The magnitude of the snow-sourced reactive nitrogen flux to the boundary layer in the Uintah Basin, Utah, USA

Abstract

Reactive nitrogen (N_r = NO, NO₂, HONO) and volatile organic carbon emissions from oil and gas extraction activities play a major role in wintertime ground-level ozone exceedance events of up to 140ppb in the Uintah Basin in eastern Utah. Such events occur only when the ground is snow covered, due to the impacts of snow on the stability and depth of the boundary layer and ultraviolet actinic flux at the surface. Recycling of reactive nitrogen from the photolysis of snow nitrate has been observed in polar and mid-latitude snow, but snow-sourced reactive nitrogen fluxes in mid-latitude regions have not yet been quantified in the field. Here we present vertical profiles of snow nitrate concentration and nitrogen isotopes (δ¹⁵N) collected during the Uintah Basin Winter Ozone Study 2014 (UBWOS 2014), along with observations of insoluble light-absorbing impurities, radiation equivalent mean ice grain radii, and snow density that determine snow optical properties. We use the snow optical properties and nitrate concentrations to calculate ultraviolet actinic flux in snow and the production of N_r from the photolysis of snow nitrate. The observed δ¹⁵N(NO₃^-) is used to constrain modeled fractional loss of snow nitrate in a snow chemistry column model, and thus the source of N_r to the overlying boundary layer. Snow-surface δ¹⁵N(NO₃^-) measurements range from -5 to 10[per thousand] and suggest that the local nitrate burden in the Uintah Basin is dominated by primary emissions from anthropogenic sources, except during fresh snowfall events, where remote NO_x sources from beyond the basin are dominant. Modeled daily averaged snow-sourced N_r fluxes range from 5.6 to 71 × 10⁷moleccm^-2s^-1 over the course of the field campaign, with a maximum noontime value of 3.1 × 10⁹moleccm^-2s^-1. The top-down emission estimate of primary, anthropogenic NO_x in Uintah and Duchesne counties is at least 300 times higher than the estimated snow NO_x emissions presented in this study. Our results suggest that snow-sourced reactive nitrogen fluxes are minor contributors to the N_r boundary layer budget in the highly polluted Uintah Basin boundary layer during winter 2014.

Atmos. Chem. Phys., 16, 1383713851, 2016 www.atmos-chem-phys.net/16/13837/2016/ doi:10.5194/acp-16-13837-2016 Author(s) 2016. CC Attribution 3.0 License.

Maria Zatko1, Joseph Erbland2,3, Joel Savarino2,3, Lei Geng1,a, Lauren Easley4,b, Andrew Schauer5, Timothy Bates7, Patricia K. Quinn6, Bonnie Light8, David Morison8,c, Hans D. Osthoff9, Seth Lyman10, William Neff11, Bin Yuan11,12, and Becky Alexander1

1Department of Atmospheric Sciences, University of Washington, Seattle 98195, USA

2Universit Grenoble Alpes, LGGE, 38000 Grenoble, France

3CNRS, LGGE, 38000 Grenoble, France

4Department of Chemistry, University of Washington, Seattle, Washington 98195, USA

5Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA

6Pacic Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, Washington 98115, USA

7Joint Institute for the Study of the Atmosphere and Oceans, University of Washington, Seattle, Washington 98195, USA

8Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, Washington 98195, USA

9Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, AB T2N 1N4, Canada

10Bingham Entrepreneurship and Energy Research Center, Utah State University, 320 Aggie Boulevard, Vernal, Utah 84078, USA

11Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, Colorado 80309, USA

12Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado 80305, USA

anow at: Universit Grenoble Alpes, LGGE, 38000 Grenoble, France, CNRS, LGGE, 38000 Grenoble, France

bcurrent address: DSG Solutions, LLC, Shoreline, WA, 98133, USA

cnow at: Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah 84112, USA Correspondence to: Becky Alexander ([email protected])

Received: 13 April 2016 Published in Atmos. Chem. Phys. Discuss.: 17 May 2016 Revised: 12 August 2016 Accepted: 1 October 2016 Published: 9 November 2016

Abstract. Reactive nitrogen (Nr = NO, NO2, HONO) and

volatile organic carbon emissions from oil and gas extraction activities play a major role in wintertime ground-level ozone exceedance events of up to 140 ppb in the Uintah Basin in eastern Utah. Such events occur only when the ground is snow covered, due to the impacts of snow on the stability and depth of the boundary layer and ultraviolet actinic ux at the surface. Recycling of reactive nitrogen from the photolysis of snow nitrate has been observed in polar and mid-latitude snow, but snow-sourced reactive nitrogen uxes in mid-latitude regions have not yet been quantied in the eld. Here we present vertical proles of snow nitrate concentration and nitrogen isotopes ( 15N) collected during the Uintah Basin Winter Ozone Study 2014 (UBWOS 2014), along

with observations of insoluble light-absorbing impurities, radiation equivalent mean ice grain radii, and snow density that determine snow optical properties. We use the snow optical properties and nitrate concentrations to calculate ultraviolet actinic ux in snow and the production of Nr from the photolysis of snow nitrate. The observed 15N(NO3) is used to constrain modeled fractional loss of snow nitrate in a snow chemistry column model, and thus the source of Nr to the overlying boundary layer. Snow-surface 15N(NO3) measurements range from 5 to 10 and suggest that the local

nitrate burden in the Uintah Basin is dominated by primary emissions from anthropogenic sources, except during fresh snowfall events, where remote NOx sources from beyond the basin are dominant. Modeled daily averaged snow-sourced

Published by Copernicus Publications on behalf of the European Geosciences Union.

The magnitude of the snow-sourced reactive nitrogen ux to the boundary layer in the Uintah Basin, Utah, USA

O3 loss through surface deposition due to snow cover (Ahmadov et al., 2015), trigger high boundary layer O3 episodes in these basins. Ozone exceedance events occur only when the ground is snow covered because snow aids in the formation and maintenance of a stable air mass and reects UV radiation upwards into the boundary layer. Ozone exceedance events end when stable boundary layers are disrupted by the passage of storm fronts, which often deposit snow. Modeling studies were used to determine whether O3 formation in these regions is NOx-sensitive or VOC-sensitive, which is necessary information for the enactment of effective regulations aimed to reduce boundary layer O3 abundance. Modeling results from Edwards et al. (2014) suggest that the Uintah Basin is in an O3 formation regime on the boundary between VOC-sensitive and NOx-sensitive, and modeling results from Ahmadov et al. (2015) suggest that the Uintah Basin regime is VOC-sensitive. Modeling results presented in Edwards et al. (2014) suggest that the dominant radical sources in the Uintah Basin are carbonyl compounds (85 %), with smaller inputs from HONO, O3, and nitryl chloride (ClNO2) photolysis.

Atmospheric measurements in the Uintah Basin during UBWOS2012, UBWOS2013, and UBWOS2014 reveal that the total reactive nitrogen abundances (NOy

=NO + NO2 + HNO3 + PAN + N2O5 + NO3 + ClNO2 +

organic nitrates) are highest (1224 ppbv) in 2013 due to persistent shallow inversion layers triggered by stagnant air masses and snow cover, lowest in 2012 (49 ppbv) when no snow covered the ground, and in between (818 ppbv) in winter 2014, with the highest NOy values generally in midday (Wild et al., 2016). In 2013, HNO3 accounted for nearly half of total NOy, while in 2012 N2O5 and ClNO2 were larger components of total NOy compared to HNO3 (Wild et al., 2016). Interestingly, atmospheric NOx mixing ratios are similar in all three years, with diurnal averages ranging from 2 ppbv during the night to 10 ppbv during the day (Wild et al., 2016). The NOx / NOy ratio, indicative of the rate of oxidation of reactive nitrogen, was highest in 2013 and lowest in 2012, with intermediate values in 2014 (Wild et al., 2016). HO2NO2 measurements range from 0 to 2.4 ppbv in 2013 and 0 to 0.4 ppbv in 2014 (Veres et

al., 2015) and are generally positively correlated with snow nitrite concentrations, suggesting that HO2NO2 deposition may be a source of snow nitrite (Veres et al., 2015).

