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Environmental Processes

Photodegradation rate constants for anthracene and pyrene are similar in/on ice and in aqueous solution Ted Hullar, Danielle Magadia, and Cort Anastasio Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02350 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 28, 2018

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Environmental Science & Technology

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Photodegradation rate constants for anthracene and pyrene are similar in/on ice and in

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aqueous solution

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Ted Hullar1, Danielle Magadia1,2, and Cort Anastasio1, *

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1

5 6 7

Department of Land, Air and Water Resources, University of California, Davis, One

Shields Avenue, Davis, CA 95616, USA 2

Now at California Department of Food and Agriculture, 3292 Meadowview,

Sacramento, CA 95832

8

*

9

Abstract

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Corresponding author, [email protected], (530) 754-6095

Snowpacks contain a variety of chemicals, including organic pollutants such as toxic

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polycyclic aromatic hydrocarbons (PAHs). While PAHs undergo photodegradation in snow and

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ice, the rates of these reactions remain in debate. Some studies report that photochemical

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reactions in snow proceed at rates similar to those expected in a supercooled aqueous solution,

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but other studies report faster reaction rates, particularly at the air-ice interface (i.e., the quasi-

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liquid layer, or QLL). In addition, one study reported a surprising non-linear dependence on

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photon flux. Here we examine the photodegradation of two common PAHs, anthracene and

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pyrene, in/on ice and in solution. For a given PAH, rate constants are similar in aqueous

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solution, in internal liquid-like regions of ice, and at the air-ice interface. In addition, we find the

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expected linear relationship between reaction rate constant and photon flux. Our results indicate

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that rate constants for the photochemical loss of PAHs in, and on, snow and ice are very similar

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to those in aqueous solution, with no enhancement at the air-ice interface.

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TOC Art

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1.0 Introduction Snow contains a wide range of chemical compounds, which can be transformed through a

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variety of mechanisms, including photochemical reactions, that alter the composition of the

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snowpack. In polar regions, snowpacks are an important location for chemical reactions,

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especially photochemical reactions, which can occur throughout the photic zone, approximately

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the top several 10s of cm of the snowpack 1-3. Photochemical transformations of inorganic

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compounds, such as the formation of NOx from nitrate photolysis or hydroxyl radical from

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hydrogen peroxide photolysis, are important in polar snowpacks 4-8. Reactions of organic

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compounds are also significant 9-12, frequently leading to more volatile components that can be

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released from the snowpack. While a large variety of organic compounds have been found in

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snowpacks 13-16, the chemical transformations and ultimate fate of these materials remains poorly

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understood.

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Chemicals in snow can be present in one or more of three different reservoirs: a quasi-

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liquid layer (QLL) at the air-ice interface, in liquid-like regions (LLRs) within the ice (e.g., at

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grain boundaries), and in the case of smaller molecules, within the bulk ice itself 7, 17-19. Solute

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location in the snowpack, while poorly understood 1, 21, is significant for several reasons. For

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example, chemicals present in QLLs can more easily partition into interstitial air in the snowpack

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and subsequently to the atmosphere. Perhaps most intriguingly, some studies have found that

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photoreaction rate constants can be enhanced in, and especially on, ice. For several PAHs,

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photodegradation rate constants are reported to be 1.3 – 5.0 times faster in LLRs and 6.3 – 9.2

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times faster in QLLs compared to rate constants in aqueous solution 22-25. In addition, for


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benzene, toluene, ethylbenzene, and xylene, photodegradation was observed in LLRs and QLLs,

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but not in aqueous solution 26, 27. These results suggest QLLs and LLRs may be a distinct

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reaction environment 22-27. In contrast, Ram and Anastasio 28 found that PAH photodegradation

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rate constants in ice LLRs were similar to rate constants in aqueous solution. Further

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complicating our understanding of organic reactions in ice, one study 24 reported that

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photodegradation rate constants of PAHs at the air-ice interface are independent of photon flux

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above a very low threshold. This finding is surprising and contrasts with the expected linear

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dependence of the rate constant on photon flux under most environmental conditions.

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There are several possible explanations for reaction rate constant differences in these

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studies. First, differences in preparation method, usually intentional but sometimes inadvertent,

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can have significant and sometimes unexpected impacts on solute location in laboratory sample

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locations 21. If the air-ice interface or LLRs are different reaction environments than aqueous

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solution, photodegradation rates may be different in each compartment, as found in some studies.

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Second, the photodegradation rate constant depends on photon flux, which can vary within a

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snowpack, with the surface snow actinic flux up to four times greater than the clear-sky actinic

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flux 29. In order to accurately measure differences in photodegradation rate constants in the

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laboratory, the local actinic flux must be measured, although this is not always done. McFall and

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Anastasio 30 recently showed that solute location and sample container can have significant

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impacts on local actinic flux, potentially biasing experimental results if photon fluxes are not

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measured. Third, several studies 26, 31, 32 have found that light absorbance by some compounds in

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QLLs and/or LLRs may red-shift slightly relative to aqueous solution, which would allow more

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photons to be absorbed and therefore result in faster decay, although other studies did not find

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red shifts in other compounds 33, 34. Finally, compounds present in QLLs and LLRs can

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aggregate or form crystals 20, 31, 33, 35, which may change their chemical properties and alter the

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kinetics of photodecay.

