Nitrate Photochemistry at the AirâIce Interface and in Other Ice

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Nitrate Photochemistry at the Air-Ice Interface and in other Ice Reservoirs Alexander S McFall, Kasey C Edwards, and Cort Anastasio Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 18, 2018

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Environmental Science & Technology

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Nitrate Photochemistry at the Air-Ice Interface and in other Ice Reservoirs

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Alexander S. McFall, Kasey C. Edwards and Cort Anastasio*

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Department of Land, Air, and Water Resources

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University of California Davis

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Davis, CA 95616

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*Corresponding Author, Department of Land, Air, and Water Resources, University of

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California - Davis; Tel: 530-754-6095; Email: [email protected]; Fax: 530-752-1552

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Submitted to Environmental Science and Technology: January 6, 2018

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Revised Version Submitted: March 16, 2018

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Word Count: 6550

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ACS Paragon Plus Environment


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TOC Abstract Art

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ABSTRACT

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The photolysis of snowpack nitrate (NO3–) is an important source of gaseous reactive nitrogen

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species that affect atmospheric oxidants, particularly in remote regions. However, it is unclear

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whether nitrate photochemistry differs between the three solute reservoirs in/on ice: in liquid-like

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regions (LLRs) in the ice; within the solid ice matrix; and in a quasi-liquid layer (QLL) at the air-

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ice interface, where past work indicates photolysis is enhanced. In this work, we explore the

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photoformation of nitrite in these reservoirs using laboratory ices. Nitrite quantum yields,

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Φ(NO2–), at 313 nm for aqueous and LLR ice samples agree with previous values, e.g., (0.65 ±

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0.07)% at –10 °C. For ice samples made via flash-freezing solution in liquid nitrogen, where

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nitrate is possibly present as a solid solution, the nitrite quantum yield is (0.57 ± 0.05)% at –10

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°C, similar to the LLR results. In contrast, the quantum yield at the air-ice interface is enhanced

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by a factor of 3.7 relative to LLRs, with a value of (2.39 ± 0.24)%. These results indicate nitrate

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photolysis is enhanced at the air-ice interface, although the importance of this enhancement in

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the environment depends on the amount of nitrate present at the interface.

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INTRODUCTION

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The photolysis of snowpack nitrate in polar regions is a major source of atmospheric nitrogen

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oxides (NOx) and nitrous acid (HONO), which can produce hydroxyl radical (•OH) and ozone.1-7

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Nitrate photolysis also affects the fates of organic contaminants in snow and snowpack records

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of past atmospheres.8-10 Recent isotopic data has shown that photolysis is a significant post-

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depositional process affecting nitrate concentrations in snow, especially in areas with low

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accumulation rates (e.g., Dome C).11, 12 A thorough understanding of nitrate reactivity is

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therefore crucial in understanding overall snow chemistry in remote regions.

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In solution and ice, nitrate photolysis with wavelengths above 280 nm proceeds via two

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channels:

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NO3– + hν → NO2 + •O–

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NO3– + hν → NO2– + O(3P) (Channel 2)

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This seemingly simple chemistry is complicated by secondary reactions which can alter apparent

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quantum yields for the two channels. These processes include photolysis of NO2– to NO,

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oxidation of NO2– by •OH (formed when •O– gains a proton) to form NO2, and reaction of

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superoxide (•O2–) with NO2 to form NO2–.13-15

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Channel 1 has been well studied,16-20 with an average aqueous quantum yield of (1.35 ± 0.3)%

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for illumination in the 302-nm band near 298 K. However, there has been significant debate

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regarding the importance of channel 2 in solution, with a wide range of reported quantum

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yields.13, 18, 21, 22 This variability is likely due to differences in experimental conditions, which

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can alter the extent of nitrite secondary chemistry.23, 24 A previous analysis of O(3P) formation

(Channel 1)



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from channel 2 in solution reported a quantum yield of 0.11%, an order of magnitude lower than

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channel 1.18 This lower value is often cited as justification for omitting channel 2 from models,3,

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10, 13, 25

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work confirms that the quantum yields for channels 1 and 2 are comparable in solution, with

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Φ(NO2–) = (1.1 ± 0.2)% at room temperature,23 while channel 2 dominates at lower

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temperatures.24

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Nitrate photochemistry in ice is complicated because solutes can be present in at least three

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different locations (Figure 1): (1) in liquid-like regions (LLRs) within the ice, generally at grain

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boundaries and at the surfaces of internal air bubbles, (2) within the bulk ice matrix, i.e., present

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in a solid solution, and (3) at the air-ice interface, which is also known as the quasi-liquid layer

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(QLL) or disordered interface.10, 26, 27 Solutes at the air-ice interface might be more reactive; Zhu

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et al. measured a channel 1 QLL quantum yield of 60% on ice films at –20 °C,28 which is 160

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times larger than the value in ice LLRs at –20 °C.19 In addition, Meusinger et al.,29 using

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Antarctic snow illuminated in the laboratory at –30 °C, found that nitrate photolysis was rapid

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and exhibited two domains of reactivity: a photolabile domain (with a quantum yield for nitrate

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loss between 12 and 44%) and a “buried” domain (with quantum yields between 0.3 and 12%).

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In contrast, the sum of the quantum yields for channels 1 and 2 in liquid-like regions of

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laboratory ice is much lower, 0.68% at –30 °C.19, 24

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There is also evidence that light absorption by nitrate is enhanced at the air-ice interface: Zhu et

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al. measured a nitrate absorption cross section nearly 50 times higher28 than the solution value at

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5 °C.19 Enhanced light absorption for nitrate has also been reported at quartz-water and air-

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sapphire interfaces.30, 31 Since Meusinger et al.29 calculated quantum yields using the aqueous

despite other measurements that show a quantum yield closer to 1%.18, 21, 22 Our recent


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nitrate molar absorptivities, it is unclear how much of their observed enhancement in reactivity is

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due to higher quantum yields or higher light absorption.

