Quantum Yields of Nitrite (NO2â) from the Photolysis of Nitrate (NO3

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Article 2–

Quantum Yields of Nitrite (NO ) from the Photolysis of Nitrate (NO ) in Ice at 313 nm 3–

Katherine Beem Benedict, and Cort Anastasio J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b08839 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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The Journal of Physical Chemistry

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Quantum Yields of Nitrite (NO2–) from the

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Photolysis of Nitrate (NO3–) in Ice at 313 nm

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Katherine B. Benedict1 ([email protected]) and Cort Anastasio*

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([email protected])

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

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Avenue, Davis, CA 95616

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now at Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523

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*Corresponding Author

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Resubmitted to Journal of Physical Chemistry A on October 13, 2017.

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ACS Paragon Plus Environment


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Abstract

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Photochemical reactions of nitrate in snow release reactive nitrogen species via two channels,

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which produce: (1) nitrogen dioxide (NO2) and hydroxyl radical (•OH), and (2) nitrite (NO2–)

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and oxygen atom (O(3P)).

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characterized, except for channel 2 in ice. In this study, we quantify Φ(NO2–) in water ices and

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examine the impacts of pH and organic scavengers of •OH. Compared to solution results, we find

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that nitrite quantum yields in ice are more sensitive to pH and that •OH scavengers are less

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effective, although 2-propanol appears to work well. The temperature dependence (−30 to 25

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°C) of Φ(NO2–) in samples containing 2-propanol is well described by a single regression line,

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ln(Φ(NO2–)) = −(1330±100)(1/T(K)) + (0.09 ± 0.39). At −10 °C the resulting quantum yield is

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4.6 times larger than the previously reported (and recommended) value without an •OH

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scavenger. Although some reports suggest nitrite is a minor product from nitrate photolysis,

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based on our current and past results, rates of photoproduction of NO2– and NO2 are similar at

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room temperature while NO2– production dominates at lower temperatures in solution and ice.

Quantum yields (Φ) for these channels are generally well

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1.0 Introduction

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The discovery that nitrate (NO3–) photolysis releases nitrogen oxides (NOx) and nitrous

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acid (HONO) from sunlit snow1 has spurred a flurry of global field measurements2–14 as well as

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complementary laboratory15–18 and modeling studies.19–23 Nitrate photolysis in snow is important

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because it alters snow and ice core records of past atmospheres24–26 and impacts the oxidative

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capacity of the remote atmosphere; e.g., NOx can produce near-surface ozone while HONO is an

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important source of hydroxyl radical. The role of photochemical and physical losses of nitrate in

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the snowpack can be separated based on isotope measurements,27–29 which reveal the greatest

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photolytic loss at sites with low snow accumulation rates where NO3– remains in the photic zone

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for longer.13,30 In solution and ice, nitrate irradiation in the troposphere leads to two reaction channels:15–

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17,31–33

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NO3– + hν (+ H+) → NO2 + •OH

(1)

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NO3– + hν → NO2– + O(3P)

(2)

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The NO2– produced in channel (2) can be protonated to form HONO, undergo photolysis to form

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NO, or oxidized (e.g., by •OH).32–35 Previously reported quantum yields in solution for the first

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channel of nitrate photolysis near 300 nm are quite consistent, with values around 1% at room

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temperature.15,17,36–38 In addition, a single temperature dependence describes the quantum yield

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of this channel both for solution and in liquid-like regions (LLRs) of ice, indicating that the

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photochemistry of nitrate is very similar in these two environments.15,16

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There is less consistency in quantum yields for the second channel of nitrate photolysis,

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which produces NO2–. This uncertainty is important because the rate of NO2– formation directly 3



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impacts our understanding of (1) the fluxes of HONO, NO, and possibly HOONO from the

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snowpack to the boundary layer, (2) the budgets of •OH and O3 in the boundary layer and firn air

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(since HONO is a source of •OH and NO is intimately tied to O3 production), and (3)

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concentrations of nitrite in snow.22,39–43 Nitrite formation from nitrate photolysis is probably the

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primary source of the nitrous acid that is released from many snowpacks and nitrite photolysis

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appears to be a major source of the NOx released to the overlying boundary layer.22,39,44

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Recently, we showed that the quantum yield for channel 2 at 313 nm and 25 °C is similar to the

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value for channel 1.32 In agreement with prior solution studies,37,45–47 Benedict et al.32 showed

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that the apparent quantum yield for channel 2 is affected by secondary chemistry, including

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destruction of nitrite by •OH and evaporation of HONO (formed by protonation of NO2–) at

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lower pH. We found that secondary chemistry in solution can be minimized by using a low

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concentration of nitrate (50 µM), relatively short illumination times (to produce only small

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amounts of nitrite), and a pH above 6. Under these conditions the nitrite quantum yield was the

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same with or without an organic scavenger of •OH (e.g., formate), likely because bicarbonate

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(formed from the dissolution of gaseous CO2) was suppressing •OH.

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It is unclear how successfully secondary reactions were minimized in previous ice studies

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that examined nitrite formation from nitrate photolysis. Dubowski et al.17 used ice made from 10

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mM NaNO3 solutions and observed an increase in the nitrite quantum yield by a factor of 5 to 6

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when 10 mM formate was added (compared to the case with no formate). This finding suggests

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either that there were significant losses of nitrite to •OH in the absence of formate or that

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secondary reactions with formate were producing nitrite (e.g., via production of superoxide from

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the reaction of •OH with formate, followed by superoxide reducing NO2 (from channel 1) to

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NO2–).32 Since pH values were not reported, it is unknown whether HONO volatilization was

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significant.

