Photodegradation Rate Constants for Anthracene and Pyrene Are Similar in/on Ice and in Aqueous Solution

Abstract

Snowpacks contain a variety of chemicals, including organic pollutants such as toxic polycyclic aromatic hydrocarbons (PAHs). While PAHs undergo photodegradation in snow and ice, the rates of these reactions remain in debate. Some studies report that photochemical reactions in snow proceed at rates similar to those expected in a supercooled aqueous solution, but other studies report faster reaction rates, particularly at the air−ice interface (i.e., the quasi-liquid layer, or QLL). In addition, one study reported a surprising nonlinear dependence on photon flux. Here we examine the photodegradation of two common PAHs, anthracene and pyrene, in/on ice and in solution. For a given PAH, rate constants are similar in aqueous solution, in internal liquid-like regions of ice, and at the air−ice interface. In addition, we find the expected linear relationship between reaction rate constant and photon flux. Our results indicate that rate constants for the photochemical loss of PAHs in, and on, snow and ice are very similar to those in aqueous solution, with no enhancement at the air−ice interface.

Photodecay of guaiacol is faster in ice, and even more rapid on ice, than in aqueous solution

“…4) Vapor deposition of gas-phase guaiacol to the top surface of frozen water ice; our method here follows the same approach as previously described. 18 First, 10 ml of MQ water was placed in a beaker, covered with PE film, and frozen in a laboratory freezer at -20 °C. Once frozen, samples were removed and uncovered, and a nitrogen stream containing gas-phase guaiacol was directed at the ice surface for 15 s. Samples were then covered and placed back in a laboratory freezer.…”

“…Sample illumination generally followed the method described for anthracene and pyrene. 18 Sample beakers were set upright in a drilled aluminum holder to maximize heat transfer and minimize the impact of sample heating from the illumination source. Dark samples were covered with aluminum foil and placed in the illumination chamber along with illuminated samples.…”

“…17 Similarly, we found that anthracene and pyrene each had similar photodegradation rates in solution, in ice LLRs, and at the air-ice interface. 18 The rate of photodecay for chemical "C" (M s -1 ) in a low-light absorbing medium (e.g., solution or ice) during sunlight illumination is: 16 !…”

The morphology of ice and liquid brine in an environmental scanning electron microscope: a study of the freezing methods

“…Our recent study of the ice spheres prepared from NaCl solution revealed that, besides the eutectic crystallization, part of the solution vitrified under the conditions of fast cooling (Imrichova et al, 2019). The differential scanning calorimetry indicated the melting of the glass at −91 • C upon progressive heating, further allowing cold crystallization of NaCl and water at −76 • C. As the ice spheres are inspected at a much higher temperature in the present study, we expect the salt to be crystalline or dissolved in liquid brine during the observations.…”

“…The FCS can be located in the lamellae and veins between the ice crystals, (micro)pockets within the ice structure, or puddles and grain boundary grooves on the ice surface. When the mixture is cooled below the eutectic temperature (T eu ), the FCS may crystallize or vitrify (Salnikova et al, 2015;Bogdan et al, 2014;Imrichova et al, 2019).…”