In addition to aiding in the formation and maintenance of a stable air mass with enhanced UV radiation, snow may also recycle reactive nitrogen oxides (Nr = NOx, HONO) be

tween the snow surface and the overlying atmosphere, effectively increasing the atmospheric lifetime of Nr. The major sink of Nr in the atmosphere is the formation and deposition of nitrate (particulate NO3 plus HNO3(g)). When nitrate is deposited to snow, its photolysis serves to recycle Nr to the overlying boundary layer (Grannas et al., 2007; Honrath et al., 2000). This snow-sourced Nr can then be re-oxidized to nitrate and re-deposited to the snow surface. The recycling

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13838 M. Zatko et al.: Snow-sourced reactive nitrogen ux in the Uintah Basin

Nr uxes range from 5.6 to 71 107 molec cm2 s1 over

the course of the eld campaign, with a maximum noontime value of 3.1 109 molec cm2 s1. The top-down emission

estimate of primary, anthropogenic NOx in Uintah and Duchesne counties is at least 300 times higher than the estimated snow NOx emissions presented in this study. Our results suggest that snow-sourced reactive nitrogen uxes are minor contributors to the Nr boundary layer budget in the highly polluted Uintah Basin boundary layer during winter 2014.

1 Introduction

Ozone (O3) has adverse respiratory effects, is an effective greenhouse gas (UNEP, 2011), and, through formation of the hydroxyl radical, inuences the oxidizing capacity of the atmosphere (Thompson, 1992). Ozone precursors include volatile organic compounds (VOCs) emitted from vegetation, biomass burning, and fossil fuel combustion (Guenther et al., 1995; Warneke et al., 2014) and nitrogen oxides (NOx = NO + NO2) emitted from fossil fuel combustion,

biomass burning, soil microbial activity, lightning, and photochemical reactions in snow (Delmas et al., 1997; Grannas et al., 2007; Logan, 1983). Maximum boundary layer O3 concentrations are typically observed during the summer in major cities, where O3 precursors are abundant, and when conditions favor efcient O3 production (high ultraviolet, UV, radiation) and air stagnation. High O3 concentrations in the boundary layer exceeding 100 ppbv were measured in winter 2005 in the Upper Green River basin in rural Wyoming (Schnell et al., 2009), which is well above the current Environmental Protection Agency (EPA) National Ambient Air Quality Standard (NAAQS) 8 h average limit of 70 ppbv. High wintertime O3 episodes have also been ob-served in the Uintah Basin in rural Utah (Martin et al., 2011), and in both basins, these O3 episodes only occur when the ground is snow covered (Oltmans et al., 2014). The Upper Green River basin and the Uintah Basin are regions of major oil and gas development, and the production of oil and natural gas in the Upper Green River basin and the Uintah Basin is expected to increase through at least 2020 (US EIA, 2014).

These wintertime high O3 episodes motivated a series of eld campaigns, including the Upper Green Winter Ozone Study (UGWOS 2011, UGWOS 2012) and the Uintah Basin Winter Ozone Study (UBWOS 2012, UBWOS 2013, UBWOS 2014). Results from these eld campaigns (Gilman et al., 2013; Helmig et al., 2014; Oltmans et al., 2014; Warneke et al., 2014; Schnell et al., 2009) and subsequent modeling studies (Ahmadov et al., 2015; Carter and Seinfeld, 2012;Edwards et al., 2013, 2014; Field et al., 2015; Rappenglck et al., 2014) reveal that emissions of NOx and VOCs from oil and gas extraction, combined with stagnant meteorological conditions, enhanced boundary layer UV radiation due to the high UV albedo of snow (Warren et al., 2006), and reduced

M. Zatko et al.: Snow-sourced reactive nitrogen ux in the Uintah Basin 13839

of nitrogen between the snow surface and boundary layer can occur many times, resulting in the continuous recycling of Nr during sunlit conditions.

The photolysis of nitrate occurs in the liquid-like region (LLR) in or on ice grains (Domine et al., 2013) in the top snow layer where UV radiation is present, which is known as the snow photic zone. Snow nitrate photolyzes at wavelengths ( ) = 290345 nm to produce aqueous-phase nitro

gen dioxide (NO2) or nitrite (NO2) according to Reactions (R1) and (R2) (Grannas et al., 2007; Mack and Bolton, 1999; Meusinger et al., 2014).

NO3(aq) + h (+H+) ! NO2(aq) + OH(aq) (R1)

NO3(aq) + h ! NO2(aq) + O(3P)(aq) (R2)

The measured quantum yields () for Reaction (R1) range from 0.003 to 0.6 molec photon1 at 253 K (Chu and Anastasio, 2003; Meusinger et al., 2014; Zhu et al., 2010) and is likely inuenced by the location of nitrate within ice grains.The NO2 produced in Reaction (R1) quickly evaporates due to its low solubility and can be transported to the overlying atmosphere. The nitrite produced in Reaction (R2) is rapidly photolyzed at longer wavelengths ( = 290390 nm) (Reac

tion R3).

NO2(aq) + h (+H+,aq) ! NO(aq) + OH(aq) (R3)

Nitrite can also react with OH or H+ in the LLR to produce aqueous-phase NO2 and HONO (Grannas et al., 2007):

NO2(aq) + OH(aq) ! NO2(aq) + OH(aq), (R4)

NO2(aq) + H+(aq) ! HONO(aq). (R5)

HONO can rapidly photolyze in the LLR to produce aqueous-phase NO and OH (Anastasio and Chu, 2009); due to its short lifetime, the aqueous-phase OH remains in the LLR, but the aqueous-phase NO can be transferred to the gas phase and ultimately be released into the boundary layer. Under acidic conditions (pka < 2.8), aqueous-phase HONO can also be transferred to the gas phase (HONO(aq) ! HONO(g)) (Anastasio and Chu, 2009) and

released into the boundary layer, where it can photolyze to produce gas-phase NO and OH (Zhou et al., 2001).

Nitrate nitrogen isotopes ( 15N(NO3)) in the air and snow can provide useful information about snow photo-chemistry, specically, the degree of photolysis-driven recycling and loss of nitrate from the snow. Nitrogen isotope ratios are expressed as 15N, where = Rsample/Rreference 1,

R=15N/14N, and N2-air is the reference material. Nitrate

photolysis in snow is a mass-dependent process and is associated with a large fractionation constant (") of 47.9

at wavelengths shorter than 320 nm (Berhanu et al., 2014).

Nitrate photolysis provides the boundary layer with a source of Nr that is highly depleted in 15N, leaving highly enriched 15N(NO3) deeper in the snow. Snow-sourced nitrate that

is redeposited to the snow surface is lighter than the remaining nitrate in the snow, leading to 15N(NO3) values that become more enriched with increasing depth within the snow photic zone. 15N(NO3) values in the atmosphere are also inuenced by the relative importance of different NOx sources (see Felix and Elliott, 2014, for a summary). For example, the atmospheric 15N signature from anthropogenic NOx sources, such as combustion of fossil fuels, ranges from

19.0 to 25.0 (Felix et al., 2012; Walters et al., 2015).

The 15N signature from soil microbial activity is generally lower than that of anthropogenic activity and ranges from

50 to 20 (Felix and Elliott, 2014). Observations of at

mospheric 15N(NO3) in non-polluted, mid-latitude regions range from 6 to 2 , while 15N(NO3) values measured

in polluted regions range from 0 to 6 (Morin et al., 2009).

In addition, atmospheric 15N(NO3) is inuenced by NOx cycling (Freyer et al., 1993; Walters et al., 2016), NO2 oxidation (Walters and Michalski, 2015), and the partitioning of nitrate between its gas and particulate phases (Heaton et al., 1997).