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Additionally, the previously reported temperature dependence of channel 2 in ice is odd:

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while the quantum yield for channel 1 (NO2 + •OH) decreases by a factor of 3 from 298 K to 263

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K,15 the quantum yield for NO2– determined with formate is essentially unchanged between these

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temperatures.17 In addition, Φ(NO2–) values for ice and solution follow two different Arrhenius

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equations,17 in contrast to the behavior seen for •OH formation from the photolysis of NO3–,

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NO2–, H2ONO+, and HOOH.15,33,39,48 These previous results suggest a thorough analysis of

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experimental conditions is necessary to understand the potential impacts of secondary chemistry

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involving •OH and pH in ice experiments of nitrate photolysis.

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Here we measure the quantum yields for nitrite from nitrate photolysis in liquid-like

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regions of ice with a focus on minimizing secondary chemistry. To do this we measure Φ(NO2–)

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as a function of pH (to understand evaporation of HONO) and with and without organic

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scavengers (to explore the importance of •OH secondary chemistry). Finally, we determine the

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temperature dependence of the nitrite quantum yield and apply our results in a simple model of

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snow photochemistry to estimate the range of NO2– concentrations in sunlit snow and the

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accompanying emissions of HONO and NOx to the boundary layer.

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2.0 Methods

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Materials

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Sodium nitrite (ACS Certified) and sodium nitrate (ACS Certified) were from Fisher; 2-

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nitrobenzaldehyde (98%), potassium nitrate (ACS Reagent >99%), magnesium nitrate (ACS

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Reagent >99%), ammonium nitrate (>99%), and sulfanilamide (>99%) were from Sigma5



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Aldrich; N-1-napthylethylene diamine (ACS Reagent 98%) was from Sigma; and calcium nitrate

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(99%) was from ARCOS. Purified water was obtained by treating house R/O water with a

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cartridge to remove organics and then a Milli-Q Plus system (>18.2 MΩ-cm). Dilutions for

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standards and illumination solutions were made daily.

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Sample Illumination

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The experimental procedures are similar to those of Benedict et al.,32 so only a brief

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description is included here, with most details specific to ice experiments.

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solutions, generally containing 50 µM NaNO3, were prepared on the day of each experiment. 1.0

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mL of illumination solution was pipetted into a 1.8-mL HPLC vial (low impurity Type I Class A

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borosilicate glass, 12 × 32 mm; Shimadzu P/N 228-45450-91), capped, and frozen using a

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custom-made ice preparation chamber (Paige Instruments) set at –20 °C. Based on micro-CT

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imaging of similarly prepared samples,49 NaNO3 in our ice samples is probably mostly present in

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liquid-like regions that are present throughout the sample but most prevalent near the top. Once

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samples were frozen, the temperature of the preparation chamber was adjusted to the

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illumination chamber temperature and the samples were equilibrated for at least 1 hour before

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illumination. Samples were transferred to the illumination chamber (which was continually

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purged with dry air to avoid water condensation on the sample vial) and allowed to equilibrate

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for at least 5 minutes before illumination. Samples were illuminated for known time intervals

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using 313 nm light from a 1000-W Hg/Xe lamp with a downstream monochromator (Spectral

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Energy). The spectral distribution of the light is shown in Benedict et al.32 Vials (each

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representing one time point) were capped during illumination and were only opened after

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illumination to add the Griess reagents.

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Illumination

Measurement of Nitrite 6


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Nitrite was determined using the Griess Method (e.g., refs. 50–53), a colorimetric method

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that forms a strongly absorbing azo-dye complex with nitrite. After illumination the sample vial

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was shielded from light and within 1 minute of stopping the illumination 25 µL of the first Griess

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Reagent (1% sulfanilamide in 10% HCl solution (w/v)) was added to the vial. After 10 minutes,

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25 µL of the second Griess reagent (0.1% N-1-napthylethylene diamine solution) was added to

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the vial. After an additional 10 minutes the nitrite-complex absorption was measured at 540 nm

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using a TIDAS II spectrophotometer (World Precision Instruments, Sarasota, FL) with a liquid

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waveguide capillary cell (LWCC; length of 100 cm, effective path length of 94 cm, 250 µL

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volume), and tungsten lamp. Absorption was measured from 350 to 700 nm to correct for any

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baseline shifts. The peak height between 530 and 550 nm was determined as the difference

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between the maximum absorbance in this wavelength range relative to a baseline determined

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from two local absorption minima: one between 400 and 500 nm and one between 550 and 700

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nm. We made fresh standards of sodium nitrite each week and calibrated the spectrophotometer

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using concentrations from 0 to 100 nM. The nitrite detection limit, based on replicate blank

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analyses, was 3 nM. The LWCC was cleaned between each sample measurement with a full cell

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volume of three separate cleaning solutions (1 M NaOH, 1 M HCl, and 50% methanol/50%

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water), each separated by an air bubble, followed by copious rinsing with Milli-Q water.

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Calculation of Quantum Yields

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The sample-surface-area normalized photon flux in the illumination chamber was measured on

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each experiment day using ice of the same volume and composition as the nitrate samples (e.g.,

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nitrate, •OH scavenger where applicable, and pH buffer) with the addition of 10 µM 2-

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nitrobenzaldehyde (2NB),54 a chemical actinometer.