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In this work we investigate the photodegradation rate constants of two common PAHs,

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anthracene and pyrene, in laboratory ice samples prepared using several freezing methods to

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preferentially place the solutes into either QLLs or LLRs. We have two main goals: (1) evaluate

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the significance of reaction location on photodegradation rate constants and (2) determine the



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dependence of anthracene photodegradation rate constant on photon flux. We also look at the

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impacts of solution ionic strength and sample exposure to laboratory air on photodegradation.

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2 Methods

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2.1 Materials

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Acetonitrile (HPLC grade) was from Acros. Anthracene (ANT, 99%), pyrene (PYR,

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99%), 2-nitrobenzaldehyde (2NB, 98%), and sodium chloride (NaCl, 99%) were from Sigma

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Aldrich. High purity water (MQ) was from house-treated R/O water that was run through a

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Barnstead International DO813 activated carbon cartridge and then a Millipore Milli-Q Plus

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system (> 18.2 MΩ cm).

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2.2 Sample preparation

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We used two different containers for our samples: 2 mL screw-top HPLC autosampler

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glass vials (12 mm outside diameter, 32 mm overall height) with plastic caps containing PTFE-

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lined septa (Fisher Scientific) or 1.6 mL glass beakers, custom made by cutting off the threads

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and neck of 2 mL glass vials. To minimize contamination with the beakers, we sealed them with

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a piece of polyethylene film (Saran Wrap, Dow Chemical) secured with an O-ring.

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We prepared four types of samples: aqueous, freezer, liquid nitrogen (LN2), and vapor

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deposited (VD) (Figure 1). We prepared aqueous samples by first using a vapor deposition

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apparatus (section 2.3) to bubble either ANT (for 10 minutes) or PYR (60 minutes) into 50 mL

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of MQ in a 100 ml glass bottle to make a working solution of PAH with a concentration of ~10

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nM. We used this solution as prepared for aqueous illumination samples. We made Freezer

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samples by freezing 800 µL aliquots of working solution in a laboratory freezer (–20 oC). We

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prepared LN2 samples by first adding 800 µL of working solution to a beaker or vial and then

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placing the beaker or vial in a container of liquid nitrogen deep enough to just reach the liquid

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level of sample in the beaker. Freezing time was typically less than 60 seconds. Based on

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imaging work with similarly prepared samples containing CsCl 21 PAHs in our Freezer and LN2

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samples are predominantly in liquid-like regions of the ices. VD samples were prepared by first

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freezing 800 µL of MQ water in a beaker or vial at –20 oC and then depositing PAH to the ice

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surface as described in the next section.


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2.3 Vapor deposition We used a custom-made apparatus to vapor deposit ANT and PYR to the surface of ice

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samples (section 2.2). To make this apparatus (Figure S1), we first filled a 9 cm long piece of

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1/16” ID Teflon tubing with either ANT or PYR; we used separate tubes for the two PAHs. The

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ends of the tubes were closed with fittings containing stainless steel frits to constrain the solid

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PAH. A 30-cm piece of 1/16” ID (1/8” OD) Teflon tubing downstream of the PAH-containing

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tubing was threaded through the narrow end of a 5 mL, 10 cm long, polypropylene pipette tip,

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with the unattached tubing end recessed 1 cm from the wider end of the pipette tip. We drilled

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four 3 mm holes 4 cm from the wide end of the pipette tip to vent the nitrogen gas. The other

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end of the PAH-containing tubing was attached to a nitrogen tank (99.998%, Praxair). To vapor

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deposit PAH to the ice, we used a nitrogen flow rate of 0.1 liter per minute and placed the wide

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end of the pipette tip over the sample container, with an interior ridge on the pipette tip resting

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on the rim of the sample container. The nitrogen stream (containing either ANT or PYR)

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impinged on the ice surface, depositing the PAH. During deposition the sample container was

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placed in a freeze chamber at –20 oC and the tip of the apparatus was held over each ice sample

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for 30 (ANT) or 60 seconds (PYR).

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The amount of chemical actually deposited to the ice surface for VD samples varied

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somewhat during each experimental day, with relative standard deviations (RSD, i.e., σ/mean) of

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melted solution concentrations ranging from 5 to 15 %. Additionally, the average PAH amount

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deposited varied from day to day, likely due to changes in the porosity of the solid PAH and in

125

laboratory temperature.

126

2.4 Sample illumination and chemical analysis

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Samples were held in a custom-built freeze chamber (Paige Instruments) set at 5 oC

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(aqueous samples) or –10 oC (frozen samples) and illuminated for 1 to 5 hours with a 1000-W

129

Xe lamp. The output of the lamp was filtered to simulate polar sunlight using a dichroic cold

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mirror (to transmit 300-500 nm light but remove other wavelengths to reduce sample heating)

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and an air mass 1.5 filter (Sciencetech) 36. Beakers were illuminated upright while being held in a

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plastic lattice grid. Vials were illuminated horizontally in the machined grooves of a polished

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aluminum plate (Figure S2) 28. We removed samples at periodic intervals for chemical analysis.



