examine O-1*(2) on ice we illuminate laboratory ices containing Rose Bengal (RB) as the source of O-1*(2), furfuryl alcohol (FFA) as the probe, and Na2SO4 to control the total solute concentration. We find that the O-1*(2)-mediated loss of FFA (and, thus, the O-1*(2) concentration) is up to 11,000 times greater on ice than in the equivalent liquid sample at the same photon flux. We attribute this large increase in the O-1*(2) steady-state concentration to the freeze-concentration of solutes into liquid-like regions (LLRs) in/on ice: compared to the initial solution, in the Tilts of ice the sources for O-1*(2) are highly concentrated, while the concentration of the dominant sink for O-1*(2) (i.e., water) remains largely unchanged. Similar to results expected in liquid solution, rates of FFA loss in ice depend on both the initial sensitizer concentration and temperature, providing evidence that these reactions occur in LLRs. However, we find that the enhancement in O-1*(2) concentrations on ice does not follow predictions from freezing-point depression, likely because experiments were conducted below the eutectic temperature for sodium sulfate, where all of the salt should have precipitated. We also explore a method for separating O-1*(2) and center dot OH contributions to FFA oxidation in laboratory ices and show its application to two natural snow samples. We find that O-1*(2) concentrations in these snows are approximately 100 times higher than observed in polluted, mid-latitude fog waters, showing that the enhancement of O-1*(2) on ice is environmentally relevant and that O-1*(2) could be a significant sink for electron-rich organic compounds in snow.

• 出版日期2013-8