In this study, we investigate the importance of snow photochemistry as a source of reactive nitrogen oxides to the boundary layer in the Uintah Basin using chemical, isotopic, and optical measurements from the snow collected during the UBWOS 2014 campaign. In Sect. 2 we describe the eld, laboratory, and modeling techniques used in this study. In Sect. 3 we present the chemical and optical measurements made during UBWOS 2014 and model-calculated uxes of snow-sourced Nr. In Sect. 4 we estimate the contribution of snow-sourced Nr to the Nr burden in the Uintah Basin boundary layer.

2 Methods

2.1 Field and laboratory observations

2.1.1 UBWOS 2014 eld site description and meteorological conditions

UBWOS 2014 occurred from 17 January to 13 February 2014 at the Horsepool eld-intensive site (40.1 N, 109.5 W) in the Uintah Basin, roughly 55 km south of Vernal, Utah. There are over 10 000 oil and natural gas wells in the basin connected by a series of dirt roads. The meteorological conditions were relatively constant for most of the campaign; wind speeds ranged from 1 to 3 m s1 and often originated from the southwest. Sky conditions were clear, temperatures ranged from 258 to 275 K, and boundary layer heights generally ranged from 25 to 150 m. There were a few cloudy days (29 January4 February, 10 February) during the campaign and the last several days experienced temperatures above freezing. Daily maximum boundary layer O3 mixing ratios ranged from 45 to 90 ppb, and the campaign-averaged daily-maximum boundary layer O3 mixing ratio was 61 ppb.

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13840 M. Zatko et al.: Snow-sourced reactive nitrogen ux in the Uintah Basin

Snow covered the ground throughout the duration of the campaign and ranged in depth from 10 to 30 cm, depending on how snow was redistributed by wind after deposition.The snow was deep enough to cover some of the lowest-lying vegetation, but branches from bushes were still visible.Three snow events occurred before the campaign, one event on 4 December, which deposited most of the snow (19 cm), and two smaller events on 8 and 19 December, which deposited roughly 3 and 1 cm of snow, respectively. There was a distinct crust layer roughly 4 cm below the snow surface, providing evidence of surface melting between the later two snowfall events. The temperature difference between the soil and the air was at least 15 K for several weeks, allowing vapor to redistribute through the snow, leading to the formation of large hoar crystals (radiation equivalent mean ice grain radii (Hansen and Travis, 1974) (re) > 1200 m) at all depths in the snow. There was one major snow event during the campaign from 30 through 31 January that deposited roughly 5 cm of fresh snow (re 100 m). There were two

smaller snow events on 4 and 10 February. On 4 February there was no measurable snow accumulation and during the early morning hours of 10 February there was 2 cm of fresh snow that subsequently melted several hours after sunrise.Figure S3b in the Supplement summarizes daily snow accumulation before and during the campaign.

2.1.2 Snow pit measurements and snow sample preparation

Twelve snow pits were dug approximately every 2 to 3 days during the campaign. Snow pits were dug from the snow surface to about 1 cm above the subniveal ground and ranged in depth from 9 to 24 cm. The snow pits were dug in a variety of directions roughly 150 m from the main Horsepool site, except for snow pit 5 (24 January), which was dug roughly 800 m away from Horsepool. The snow pits were dug wearing clean, nitrate-free gloves using a stainless steel spatula.For each snow pit, vertical proles (1 cm depth resolution) of snow density (snow), temperature, and radiation equivalent ice grain radii (re) were measured using a TaylorLaChapelle snow density kit, a dial stem thermometer, and a laminated snow grid card with 1 mm grid spacing, respectively. Snow grains from each distinct snow layer were placed on the snow grid card and a photograph was taken. The photographs were projected onto a larger screen and the shortest dimension of each snow crystal was estimated. The shortest dimension of a snow grain is the most optically important dimension (Gren-fell and Warren, 1999), and in this study, it is used to represent re. For hoar crystals, the smallest dimension is the width of the crystal wall and for freshly fallen crystals, the smallest dimension is the radius of the rounded crystal. For each snow pit, approximately 1 kg of snow was collected at 1 cm depth intervals and placed into Whirl-Pak plastic bags. The bags were kept covered while in the eld and then immediately placed into a freezer once back at the Utah State University

(USU) Uintah Basin campus in Vernal, Utah. Section A in the Supplement shows detailed information on each snow pit.

2.1.3 Optical measurements

The snow from each plastic bag was spooned into a clean glass beaker and melted in a microwave oven at USU. The meltwater was transferred to a stainless steel funnel and passed through a 0.4 m Nuclepore lter, using an electric diaphragm vacuum pump to create a partial vacuum in a volumetric ask. The Nuclepore lter collects insoluble light absorbing impurities (LAI) in snow, including black carbon (BC) and non-black carbon (non-BC) species, the latter of which encompass brown carbon, dust, and organics. The volume of ltrate was measured, which ranged from 40 to 750 ml depending on impurity content. After the Nuclepore lters dried overnight, the lters were frozen until further analysis at the University of Washington (UW).

The absorption spectrum of each Nuclepore lter was measured using an ISSW spectrophotometer (Grenfell et al., 2011) in the Arctic Snow Laboratory at UW. The Nucle-pore lter is placed between two integrating spheres lined with Spectralon material to create a fully diffuse medium. An Ocean Optics USB-650 spectrophotometer is used to measure the absorption spectrum in units of optical depth, ( ) (dimensionless, e.g., cm2 cm2), from = 3501000 nm in

10 nm intervals. A set of standard lters containing known loadings of black carbon (Fullerene) is used to calibrate the ISSW spectrophotometer. The spectral absorption measured by the spectrophotometer for each lter is characterized by an ngstrm exponent (), which represents the total absorption by both BC and non-BC LAI on the lter between two visible wavelengths. is calculated in Eq. (1):

( 1 to 2) =

ln(( 1)( 2)) ln( 2 1 )

, (1)

where 1 = 450 nm and 2 = 600 nm. The = 450600 nm

range is chosen because the ISSW spectrophotometer signal is most stable over this wavelength range. The total absorption ngstrm exponent on each lter, along with assumed ngstrm exponents for BC ( = 1) and non-BC ( = 5), are

used to estimate snow BC concentrations and the fraction of ultraviolet ( = 300350 nm) absorption by non-BC ma

terial (see Doherty et al., 2010; Grenfell et al., 2011; Zatko et al., 2013; Zatko and Warren, 2015). Triplicate measurements were performed for all samples.

Surface upwelling and downwelling irradiance was measured using a commercial spectral radiometer equipped with a photodiode array (Metcon GmbH & Co. KG, http://www.metcon-us.com

Web End =http: http://www.metcon-us.com

Web End =//www.metcon-us.com http://www.metcon-us.com

Web End = ). Upwelling and downwelling UVA and UV-B were measured with Kipp and Zonen Model UV-S-AB-T radiometers. Radiometers were placed at 2 m above ground (one up-facing and one down-facing) and were cleaned and checked weekly to ensure that the radiometers

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M. Zatko et al.: Snow-sourced reactive nitrogen ux in the Uintah Basin 13841

remained directly perpendicular to the ground. Detailed irradiance data are provided in the Supplement.

2.1.4 Chemical concentration and nitrate isotopic measurements

In a laboratory on the USU campus in Vernal, UT, a 50 L aliquot of snow meltwater that was passed through the Nuclepore lter was used to measure ion (Cl, Br, NO3,

SO24, Na+, NH+4, K+, Mg+2, Ca+2, oxalate) concentrations using a Metrohm 761 Compact Ion Chromatograph Analyzer (Quinn et al., 1998). The nitrate in the remaining ltrate was pre-concentrated for isotopic analysis. Nitrate was pre-concentrated by passing the meltwater through an anion exchange resin (Bio-Rad AG 1-X8) using an electric diaphragm pump. The sample anions in the resin were eluted with 5 2 mL 1 M sodium chloride (NaCl/Milli-Q water) so

lution into a 30 mL pre-cleaned sample bottle. This method has been shown to ensure full recovery of nitrate (Silva et al., 2000; Frey et al., 2009) The solution was kept frozen in the dark until analysis in the University of Washington IsoLab (http://isolab.ess.washington.edu/isolab/

Web End =http://isolab.ess.washington.edu/isolab/ ).