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Dark samples were covered with aluminum foil and placed in the illumination chamber along

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with illuminated samples.

136

We measured PAH concentrations using a Shimadzu HPLC with a SIL-20A HT

137

autosampler, LC-20AB pump, and SPD-M20A photodiode array detector with a Thermo

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Scientific BetaBasic-18 250 mm × 3 mm diameter column (5 µm particle size). We used an

139

eluent of 80:20 acetonitrile:MQ water at a flow rate of 0.700 mL min–1 and a detection

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wavelength of 250 nm (ANT) or 335 nm (PYR). We analyzed all samples in the same container

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used for sample illumination. Frozen samples were melted prior to analysis. For samples in

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beakers, the polyethylene film cover and O-ring were removed (since the film might clog the

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autosampler needle) and replaced with aluminum foil formed tightly around the beaker opening

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to prevent evaporation or contamination.

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2.5 Actinometry

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We used 2-nitrobenzaldehyde (2NB), a chemical actinometer, to normalize for

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differences in photon fluxes between samples and sample treatments 28, 37. On each experiment

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day we prepared 2NB samples with the same volume, freezing method, and sample container as

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the corresponding PAH sample. A stock of 10 µM 2NB was stored in the refrigerator and used

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for aqueous, freezer, and LN2 experiments. Vapor deposition 2NB samples were prepared in the

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same manner as VD PAH samples, using a separate 2NB vapor deposition apparatus. We then

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illuminated the 2NB samples at a reference position (e.g., “B2” for vertically illuminated

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beakers) in our illumination system for times ranging from 0 to 150 s.

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We illuminated each 2NB sample on each day using the same HPLC as for the PAH

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analyses, with an eluent of 60:40 acetonitrile:MQ water at a detection wavelength of 258 nm. To

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determine the original deposited 2NB concentration in vapor-deposited samples, 2NB was

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analyzed using an acidic (pH 2) eluent of 60:40 acetonitrile:MQ water so that we could measure

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concentrations of both 2NB and its only photoproduct, nitrosobenzoic acid (NBA), using a

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detection wavelength of 277 nm 30. Relative standard deviations for VD 2NB concentrations

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deposited on a given day ranged from 9 to 19 %.

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Once analyzed, we calculated the j2NB value from a linear regression of


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[2NB] ln = − (1) [2NB]

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where [2NB]t and [2NB]0 are the concentrations at times t and 0, respectively. Because the

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actinic flux in our illumination system varies slightly between illumination positions, we also

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performed a “mapping” to quantify how illumination varies with position: on a single day we

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measured j2NB in aqueous solution in the reference position (e.g., B2) and at each other position

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(x). The ratio of these values is the correction factor F2NB, x , i.e., the ratio of photon flux at a

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given sample position relative to the reference position 28: , =

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, (2) ,

On a given PAH experiment day we measured j2NB in the B2 reference position and then used

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the correction factors at every other sample position (Table S1) to account for variations in

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actinic flux across the series of samples (section 2.6).

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2.6 Determining rate constants for PAH loss

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We determined PAH photodegradation rate constants (jPAH) following the same general

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approach found in Ram and Anastasio 28. For each experiment, we first illuminated the samples

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(or placed the samples in the illumination system under aluminum foil for dark controls) and

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then analyzed them to determine the PAH concentration (section 2.4). For a given experiment

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we used the slope of the natural logarithm of the ratio of the measured concentration at each

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illumination time t, [PAH]t, to the initial concentration, [PAH]0, in conjunction with the photon

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flux correction factor for each sample position (section 2.5) to determine the photodegradation

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rate constant, jPAH: [] ln , = − (3) []

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We followed a similar procedure for dark controls to determine the rate constant for dark loss

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(k’PAH, dark), without applying any correction factor for spatial variations in actinic flux. In those

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cases where there was loss of PAH in the dark samples, we corrected the measured jPAH by

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subtracting k’PAH, dark to give the dark-corrected photodegradation rate constant jPAH, exp. Finally,

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to normalize the results to daily differences in photon flux, we divided jPAH, exp by the measured



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daily j2NB value for the same sample preparation method to give the photon flux-normalized rate

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constant, j*PAH: "#$,%&' ∗ = "#$ (4)

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3 Results and Discussion

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3.1 Actinometry and example illumination experiment

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Because our different sample preparation methods may lead to variations in local photon

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flux, which affects the photodegradation rate, we measured the 2-nitrobenzaldehyde photolysis

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rate constant (j2NB) for each sample type on each experiment day as a proxy for photon flux.

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Figures 2a and 2b show the j2NB values measured for each anthracene illumination experiment

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conducted under our four experimental conditions (aqueous, freezer, LN2, and vapor deposition)

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in vertically and horizontally illuminated beakers, respectively. By measuring j2NB we can

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normalize the PAH photodegradation rate constants to account for varying photon flux in each

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experimental condition. As seen in Figures 2a and 2b, actinic fluxes within each treatment vary

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somewhat from day to day, with RSD values of 13, 16, 17, and 21% for vertically-illuminated

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aqueous, freezer, LN2, and VD samples, respectively; for horizontally-illuminated samples, RSD

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values are lower (4, 3, 3, and 14%, respectively). These differences are likely caused by

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variations in lamp output and sample preparation. There is generally good agreement of j2NB

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values within a particular sample type for each illumination orientation, as well as across sample

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types, consistent with McFall and Anastasio 30, who found sample container and preparation

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method can create modest (< 50%) but statistically significant differences in j2NB values.