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To help resolve the uncertainty over nitrate photolysis at the air-ice interface, here we measure

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the quantum yield for channel 2 (i.e., nitrite formation) for photolysis of nitrate in each of the

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three ice solute reservoirs while optimizing experiments to minimize secondary chemistry. Our

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goal is to determine whether the nitrite quantum yield for nitrate at the air-ice interface (i.e., in a

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QLL) is greater than that for nitrate in ice LLRs or in a solid solution. In addition, we determine

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the temperature dependence of Φ(NO2–) at the air-ice interface and compare it to the recently

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measured temperature dependence for nitrate in LLRs. Lastly, we examine the role of an

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enhanced nitrate quantum yield in understanding field measurements of NOx and HONO.

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METHODS

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Sample preparation

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Details on chemicals, stock use, and stability are given in Supplemental Section S1. Aqueous

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samples were prepared as 0.75 mL aliquots in 2.0 mL glass autosampler vials (Shimadzu). A

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Teflon stir bar was added to each sample, which was then capped and placed in the illumination

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system. We prepared ice samples in custom-made 1.0 mL Teflon molds (Supplemental Figure

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S1) using three different methods designed to place nitrate in one of the three ice reservoirs. To

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prevent condensation of trace gases, including water, ice pellet samples were covered with foil

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during freezing and before illumination, and with clear polyethylene wrap (Glad) during

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illumination and thawing. Samples were allowed to equilibrate in the illumination chamber for 5

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to 7 minutes to reach the desired temperature. To concentrate solutes in LLRs, we used a custom-

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designed, Peltier-cooled freeze chamber (Paige Instruments) at –20 °C to freeze our samples over



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a two-hour period. Micro computerized tomography (micro-CT) indicates that this slow freezing

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places a majority of solutes in LLRs, especially wrapped around internal air bubbles in the ice.27

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In our next method, we added solution to a mold, covered it with foil, and placed it in liquid

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nitrogen (LN2) for 60 seconds to flash-freeze. Imaging of this type of sample by micro-CT

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showed some evidence of very small LLRs, but it was unclear what fraction of solutes were in

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LLRs versus present as a solid solution in the ice crystal lattice.27

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To place solutes at the air-ice interface, we used gas-phase solute deposition. 750 µL of Milli-Q

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water was pipetted into a Teflon mold and frozen in our freeze chamber at –20 °C. After samples

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were frozen, we initially used a calibrated permeation tube (KinTek) for nitric acid experiments

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(~13 nmol min-1 at a 250 mL min-1 flow rate), which was later replaced with a mini-bubbler (Ace

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Glass, No. 7533-27) containing 6.4 M HNO3 (i.e., 1 mL of concentrated nitric acid and 1.5 mL

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of Milli-Q) and delivering ~20 nmol min–1 at a flow rate of 100 mL min–1 (Supplemental Figure

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S2). Our typical deposition time was 45 seconds, which resulted in an average (±1 σ) nitrate

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concentration of 19 ± 6 µM in the melted ice pellet, corresponding to a surface coverage of

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approximately 4 × 1015 mlc cm–2 of HNO3 (see Section S2). Since monolayer thickness

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corresponds to a nitric acid surface coverage of 1.1 × 1014 mlc cm–2, our samples represent

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approximately 39 layers of nitrate at the surface.28

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Illumination

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All samples were illuminated with 313-nm light from a 1000 W Hg/Xe arc lamp with a

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downstream monochromator (Spectral Energy) and four aluminum screens upstream of the

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sample to attenuate the beam. Aqueous samples were illuminated vertically in glass vials held in

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a custom-built, Peltier-cooled aluminum housing with a magnetic stirrer (Paige Instruments). Ice


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samples were illuminated in a temperature-controlled freeze chamber (Paige Instruments) with a

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dry air flow plumbed into the chamber to prevent condensation of water vapor on the aperture

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window. The samples were placed in the chamber vertically, with the illumination beam

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perpendicular to the ice surface.

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Analysis of Nitrite and Nitrate

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After illumination, ice samples were thawed in the dark at room temperature, and then Griess

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reagents were added to form a strongly-absorbing azo-dye complex with nitrite.32, 33 Though the

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Griess method can also react with dissolved NO2,34 this interference appears to be negligible in

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our samples.23 Additional details are provided in Supplemental Section S3.

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Since the amount of HNO3 deposited to the ice surface varies between pellets, we measured

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nitrate in each pellet after illumination:35 950 µL of a vanadium (III) chloride solution (which

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reduces NO3– to NO2–) and the Griess reagents were added to 50 µL of thawed sample in a

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HPLC vial, which was sealed and allowed to sit for 18 hours prior to measurement. Since nitrate

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concentrations were typically 25-50 µM, we used a UV/Vis spectrophotometer (Shimadzu

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UV2501PC) to measure absorbance: 900 µL of the final sample volume and 1.5 mL of Milli-Q

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water were mixed and measured in a 1-cm quartz cuvette. All UV/Vis runs were accompanied by

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a Milli-Q blank and 25 µM nitrate check standard.