The degradation of 2NB during 7



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illumination was monitored using a HPLC with a C18 Column, 60%/40% acetonitrile/water

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eluent, and UV detection at 258 nm. Under our low light-absorbing sample conditions, the

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measured rate constant for 2NB loss (j2NB,λ) is equal to j2NB,λ =2.303 × 10 Iλ × , Φ ,

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(4)

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

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(640 M-1 cm-1) is the product of the base-10 molar absorptivity and quantum efficiency for 2NB

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at 313 nm in solution,55 and 2.303 converts from base-10 to base-e. While we are using the

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value for the product , , determined in solution, it appears to be essentially the

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same in ice.54

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For our conditions we can write a similar equation to describe the rate constant for nitrite formation from nitrate photolysis: NO → NO = 2.303 × 10 λ × ! , ΦNO (5)

155 –

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where Φ "NO $ is the quantum yield of nitrite from nitrate photolysis and !, is the base-10

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molar absorptivity of nitrate at the photolysis wavelength (5.29 and 0.37 M–1cm–1 at 313 and 334

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nm, respectively, at 278 K).15

%

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During each illumination experiment we determine the rate of nitrite formation, d[NO2–]/dt,

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from the linear regression slope of nitrite concentration versus illumination time. This rate is

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equal to:

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&[! (] &*

= NO → NO [NO ]

(6)

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We illuminate samples over short times (typically 0 to 16 minutes, corresponding to typical

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nitrite concentrations of 0 to 40 nM) so that the change in nitrite concentration is linear.

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Combining equations 4 through 6 allows us to solve for the quantum yield of NO2–: 8


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ΦNO

=

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&[! (] &*

×

+(,-,. /(01,.

2(,-,. +,3! ,. [! ]

(7)

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Errors on individual quantum yields (e.g., in figures) represent ±1 standard error determined

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by propagating measured or reported errors in all terms in equation 7. Errors on stated mean

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values are ±1 standard deviation, which were determined from multiple individual experiments.

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For each experiment, several vials with frozen illumination solution were prepared and treated is

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if they were samples but were stored in the dark in the freezing chamber and analyzed

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periodically. There was no nitrite formation in these dark controls.

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Nitrite Formation in Snow Samples

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Samples of snow were collected at Summit, Greenland in 2007 then stored in the dark at

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–20 °C until experiments were performed in 2011 and 2012. Prior to each experiment, the snow

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was melted, spiked with 50 µM NaNO3, aliquoted into illumination vials and frozen. Nitrite

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photoformation was measured in these snow samples using the same procedures as for the

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laboratory samples.

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chromatography (IC) with conductivity detection (Metrohm 881 Compact IC Pro) and for total

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organic carbon (TOC) using a Shimadzu TOC-VCPH with high sensitivity catalyst. More details

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on the TOC and IC analysis can be found in Yu et al.56

The original and NO3–-spiked snow samples were analyzed by ion

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3. Results and Discussion

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3.1 The influence of buffers and organic compounds

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We first examined how Φ(NO2–) in ice is affected by acidity, by performing experiments

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with solutions of different pH that were frozen and illuminated at –5 °C (Figure 1). With no

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organic scavenger added, the quantum yield is approximately constant above pH 6.5, at 9



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(0.79±0.08)%, and decreases non-linearly in more acidic samples. This is different behavior

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than what we observed in solution,32 where the quantum yield was constant above pH 5 and

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decreased more slowly at lower pH (Figure S1a). The higher apparent sensitivity in ice is likely

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due to proton concentration enhancements in the liquid-like regions (LLRs) of ice. At a given

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temperature, the freeze-concentration factor (F) for a species in LLRs increases with decreasing

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total solute concentration in the solution prior to freezing.57 Based on dissociation constants of

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H2SO4 and the freeze-concentration factors of our samples, the proton concentration in LLRs at –

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5 °C is higher by a factor of 660 for an initial pH 2.5 solution and by a factor of 24,000 for an

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initial pH 5.4 solution. The result is a relatively narrow range in LLR pH for the acidic solutions

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in our Figure 1 experiments, ranging from a pH 0.1 (for a pH 2.5 solution) to pH 0.9 (for a pH

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5.4 solution). Thus the apparent higher sensitivity to acidity in ice compared to solution is driven

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by the fact that the pH values in ice LLRs are much lower than the corresponding solution pH

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values plotted in Figure 1.

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10


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Figure 1. Quantum yields of nitrite as a function of solution pH (prior to freezing) in −5 °C

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ice for samples containing 50 µM NaNO3, without or with an organic scavenger (50 µM 2-

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propanol or 50 µM formate). Prior to freezing, samples were buffered using sulfuric acid

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(pH ≤ 5) or phosphate (pH > 5.6). The average quantum yield in solution at 25 °C (pH 5-9

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with and without buffers)32 is indicated by the horizontal grey dotted line; the average at

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−5 °C for samples with pH > 6.8 and no organic scavenger is indicated by the horizontal

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black line.

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An additional factor that may contribute to the differences in the –5 and 25 °C

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experiments is the behavior of carbonate and bicarbonate. In solutions containing 50 µM nitrate

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(and 20 nM NO2–) but no organic scavenger, carbonate and bicarbonate are the most important

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sinks of •OH above pH ~6.4.32

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ice then the nitrite quantum yield would be the same with and without an organic •OH scavenger

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at high pH. As shown in Figure 1, the quantum yield of nitrite in the presence of •OH scavenger

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(formate or 2-propanol) is higher than the case without scavenger, but still decreases below

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approximately pH 6. Above pH 6 we observe a consistent quantum yield, with no statistically

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significant difference between the two scavengers (p > 0.05), and an average Φ(NO2–) of (0.96 ±

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0.05)%. Compared to the case without scavenger, this is a 25% increase in Φ(NO2–) in the

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presence of formate or 2-propanol, indicating these species are suppressing •OH in LLRs and