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However, while differences within the vertically- or horizontally-illuminated samples are

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modest, we see larger differences between these two approaches. Figure 2c shows the ratio of

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the average horizontally- to vertically-illuminated j2NB value for each sample treatment. For

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aqueous, freezer, and LN2 samples, j2NB values in the horizontally illuminated samples are

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approximately 3 times larger than in vertically illuminated samples. This is likely because the

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polished aluminum sample holder reflects more light onto the horizontal samples than the dull

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copper plate does for the vertical samples. For vapor deposited samples, however, j2NB values

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are only approximately two times greater for horizontal illumination compared to vertical

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illumination. In this case, the 2NB was deposited as a layer on top of the frozen water ice


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surface. When turned horizontally, this layer would be the thin side of a “disk” relative to the

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light source, reducing its effective cross-section to the light beam and resulting in a lower

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calculated j2NB value.

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Figure 3 shows a typical illumination experiment, where samples of a solution of ANT

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were frozen in sealed glass vials in a laboratory freezer and illuminated horizontally. Dark

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controls show very little loss, probably attributable to volatilization, with a measured rate

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constant (k’ANT, dark ± 1 SE) of 0.0022 ± 0.0009 s–1 (R2 = 0.67). ANT concentrations in

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illuminated samples, however, show that photodegradation is much faster than dark loss, with a

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calculated photodecay rate constant (jANT ± 1 SE) of 0.024 ± 0.0015 s–1 (R2 = 0.96).

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3.2 ANT photodegradation rate constants with varying sample orientation

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As described in section 2.4, we used two methods for illumination experiments: samples

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in beakers illuminated vertically, i.e., through the top (diagram Figure 4b, photograph Figure

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S2a), and samples in capped vials illuminated horizontally, i.e., through the side (diagram Figure

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4d, photograph Figure S2b).

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Figure 4a compares the ANT illumination experiments for each sample preparation

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method, with all samples illuminated vertically in beakers. Rate constants are normalized to the

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j2NB value measured for each experimental day. Here, we see no evidence of faster anthracene

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photodegradation at the air-ice interface (i.e., for vapor deposited samples) compared to aqueous

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solution, as was suggested by some previous studies 22-25: in fact, the vapor deposited sample

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mean rate constant is 21 % lower than the aqueous mean, although the difference is not

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statistically significant. Applying a single-factor ANOVA test shows that the means of all four

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sample treatments for each illumination orientation are not the same. Applying the Tukey-

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Kramer test for multiple comparisons (P < 0.05) shows two sample treatment pairs had the same

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mean: aqueous and VD, and freezer and LN2. Sample preparation therefore appears to have a

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small effect on j*ANT, with solutions frozen with a laboratory freezer or liquid nitrogen having a

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j2NB-normalized photodegradation rate constant approximately 40 or 55% faster than aqueous

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samples, respectively. Variability within each sample preparation method, as measured by RSD,

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is 15, 18, 7, and 24% for the solution, freezer, LN2, and VD samples, respectively. Our

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maximum VD ANT concentration (70 nM) corresponds to a surface loading of 1.1 ×1013

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molecules cm–2, or roughly equal to a monolayer surface coverage if the ANT was spread evenly



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across the surface 23, 38; all other VD concentrations were lower, indicating sub-monolayer

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conditions.

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Comparing our results here to previous work requires an understanding of whether

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solutes are in LLRs or at the air-ice interface. Some previous studies have crushed ice cubes into

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granules to study compounds at the air-ice interface, stating that the crushing process increases

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the surface area to volume ratio sufficiently to expose much of the (previously LLR-residing)

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solute at the air-ice interface and that the increased surface area to volume ratio provides a

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mechanism to study photolysis at the air-ice interface 24, 25, 27. While we agree that increasing the

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surface area to volume ratio will increase the fraction of solute present at the air-ice interface, a

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mathematical evaluation suggests that even in a finely ground powder most of the ice volume is

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not in contact with the surface air, and that a uniformly distributed solute would be found almost

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exclusively within the granule. Based on our evaluations of this sample preparation approach

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(Supplemental section S1), we have chosen to identify the solute location in crushed granules as

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LLRs, rather than at the air-ice interface as previously claimed.

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Our finding that ANT at the air-ice interface is not more photochemically reactive than

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anthracene in solution is contrary to the one previous result showing an enhancement (relative to

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solution) of a factor of 6.2 23. Similarly, while past work reported that rate constants for ANT

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photodegradation within ice cubes or granules (for solutes presumably in LLRs) are faster by

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factors of 1.3 – 5.0 times compared to in solution 22-25, we see only a 40% increase in the LLR

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(Freezer) samples compared to solution (Figure 4a). The faster reaction rate constants in these

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past works could be partially explained by higher local photon fluxes, which were not measured.