The denitrier method (Casciotti et al., 2002; Kaiser et al., 2007; Sigman et al., 2001) was used to determine the nitrogen isotopic signature ( 15N) in each snow sample. Denitrifying bacteria, Pseudomonas aureofaciens, convert nitrate to nitrous oxide (N2O) gas in anaerobic conditions (Casciotti et al., 2002; Sigman et al., 2001), and N2O is transported via helium gas through a heated gold tube (800 C), where it thermally decomposes into O2 and N2. After separation by gas chromatography, the O2 and N2 are run through a Thermo Finnigan Delta Plus Advantage isotope ratio mass spectrometer (IRMS), equipped with a Precon and Gas-Bench II. The 15N values were calculated with respect to N2 (air) via two international reference materials, USGS32 ( 15N = 180 ) and USGS34 ( 15N = 1.8 ), with IAEA

( 15N = 4.7 ) as a quality control standard. For many sam

ples, the NaCl / NO3 solution was diluted with Milli-Q water to obtain the optimal nitrate concentration (200 nmol in 2 mL) for each sample run on the IRMS. Triplicate measurements were performed for all samples. The analytical uncertainty of 15N(NO3) (1) was 0.75 based on repeated measurements of the quality control standard.

Aerosol nitrate was collected throughout the campaign in 12 h intervals. Aerosol nitrate was sampled from an inlet 13 m above ground and drawn through a heated (283 K) pipe, where it was then collected on a two-stage, multi-jet cascade impactor. The impactor Tedlar lms separates aerosols with diameters less than 2.5 m from those with diameters between 2.5 and 12.5 m. The aerosols were extracted from the lters and analyzed using ion chromatography, following methods described in Quinn et al. (2000). Gas-phase nitric acid was measured using an Acetate HR-ToF-CIMS instrument throughout the campaign with 1 min time resolution, as described in Yuan et al. (2016).

2.2 Calculations

2.2.1 Snow radiative transfer model

A four-stream, plane-parallel radiative transfer model using the discrete ordinates method with a -M transformation originally described in Grenfell (1991) was used to calculate vertical proles of UV actinic ux in each snow pit. This model properly treats layers with differing refractive indices and the 4-stream model produces albedo and absorptivity results that agree to within 1 % of higher-order models representative of snow (Wiscombe, 1977), including DISORT (Stamnes et al., 1988). Vertical proles of the snow, re, and

LAI absorption are used to calculate vertical proles of inherent optical properties (IOPs) in snow at the wavelengths relevant for photochemistry (UV). These wavelength-dependent IOPs include the bulk extinction coefcient in snow (Kexttot)

and the co-albedo of single scattering (c[pi1]eff); see Zatko et al. (2013) for more details about the IOP calculations. Kexttot and c[pi1]eff, along with observations of downwelling surface UV irradiance, solar zenith angle, cloud fraction, and soil albedo (0.1) (Markvart and Castalzer, 2003; Matthias et al., 2000), are used to calculate 1 cm resolution vertical proles of UV actinic ux for each snow pit, following methods described in Zatko et al. (2013). The UV actinic ux proles are used to calculate depth-dependent photolysis rate constants for nitrate photolysis in snow as described below.

2.2.2 Snow-sourced reactive nitrogen ux calculations

The modeled vertical proles of actinic ux and observed snow nitrate concentrations are used to calculate daily-average uxes of snow-sourced Nr from each snow pit according to Eq. (2).

FNr(z) =

1

[integraldisplay]

0

NO

3 ( ) (T ,pH) I ( ,z)

NO3[bracketrightbig]

(z)d (2)

FNr(z) is the ux of snow-sourced Nr (molec cm2 s1) at 1 cm depth (z) increments in the snow, NO3 is the wavelength ( )-dependent absorption crosssection for nitrate photolysis (cm2) from Berhanu et al. (2014), is the temperature- and pH-dependent quantum yield for nitrate photolysis (, molec photon1) from Chu and Anastasio (2003) (4.6 103 molec photon1 at T = 267 K), I

is the depth (z)- and -dependent actinic ux in the snow photic zone (photons cm2 s1 nm1), and [NO3](z) is the

observed nitrate concentration (ng g1) in each snow layer.

Equation (2) is integrated over the UV wavelength region ( = 298345 nm). The snow photic zone is dened as 3

times the e-folding depth of UV actinic ux in snow (Zatko et al., 2016). The total ux of Nr to the boundary layer,

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13842 M. Zatko et al.: Snow-sourced reactive nitrogen ux in the Uintah Basin

FNr, is calculated according to Eq. (3).

FNr =

z3e

Xz0FNr(z) (3)

Observed surface downwelling irradiance values for a solar zenith angle of 65 , the average solar zenith angle from mid-December to mid-February, are used for calculation of I ( ,z) in Eq. (3). Therefore the calculated FNr values represent daily averaged FNr values. It is assumed that all Nr escapes into the boundary layer due to its low solubility.

2.2.3 Snow photochemistry column model (TRANSITS)

The ux of snow-sourced Nr from each snow pit is also calculated using a snow photochemistry column model, TRansfer of Atmospheric Nitrate Stable Isotopes To the Snow (TRANSITS)(Erbland et al., 2015). TRANSITS is a multilayer, one-dimensional model that simulates nitrate photo-chemistry in the snow and allows for chemical exchange between the air and snow and calculates the isotopic composition of snow nitrate. The model was originally developed to simulate snow nitrate photolysis and subsequent nitrogen recycling at the airsnow interface on the eastern Antarctic Plateau (Dome C), and has been adapted to mid-latitude, shallow-snowpack conditions for this study. The model has a well-mixed, atmospheric boundary layer with a height of 50 m and a snow compartment containing up to fty 1 cm thick layers. In the atmosphere and in each snow layer, the model solves a general mass-balance equation for nitrate concentration and isotopic composition (Erbland et al., 2015) at each time step (1 h).

In TRANSITS, nitrate is deposited to the snow surface via dry deposition. Nitrate dry deposition is calculated using the campaign-averaged observed boundary layer mixing ratios for HNO3 (5784 ng m3) and NO3 (5777 ng m3), and an assumed dry-deposition velocity of 0.03 cm s1, which is similar to the dry-deposition velocity used in Edwards et al. (2013, 2014) (0.02 cm s1) (see Table 1B in the Supplement for nitrate dry-deposition uxes). Nitrate diffuses through the snowpack based on a diffusion coefcient that is dependent on temperature, pressure, snow specic surface area, snow density, and tortuosity (Crowley et al., 2010; Durham and Stockburger, 1986; Massman, 1998).

We include only the major channel for the production of Nr from nitrate photolysis (Reaction R1) in TRANSITS. The minor channels, Reactions (R2)(R5), all consist of chemistry of the intermediate in nitrate photolysis, i.e., nitrite, which will photolyze or react rapidly once produced to form Nr. We assume no export of snow-sourced Nr out of the atmospheric box, which is consistent with the low wind speeds and stable boundary layer conditions observed during the campaign. In this way there is no net loss of nitrate from the snow; however, vertical redistribution of snow nitrate

can occur, which would result in distinctive vertical proles of nitrate concentration and 15N(NO3) in the snow column. In addition to calculating the ux of snow-sourced

Nr, TRANSITS calculates vertical proles of nitrate concentration and isotopes ( 15N(NO3)) in the snow. To calculate 15N(NO3) in the snow, the nitrate photolysis fractionation factor (15"pho) is calculated at each time step and is dependent upon the spectral distribution of the UV irradiance at the snow surface (Bernhau et al., 2014; Erbland et al., 2015). Calculated 15"pho values range from 88 to 35 between

the snow pits and are constant with snow depth.