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that carbonate/bicarbonate are insufficient scavengers; it is also possible that these organic

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scavengers form some secondary nitrite, although we did not observe this behavior in solution.32

If carbonate and bicarbonate also effectively scavenge •OH in

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We next examined results at a lower temperature, –10 °C, for a range of conditions and

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compared them to the –5 °C results (Figure 2). While quantum yields at –5 °C are generally

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higher than those at –10 °C, the two results are not statistically different under any condition, in

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large part because the results at –10 °C are quite variable. For the experiment with no buffer or

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scavenger, quantum yields are very similar at –5 and –10 °C, but it is difficult to interpret these

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results since both the sample acidity and lack of an •OH scavenger are affecting the results

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(Figure 1). As shown in the second set of bars, buffering the initial solutions to pH 7 with

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phosphate (max [HPO42-] = 1mM) enhances the quantum yield at –5 °C but not at –10 °C,

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suggesting that the LLRs at the colder temperature are not buffered; based on the eutectic

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temperature for potassium dihydrogen phosphate (–2.7 °C),58 the buffer is more likely to have

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precipitated at the colder temperature and thus no longer buffered effectively (the Na+ in the

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solutions likely moved the eutectic temperature slightly colder).59 In contrast, adding either 2-

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propanol or formate to scavenge •OH enhances Φ(NO2–) at both temperatures compared to the

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no-buffer, no-scavenger case (Figure 2).

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compared to with 2-propanol, but the results are not statistically different. Finally, using TRIS

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(tris(hydroxymethyl)aminomethane) as an •OH scavenger gives even higher quantum yields than

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2-propanol or formate (Figure 2). While it is difficult to determine, we suspect that the higher

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TRIS result is artificially high because it is converting NO2 (formed in channel 1) to NO2– via

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superoxide, which is formed from the reaction of TRIS with •OH.60 While •OH reaction with

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formate and 2-propanol can also form superoxide,61,62 based on our solution data these

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scavengers do not appear to lead to secondary nitrite;32 thus it is unclear why TRIS appears to

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cause secondary formation of NO2–. We do not see an enhancement in the nitrite quantum yield

Quantum yields are slightly higher with formate

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with TRIS for NaNO3 solutions (Figure S3), suggesting that concentration of TRIS into LLRs is

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responsible for the higher values of Φ(NO2–) in ice.

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249 250

Figure 2. Nitrite quantum yield at −5 (grey) and −10 °C (white) for ice samples made from

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solutions containing 50 µM NaNO3. The top label for each set of bars indicates whether

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solutions were buffered to near pH 7, while the bottom label indicates whether solutions

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contained 50 µM of a single organic to scavenge •OH. The solutions in the second set of

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bars were buffered to pH 7 with phosphate. Since TRIS acts as both an •OH scavenger and

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a buffer, the TRIS solution contained no phosphate but was pH 6.9 on average. Bars

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represent average values for at least 2 experiments. Statistically significant differences

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compared to the 2-propanol case at the same temperature are indicated by * (p ≤ 0.1) and

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** (p ≤ 0.01).

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3.2 Temperature dependence

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In ordered to characterize the temperature dependence of nitrite formation we first carried

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out experiments between +25 and –30 °C for ices containing 50 µM NaNO3 and either nothing

263

else (“No Buffer or Scavenger”) or a variety of pH buffers. As shown in Figure 3a, the No

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Buffer or Scavenger samples and buffered samples behave similarly: a weak temperature

265

dependence in solution and then a steep drop in Φ(NO2–) in ice. The quantum yield for the No

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Buffer or Scavenger samples drops off at the first subzero temperature (–5 °C), while in the

267

buffered (no organic) experiments Φ(NO2–) drops off at somewhat colder temperatures, but both

268

have significant variability (Figure 3a). In addition to the species in Figure 3a, we tested several

269

other buffers, but these attempts yielded very similar results (Figure S2). It is unclear why the

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buffered samples have a sharp drop in quantum yield compared to the solution temperature

271

dependence. It does not seem to be due solely to the eutectic temperature (Teu) of the buffer,

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since these span a wide range, from relatively high (e.g., potassium dihydrogen phosphate, –2.7

273

°C,58 and Na2CO3, –4 °C63) to very low (e.g., CaCl2, –54 °C,64 and KOH, –66 or –82.5 °C65).

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While precipitation of buffer does not appear to be responsible for the Figure 3a ice results, the

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inability of buffers to work properly in frozen solutions has been well documented.59,66,67

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Precipitation of NaNO3 does not appear to be driving our Figure 3a results: its eutectic

277

temperature is –17.7 °C,68 but there is no significant change in quantum yield above and below

278

this threshold (Figure 3a). Finally, it is also possible that our results in Figure 3a are affected by

279

freezing potential,69 i.e., the electrical potential that can form due to preferential incorporation of

280

some ions into the ice lattice during freezing, since this can alter the LLR pH.70,71

281

We next performed experiments with an organic scavenger of •OH but no pH buffer. As

282

shown in Figure 3b, these results are more consistent at colder temperatures: they generally

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match the temperature dependence observed in solution and show less variability between 14


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284

different scavengers compared to the different buffer results. However there are some subtle

285

differences, including a drop off in the quantum yield with formate at the coldest temperatures

286

and an enhancement in Φ(NO2–) for TRIS from –5 to –20 °C. The scavenger that shows the

287

most consistency between solution and ice is 2-propanol, where the quantum yield follows the

288

same trend line in solution and ice (Figures 3a and b): ln(Φ(NO2–)) = –(1330±100)(1/T) + (0.09 ±

289

0.39), where T is in Kelvin. This experimental condition appears to be the best measure of the

290

quantum yield for channel 2 while minimizing secondary chemistry. In contrast, the other

291

conditions that minimized secondary chemistry in solution (i.e., pH buffering with or without

292

formate)32 do not closely match the propanol results in ice (Figure 3b).