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Using 2NB as a chemical actinometer to evaluate photon fluxes in crushed ice samples, McFall

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et al. 30 found a 1.8 ± 0.1 enhancement of photon flux relative to aqueous solution.

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Figure 4c also presents ANT photodegradation data, but for samples prepared in capped

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vials and illuminated horizontally, rather than the vertically-oriented beakers of Figure 4a. All

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the horizontally illuminated j2NB-normalized rate constants were very similar: ANOVA indicates

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no significant difference between the average rate constants for any of the sample preparations.

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This is generally consistent with the vertically-illuminated ice samples, although the LLR

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samples showed a modest rate constant enhancement of up to 55% relative to solution. Thus the

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horizontally illuminated samples give additional evidence that anthracene at the air-ice interface


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is not significantly more photoreactive than ANT in solution. They also suggest that the modest

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enhancement seen in the vertically-illuminated LLR samples in Figure 4a might not be real.

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However, it is surprising that the dark-corrected and j2NB -normalized rate constants for

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ANT photodecay (i.e., j*ANT) for the horizontal samples are lower than values from samples

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illuminated vertically in beakers (Figures 4 and S3). To understand these differences, we

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examine both components of j*ANT: 1) the values for jPAH, exp, the anthracene photodegradation

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rate constants prior to photon-flux normalization (but after dark correction), and 2) the j2NB

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values. First, Figures S4a and S4b present jPAH, exp values for samples illuminated vertically and

281

horizontally, respectively, while Figure S4c shows the ratio between these rate constants for each

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sample treatment. While there is some variation in jANT, exp for each sample preparation method,

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the vertical and horizontal results are roughly equivalent. However, this result is different than

284

the j2NB results (Figure 2), which show 2-3-fold higher photon fluxes in the horizontal samples

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compared to the corresponding vertical samples. These higher photon fluxes should result in

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higher photodegradation rate constants for the horizontally illuminated ANT samples, but this is

287

not seen in Figure S4c. One possible explanation for the apparent discrepancy is that the

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horizontal samples were illuminated through the glass walls of the vials, while the vertical

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samples were illuminated through a much thinner sheet of polyethylene film. Figure S5 shows

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that glass vials pass very little light below 300 nm, where the polyethylene film passes most of

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the incident light. But Figures S6a and S6b show that a significant fraction of ANT light

292

absorbance in our system occurs around 250 nm, while 2NB absorbance is more widely

293

distributed. While the j2NB results may be accurately reflecting an increased photon flux in the

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vials relative to beakers at wavelengths above 300 nm, the quantum yield for ANT

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photodegradation may be significantly greater around 250 nm than at longer wavelengths,

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resulting in similar photodegradation rate constants in vertically illuminated beakers and

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horizontally illuminated vials. While several studies have examined wavelength-dependent

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quantum yields for PAHs in aqueous solution at wavelengths greater than 300 nm 39-41,only one

299

measured quantum yields below that value, at 254 nm 42. For the single compound examined at

300

wavelengths both above and below 300 nm, phenanthrene, the quantum yield was approximately

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50% faster at 313 nm than 254 nm. While this finding does not support our explanation of the

302

ANT results seen, further research would be needed to determine the true wavelength

303

dependence of ANT quantum yield. Despite the differences in the measured photodegradation



304

rate constants between horizontally- and vertically-illuminated samples, both sets of data support

305

the conclusion that photodegradation rate constants are similar in aqueous solution, LLRs, and at

306

the air-ice interface. Our finding that anthracene photodegradation is similar in aqueous solution and at the

307 308

air-ice interface contradicts previous work 22-25, which found much faster anthracene

309

photodegradation at the air-ice interface and in LLRs, relative to in solution. While we have not

310

measured photoproducts, our kinetic results suggest (but of course do not prove) that the reaction

311

environment and photodegradation mechanism at the interface, as well as in liquid-like regions

312

of ice, are broadly similar to aqueous solution, at least for this PAH. However, our small but

313

statistically significant enhancement in ANT photodecay in Freezer and LN2 samples compared

314

to aqueous solution or vapor deposited samples (Figure 4a) suggests additional processes might

315

impact the overall chemical reaction rate constant, although we do not see this in the

316

horizontally-illuminated samples (Figure 4c). In both the Freezer and LN2 samples, reactants

317

should be in highly concentrated LLRs within the ice matrix 21, 43. This concentration effect can

318

increase the steady-state concentration of oxidants such as singlet oxygen 43-45, which (in

319

solution) is both formed by PAHs and acts as a sink for PAHs 46. It is possible that such an

320

effect is enhancing PAH loss in some ice cases.

321

3.3 Pyrene experiments

322

We next performed experiments using pyrene (PYR) instead of anthracene. Recent work

323

has reported some small enhancements in PYR photodecay for ice samples made by freezing 100

324

nM pyrene solution into 10 ml pellets and illuminating at –15 °C using the output of a 450 W arc

325

lamp filtered through water. For illuminated pellets (where solutes are expected to be in LLRs)

326

the rate constant was 1.9 (± 0.45, 1 SD) times faster than in solution at 23 °C, while pellets

327

crushed to 2 mm spheres had an enhancement of 1.3 (± 0.52, 1 SD) relative to solution 25. In the

328

latter case the authors suggested that PYR would be preferentially found at the air-ice interface

329

25

330

also had higher average photon fluxes as a result of increased reflection 30.