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Chemical Actinometry and Quantum Yield Calculation

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We used 2-nitrobenzaldehyde as a chemical actinometer, as described in Supplemental Section

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S4. Actinometry samples were prepared under identical conditions as the corresponding nitrate

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samples to ensure accurate measurement of the photon flux in each solute location.36 Under the



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low light-absorbing conditions of our actinometer, the measured rate constant for loss of 2NB

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(j2NB,313) is related to the photon flux via:

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, = 2.303 × 10 ( )(, Φ,)

(1)

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where I313l is the surface-area-normalized photon flux (mol-photon cm–2 s–1 nm–1), and

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ε2NB,313Φ2NB,313 (640 M–1cm–1) is the product of the base-10 molar absorptivity and quantum

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yield for 2NB at 313 nm.37 Similarly, the rate constant of nitrite formation is given by:

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(NO )(Φ(NO → NO ) = 2.303 × 10 ( )( ) ) ,

(2)

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where Φ(NO2−)313 is the quantum yield of nitrite formation from nitrate photolysis and εNO3−,313

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is the base-10 molar absorptivity of solution nitrate at 313 nm (5.33 and 5.29 M−1 cm−1 at 25 and

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5 °C, respectively).19 We use aqueous molar absorptivities since past work from Dubowski et al.

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and Matykiewiczova et al. observed no difference in the nitrate and nitrite absorption profiles in

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ice relative to aqueous samples.20, 38 Thus, any observed rate enhancement should be due to an

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increase in the quantum yield. Since nitrate photolysis is a first-order process, the rate of nitrite

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formation is equal to:

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[ ]

= (NO → NO ) [NO ]

(3)

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Our experiments used short time scales (typically no longer than 9 min) to ensure the increase in

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nitrite concentration was linear and the formation rate could be determined via simple linear

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regression. Combining equations 1-3, we solve for the quantum yield of nitrite formation:

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Φ(NO ) =

[ ]

×&

!"#,$ %"#,$

[ '(,$ !") ] ,$


(4)

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However, while equation 4 accounts for any variation in the photon flux, it assumes that the

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initial concentration of nitrate is the same in all samples. While this is the case for aqueous, LLR,

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and LN2 samples, it is not for samples with nitrate at the interface. To account for variability in

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nitrate concentration, we normalize the nitrite concentration at each time point by the nitrate

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concentration:

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Φ(NO ) =

+', -. +', -

*

×

!"#,$ %"#,$ &'(,$ !"),$

(5)

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Since we use the aqueous molar absorptivity in Equations 4 and 5, we are implicitly assuming

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that light absorption by nitrate in each of the ice locations is the same as in solution at 5 °C, i.e.,

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that any difference in the rate of nitrite formation is due to changes in the quantum yield.

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RESULTS AND DISCUSSION

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Aqueous Comparison to Past Results

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To compare our results to published literature, we first measured the nitrite quantum yield under

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aqueous conditions, with 50 µM NaNO3, 500 µM 2-propanol as an •OH scavenger, pH 5.2 and

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25 °C. Our average value of Φ(NO2–), (0.93 ± 0.10)%, is similar to those of Warneck and

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Wurzinger (1.0%),18 Goldstein and Rabani (0.94%),22 and Benedict et al. (1.1%).23 We also

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performed some control experiments where we bubbled humidified N2 gas through samples

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during illumination to purge any photoformed NO2, with no significant change in quantum yield.

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Liquid-Like Regions and LN2 Samples

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In LLRs of ice samples made from solutions containing 50 µM NaNO3 and 500 µM 2-propanol,

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our average (±1 σ) measured quantum yield is (0.65 ± 0.07)% at –10 °C. This result matches



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observations from Benedict and Anastasio, and is approximately 33% lower than our aqueous

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results at 25 °C, in agreement with the temperature dependence determined in Benedict and

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Anastasio.24 Our LLR ice value is lower than past measurements from Dubowski et al. (0.90%)

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with formate as an •OH scavenger (10 mM nitrate, 302 nm, –10°C),20 likely because of their

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larger nitrate concentrations, which can augment secondary chemistry.23

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Our flash-frozen (LN2) samples, where nitrate is present in very small LLRs and/or as a solid

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solution, give a Φ(NO2–) value of (0.57 ± 0.05)%. This result is lower but not statistically

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different from our LLR measurements, suggesting either that nitrate in a solid solution has a

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similar photochemical reactivity to nitrate in LLRs or that nitrate in our LN2 samples is primarily

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present in small LLRs rather than as a solid solution. Our micro-CT imagery does not have the

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resolution necessary to discern the solute distribution in LN2 samples,27 although it suggests at

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least some of the solutes are in very small liquid-like regions. If nitrate was present as a solid

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solution, one might expect a lower nitrite quantum yield due to a stronger cage effect. However,

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it is possible that even a solid solution of nitrate has very small liquid-like domains since the

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dissociated salt ions (Na+ and NO3–) might be solvated with liquid-like water molecules, as

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occurs for HNO3 deposited to the air-ice interface39 as well as for salt solutions even below their

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eutectic temperatures.40, 41

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Air-Ice Interface Samples

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Compared to the tests above, measuring nitrate reactivity at the air-ice interface is more

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complicated because of variability in both the amount of HNO3 we deposited and the surface pH,

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which affects nitrite secondary chemistry. Surface concentrations of HNO3 varied by up to a

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factor of four on a given experiment day, although the typical range was smaller, with an average


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daily relative standard deviation of 14%. We modified the method of calculating Φ(NO2–) that

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we used for the other sample types, where nitrate is constant (equation 4), to account for this

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variability (equation 5). An example of this data treatment, in which nitrate concentrations were

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especially variable, is shown in Figure 2 and illustrates the improvement in kinetic data after

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normalization.