In this study, TRANSITS is run at hourly resolution and is spun up beginning 27 days before the start of the campaign using available atmospheric chemical (boundary layer, gas-phase, and aerosol-phase nitrate) and meteorological data (air, temperature, and pressure). A constant model boundary layer height of 50 m is assumed, which is a rough estimate of daily averaged boundary layer heights based on sodar facsimile data from NOAA. The campaign-averaged observed boundary layer total nitrate (HNO3 + NO3) mix

ing ratio (11.56 g m3) was used to spin up the model. We collected and measured atmospheric 15N(NO3) throughout the campaign using a high-volume air sampler with Nylasorb lters. However, comparison with the aerosol nitrate (NO3) concentration measurements from the PMEL (Pacic

Marine Environmental Laboratory) two-stage, multi-jet cascade impactor measurements revealed incomplete trapping. Since non-quantitative collection of nitrate may inuence the observed 15N(NO3) values, the data were not used in this study. We instead use surface snow 15N(NO3) observations to represent atmospheric 15N(NO3) (Fig. 1a). The TRAN

SITS snowpack is initialized by setting the snow height equal to 50 cm, the snow photic zone to 6 cm (average photic zone depth for all snow pits), and using the measured snow nitrate concentration and 15N(NO3) vertical proles from the rst snow pit of the campaign (15 January). The snowfall event on31 January is simulated in the model, but the other smaller events are not included. As the model evolves, snapshots of the top 25 cm of snow are taken on days corresponding to each snow pit, and modeled proles of nitrate concentration and 15N(NO3) are compared to observed proles for each snow pit. Since vertical proles of snow 15N(NO3)

are highly sensitive to photochemical-driven redistribution of Nr in the snowpack (Erbland et al., 2013, 2015), observed 15N(NO3) provides a metric to assess model-calculated

FNr.

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M. Zatko et al.: Snow-sourced reactive nitrogen ux in the Uintah Basin 13843

(a)

5

presented in the Supplement, Veres et al. (2015) also show decreases in the daily maximum HO2NO2 mixing ratios on 30 and 31 January during UBWOS2014. The decrease in HO2NO2 mixing ratios corresponds to a sharp decrease in snow nitrite concentrations (see Fig. 7 in Veres et al., 2015).

Generally, the surface-snow 15N(NO3) values fall within the range of primary anthropogenic 15N values (425 )

(Felix and Elliott, 2014; Walters et al., 2015). During snow events the boundary layer is less stable, possibly allowing for the transport of nitrate from remote sources outside the basin.In unpolluted, mid-latitude environments, background atmospheric 15N(NO3) ranges from 6 to 2 (Morin et al.,

2009). During the major snowfall event on 3031 January, surface-snow 15N values were 10 lower compared to

the rest of the campaign, suggesting that nitrate from beyond the basin deposits to the snow surface. Two-day NOAA HYSPLIT back trajectories (Rolph, 2016; Stein et al., 2015) show that the air mass on 31 January in the Uintah Basin originated in the Pacic Ocean, which is distinctly different from the other air masses that reached the Uintah Basin during UBWOS2014 (see Supplement, Figs. S4bS15b). Uintah Basin boundary layer air masses typically originated in the inter-mountain western region and often centered over eastern Utah for several days.

3.1.2 Snow depth proles of snow optical properties, nitrate concentrations, and 15N(NO3)

In this section and the following sections, we focus on three snow pits (22, 31 January, and 4 February) as being representative of the time period before, during, and after the largest snow event. The other nine snow pits will not be discussed in detail, but observed and modeled vertical proles of chemical and optical measurements for all 12 snow pits can be found in the Supplement Sect. A.

Figure 2a and b show vertical proles of snow optical properties from an 18 cm deep snow pit dug on 22 January, which represents typical proles from the beginning of the eld campaign until before the rst snow event. Black carbon concentrations (CBC, ng g1) range from 3 to 100 ng g1 with the highest concentrations in the top several centimeters of snow. Below 3 cm snow depth, CBC decreases dramatically. Figure 2b shows the average absorption ngstrm exponent () from = 450600 nm. Over this wavelength

range, the dominant absorber at the snow surface is non-BC material ( is nearly 5), and both BC and non-BC contribute to absorption in sub-surface snow layers ( ranges from 2 to2.7). Although BC and non-BC material are both responsible for the absorption of radiation at = 450600 nm, non-

BC material is responsible for between 99.6 and 100 % of UV ( = 300350 nm) absorption at all depths in this and all

snow pits measured during the eld campaign. The top 3 cm of snow contains the highest concentration of both BC and non-BC material; we dene this layer as the dusty layer and it is represented as a brown shaded region in Fig. 2.

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15

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5000

Figure 1. (a) Mean surface snow (top 1 cm) 15N(NO

3 ) ob

servations () for triplicate measurements from each snow pit (close circles). The full range of triplicate measured surface snow 15N(NO

3 ) for each snow pit is also indicated (vertical black lines). (b) Surface snow nitrate concentration measurements (ng g1) for each snow pit. The uncertainty in the concentration measurements is 0.75 . The vertical blue lines indicate snowfall events.

3 Results and discussion

3.1 Observations

3.1.1 Nitrate concentrations and 15N(NO3) in the surface snow

Figure 1a shows mean surface snow 15N(NO3) values for each snow pit, which range from 5.5 to 11.1 . The low

est observed surface snow 15N(NO3) occurred immediately after the only signicant fresh snowfall event on 3031 January (5.5 ). All other surface snow samples were over

10 higher (5.2 to 11.1 ).

Figure 1b shows surface snow nitrate concentration measurements for each snow pit, which range from 800 to 18 000 ng g1. Similar to 15N(NO3), surface-snow nitrate concentrations are lowest during the snowfall event on 30

31 January, with the exception of 11 February when the snow was rapidly melting. Similarly, boundary layer gas (HNO3)

and aerosol-phase (NO3) nitrate mixing ratios decrease by a factor of 6 between 30 and 31 January (Fig. S1b in the

Supplement) compared to the rest of the eld campaign. In addition to the gas and aerosol phase nitrate mixing ratios

13844 M. Zatko et al.: Snow-sourced reactive nitrogen ux in the Uintah Basin

(a)

(c)

(e)

0 25 50 75 100 125

22 January

(b)

1 2 3 4 5

31 January

CBC (ng g1)

(d)

1 2 3 4 5

4 February

(f)

1 2 3 4 5

0 5 10 15 20 25

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0 25 50 75 100 125

0 25 50 75 100 125

0 5 10 15 20 25

Depth (cm)

ngstrm exponent

Figure 2. Snow optical properties measured on 22 January (left),31 January (middle), and 4 February (right). (top) Vertical proles of mean snow black carbon (CBC, ng g1) measurements and the full range of CBC measured at each depth (horizontal black lines), (bottom) mean ngstrm exponent (, unitless) measurements and the full range of measured at each depth (horizontal black lines). The brown shaded region represents the dusty layer as dened in the text.

Figure 2c and d show vertical proles of snow optical properties from a 14 cm deep snow pit dug on 31 January. It snowed 5 cm between the afternoon of 30 January and the morning of 31 January, and this new snow layer is evident in Fig. 2c and d because the dusty layer is now located roughly 5 cm below the snow surface. Figure 2c shows that CBC ranges from 5 to 100 ng g1; the maximum CBC value has been buried deeper in the snow. Figure 2d shows that is close to 1 at the snow surface, indicating that BC material dominates visible absorption at the snow surface immediately following the fresh snowfall event. Figure 2e and f show vertical proles of snow optical properties from a 24 cm deep snow pit dug on 4 February, 5 days after the snow event. In this snow pit, CBC ranges from 4 to 100 ng g1 and ranges from 1.7 to 3.4. Figure 2e and f show that the original dusty layer is still located roughly 5 cm below the snow surface and that a new dusty layer has formed at the snow surface.