331

, although we expect that the solutes would still be in LLRs. The crushed samples probably

In Figure 5 we show results for our experiments using pyrene in vertically illuminated

332

beakers under conditions identical (except for PAH) to Figure 4a. While we see a slight (~10%)

333

enhancement in vapor-deposited samples relative to aqueous samples, neither of the mean rate


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334

constants in/on ice are statistically different from the solution value. As with ANT, and again in

335

contrast with previous work, we see no evidence for an enhancement in pyrene photodegradation

336

at the air-ice interface, or in liquid-like regions of ice, compared to aqueous solution.

337

Ram and Anastasio 28 also examined PYR photodegradation, in this case in slowly frozen

338

ice (where PYR should be in LLRs) illuminated in horizontally-oriented quartz tubes in/on the

339

same aluminum sample holder used in our current work for horizontal samples. They measured

340

a rate constant for photodegradation at –10 °C, normalized to noon conditions at Summit

341

Greenland on the summer solstice, of 28 × 10–5 ± 3 × 10–5 (± 1 SE) s–1. Converting units and

342

correcting for the Summit summer j2NB value used in Ram and Anastasio (2.2 × 10 –2 s–1) gives a

343

j2NB-normalized rate constant of 0.76 ± 0.11 min–1/s–1 for this past LLR work. In comparison,

344

our average Freezer (LLR) result in vertically-illuminated samples of frozen PYR solution is 1.3

345

± 0.3 (± 1 σ) min–1/s–1 (Figure 5). We note Ram and Anastasio’s sample illumination method

346

was different than ours, employing quartz tubes illuminated horizontally, and could account for

347

the differing values. But it is interesting that for both pyrene (comparing results from Ram and

348

Anastasio to ours here) and anthracene the photon-flux-normalized rate constant was greater in

349

vertically-illuminated samples.

350

Using their experimental results – combined with information from previous studies of

351

pyrene photodecay in aqueous solution – Ram and Anastasio 28 estimated an activation energy

352

(Ea) for pyrene photodegradation in LLRs of 30 ± 4 kJ mol–1 (which was incorrectly listed as

353

negative in their paper). Based on this value, the rate constant j*PYR should be smaller by a

354

factor of two in/on ice at – 10 °C compared to in solution at 5 °C. Instead, we find

355

photodegradation rate constants are roughly equivalent in solution and ice (Figure 5). Unlike

356

Ram and Anastasio, we measured photodegradation for both frozen and aqueous samples using

357

the same experimental system; since this approach is better, our results suggest that pyrene

358

photodegradation has a very small temperature dependence. Using our aqueous and frozen

359

(Freezer) rate constants we estimate that the apparent activation energy for PYR photodecay is 9

360

± 15 kJ mol–1, i.e., that this reaction is independent of temperature.

361

3.4 Rate constant dependence on photon flux



362

One study reported that the photodegradation rate constant of anthracene is largely

363

independent of photon flux, with additional light past a certain intensity causing no increase in

364

photodegradation 24. This is unexpected. Because the photodegradation rate constant should be

365

directly proportional to the rate of photon absorption by a chemical, we expect photodegradation

366

rate constants to be proportional to photon flux. To test this, we measured rate constants for

367

PAH loss as a function of photon flux. To reduce the light intensity from our standard condition,

368

we inserted either 2 or 4 galvanized steel mesh screens (with approximately 1 mm grid spacing)

369

into the light path of the illumination system.

370

Figure 6 shows our results for jANT (here, not dark corrected) versus the j2NB value

371

measured in each experiment for solution and ice samples. Results from the previous study 24,

372

which showed fast reactivity and a weak dependence on photon flux, are bounded by the narrow

373

vertical box along the y axis; to better visualize this past data, we also present this figure in a log-

374

log plot in Figure S7. As expected, our data shows a linear relationship between the

375

photodegradation rate constant and photon flux for all three of the sample types tested,

376

contradicting the previous work. Y-intercepts for all three sample treatments are statistically

377

indistinguishable from zero, although there does appear to be dark loss in some samples. The

378

slopes of the regression lines for VD and aqueous samples are statistically indistinguishable,

379

while the slopes for the aqueous and freezer lines are different; the slopes for the freezer and VD

380

lines are indistinguishable. For each sample treatment, the regression line slope in Figure 6

381

corresponds closely to and is statistically indistinguishable from the average photon flux-

382

normalized rate constant determined with full illumination (without metal screens) presented in

383

Figure 4a.

384

Taken together, these results suggest that photodegradation in all sample treatments

385

proceeds by a similar mechanism, but product measurements are needed to better constrain this.

386

In addition, we note that the photon fluxes in our work are dramatically higher than those

387

reported in the previous work, and are closer to (though still lower than) values representative of

388

midday summer conditions at Summit, Greenland (with a calculated j2NB value of 0.022 s–1) 28.