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We first tested simple depositions of nitric acid to a pure ice surface, but found that the measured

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quantum yield, (0.21 ± 0.14)%, was far lower than our other ice results (Supplemental Figure

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S3). This is likely due to the high acidity of the quasi-liquid layer, which protonates photoformed

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nitrite to HONO that is then lost to the gas phase. As described in Supplemental Section S5, we

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next tested a series of conditions to minimize loss of nitrite due to HONO volatilization and

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oxidation by •OH. We found that gas-phase deposition of nitric acid and then ammonia appears

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to both increase pH sufficiently to minimize HONO formation and provide NH3 as a scavenger

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for •OH.42, 43

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As shown in Figure 3, under these conditions the average (± 1σ) interface quantum yield at –10

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°C is (2.39 ± 0.24)%, which is 3.7 ± 0.5 times greater than the average LLR result at this

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temperature. Despite the colder temperature, the interface quantum yield is also 2.6 ± 0.4 times

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higher than the solution value at 25 °C. As we describe in the Methods, we are attributing the

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entire enhancement in the rate of nitrite formation at the interface to an increase in the nitrite

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quantum yield (i.e., we use the aqueous molar absorptivity of nitrate for all quantum yield

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calculations; equation 5). While some of the enhancement in the QLL might be due to an

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increase in the absorption cross-section of nitrate relative to in solution, if we use the

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enhancement reported by Zhu et al.28 then our quantum yield reduces to 0.05%, well below the



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LLR value. Enhancements in nitrate reactivity at other interfaces – including metal sheets,

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construction materials, and plant surfaces – have also been reported and attributed to sample

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properties (e.g., surface properties, relative humidity, stabilizing role of other species) or

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increased photolability of NO3– at the surface.28, 29, 44-48

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Temperature Dependence at the Air-Ice Interface

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We next examined the temperature dependence of ΦQLL in ice pellets with HNO3 and NH3

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deposited to the interface. As shown in Figure 4a, the data form a linear Arrhenius plot, with an

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activation energy (Ea) of 10 ± 1 kJ mol–1 and a change in entropy (∆S) of 7 ± 6 J mol–1 K–1.

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Based on a regression of the average quantum yield at each temperature, the temperature

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dependence of the quantum yield at the interface is

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ln1Φ233 4 = −

(78 ± :) ;

+ (0.84 ± 0.66)

(6)

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where T is the temperature in Kelvin. Figure 4A also shows the previously reported temperature

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dependences for channels 1 and 2 of nitrate photolysis in LLRs.19, 24 For each channel there is a

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smooth, continuous temperature dependence across the range of aqueous and ice temperatures,

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suggesting that the ice LLR environment is similar to that of aqueous nitrate. While it was

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previously thought that Chu and Anastasio measured photolysis in QLLs,19 we now believe that

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the nitrate was present in LLRs based on the slow-freezing conditions used to prepare the

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samples and our recent imaging work.27 The continuous linear temperature dependence is

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consistent with past work from Guzman et al. that observed hydration of acid molecules on ice

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films far below the eutectic temperature, and measured a similar linear dependence on

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temperature for pyruvic acid photolysis.49, 50 The air-ice interface results for channel 2 are offset


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higher relative to the LLR and solution results. However, the activation energies for the channel

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2 QLL and LLR quantum yields are statistically indistinguishable (Figure 4A). ∆S for nitrate

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photolysis in the QLL is higher than in LLRs, but the difference is not statistically significant.24

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While the QLL is clearly distinct from LLRs as a reaction environment, they may share some

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similar liquid-like behavior. Work from Kahan et al. examined the surface of freshwater and

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saltwater ices and found that, in the presence of inorganic salts, solutes behaved as if in a liquid-

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like environment, unlike on a freshwater ice surface.51 Though we start with a pure water ice

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pellet, the HNO3 and NH3 deposited should form a disordered brine layer, which could explain

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the similar activation energies observed in our LLR and QLL results. While related work from

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Morenz and Donaldson has indicated that surface-excluded nitrate can form crystals at the air-ice

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interface close to the eutectic temperature, we see no change in behavior below the eutectic

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temperature (~255K) for our results.52

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Figure 4B shows the experimental ratios of ΦQLL relative to ΦLLR for channel 2 (NO2– formation)

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in the ice samples from Figure 4A along with a line representing the ratio of the Arrhenius

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equations obtained from each data set. The ratio of the two values is essentially independent of

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temperature in the range studied (–5 to –25 °C), with an average QLL enhancement relative to

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LLRs of 3.4 ± 0.3, determined as the average (± 1σ) value of the Figure 4B data. Thus, the

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enhancement in surface reactivity is not altered by temperature in this range, consistent with our

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finding of similar activation energies for both solute locations.

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Comparison with Past Laboratory Work

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Our measured 3.4-fold enhancement in the quantum yield of nitrite (channel 2) from illumination

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of nitrate in the QLL (relative to in the LLR) is much lower than the 160-fold QLL enhancement



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for the NO2 quantum yield (channel 1) of Zhu et al.28 relative to the LLR value of Chu and

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Anastasio at –20 °C.19 The difference is even starker if we consider the product of the quantum

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yield and molar absorptivity, which is proportional to the rate constant for nitrate photolysis,

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jNO3– (e.g., equation 2). Zhu et al. determined that the molar absorptivity of nitrate at the air-ice

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interface (308 nm, –20 °C) is 48 times higher than in solution, which means that the product

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Φ(NO3– → NO2)×εNO3- in their work is a factor of 7700 times higher than the product we

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determined for channel 2 in LLRs.