Figure 3ac show observed vertical proles of nitrate in snow from snow pits dug on 22, 31 January, and 4 February. Prior to the fresh snowfall event, snow nitrate concentrations were highest at the surface (13 900 ng g1), and decreased exponentially in the top 10 cm to a low of 90 ng g1 at 18 cm depth (Fig. 3a). Immediately following the fresh snowfall event, the highest nitrate concentrations (12 200 ng g1) are buried below 5 cm of fresh snow within the dusty layer at 5 7 cm depth. The measured nitrate concentrations in the fresh snow layer range from 1280 to 4640 ng g1, which is up to 10 times lower than nitrate concentrations in the dusty layer (Fig. 3b). Five days after the fresh snowfall event, the highest

Figure 3. Measured (black) and modeled ([Phi1] = 4.6 10

3, blue;

[Phi1] = 0.2, red) vertical proles of snow nitrate concentration (top)

and 15N(NO

3 ) (bottom) on 22 January (left), 31 January (center), and 4 February (right). Modeled 15N(NO

3 ) proles are calculated using variable quantum yields ([Phi1] = 4.610

3, blue; [Phi1] = 0.2, red;

[Phi1] = 0, magenta). The brown shaded region represents the dusty

layer.

nitrate concentrations are still located roughly 7 cm below the snow surface within the dusty layer, but surface nitrate concentrations are a factor of 2 higher compared to immediately after the fresh snowfall event (Fig. 3c).

Figure 3df show measured snow 15N(NO3) in each of the snow pits, which ranges from 5.5 to 13 . In the

22 January snow pit, measured 15N(NO3) is highest near the top and bottom of the snow pit and lowest from 12 to 16 cm depth (Fig. 3d). Following the fresh snowfall event on 3031 January, snow 15N(NO3) values are lowest at the snow surface and increase with depth in the fresh snow layer until the top of the dusty layer, below which they decrease to 3.5 (Fig. 3e). Five days after the fresh snowfall event,

measured 15N(NO3) is most enriched in the dusty layer and at the snow surface (Fig. 3f).

The last snowfall event prior to the start of the campaign occurred on 19 December and resulted in roughly 1 cm of snow accumulation (Supplement Fig. S5a). The high concentrations of LAI and nitrate in surface snow on 22 January, combined with the prolonged lack of snowfall, suggest continual dry-deposition of LAI to the surface snow. We speculate that the major source of LAI originates from truck trafc on the dirt roads in the area of the eld site due to high values of (Fig. 2). The factor of 150 and 17 decrease in nitrate and black carbon concentrations, respectively, from the surface to 18 cm depth on 22 January suggests that minimal nitrate and LAI are transported (via, e.g., diffusion or meltwater transport) from upper to lower snow layers. Immediately after the snowfall event on 31 January, nitrate and black carbon concentrations are 10 and 3 times lower, respectively, in the sur-

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M. Zatko et al.: Snow-sourced reactive nitrogen ux in the Uintah Basin 13845

face snow layers compared to earlier in January, because the fresh snow has lower concentrations of these species. Even just ve days after the snowfall event on 3031 January, concentrations of nitrate and the ngstrm exponent () in the snow surface layer have increased by a factor of 2, which is likely due to dry deposition of these species to the surface in the absence of snowfall.

The 15N(NO3) proles in snow do not immediately suggest signicant photolysis-driven redistribution of nitrate in the snowpack, which would result in the lowest values at the surface, increasing exponentially with depth as observed in Antarctica (Erbland et al., 2013). Prior to the rst snowfall event on 3031 January, the surface dusty layer contains the highest values of measured 15N(NO3), which are similar to those expected from primary emission of NOx from anthropogenic sources (Felix and Elliott, 2014; Walters et al., 2015). We speculate that the depleted 15N(NO3) values towards the bottom of the snow pit correspond to remote-sourced atmospheric nitrate that was deposited during the large snow event ( 20 cm of snow) on 4 December. Emis

sions of microbial NO from subniveal soil could also lead to depleted 15N(NO3) if this NO is oxidized to nitrate in the snowpack and deposited to the surface of snow grains before escaping to the atmosphere. However, the depleted 15N(NO3) would also likely correspond with enhanced nitrate concentrations, which is not observed (Fig. 3ac). Additionally, calculations by Zatko et al. (2013) suggest that the lifetime of NOx against oxidation to HNO3 in snow inter-stitial air is long enough so that most NO emitted from soil microbial activity would likely be transported to the atmospheric boundary layer prior to oxidation. On 31 January, depleted 15N(NO3) measurements at the snow surface suggest that there is deposition of nitrate from less polluted regions surrounding the basin during the snow event. The increase in surface snow 15N(NO3) values after 31 January is likely due to deposition of primary-sourced nitrate from anthropogenic NOx sources in the basin. In the following section, we examine the inuence of photolysis of snow nitrate on the proles of 15N(NO3) in snow.

3.2 Calculations

3.2.1 Calculations of snow actinic ux proles and ux of snow-sourced Nr

Figure 4ac show calculated vertical proles of UV actinic ux normalized to surface downwelling irradiance for the three snow pits. On 22 January, the normalized actinic ux ratio is nearly 4 at the snow surface because actinic ux is calculated by integrating irradiance over a sphere (surface area of 4r2) and also because scattering in snow dominates over absorption. In Fig. 4a, the actinic ux decreases to 2.9 within the top centimeter of snow due mainly to UV absorption by non-BC in the surface snow layer. The actinic ux is rapidly extinguished in the dusty layer and contin-

0 1 2 3 4

22 January

31 January

4 February

Depth (cm)

0 5 10 15 20 25

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Actinic flux (normalized)

0 1 2 3 4

January 22

January 31

February 4

FNr = 5.6x108

d

a

= 3.2x108

c

f

0 5 10 15 20 25

FNr = 1.9x108

e

b

Depth (cm)

0 1 2 3 4 x 10

0 1 2 3 4 x 10

FNr(z)

(Molec cm s )

2 1

0 1 2 3 4 x 10

Figure 4. (ac) Modeled vertical proles of UV actinic ux (I, photons cm2 s1) normalized to surface downwelling irradiance (Io, photons cm2 s1). Also presented is measured total UV Io ( = 300350 nm) for a solar zenith angle of 60 on each day.

(df) Modeled vertical proles of snow-sourced Nr uxes (FNr, molec cm2 s1) calculated using Eq. (3). Also shown is total FNr, which is the depth-integrated FNr over the photic zone. The blue shaded region represents the snow photic zone.

ues to decrease with increasing snow depth, reaching a value of 0.01 at 18 cm depth. The blue shaded region represents the snow photic zone (top 5 cm of snow) on 22 January. The snow photic zones calculated in this study (47 cm) are much shallower compared to calculated snow photic zones in polar regions (72207 cm in Antarctica, 651 cm in Greenland) (Zatko et al., 2016) because UV absorption by LAI in the snow photic zone is at least 5 orders of magnitude higher in Utah compared to Antarctica and Greenland.

In the snow pits following the fresh snowfall event, the existence of the dusty layer deeper in the snow inuences the vertical actinic ux prole and increases the photic zone depth from 5 to 7 cm. The fresh snow at the surface contains less LAI compared to the dusty layer. Therefore, actinic ux values are higher in the top several centimeters of snow compared to actinic ux values measured before the snowfall event, even though re values in the new snow are a factor of 3.38.3 times smaller than the underlying depth hoar grains. Smaller re values lead to more scattering in the snow, which increases the probability of absorption by LAI.Although actinic ux values are highest at the surface on31 January, Fig. 4b illustrates that UV radiation is rapidly attenuated below the fresh snow layer because radiation is forward-scattered into the highly absorbing dusty layer. As a result, there is roughly an order of magnitude less actinic ux at 14 cm depth on 31 January compared to 22 January.