389

Our typical experimental j2NB value was 0.0020 s–1, corresponding to an incident photon flux of

390

1.8 × 1015 photons cm–2 s–1 over a wavelength range of 291 to 400 nm. On the other hand,

391

Kahan et al. 24 report photon fluxes ranging from 1.1 ×1012 to 2.2 × 1013 photons cm–2 s–1,


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392

approximately 80-1600 times lower than our typical experimental photon flux and another factor

393

of 10 lower than summertime photon fluxes in polar regions.

394

3.5 Impact of solutes and exposure to lab air Because the total solute amount in an ice sample impacts the volumes of LLRs and QLLs

395 396

47-49

397

PAH photodegradation rate constants. As shown in Supplemental Figure S8, the presence of

398

NaCl tends to increase j*ANT for all sample preparation methods, including solution, but this

399

behavior does not correlate well with NaCl concentration. The lowest concentration of NaCl

400

used (0.02 M) gives results similar to solutions made in pure water, but at higher concentrations

401

(0.2 M and above), the reaction rate constant is often faster than pure-water values. The ratios of

402

the average j*ANT value for salt concentrations above 0.02 M relative to the average j*ANT for

403

NaCl concentrations equal to or below 0.02 M are 2.2, 1.8, and 2.2 for aqueous solution, frozen

404

solution, and vapor-deposited samples, respectively. It is unclear why higher total solute

405

concentrations would increase PAH photodegradation rates in aqueous solution, but the effect

406

might be due to trace impurities in the salt. For ice samples made from solutions containing

407

higher total solute concentrations, the volume of the QLL or LLR should be larger, and therefore

408

any reactive solutes should be less concentrated compared to ice made from solution containing a

409

lower initial total solute concentration. For first-order reactions, such as direct photolysis, we

410

would expect the same reaction rate constant regardless of salt concentration. But for second

411

order reactions, the addition of salt should decrease the measured pseudo-first-order rate constant

412

(e.g., 44). Instead, in Figure S8 for ANT photodecay we see a slight increase in the measured rate

413

constant with higher salt concentration, inconsistent with either expectation. Although this effect

414

seems consistent across sample types, we do not have an explanation. Interestingly, our results

415

are the opposite of those found in another study using harmine as the test compound and either

416

sodium chloride or sodium bromide to control total solute concentration, which showed slower

417

photodecay with increasing salt concentration 50.

418

, we also examined whether varying the concentration of sodium chloride (NaCl) affects

Finally, we also examined whether contaminants present in laboratory air might partition

419

to the air-ice interface of our frozen samples and enhance the rate of PAH photodecay by making

420

singlet molecular oxygen, which can react rapidly with PAHs 46. We have previously found that

421

small amounts of contaminants in solution get concentrated during freezing and can generate



422

high concentrations of 1O2* during illumination 43. We suspect that a similar mechanism might

423

occur at the air-ice interface. To test this, we first made ice samples (in our homemade beakers)

424

and then exposed the samples to laboratory air while keeping the samples at –18 oC in a

425

temperature-controlled chamber with the lid raised approximately 1 cm above the top edge of the

426

chamber. After exposing the ice sample to lab air we vapor-deposited ANT onto the ice surface

427

and illuminated with simulated sunlight. As shown in Supplemental Figure S9, there is a general

428

trend toward higher rate constants for ANT loss for samples exposed to lab air: three of the five

429

exposed samples have higher photodegradation rate constants, with enhancements ranging from

430

1.3- to 4.0-fold compared to the average j*ANT value for samples not exposed to lab air.

431

However, the rate constants are generally noisy, the enhancement does not correlate with

432

exposure time, and experiments on the same day show different results. Thus, while we see

433

some evidence that gaseous contaminants can enhance the photodegradation of anthracene at the

434

air-ice interface, more work is be needed to quantify this effect.

435

4 Implications

436

Our results show no enhancement in photodegradation rate constants for anthracene or

437

pyrene at the air-ice interface compared to in aqueous solution. Further, we find either no

438

enhancement, or only small enhancements (40-55%), in LLRs relative to aqueous solution. This

439

is in contrast to previous results for anthracene and naphthalene 23. Further, our findings support

440

the expected relationship that photodegradation rate constants are proportional to photon flux;

441

this is also in contrast to previous work 24. The reasons for the difference in our current results

442

compared to previous work are unclear, but the anthracene photodegradation loss rate constants

443

normalized for photon flux are far greater in the Kahan et al. study 24 than measured here (and in

444

another study 28), suggesting loss mechanisms other than photodegradation might have been

445

important in past work. We also note that our methods here are different than used in previous

446

work: our lamp intensities were substantially higher (and more similar to ambient actinic flux),

447

our PAH concentrations were lower (and more similar to ambient snow levels), and a much

448

smaller fraction of our sample surface area was exposed to air. While it is difficult to assess the

449

individual significance of these methodological differences, they could contribute to the overall

450

difference in results.