293

experiments with 2-propanol and 50 µM Ca(NO3)2, which has a eutectic temperature (−33.1

294

°C68) that is lower than that of NaNO3. While there is good agreement in the Ca(NO3)2 and

295

NaNO3 data between –5 and –20°C, the quantum yield with calcium nitrate actually increases at

296

lower temperatures. We discuss possible reasons for this behavior, and results from other nitrate

297

salts, in section 3.3.

We also tried

15



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Page 16 of 41

298

.

299

Figure 3. Temperature dependence of the nitrite quantum yield. Panel (a) shows results for

300

50 µM NaNO3 samples with various pH buffers, with no buffer and for Summit snow

301

samples (section 3.4). Panel (b) shows results for samples containing organic scavengers

302

and no pH buffer. In each panel the solid line is the regression fit of the 2-propanol (2-

303

PrOH) data points while the dashed line is the fit to the No Buffer or Scavenger data in ice.

16


Page 17 of 41

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304 305

In Figure 4 we compare our nitrite quantum yields for samples containing 2-propanol to

306

previous results, including nitrite results from Dubowski et al16,17 and channel 1 (NO2 + •OH)

307

results from Chu and Anastasio.15 While our nitrite results at room temperature are in good

308

agreement with most of the past results that used an •OH scavenger, our ice results are generally

309

higher than the previous nitrite results (with scavenger) of Dubowski,17 and have a weaker

310

temperature dependence. As shown in Table 1, the activation energy (Ea) for our nitrite results is

311

approximately half of that observed by Dubowski et al.17 Of the previously reported nitrite

312

quantum yields determined without an •OH scavenger, the initial (2001) ice value16 is closer to

313

our current 2-propanol results, while the later (2002) values17 are approximately an order of

314

magnitude lower (Figure 4). It is possible that part of the difference in quantum yields is because

315

of different ice preparation techniques: while we slowly froze 50 µM NaNO3 solutions,

316

Dubowski et al.17 flash-froze 1 or 10 mM NaNO3 solutions into liquid nitrogen, ground up the

317

ice, and then pressed it into pellets. This might lead to differences in the location of nitrate in

318

ice, which might alter the efficiency of photolysis. However, our solution results for Φ(NO2–)

319

(Figure 4 and Benedict et al.,32) are also very different from the solution data of Dubowski et

320

al.,17 indicating that the ice differences are not solely due to differences in ice sample

321

preparation. We believe that a major factor is the very different nitrate concentrations, since

322

high nitrate concentrations appear to increase the importance of unwanted secondary processes

323

that alter nitrite (Benedict et al.32). Finally, we note that while our ice results with formate as

324

scavenger are lower than our recommended results with 2-propanol (Figure 3), our formate

325

results are similar to the formate results from Dubowski et al. 17 (Figure 4).

17



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Page 18 of 41

326

Figure 4 also compares our current nitrite quantum yields (channel 2) with past

327

measurements of the NO2 + •OH quantum yield (channel 1). As discussed in Benedict et al.,32

328

Φ(NO2–) is similar to Φ(NO2) at 25 °C, which indicates nitrite formation is not a minor pathway

329

(as is sometimes claimed), but rather is as important as NO2 (and •OH) formation. Furthermore,

330

the nitrite channel has a weaker temperature dependence compared to the NO2 channel, so that

331

the relative importance of nitrite production increases with decreasing temperature: the ratio

332

Φ(NO2–)/Φ(NO2) ranges from slightly above 1 at 300 K to 2.6 at 240 K (Figure S4). Finally, we

333

note that the our past ice and solution results for channel 1 gave an activation energy that is

334

approximately half of the value from the ice work of Dubowski et al.16, which is similar to the

335

difference we see for channel 2 between our current results and Dubowski’s past work17 (Table

336

1).

337

338 18


Page 19 of 41

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339

Figure 4. Our recommended quantum yields for nitrite formation (based on samples made

340

from 50 µM NaNO3 and 50 µM 2-propanol) compared with previously reported values for

341

both channels of nitrate photolysis. Past data are from Dubowski et al.16,17 (who measured

342

nitrite), Chu and Anastasio15 (who measured •OH), as well as 25 °C nitrite data from

343

Goldstein and Rabani,45 Roca et al.,46 and Warneck and Wurzinger37 for 10 mM NO3–

344

solutions. “Fo” indicates formate was used. The “O(3P) Msmt.” point is an oxygen atom

345

measurement of channel 2 from Warneck and Wurzinger.37 Solid lines are regression fits

346

to data from measurements of channel 2 with an •OH scavenger, the dashed line is for

347

channel 2 without an •OH scavenger, and the dotted lines are measurements of channel 1.

348 349

350 351 352 353 354 355 356

Table 1. Activation energy and change in entropy for both channels of nitrate photolysis.