The presence of a new dusty layer on the snow surface ve days after the fresh snowfall event does not signicantly alter the vertical prole of normalized UV actinic ux, likely

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13846 M. Zatko et al.: Snow-sourced reactive nitrogen ux in the Uintah Basin

0 0 4 8 12 16 20 24

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2 s1 )

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Figure 5. Modeled diurnal proles of snow-sourced Nr uxes (FNr, molec cm2 s1) calculated using TRANSITS on 22 January (red),31 January (magenta), and 4 February (blue).

because UV absorption by LAI in the surface layer is at least ve times lower than UV absorption by LAI in the original dusty layer (surface snow from 22 January snow pit). Surface snow UV albedo is strongly inuenced by the presence of LAI, and Fig. S2b in the Supplement shows that snow UV albedo is lowest right before the snowfall event on 3031 January and highest immediately afterwards.

We use these actinic ux proles and the observed snow nitrate concentrations (Fig. 3ac) to calculate daily averaged uxes of snow-sourced Nr (molec cm2 s1) at 1 cm depth (z) increments in the snow (FNr(z)), and total uxes of Nr to the boundary layer (FNr) according to Eqs. (2) and (3)

for each of the three snow pits (Fig. 4df). Prior to the fresh snowfall event, FNr(z) decreases exponentially with depth in the photic zone. FNr(z) is highest at the snow surface because that is where both actinic ux and snow nitrate concentrations are highest. Daily average FNr summed over the snow photic zone is 5.6 108 molec cm2 s1 on 22 Jan

uary (Fig. 4d and Table 1). Immediately following the fresh snowfall event, FNr(z) decreases by a factor of 3 at the surface because of the factor of 4 decrease in surface snow nitrate concentrations, which is partially compensated for by the higher UV actinic ux in the top of the snow photic zone (Fig. 4b). The daily averaged FNr on 31 January is1.9 108 molec cm2 s1, which is a factor of 3 lower than

total FNr on 22 January. Five days later, FNr(z) has increased by a factor of 2 at the surface due to the factor of 2 increase in surface nitrate concentrations (Figs. 3c and 4f). The daily averaged FNr on 4 February is 3.2 108 molec cm2 s1,

which is a factor of 1.7 higher than total FNr on 31 January.

Figure 6. Modeled snow-sourced Nr uxes (molec cm2 s1) for each hour during the campaign from 15 January to 11 February.

3.2.2 Snow photochemistry column model

The snow chemistry column model is used to calculate the time-dependent ux of snow-sourced Nr (FNr) and the depth prole of nitrate concentration and 15N(NO3). Figure 5 shows the diurnal FNr values on 22, 31 January, and4 February. The daily averaged snow FNr on 22 January is6.3 108 molec cm2 s1. Immediately following the snow

event, the daily averaged snow FNr decreases by a factor of 11 compared to 22 January (5.6 107 molec cm2 s1). The

dramatic difference in FNr is due to the differences in nitrate concentrations in the top several centimeters of snow. Modeled snow nitrate concentrations in the fresh snow layer on31 January are between 30 and 300 times lower compared to nitrate concentrations in the dusty layer. Five days after the snow event, the daily averaged snow FNr has increased by a factor of 2 (1.2 108 molec cm2 s1) because depo

sition of nitrate to the snow surface layer enhances surface nitrate concentrations and thus FNr. Calculated daily average

FNr using observed (Sect. 3.2.1) and modeled (TRANSITS) snow nitrate concentrations agree within a factor of 2 (Ta

ble 1); modeled FNr tends to be lower because modeled snow nitrate concentrations are lower than observed (Fig. 3).

Figure 6 shows hourly FNr values calculated for the entire UBWOS2014 campaign using TRANSITS. From the start of the campaign until the fresh snow event on 31 January, the daily maximum FNr values increase as surface snow nitrate concentrations increase due to continual dry-deposition of atmospheric nitrate to the snow surface. Immediately after the snow event on 31 January, daily maximum FNr values are lowered by more than a factor of 10 due to decreased nitrate concentrations in the snow photic zone. Following the snow event, the ux of snow-sourced Nr gradually increases again due to dry-deposition of nitrate to the surface layer, although daily maximum FNr values remain lower throughout the remainder of the eld campaign compared to values before the snow event.

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Vernal local time (UTC+7)

M. Zatko et al.: Snow-sourced reactive nitrogen ux in the Uintah Basin 13847

Table 1. Snow photic zone depth and daily averaged modeled FNr calculated using Eq. (3) and the TRANSITS model on 22, 31 January, and4 February.

Pit date Photic zone Daily-averaged FNr depth (cm) (molec cm2 s1)

Eq. (3) TRANSITS TRANSITS( = 4.6 10

3) ( = 0.2)

22 January 5.0 5.6 10

8 6.3 10

8 2.9 10

10

31 January 7.0 1.9 10

8 5.6 10

7 2.7 10

9

4 February 7.0 3.2 10

8 1.2 10

8 5.6 10

9

at all depths. Despite the large nitrogen isotope fractionation (" = 88 to 35 ) resulting from the photolysis of snow

nitrate, the difference in modeled 15N(NO3) when snow nitrate is turned on ( = 4.6 103) and off ( = 0) is small

because of the very small fraction of nitrate photolyzed.

In another sensitivity study, we calculate the maximum possible FNr in the Uintah Basin by increasing the value of

until modeled snow 15N(NO3) falls outside the full range of observations. Above = 0.2, there is signicant disagree

ment (when the maximum change in 15N(NO3) is > 1 of the mean 15N(NO3) in all snow pits) between modeled and measured 15N(NO3) values. Using = 0.2 results in more

enriched 15N(NO3) at depth due to enhanced photolytic loss, and more depleted 15N(NO3) at the snow surface due to the deposition of isotopically light snow-sourced nitrate.

Using = 0.2 results in a maximum possible FNr at least

45 times larger than when using = 4.6 103 for all snow

pits (see Table 1).

4 Impact of snow-sourced Nr on the boundary layer reactive nitrogen budget

4.1 NOx

We rst assume that all Nr is NOx and use FNr values calculated using the snow photochemistry column model to estimate the impact of FNOx on the NOx budget in the Uintah Basin. Using the best estimate for the quantum yield of nitrate photolysis ( = 4.6 103), the modeled daily av

eraged ux of snow-sourced NOx ranges from 5.6 107

to 7.2 108 molec cm2 s1 and the maximum FNr value

is 3.1 109 molec cm2 s1 for the entire campaign (Sup

plement Table S4b). The top-down NOx emission inventory for oil, gas, and all other sources, excluding the Bonanza power plant in Duchesne and Uintah counties, is 6.5

106 kg NOx year1 (Ahmadov et al., 2015). The power plant is excluded because its emissions occur above the boundary layer due to the plumes positive buoyancy. Assuming a constant NOx emission rate and using the area of Duchesne (8433 km2) and Uintah counties (11 658 km2), the top-down NOx emission estimate for the Uintah and Duchesne coun-

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Figure 3 shows modeled snow nitrate concentrations and 15N(NO3) from TRANSITS compared to the observations.

The general shapes of the modeled and measured vertical proles of nitrate concentration are in agreement for all three snow pits; both modeled and measured nitrate concentrations are highest in the dusty layer and lowest near the bottom of the snow pit (Fig. 3ac). Both the model and the observations show increased snow nitrate concentrations at the surface following the fresh snowfall event, but the model tends to underestimate surface snow nitrate concentrations after the snow event.

Modeled 15N(NO3) is also within the range of observations (Fig. 3df). Modeled 15N(NO3) at the top surface snow layer becomes more depleted from the 22 to31 January snow pit, reecting the decrease in atmospheric 15N(NO3) in the model based on surface snow observations (Fig. 1a). Without additional snowfall between 31 January and 4 February, surface snow 15N(NO3) becomes more enriched in the model during this time because model atmospheric 15N(NO3) becomes more enriched (Fig. 1a). In contrast, the observations retain this low 15N(NO3) at a depth of 2 cm until the 11 February snow pit (see Sup

plement A). The difference between modeled and observed 15N(NO3) at 2 cm depth after 31 January may be due to the redistribution of surface snow by wind, and the fact that each snow pit was dug in a slightly different location. Blowing snow will bury the surface snow with low 15N(NO3), and subsequent atmospheric deposition of more enriched 15N(NO3) will occur onto this new, wind-blown snow surface, retaining the light 15N(NO3) at 2 cm depth. In contrast to the observations, the model does not account for wind-blown redistribution of snow and calculates the time evolution of nitrate concentration and 15N(NO3) gradients of a single snow pit.