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Page 17 of 27


Overall, our current work suggests that the kinetics of PAH photochemistry in/on snow

451 452

and ice are very similar to kinetics in aqueous solution. While the presence of other contaminants

453

in environmental snows and ices might alter PAH kinetics (e.g., 22, 25), the puzzle of polar PAHs

454

is not that they decay more quickly in snow than expected, but the opposite: PAHs in snow have

455

longer-than-expected lifetimes, probably because they are embedded in light-absorbing particles

456

28

457

molecular oxygen can be enormously enhanced in ice samples compared to solution 18, 43, 45, our

458

finding that PAHs in ice and solution have similar lifetimes suggests that 1O2* is not an

459

important sink for ANT and PYR in the ice samples studied here.

460

Supporting information

461 462

Supporting information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.??????.

463 464 465

. Finally, since previous work has found that the steady-state concentrations of singlet

PAH location in crushed ice granules, summary of previous work, diagrams and pictures of experimental equipment, details on some experimental results, Figures S1 – S9, and Tables S1 and S2 (PDF)

466

Acknowledgments

467 468

We thank the National Science Foundation for funding (ECS-1214121 and AGS-PRF 1524857) and Ricky Obregon for experimental assistance.



469

Figures

470

471 472 473 474

Figure 1. Diagram of the four different sample preparation methods: aqueous solution, solution frozen in a laboratory freezer, solution frozen in liquid nitrogen, and PAH vapor-deposited to a water ice surface.

475


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Page 19 of 27


476 477

478 479 480

481

482 483 484 485 486 487 488 489 490 491 492 493

Figure 2. a) 2-nitrobenzaldehyde photolysis rate constants (j2NB) for the vertical orientation of each of the four experimental conditions (aqueous solution, samples frozen in freezer, samples frozen in liquid nitrogen, and samples with 2NB deposited to the air-ice interface). Each bar is an individual actinometry experiment, ordered chronologically within a particular experimental condition. In each case the 2NB sample was illuminated in a glass beaker oriented vertically (i.e., with the long axis of the beaker parallel to the incident light). Individual error bars are ± 1 standard error. Shaded areas represent the mean value for each treatment (dark line) with 95% upper and lower confidence intervals (UCL and LCL) above and below. Sample treatments with statistically indistinguishable average rate constants (P < 0.05) are labelled with the same capital letter (“A”, “B”, etc.); treatments with different letters are statistically different. b) Identical to



494 495 496 497

a), but for samples illuminated horizontally in 2 ml vials. c) Ratio of average horizontal to average vertical j2NB values for a given experimental condition, using the individual data points shown in Figures 2a and b. Error bars are 95% confidence intervals for each ratio.

498

499 500 501 502 503 504 505 506

Figure 3. Illumination experiment for anthracene ice samples in horizontally-oriented glass vials prepared in the freezer with an initial ANT concentration of 1 nM. Illuminated samples are given by filled blue diamonds, while open diamonds are dark control samples. Each data point is from an individual sample container (we illuminated two samples for each time point). The value for j2NB is 0.0067 s–1.


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507

b)

508

d)

509 510 511 512 513 514 515 516 517 518 519 520 521 522

Figure 4. Results of anthracene photodegradation experiments. a) Dark-corrected (and j2NBnormalized) ANT photodegradation rate constants (j*ANT) for vertically illuminated samples. Results are given in chronological order within each sample preparation method; each vertical bar indicates a separate experiment. Individual error bars are ± 1 standard error. Shaded areas represent the mean value for each treatment (dark line) with 95% upper and lower confidence intervals (UCL and LCL) above and below. Sample treatments with statistically different average rate constants (P < 0.05) are labelled with different capital letters. b) Schematic of the vertical illumination setup with a beaker covered with polyethylene film. c) Identical to a), but for horizontally illuminated samples. Means for all four sample preparation methods were statistically indistinguishable (P > 0.05). d) Horizontal illumination setup showing capped vial; see Figure S2c for a diagram of vials in the polished aluminum sample holder.



523

524 525 526 527 528 529 530 531 532

Figure 5. Comparison of dark-corrected pyrene (PYR) photodegradation rate constants for vertically-oriented samples. Results are individual experiments given in chronological order within each sample preparation method. Individual error bars are ± 1 standard error. Shaded areas represent the mean value for each treatment (dark line) with 95% upper and lower confidence intervals (UCL and LCL) above and below. Means for all three sample types were statistically indistinguishable (P > 0.05).


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533 534

535 536 537 538 539 540 541 542 543 544 545 546 547 548

Figure 6. Dependence of the ANT photodegradation rate constant (jANT, not dark corrected) on the photon flux, measured with j2NB, using various sample treatments in vertically-oriented beakers. Each point indicates a separate experiment. For comparison, the lower range of the data reported in Kahan et al. 24 is delineated by the narrow grey rectangle on the upper left portion of the plot. Bars on our data points are ± 1 standard error, determined from propagated errors. For our three datasets, the slope ± 95% confidence interval (CI) is given on the graph. The corresponding y-intercept (min–1) ± 95% CI and R2 values are: -0.00053 ± 0.00077, 0.97; 0.00074 ± 0.0016, 0.95; and 0.0021 ± 0.0022, 0.97 for aqueous (red diamonds), freezer (blue squares), and VD (black triangles) samples respectively. None of the y-intercept values are statistically different from zero.



549

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