Direct Observations of Anthracene Clusters at Ice Surfaces

Abstract

Heterogeneous processes can control atmospheric composition. Snow and ice present important, but poorly understood, reaction media that can greatly alter the composition of air in the cryosphere in polar and temperate regions. Atmospheric scientists struggle to reconcile model predictions with field observations in snow-covered regions due to experimental challenges associated with monitoring reactions at air-ice interfaces, and debate regarding reaction kinetics and mechanisms has persisted for over a decade. In this work, we use wavelength-resolved fluorescence microscopy to determine the distribution and chemical speciation of the pollutant anthracene at the surfaces of environmentally relevant frozen surfaces. We show that anthracene adsorbs to frozen surfaces in monomeric form, but that following lateral diffusion, molecules ultimately reside within brine channels at saltwater ice surfaces, and in micron-sized clusters at freshwater ice surfaces; emission profiles indicate extensive self-association. We also measure anthracene photodegradation kinetics in aqueous solution and artificial snow prepared from frozen freshwater and saltwater solutions and use the micro-spectroscopic observations to explain the rate constants measured in different environments. These results resolve long-standing debates and will improve predictions of pollutant fate in the cryosphere. The techniques used can be applied to numerous surfaces within and beyond the atmospheric sciences.

Chemical Morphology Controls Reactivity of OH Radicals at the Air–Ice Interface

“…The lack of blue-shift in coumarin emission at air–ice interfaces suggests that it is in monomeric form there. We have previously reported that anthracene emission spectra at air–ice interfaces contain features indicative of self-association, and we have recently demonstrated that this is also true for spectra of anthracene clusters at ice surfaces as detected by fluorescence microscopy …”

“…It is apparent that whereas the distribution of coumarin at the air–ice interface is fairly homogeneous, that of anthracene shows significant clumping in specific locations. We have recently reported similar clumping of anthracene at air–ice interfaces when anthracene is deposited to the surface from the gas phase, at deposition times corresponding to fractional surface coverages as low as 6% of a formal monolayer …”

“…For example, we have shown that several PAHs undergo more rapid photodegradation at ice surfaces than in liquid aqueous solution or in liquid regions within bulk ice and have inferred that this enhanced reactivity is due to their different distribution at the interface; emission spectra of the PAHs at ice surfaces indicate the formation of PAH–PAH complexes. ,− Malley et al recently demonstrated that anthracene photodegradation in room-temperature aqueous solution becomes more rapid in saturated solutions where emission spectra indicate self-association; this increase was accompanied by a shift from first-order to second-order kinetics, which suggests that anthracene’s self-association results in a different, bimolecular, photodegradation mechanism, possibly forming products different from those observed in aqueous solution . Recently, Chakraborty et al used fluorescence microscopy to demonstrate that anthracene forms micrometer-sized clusters at ice surfaces …”

“…The surface fluorescence map of anthraceneexpected to behave in a similar manner to pyreneconfirms this conclusion. Anthracene is seen to exist in clumps at the interface, rather than being evenly distributed there, covering only 7% of the surface at amounts that would nominally form a monolayer . The observation that there is not significant fluorescence quenching of pyrene by hydrogen peroxide at the ice surface further suggests that the highly soluble hydrogen peroxideexpected to be homogeneously distributed at the interfaceis not in effective contact with the much less soluble pyrene.…”

“…Water ice contains liquid inclusions and liquid-filled grain boundaries, which may trap reagents within them. Solutes excluded from the ice crystal during a slow freezing process also create a concentrated solution at the air–ice interface, which can lead to altered reaction mechanisms. − Moreover, recent work has shown that the distribution of excluded molecules in the quasi-liquid layer at the ice surface is not always uniform but that solutes may be arranged in uneven patches and self-associate, with liquid or frozen regions between them. − Thus, reagents may either be solvated or exist in solid form at the ice surface, even in the case of highly soluble species like nitrate; this inhomogeneous distribution may play a crucial role in the reaction dynamics at the surface.…”