To examine the sensitivity of snow nitrate to photolysis, we turn off photolysis of snow nitrate in the model by setting = 0. When snow nitrate photolysis is turned off, snow

nitrate concentrations change by less than 0.5 % in all snow pits, resulting in relatively little sensitivity of modeled snow nitrate concentration to snow photochemistry because only this small fraction (< 0.5 %) of nitrate is lost via photolysis

13848 M. Zatko et al.: Snow-sourced reactive nitrogen ux in the Uintah Basin

ties is 1.2 1012 molec cm2 s1. The emission of primary

NOx in these two counties is thus at least 300 times higher than the estimated snow NOx emissions, implying that snow-sourced NOx uxes likely do not inuence the NOx boundary layer budget in the highly polluted Uintah Basin. If the upper limit of = 0.2 is used, snow-sourced NOx emissions

are still at least seven times smaller than primary NOx emissions. Although reactive nitrogen is likely being emitted from the snow into the boundary layer, the snow-sourced NOx signal is swamped by emissions from primary anthropogenic sources in the Uintah Basin.

4.2 HONO

Only the major channel for snow nitrate photolysis (Reaction R1) is simulated in the TRANSITS model, although nitrate can also photolyze via Reaction (R2) and form both NOx and HONO (Reactions R3R5). The surface snow pH ranged from 2 to 4 during the campaign (see Fig. S3a in the Supplement), which is low enough to enable direct volatilization of HONO from the snow. We estimate the maximum possible inuence of the snow-photolytic source of boundary layer HONO by assuming that all snow-sourced Nr is in the form of HONO. If we assume that the campaign-maximum FNr value (3.1109 molec cm2 s1) is all HONO

that escapes from the snow into the boundary layer, with a layer height of 50 m and a lifetime of HONO of 18 min (at solar noon) (Edwards et al., 2013), snow nitrate photolysis would contribute a maximum of 25 pptv of HONO to the boundary layer at solar noon. The modeled and observed Uintah Basin boundary layer HONO mixing ratios presented in Edwards et al. (2014) range from 20 pptv at night to

up to 150 pptv during the day, which suggests that the daytime uxes of reactive nitrogen are not a signicant source of HONO to the boundary layer compared to other HONO sources in the basin. Our estimated maximum HONO ux is comparable to snow-sourced HONO uxes measured at another polluted, mid-latitude location (Paris, France), estimates of which ranged from 0.7 to 3.11010 molec cm2 s1

(assuming a snow density of 0.36 g cm3 and snow photic zone depth of 6 cm) (Michoud et al., 2015). If the upper limit of = 0.2 is used (campaign-maximum FNr = 1.4

1011 molec cm2 s1), the maximum boundary layer HONO mixing ratio calculated using this approach is 1.1 ppbv at solar noon, which would signicantly impact boundary layer HONO mixing ratios in the Uintah Basin. Given that HONO is thought to be only a minor fraction of total Nr emitted from snow (Beine et al., 2008), we consider this to be an overestimate.

5 Conclusions

This study estimates the inuence of snow nitrate photolysis on the boundary layer reactive nitrogen (Nr) budget in

the Uintah Basin, which is a region with heavy oil and natural gas extraction processes. Observations of snow optical properties, including ultraviolet (UV) light-absorbing impurities (e.g., black carbon, dust, and organics), radiation equivalent ice grain radii, and snow density from 12 snow pits measured during the Uintah Basin Winter Ozone Study (UBWOS) 2014 are incorporated into a snowpack radiative transfer model to calculate vertical proles of UV actinic ux in 12 snow pits dug during the campaign. The calculated UV actinic ux proles along with measurements of nitrate concentration are used to calculate snow-sourced Nr uxes associated with snow nitrate photolysis using both a simple Eq. (3) and a more complex snow photochemistry column model, which yield similar results. Snow nitrate photolysis in the column model is constrained by 1 cm depth-resolved observations of 15N(NO3) in the snow pits, which is highly sensitive to UV photolysis (Erbland et al., 2015).

The daily averaged ux snow-sourced Nr (FNr) to the boundary layer ranges from 5.6 107 to 7.2

108 molec cm2 s1 and the modeled campaign-maximum FNr is 3.1109 molec cm2 s1. The top-down emission es

timate of primary NOx in Uintah and Duchesne counties reported in Ahmadov et al. (2015) is at least 300 times higher than estimated snow NOx emissions, assuming that all Nr is emitted as NOx. This suggests that snow-sourced NOx uxes likely have little inuence on the boundary layer NOx budget in the highly polluted Uintah Basin. Assuming that all Nr is emitted as HONO also suggests that the snow-sourced reactive nitrogen uxes associated with snow nitrate photolysis do not signicantly contribute to boundary layer HONO mixing ratios in the Uintah Basin. The relative importance of the ux of NOx and HONO will inuence the impact of the recycling of Nr in snow on the chemistry of the boundary layer in snow-covered regions but is unknown. Knowledge of the chemical speciation of snow-source Nr is required for a better understanding of the full impact of snow on local oxidant budgets. However, in the Uintah Basin, we conclude that air quality models can safely neglect the recycling of reactive nitrogen in snow when identifying the most effective strategies for reducing wintertime ozone abundances.

6 Data availability

Our eld and laboratory measurements are permanently archived in the University of Washington Libraries ResearchWorks Archive: https://digital.lib.washington.edu/researchworks/handle/1773/37311

Web End =https://digital.lib.washington.edu/ researchworks/handle/1773/37311.

The Supplement related to this article is available online at http://dx.doi.org/10.5194/acp-16-13837-2016-supplement

Web End =doi:10.5194/acp-16-13837-2016-supplement .

Atmos. Chem. Phys., 16, 1383713851, 2016 www.atmos-chem-phys.net/16/13837/2016/

M. Zatko et al.: Snow-sourced reactive nitrogen ux in the Uintah Basin 13849

Acknowledgements. We gratefully acknowledge support from 155 backers from http://www.experiment.com

Web End =http://www.experiment.com , NSF PLR 1244817, PMEL contribution number 4468, and an EPA STAR fellowship to M. C. Zatko. The Uintah Basin Winter Ozone Studies were a collaborative project led and coordinated by the Utah Department of Environmental Quality (UDEQ) with support from the Uintah Impact Mitigation Special Service District (UIMSSD), the Bureau of Land Management (BLM), the Environmental Protection Agency (EPA), and Utah State University. The authors acknowledge the NOAA/ESRL Chemical Sciences Division and Questar Energy Products for site preparation and support.We thank Kristen Shultz, Jim Johnson, Drew Hamilton, and Derek Coffman for all of their help before, during, and after the eld campaign. We would also like to thank Dean Hegg for advice on aerosol sampling, Angela Hong and Jennifer Murphy for helpful discussions about NOy vertical gradients, and Chad Mangum for laboratory assistance at USU. We thank Sarah Doherty for the use of the ISSW spectrophotometer and Stephen Warren for graciously allowing M. C. Zatko to borrow snow sampling instruments and gear and providing comments about this work. We thank Jonathan Raff for helpful discussions about soil microbial activity as well as Joost de Gouw and Gail Tonnesen for useful discussions about boundary layer HONO. Finally, we thank Lyatt Jaegl, Joel Thornton, and Thomas Grenfell for their helpful comments.Joel Savarino and Joseph Erbland have been partly supported by a grant from Labex OSUG@2020 (Investissements davenir ANR10 LABX56) during the development of the TRANSITS model.

Edited by: P. B. ShepsonReviewed by: two anonymous referees

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