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We can also compare our results with those of Meusinger et al., who reported a mean quantum

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yield of 26% (range 12% – 45%) for the sum of channels 1 and 2 in the “labile” domain of

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natural snow during laboratory illumination at –30 °C.29 If we assume that the Chu and

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Anastasio LLR quantum yield for NO2 formation19 is enhanced at the air-ice interface (relative to

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LLRs) by the same factor of 3.4 that we determined for channel 2, we calculate an overall QLL

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quantum yield for nitrate loss of 2.3% at –30 °C. This makes the 26% measurement of

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Meusinger et al. for nitrate loss in the labile domain 11 times higher than our observations.29

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To further investigate this difference, we prepared ice samples with only nitric acid deposited to

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the surface, and measured the total quantum yield for loss of nitrate, Φ(NO3–), during 24 hr of

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illumination. We did not neutralize HNO3 so that photoformed NO2– would be protonated and

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lost as HONO. To enhance removal of HONO and NO2, samples were not covered during

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illumination and were flushed with a slow stream of cooled, humidified N2. As shown in Figure

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5, for the dark control there is rapid intial loss of nitrate between 0 and 6 hr followed by no loss

289

between 6 and 24 hr. The illuminated ice pellets at –10 °C show a similar behavior, with an

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initial, dark-loss-corrected rate constant (±SE) for nitrate loss of 2.5 ± 0.4 × 10–5 s–1, which


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corresponds to a quantum yield (±SE) of (9.3 ± 4.3)%. This is not statistically different (at 95%

292

confidence) from the value of (3.9 ± 0.37)% that we predict assuming similar QLL reactivity

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enhancements for channels 1 and 2 at –10 °C. Further, our measurement of (2.0 ± 1.9)% for the 6

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– 24 hr period, while noisy, also encompasses our predicted quantum yield. The purple lines in

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Figure 5 are calculated decay curves based on the average “labile domain” quantum yield of 24%

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for surface HNO3 photolysis from Meusinger et al.29 Under this condition, 92% of HNO3 is

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depleted within 10 hours, far faster than the loss rate we observe in our samples.

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Our quantum yield for channel 2 at the interface might be lower than those of Zhu et al. and

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Meusinger et al. because we have more nitrate at the air-ice interface. While our typical sample

300

has a loading of HNO3 that represents almost 40 layers, Zhu et al.28 report monolayer coverage,

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and Meusinger et al.29 had less than monolayer coverage (0.3 layers, assuming a specific surface

302

area of 316 cm2 g–1 in the top 10 cm of snow)53 if we assume that their entire average 18 µM of

303

NO3– was present on the grain surfaces of their homogenized snow. As additional solutes are

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deposited to the QLL it thickens,40 and the layer behaves more like a concentrated solution.

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However, as shown in Supplemental Figure S4, we see no relationship between sample nitrate

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concentration and quantum yield in our range. Nitrate at the interface may also move into the

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bulk ice,39, 54-56 especially in the presence of the ammonium cation,57 which would lower the

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apparent quantum yield.

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Using Field Data to Constrain Nitrate Photochemistry

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Given the enormous ranges in reported quantum yields and molar absorptivities for nitrate

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photolysis, our goal in this section is to use field measurements of reactive nitrogen fluxes from

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sunlit snow to roughly constrain nitrate photochemistry. Our group recently measured the LLR



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quantum yield for nitrite formation (0.69% at –10 °C) and, along with the LLR quantum yield

314

for channel 1, updated our simple box model of nitrate photochemistry in snow.4, 24 Our modeled

315

fluxes of NOx compare well with most reported field measurements, although several field

316

measurements are lower and several are 2 – 4 times higher.25 For HONO, the results are

317

generally comparable, but the modeled HONO flux is very sensitive to the LLR/QLL pH, which

318

is unconstrained in our model and unmeasured in the field. Overall, while there are a number of

319

important uncertainties, these recent results suggest that photolysis of LLR nitrate can largely

320

explain the fluxes of NOx and HONO.24

321

To explore the potential role of QLL nitrate photolysis in the field, we first define the apparent

322

j(NO3–) enhancement, which is the fold-increase in the apparent nitrate photolysis rate constant

323

for a snow sample containing nitrate at the interface and in LLRs, relative to the rate constant

324

expected if nitrate was only in LLRs:

325

@AABCDEF GEℎBEIDJDEF =

KLMM &LMM N KMMO &MMO &MMO

(7)

326

Here f represents the fraction of total snow nitrate present in the QLL or LLR and j is the rate

327

constant for nitrate photolysis in each reservoir. Details on this calculation are in Supplemental

328

Section S6. Figure 6 shows the enhancement in j(NO3–) in snow relative to its value in LLRs as a

329

function of the fraction of nitrate present at the interface. Each of the lines represents different

330

combinations of possible interface values of Φ and ε, ranging from the lower values determined

331

by our research group (Chu and Anastasio19 and this work) to the higher values determined by

332

Zhu et al.28 The blue and green horizontal bands represent upper bounds of the fluxes measured

333

from the field relative to our previous box model result. While it is unclear what fraction of snow

334

nitrate is at the air-ice interface,26 Figure 6 suggests that the reactivity and abundance of nitrate at


Page 17 of 29


335

the interface are inversely related. For example, if interface nitrate is highly reactive (i.e., molar

336

absorptivity and quantum yield from Zhu et al.28), then only a small amount can be present at the

337

interface to account for the differences between model and field results;24 in this case, the

338

enhancement reaches a factor of two if 1.8 × 10–4 of nitrate is at the air-ice interface. As

339

described in Supplemental Section S6, this scenario uses only the channel 1 quantum yield

340

measured by Zhu et al.; if both channels are considered with equal enhancements, the quantum

341

yield exceeds 100%, a nonsensical result. In contrast, using the aqueous nitrate absorptivity from

342

Chu and Anastasio,19 and assuming that both channels are enhanced by a factor of 3.4 above our

343

LLR results gives a total quantum yield of 3.2% at -20 °C, and requires that 63% of nitrate be at

344

the interface to double the overall nitrate reactivity. Our results suggest that nitrate at the air-ice

345

interface is either not enormously more reactive than LLR nitrate or that only a small portion of

346

nitrate in the field is present at the interface. Laboratory work has indicated that ice samples

347

prepared to mimic natural ices show exclusion of nitrate to the air-ice interface, though less than

348

expected via thermodynamic modeling.58 Other work has shown that the surface and bulk of ice

349

samples exposed to gaseous solutes may experience a change in pH consistent with the final

350

measured bulk pH.51 Better constraining this range of possibilities requires field measurements

351

of important (but largely unconstrained) modeling parameters, including snow pH, mass-transfer

352

coefficients for the release of gases from snow, and the fraction of nitrate present at the air-ice

353

interface.

