Effects of Chromophoric Dissolved Organic Matter on Anthracene Photolysis Kinetics in Aqueous Solution and Ice.

We measured photolysis kinetics of the PAH anthracene in aqueous solution, in bulk ice, and at ice surfaces in the presence and absence of chromophoric dissolved organic matter (CDOM). Self-association, which occurs readily at ice surfaces, may be responsible for the faster anthracene photolysis observed there. Photolysis rate constants in liquid water increased under conditions where anthracene self-association was observed. Concomitantly, kinetics changed from first-order to second-order, indicating that the photolysis mechanism at ice surfaces might be different than that in aqueous solution. Other factors that could lead to faster photolysis at ice surfaces were also investigated. Increased photon fluxes due to scattering in the ice samples can account for at most 20% of the observed rate increase, and other factors including singlet oxygen (1O2*) production and changes in pH and polarity were determined not to be responsible for the faster photolysis. CDOM (in the form of fulvic acid (FA)) did not affect anthracene photolysis kinetics in aqueous solution but suppressed photolysis in ice cubes and ice granules (by 30% and 56%, respectively). This was primarily due to competitive photon absorption (the inner filter effect). Freeze-concentration (or "salting out") appears to slightly increase the suppressing effects of FA on anthracene photolysis. This may be due to increased competitive photon absorption or to physical interactions between anthracene and FA.

Nonchromophoric organic matter suppresses polycyclic aromatic hydrocarbon photolysis in ice and at ice surfaces.

We have investigated the effects of organic matter (OM) that does not absorb sunlight ("nonchromophoric") on the reactive environment presented by bulk ice and ice surfaces. Fluorescence spectroscopy showed that the presence of as little as 2.5 × 10(-4) M octanol or decanol reduces the extent to which naphthalene self-associates at ice surfaces, which indicates that naphthalene partitions between ice and organic phases present there. We also measured photolysis kinetics of the polycyclic aromatic hydrocarbons (PAHs) anthracene, pyrene, and phenanthrene in bulk ice and at ice surfaces containing 2.5 × 10(-5) M to 7.5 × 10(-3) M OM. In bulk ice, even the lowest concentrations of OM reduced photolysis kinetics to below our detection limits. Organic matter also reduced measured photolysis kinetics of PAHs at ice surfaces, but generally to a lesser extent than in bulk ice. Our results support previous reports that bulk ice and ice surfaces present distinct reaction environments, and show for the first time that OM can affect PAH photolysis kinetics by altering the physical environment within bulk ice and at ice surfaces.