Ea

∆S

(kJ mol-1)

(J mol-1 K-1)

-30 to 25°C

11±1

1±3

This Study

2. NO2– + O(3P)

-35 to 20°C

23±1

34±5

Dubowski et al.17 (2002)

1. NO2 + OH

-34 to 45°C

20±1

31±3

Chu and Anastasio15 (2003)

1. NO2 + OH

-25 to - 5°C

42±6

103±22

Dubowski et al.16 (2001)

Channel

T Range

2. NO2– + O(3P)

Study

ǂ

The activation energy and change in entropy for our nitrite data were calculated based on the linear regression of ln(Φ(NO2–)) versus 1/T for liquid and ice samples made from solutions containing 50 µM NaNO3 and 50 µM 2-propanol (Figure 3b): Ea = −slope·R·10-3 kJ J-1 and ∆S= y-intercept·R, where R is the gas constant (8.314 J mol-1 K-1). Values of Ea and ∆S from Dubowski et al.17 were calculated based on their Figure 6 for samples containing formate. The Dubowski et al.16 Ea and ∆S were taken from Table 2 of Chu and Anastasio.15 Values of Ea and ∆S for Chu and Anastasio15 were calculated from tables of their data. 19



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Page 20 of 41

357 358 359

3.3 Dependence on salt

360

Previously we tested five different nitrate salts at 25 °C and saw no significant difference

361

in Φ(NO2–).32 Here we extend this by measuring Φ(NO2–) at –10 and –30 °C for the same five

362

salts. Several factors may contribute to quantum yield differences among the salts, including

363

eutectic temperature, solubility, and the extent that the cations are incorporated into the ice

364

lattice. As shown in Figure 5, there is no clear pattern of behavior between the nitrite quantum

365

yield and eutectic temperature. For example, at –10 °C KNO3 is below Teu but its quantum yield

366

is similar to those for the Na, Ca, and Mg salts, which are all above their eutectic, while NH4NO3

367

(which is also above its eutectic) has the lowest quantum yield. This latter observation might be

368

related to the finding of Gross72 that of the salts KNO3, NaNO3, and NH4NO3, ammonium

369

enhances uptake of nitrate into ice by a factor of 27, on average, over sodium and potassium. In

370

contrast to the –10 °C data, there is some suggestion of an influence of Teu at –30 °C, with the

371

highest Φ(NO2–) values for the two salts that are still above their eutectics and lower (and quite

372

variable) quantum yields for the three salts below their eutectics. The influence of eutectic

373

temperature on Φ(NO2–) is undoubtedly complicated by the fact that not all of a salt precipitates

374

below its eutectic, but some remains in liquid-like regions.57,73

375

20


Page 21 of 41

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376 377

Figure 5. Measured Φ(NO2–) at 25, -10, and -30 °C for 50 µM solutions of KNO3 (green;

378

Teu= −2.9 °C), NH4NO3 (orange; Teu= −16.9 °C), NaNO3 (red; Teu= −17.7 °C), Ca(NO3)2

379

(blue; Teu= −33.1 °C), and Mg(NO3)2 (purple; Teu= −72.5 °C). Quantum yields at 25 °C

380

were taken from Benedict et al.32 for experimental solutions with pH above 7.

381

Experimental solutions at -10 and -30 °C contained 50 µM 2-propanol and no pH buffer.

382

Experiments below the eutectic temperature of a given salt are indicated by striped bars.

383

Eutectic temperatures were calculated using the FrezChem68 computer model.

384 385 386

3.4 Nitrite quantum yields in Arctic snow

387

To see how components of real snow might affect the nitrite quantum yield, we took two

388

archived samples (and one field blank) from Summit, Greenland collected in 2007, spiked them

389

with 50 µM NaNO3 so that we could measure nitrite production, then froze and illuminated them

390

as with the lab samples. The samples are very clean, with low concentrations of ions and organic

391

carbon, to the extent that the blank (Milli-Q collected in the field) has comparable levels of all

392

species except nitrate and ammonium (Table S2). 21



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Page 22 of 41

393

As shown in Figure 3a, Φ(NO2–) in melted snow at 25 °C is similar to results with

394

laboratory solutions. In ice, Φ(NO2–) for the Summit samples is lower than for the lab solutions

395

containing 2-propanol but is similar to those without an •OH scavenger (Figure 3a). An

396

exception is observed at –30 °C, where the snow Φ(NO2–) is higher than the “No Buffer or

397

Scavenger” and buffered samples but not as high as the 2-propanol results. These lower Φ(NO2–

398

) values in ice suggest NO2– is the dominant sink of •OH in the samples, which implies that snow

399

organics are relatively unreactive toward •OH (Figure S5). However, even though the oxidation

400

of NO2– by •OH suppresses the apparent nitrite quantum yield in our laboratory ice samples

401

made from Summit snow, this does not mean that this secondary chemistry is similarly important

402

in the field. Since snow at Summit has a much higher surface area than our ice pellets, most

403

photoproduced NO2– in the field is likely quickly protonated and released to the firn air as

404

HONO.

405 406

4.0 Implications

407

Models of reactive nitrogen fluxes from snowpacks sometimes do not include nitrite

408

production from nitrate photolysis,14,74 which should lead to underestimates of NO and HONO

409

fluxes relative to observations. Our finding that Φ(NO2–) is larger than previously described,

410

and has a weaker temperature dependence than NO2 production, indicates that nitrite production

411

is more important than previously believed. For example, while we had previously

412

recommended15 the Dubowski et al.17 quantum yield of 0.15% at –10 °C, our newly measured

413

value is (0.69±0.12)%, which is 4.6 times higher.