354

SUPPORTING INFORMATION

355

Details on chemicals used, nitric acid surface coverage calculations, further method details, and

356

experimental data are provided in the Supporting Information, Figures S1-S4 and Table S1.



357

ACKNOWLEDGEMENTS

358

This research was made possible by funding from the NSF (ANS 1204169) and a Jastro Shields

359

Research Award and Donald G. Crosby Research Fellowship from UC Davis. We thank Zachary

360

Redman for helpful discussions of chromatography.

361

REFERENCES

362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395

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12. Berhanu, T.; Savarino, J.; Erbland, J.; Vicars, W.; Preunkert, S.; Martins, J.; Johnson, M. S., Isotopic effects of nitrate photochemistry in snow: a field study at Dome C, Antarctica. Atmospheric chemistry and physics 2015, 15, (19), 11243-11256. 13. Scharko, N. K.; Berke, A. E.; Raff, J. D., Release of nitrous acid and nitrogen dioxide from nitrate photolysis in acidic aqueous solutions. Environmental Science & Technology 2014, 48, (20), 11991-12001. 14. Chu, L.; Anastasio, C., Temperature and Wavelength Dependence of Nitrite Photolysis in Frozen and Aqueous Solutions. Environmental Science & Technology 2007, 41, (10), 3626-3632. 15. Fischer, M.; Warneck, P., Photodecomposition of nitrite and undissociated nitrous acid in aqueous solution. The Journal of Physical Chemistry 1996, 100, (48), 18749-18756. 16. Zellner, R.; Exner, M.; Herrmann, H., Absolute OH quantum yields in the laser photolysis of nitrate, nitrite and dissolved H2O2 at 308 and 351 nm in the temperature range 278– 353 K. Journal of Atmospheric Chemistry 1990, 10, (4), 411-425. 17. Zepp, R. G.; Hoigne, J.; Bader, H., Nitrate-induced photooxidation of trace organic chemicals in water. Environmental Science & Technology 1987, 21, (5), 443-450. 18. Warneck, P.; Wurzinger, C., Product quantum yields for the 305-nm photodecomposition of nitrate in aqueous solution. The Journal of Physical Chemistry 1988, 92, (22), 6278-6283. 19. Chu, L.; Anastasio, C., Quantum Yields of Hydroxyl Radical and Nitrogen Dioxide from the Photolysis of Nitrate on Ice. The Journal of Physical Chemistry A 2003, 107, (45), 95949602. 20. Dubowski, Y.; Colussi, A. J.; Boxe, C.; Hoffmann, M. R., Monotonic Increase of Nitrite Yields in the Photolysis of Nitrate in Ice and Water between 238 and 294 K. The Journal of Physical Chemistry A 2002, 106, (30), 6967-6971. 21. Roca, M.; Zahardis, J.; Bone, J.; El-Maazawi, M.; Grassian, V. H., 310 nm Irradiation of Atmospherically Relevant Concentrated Aqueous Nitrate Solutions: Nitrite Production and Quantum Yields. The Journal of Physical Chemistry A 2008, 112, (51), 13275-13281. 22. Goldstein, S.; Rabani, J., Mechanism of Nitrite Formation by Nitrate Photolysis in Aqueous Solutions: The Role of Peroxynitrite, Nitrogen Dioxide, and Hydroxyl Radical. Journal of the American Chemical Society 2007, 129, (34), 10597-10601. 23. Benedict, K. B.; McFall, A. S.; Anastasio, C., Quantum Yield of Nitrite from the Photolysis of Aqueous Nitrate above 300 nm. Environmental Science & Technology 2017, 51, (8), 4387-4395. 24. Benedict, K. B.; Anastasio, C., Quantum Yields of Nitrite (NO2-) from the Photolysis of Nitrate (NO3-) in Ice at 313 nm. The Journal of Physical Chemistry A 2017, 121, (44), 84748483. 25. Shi, G.; Buffen, A.; Hastings, M.; Li, C.; Ma, H.; Li, Y.; Sun, B.; An, C.; Jiang, S., Investigation of post-depositional processing of nitrate in East Antarctic snow: isotopic constraints on photolytic loss, re-oxidation, and source inputs. Atmospheric Chemistry and Physics 2015, 15, (16), 9435-9453. 26. Bartels-Rausch, T.; Jacobi, H. W.; Kahan, T. F.; Thomas, J. L.; Thomson, E. S.; Abbatt, J. P. D.; Ammann, M.; Blackford, J. R.; Bluhm, H.; Boxe, C.; Domine, F.; Frey, M. M.; Gladich, I.; Guzmán, M. I.; Heger, D.; Huthwelker, T.; Klán, P.; Kuhs, W. F.; Kuo, M. H.; Maus, S.; Moussa, S. G.; McNeill, V. F.; Newberg, J. T.; Pettersson, J. B. C.; Roeselová, M.; Sodeau, J. R., A review of air–ice chemical and physical interactions (AICI): liquids, quasi-liquids, and solids in snow. Atmos. Chem. Phys. 2014, 14, (3), 1587-1633.



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27. Hullar, T.; Anastasio, C., Direct visualization of solute locations in laboratory ice samples. The Cryosphere 2016, 10, (5), 2057-2068. 28. Zhu, C.; Xiang, B.; Chu, L. T.; Zhu, L., 308 nm Photolysis of Nitric Acid in the Gas Phase, on Aluminum Surfaces, and on Ice Films. The Journal of Physical Chemistry A 2010, 114, (7), 2561-2568. 29. Meusinger, C.; Berhanu, T. A.; Erbland, J.; Savarino, J.; Johnson, M. S., Laboratory study of nitrate photolysis in Antarctic snow. I. Observed quantum yield, domain of photolysis, and secondary chemistry. The Journal of Chemical Physics 2014, 140, (24), 244305. 30. Sangwan, M.; Stockwell, W. R.; Stewart, D.; Zhu, L., Absorption of Near UV Light by HNO3/NO3–on Sapphire Surfaces. The Journal of Physical Chemistry A 2016, 120, (18), 28772884. 31. Hayes, P. L.; Malin, J. N.; Konek, C. T.; Geiger, F. M., Interaction of nitrate, barium, strontium and cadmium ions with fused quartz/water interfaces studied by second harmonic generation. The Journal of Physical Chemistry A 2008, 112, (4), 660-668. 32. Moorcroft, M. J.; Davis, J.; Compton, R. G., Detection and determination of nitrate and nitrite: a review. Talanta 2001, 54, (5), 785-803. 33. Huang, G.; Zhou, X.; Deng, G.; Qiao, H.; Civerolo, K., Measurements of atmospheric nitrous acid and nitric acid. Atmospheric Environment 2002, 36, (13), 2225-2235. 34. Villena, G.; Bejan, I.; Kurtenbach, R.; Wiesen, P.; Kleffmann, J., Interferences of commercial NO2 instruments in the urban atmosphere and in a smog chamber. Atmospheric Measurement Techniques 2012, 5, (1), 149. 35. Doane, T. A.; Horwáth, W. R., Spectrophotometric determination of nitrate with a single reagent. Analytical Letters 2003, 36, (12), 2713-2722. 36. McFall, A. S.; Anastasio, C., Photon flux dependence on solute environment in water ices. Environmental Chemistry 2016, 13, (4), 682. 37. Galbavy, E. S.; Ram, K.; Anastasio, C., 2-Nitrobenzaldehyde as a chemical actinometer for solution and ice photochemistry. Journal of Photochemistry and Photobiology A: Chemistry 2010, 209, (2–3), 186-192. 38. Matykiewiczová, N.; Kurková, R.; Klánová, J.; Klán, P., Photochemically induced nitration and hydroxylation of organic aromatic compounds in the presence of nitrate or nitrite in ice. Journal of Photochemistry and Photobiology A: Chemistry 2007, 187, (1), 24-32. 39. Moussa, S. G.; Kuo, M. H.; McNeill, V. F., Nitric acid-induced surface disordering on ice. Physical Chemistry Chemical Physics 2013, 15, (26), 10989-10995. 40. Cho, H.; Shepson, P. B.; Barrie, L. A.; Cowin, J. P.; Zaveri, R., NMR investigation of the quasi-brine layer in ice/brine mixtures. The Journal of Physical Chemistry B 2002, 106, (43), 11226-11232. 41. Bower, J. P.; Anastasio, C., Using singlet molecular oxygen to probe the solute and temperature dependence of liquid-like regions in/on ice. The Journal of Physical Chemistry A 2013, 117, (30), 6612-6621. 42. Hickel, B.; Sehested, K., Reaction of hydroxyl radicals with ammonia in liquid water at elevated temperatures. International Journal of Radiation Applications and Instrumentation. Part C. Radiation Physics and Chemistry 1992, 39, (4), 355-357. 43. Men'kin, V.; Makarov, I.; Pikaev, A., Pulse radiolysis study of reaction rates of OH and O radicals with ammonia in aqueous solutions. High Energy Chemistry (English Translation) 1989, 22, (5), 333-336.