414

To examine how this revised quantum yield impacts our understanding of nitrate

415

photolysis in polar regions, we employed a kinetic model of nitrogen chemistry previously used 22


Page 23 of 41

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416

to calculate steady-state concentrations of aqueous N(III) (i.e., H2ONO+, HONO, and NO2–) and

417

the fluxes of HONO and NOx from sunlit snow.39 We used temperature (–10 °C), nitrate

418

concentration (4.0 µM melted equivalent), and sunlight conditions for midday, clear-sky,

419

summer at Summit, Greenland. Nitrite in the model is produced by channel 2 of nitrate

420

photolysis and also by hydrolysis of a portion of the NO2 formed from channel 1.39

421

With the new quantum yield for nitrite, the steady-state N(III) concentration increases by

422

a factor of 3.6 (for all pH values), resulting in 2-15 nM of nitrite in the snowpack, with the value

423

highly dependent upon the pH of the LLR (Figure 6). The snowpack nitrite concentration also

424

depends upon the mass transfer coefficient for evaporation of HONO (kMT). Because this value

425

is unknown, we consider a range in our model, which is represented by the shading in Figure 6:

426

as kMT increases, the snowpack nitrite concentration decreases because more N(III) is lost as

427

HONO to the gas phase. Compared to our past modeling work,39 using the new, higher value of

428

Φ(NO2–) gives nitrite concentrations in snow that are closer to polar observations, which are

429

typically 10 – 50 nM.44,75 However, this comparison suggests there are still unexplained

430

sources of nitrite in polar snow since the maximum concentration from our model remains

431

lower than the maximum measured values.

432

23



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Page 24 of 41

433 434

Figure 6. Modeled steady-state N(III) concentrations in a polar snowpack (–10 °C, solar

435

zenith angle 49.1, [NO3–] = 4 µM) as a function of liquid-like region pH using the Anastasio

436

and Chu39 snow photochemistry model (with j-values averaged over the top 20 cm of

437

snowpack). The solid line and dark shading values are calculated for the Φ(NO2¯ ) value

438

determined in this study (0.69%) while the dashed line and light shading are calculated for

439

the previously recommended value from Dubowski et al.17 (0.15%). Lines represent N(III)

440

concentrations determined with a mass transfer coefficient (kMT) of 2E-4 s–1 while the

441

shaded regions represent concentrations for a range of kMT from 2E-3 to 2E-5 s–1.

442

In Figure 7 we plot the fluxes of NO, HONO, and NO2 to the atmosphere in the model with

443

our new, higher quantum yield and a mass transfer coefficient of 2E-4 s–1. Our modeled fluxes

444

for HONO and NOx at Summit reasonably overlap with the wide range of measured fluxes

445

(Figure 7), which represent observations at Summit and a variety of other locations and

446

conditions. Our simple model predicts higher fluxes of NOx compared to HONO, which is

447

generally seen in the field. However, the model result is sensitive to the assumed value of kMT:

448

as mass transport from the snow increases, the flux of HONO increases while NO decreases 24


Page 25 of 41

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449

(data not shown). In addition, our current modeling reiterates the point39 that HONO fluxes (and

450

NO2– concentrations) are very sensitive to the pH of snow LLRs, which is as-yet unmeasured. In

451

addition, the snowpack flux of reactive nitrogen species is affected by meteorological conditions

452

and snow properties like grain size, permeability, and composition.72,76

453

Finally, some laboratory studies77–79 indicate that the nitrite and/or NO2 quantum yields

454

from nitrate photolysis can be orders of magnitude higher than our measurements here and

455

elsewhere,15 possibly because of reactivity enhancements at the air-ice interface. However, the

456

general agreement between our simple model and past field measurements of snow nitrite and

457

snow-to-air fluxes of HONO and NOx (Figures 6 and 7) suggests that quantum yields for nitrate

458

photolysis are not significantly larger than what we measure. Of course there are numerous

459

important uncertainties in our simple model, in field flux measurements, and in our

460

understanding of how secondary chemistry alters the forms and amounts of reactive nitrogen in

461

the snowpack. But the current agreement between model and measurements suggests we have at

462

least a reasonably quantitative understanding of nitrate photolysis in snow.

463

25



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Page 26 of 41

464 465 466

Figure 7. Comparison of model results (left panel) to measured fluxes from a variety of

467

sites in the Arctic and Antarctic (right panel) for HONO (black), NO2 (orange), NO (light

468

blue), and NOx (blue). The steady-state model39 was run for clear sky conditions at midday

469

on the summer solstice at Summit (–10 °C) using the value of Φ(NO2–) from this work,

470

photolysis rate constants averaged over the top 20 cm of snow, a nitrate concentration of 4

471

µM (liquid equivalent), and a mass transport rate constant (kMT) of 2 × 10–4 s–1.

472

Observations of a range of fluxes are represented by points at either end of the range with

473

a line connecting the points. Observational data are from the following sources: Alert-18,

474

Alert-25, Kuujjuarapik11, Ny-Alesund-180, Ny-Alesund-29, Summit4, Dome C-181, Dome C-

475

282, Halley-183, Halley-284, South Pole10, Neumayer6, and Concordia.85

476

26


Page 27 of 41

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477


5. Conclusions

478

Our results show that quantum yields for the photoproduction of NO2– and NO2 are

479

similar at room temperature, while nitrite production is more efficient at lower temperatures.

480

The temperature dependence of Φ(NO2–) in samples containing 2-propanol is described by a

481

single regression line, ln(Φ(NO2–)) = −(1330±100)(1/T) + (0.09 ± 0.39), where T is in Kelvin; at

482

– 10 °C, this relationship gives a quantum yield that is nearly 5 times larger than the value we

483

had previously recommended. Our experimental measurements show that the nitrite quantum

484

yield in ice is more sensitive to pH and •OH compared to in solution. Including our new

485

quantum yield of nitrite in a kinetic model of snowpack nitrogen chemistry improves agreement

486

with observations, although there remain many uncertainties in snowpack physical processes and

487

secondary chemistry.

488

Supporting Information. Additional experiment details. Composition of real snow samples.

489

Figures S1-S5. Tables S1-S3.

490 491

Acknowledgements

492

This work was funded by the Arctic Natural Sciences at the National Science Foundation (ANS-

493

1204169). We thank Alex Funderburk and Emily Lucic for assistance with experiments; Barry

494

Lefer for the Summit snow samples; and Richie Kaur and Qi Zhang for snow sample

495

measurements of ions and TOC/IC.

27



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Page 28 of 41

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Figure 1. Quantum yields of nitrite as a function of solution pH (prior to freezing) in -5 °C ice for samples containing 50 µM NaNO3, without or with an organic scavenger (50 µM 2-propanol or 50 µM formate). Prior to freezing, samples were buffered using sulfuric acid (pH ≤ 5) or phosphate (pH > 5.6). The average quantum yield in solution at 25 °C (pH 5-9 with and without buffers)32 is indicated by the horizontal grey dotted line; the average at -5 °C for samples with pH > 6.8 and no organic scavenger is indicated by the horizontal black line. 88x69mm (300 x 300 DPI)



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Figure 2. Nitrite quantum yield at -5 (grey) and -10 °C (white) for ice samples made from solutions containing 50 µM NaNO3. The top label for each set of bars indicates whether solutions were buffered to near pH 7, while the bottom label indicates whether solutions contained 50 µM of a single organic to scavenge •OH. The solutions in the second set of bars were buffered to pH 7 with phosphate. Since TRIS acts as both an •OH scavenger and a buffer, the TRIS solution contained no phosphate but was pH 6.9 on average. Bars represent average values for at least 2 experiments. Statistically significant differences compared to the 2-propanol case at the same temperature are indicated by * (p ≤ 0.1) and ** (p ≤ 0.01). 88x69mm (300 x 300 DPI)


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Figure 3. Temperature dependence of the nitrite quantum yield. Panel (a) shows results for 50 µM NaNO3 samples with various pH buffers, with no buffer and for Summit snow samples (section 3.4). Panel (b) shows results for samples containing organic scavengers and no pH buffer. In each panel the solid line is the regression fit of the 2-propanol (2-PrOH) data points while the dashed line is the fit to the No Buffer or Scavenger data in ice. 203x246mm (300 x 300 DPI)



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Figure 4. Our recommended quantum yields for nitrite formation (based on samples made from 50 µM NaNO3 and 50 µM 2-propanol) compared with previously reported values for both channels of nitrate photolysis. Past data are from Dubowski et al.16,17 (who measured nitrite), Chu and Anastasio15 (who measured •OH), as well as 25 °C nitrite data from Goldstein and Rabani,45 Roca et al.,46 and Warneck and Wurzinger37 for 10 mM NO3– solutions. “Fo” indicates formate was used. The “O(3P) Msmt.” point is an oxygen atom measurement of channel 2 from Warneck and Wurzinger.37 Solid lines are regression fits to data from measurements of channel 2 with an •OH scavenger, the dashed line is for channel 2 without an • OH scavenger, and the dotted lines are measurements of channel 1. 101x58mm (300 x 300 DPI)


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Figure 5. Measured Φ(NO2–) at 25, -10, and -30 °C for 50 µM solutions of KNO3 (green; Teu= -2.9 °C), NH4NO3 (orange; Teu= -16.9 °C), NaNO3 (red; Teu= -17.7 °C), Ca(NO3)2 (blue; Teu= -33.1 °C), and Mg(NO3)2 (purple; Teu= -72.5 °C). Quantum yields at 25 °C were taken from Benedict et al.32 for experimental solutions with pH above 7. Experimental solutions at -10 and -30 °C contained 50 µM 2-propanol and no pH buffer. Experiments below the eutectic temperature of a given salt are indicated by striped bars. Eutectic temperatures were calculated using the FrezChem68 computer model. 100x60mm (300 x 300 DPI)



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Figure 6. Modeled steady-state N(III) concentrations in a polar snowpack (–10 °C, solar zenith angle 49.1, [NO3–] = 4 µM) as a function of liquid-like region pH using the Anastasio and Chu39 snow photochemistry model (with j-values averaged over the top 20 cm of snowpack). The solid line and dark shading values are calculated for the Φ(NO2–) value determined in this study (0.69%) while the dashed line and light shading are calculated for the previously recommended value from Dubowski et al.17 (0.15%). Lines represent N(III) concentrations determined with a mass transfer coefficient (kMT) of 2E-4 s–1 while the shaded regions represent concentrations for a range of kMT from 2E-3 to 2E-5 s–1. 112x82mm (300 x 300 DPI)


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Figure 7. Comparison of model results (left panel) to measured fluxes from a variety of sites in the Arctic and Antarctic (right panel) for HONO (black), NO2 (orange), NO (light blue), and NOx (blue). The steadystate model39 was run for clear sky conditions at midday on the summer solstice at Summit (–10 °C) using the value of Φ(NO2–) from this work, photolysis rate constants averaged over the top 20 cm of snow, a nitrate concentration of 4 µM (liquid equivalent), and a mass transport rate constant (kMT) of 2 × 10–4 s– 1 . Observations of a range of fluxes are represented by points at either end of the range with a line connecting the points. Observational data are from the following sources: Alert-18, Alert-25, Kuujjuarapik11, Ny-Alesund-180, Ny-Alesund-29, Summit4, Dome C-181, Dome C-282, Halley-183, Halley-284, South Pole10, Neumayer6, and Concordia.85 94x54mm (300 x 300 DPI)


Quantum Yields of Nitrite (NO2â) from the Photolysis of Nitrate (NO3

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