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44. Zhou, X.; Gao, H.; He, Y.; Huang, G.; Bertman, S. B.; Civerolo, K.; Schwab, J., Nitric acid photolysis on surfaces in low-NOx environments: Significant atmospheric implications. Geophysical Research Letters 2003, 30, (23), 2217. 45. Zhou, X.; Zhang, N.; TerAvest, M.; Tang, D.; Hou, J.; Bertman, S.; Alaghmand, M.; Shepson, P. B.; Carroll, M. A.; Griffith, S.; Dusanter, S.; Stevens, P. S., Nitric acid photolysis on forest canopy surface as a source for tropospheric nitrous acid. Nature Geoscience 2011, 4, (7), 440-443. 46. Zhou, X.; Beine, H. J.; Honrath, R. E.; Fuentes, J. D.; Simpson, W.; Shepson, P. B.; Bottenheim, J. W., Snowpack photochemical production of HONO: A major source of OH in the Arctic boundary layer in springtime. Geophysical Research Letters 2001, 28, (21), 4087-4090. 47. Baergen, A. M.; Donaldson, D. J., Formation of reactive nitrogen oxides from urban grime photochemistry. Atmospheric Chemistry and Physics 2016, 16, (10), 6355-6363. 48. Ye, C.; Gao, H.; Zhang, N.; Zhou, X., Photolysis of Nitric Acid and Nitrate on Natural and Artificial Surfaces. Environmental Science & Technology 2016, 50, (7), 3530-3536. 49. Guzmán, M. I.; Hildebrandt, L.; Colussi, A. J.; Hoffmann, M. R., Cooperative Hydration of Pyruvic Acid in Ice. Journal of the American Chemical Society 2006, 128, (32), 10621-10624. 50. Guzmán, M.; Hoffmann, M.; Colussi, A., Photolysis of pyruvic acid in ice: Possible relevance to CO and CO2 ice core record anomalies. Journal of Geophysical Research: Atmospheres (1984–2012) 2007, 112, (D10). 51. Kahan, T. F.; Wren, S. N.; Donaldson, D. J., A Pinch of Salt Is All It Takes: Chemistry at the Frozen Water Surface. Accounts of Chemical Research 2014, 47, (5), 1587-1594. 52. Morenz, K. J.; Donaldson, D. J., Chemical Morphology of Frozen Mixed Nitrate–Salt Solutions. The Journal of Physical Chemistry A 2017, 121, (10), 2166-2171. 53. Gallet, J. C.; Domine, F.; Arnaud, L.; Picard, G.; Savarino, J., Vertical profile of the specific surface area and density of the snow at Dome C and on a transect to Dumont D'Urville, Antarctica – albedo calculations and comparison to remote sensing products. The Cryosphere 2011, 5, (3), 631-649. 54. Křepelová, A.; Newberg, J.; Huthwelker, T.; Bluhm, H.; Ammann, M., The nature of nitrate at the ice surface studied by XPS and NEXAFS. Physical Chemistry Chemical Physics 2010, 12, (31), 8870-8880. 55. Marchand, P.; Marcotte, G.; Ayotte, P., Spectroscopic study of HNO3 dissociation on ice. The Journal of Physical Chemistry A 2012, 116, (49), 12112-12122. 56. Marcotte, G.; Ayotte, P.; Bendounan, A.; Sirotti, F.; Laffon, C.; Parent, P., Dissociative adsorption of nitric acid at the surface of amorphous solid water revealed by X-ray absorption spectroscopy. The Journal of Physical Chemistry Letters 2013, 4, (16), 2643-2648. 57. Gross, G. W., Nitrates in ice: uptake; dielectric response by the layered capacitor method. Canadian Journal of Physics 2003, 81, (1-2), 439-450. 58. Wren, S. N.; Donaldson, D. J., Exclusion of Nitrate to the Air–Ice Interface During Freezing. The Journal of Physical Chemistry Letters 2011, 2, (16), 1967-1971.

525

DISCLOSURES

526

The authors declare no competing financial interest.

527



Page 22 of 29

528 529

FIGURES

Quasi-Liquid Layer (QLL) at Air-Ice Interface

Solid Solution

Liquid-Like Regions (LLRs)

530 531

Figure 1: Potential solute reservoirs in, and on, our ice samples. Since samples are prepared and

532

illuminated in Teflon molds (not shown), the only air-ice interface is at the top of the sample.

533 534 535


Page 23 of 29


536 537

Figure 2: Correction for the variability in concentrations of surface-deposited nitrate in a given

538

experiment. The left panel shows the measured concentrations of nitrite and nitrate in each ice

539

pellet after a given illumination time. The right panel shows the ratio of nitrite to nitrate in each

540

pellet. Correlation coefficients for the linear regressions of [NO2–] (left panel) and [NO2–]/[NO3–

541

] (right panel) versus time are 0.32 and 0.97, respectively.



542 543

Figure 3: Measured values of Φ(NO2–) for nitrate in each solute location (313 nm, –10 °C). The

544

asterisk denotes that the QLL quantum yield is greater than the other measured values (p
0.05), consistent with the QLL

568

and LLR activation energies not being statistically different (Panel A).


Page 26 of 29

Page 27 of 29


569 570

Figure 5: Photolytic loss of HNO3 at the interface during 313-nm illumination at –10°C. Solid

571

points represent measurements with lines representing first-order decay curves fit to either the

572

first two data points (0 and 6 hr) or the last 5 data points (6 – 24 hr) for dark controls (red) and

573

samples (blue). Calculated rate constants (±SE) for nitrate loss in these two time periods

574

(respectively) are (1.6 ± 0.4) × 10–5 s–1 and zero for the dark control, and (4.1 ± 0.3) × 10–5 and

575

(5.3 ± 4.9) × 10–6 s–1 for the illuminated samples. Purple lines represent the expected loss of

576

HNO3 in our system based on the recommended photolabile domain quantum yield of 24% from

577

Meusinger et al.29 using our measured j(2NB) value, both with (solid purple line) and without

578

(dashed purple line) the rate constant for HNO3 loss in our dark samples; the corresponding rate

579

constants for loss are 9.0 × 10–5 and 7.0 × 10–5 s–1 s–1, respectively. Standard errors for initial



580

rate constants were determined via calculating the SE of the blank concentrations between 6 and

581

24 hours, and using this uncertainty to construct three initial slopes for which an average and SE

582

was determined.

583


Page 28 of 29

Page 29 of 29


584 585

Figure 6: Apparent enhancement in the overall rate constant at –20 °C for nitrate photolysis in

586

snow (relative to the reactivity in LLRs) as a function of the fraction of snow nitrate that is

587

present at the air-ice interface. This enhancement due to interface chemistry is calculated using

588

Equation 7 with different combinations of the molar absorptivity (ε) and quantum yield (Φ) for

589

nitrate at the interface, based on data from Zhu et al.,28 Chu et al.,19 and this work (“McFall”).

590

The three lines represent different scenarios for the photochemistry of interface nitrate, based on

591

different combinations of Φ and ε (see Section S6). Modeled snow-to-air fluxes of NOx based

592

on LLR chemistry generally match field observations, although some observations are twice as

593

high (blue shading) and a few are up to four times as high (green shading).24


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