# On the origin of the occasional spring nitrate peak in Greenland snow

L. Geng, J. Cole-Dai, B. Alexander, J. Erbland, Joel Savarino, A. Schauer, E.

Steig, P. Lin, Q. Fu, M. Zatko

► To cite this version:

L. Geng, J. Cole-Dai, B. Alexander, J. Erbland, Joel Savarino, et al. On the origin of the occasional spring nitrate peak in Greenland snow. Atmospheric Chemistry and Physics, 2014, 14 (24), pp.13361-13376. 10.5194/acp-14-13361-2014. hal-04765516

HAL Id: hal-04765516

https://hal.science/hal-04765516v1

Submitted on 8 Nov 2024

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

# On the origin of the occasional spring nitrate peak in Greenland snow

L. Geng<fnref n="1" />,*, J. Cole-Dai<fnref n="1" />, B. Alexander<fnref n="2" />, J. Erbland<fnref n="3" />,<fnref n="4" />, J. Savarino<fnref n="3" />,<fnref n="4" />, A. J. Schauer<fnref n="5" />, E. J. Steig<fnref n="5" />, P. Lin<fnref n="2" />,**, Q. Fu<fnref n="2" />,
and M. C. Zatko<fnref n="2" />

*Correspondence to:* L. Geng (leigeng@uw.edu)

Received: 20 January 2014 – Published in Atmos. Chem. Phys. Discuss.: 7 April 2014

Revised: 13 November 2014 – Accepted: 13 November 2014 – Published: 16 December 2014

**Abstract.** Ice core nitrate concentrations peak in the summer in both Greenland and Antarctica. Two nitrate concentration peaks in one annual layer have been observed some years in ice cores in Greenland from samples dating post-1900, with the additional nitrate peak occurring in the spring. The origin of the spring nitrate peak was hypothesized to be pollution transport from the mid-latitudes in the industrial era. We performed a case study on the origin of a spring nitrate peak in 2005 measured from a snowpit at Summit, Greenland, covering 3 years of snow accumulation. The effect of long-range transport of nitrate on this spring peak was excluded by using sulfate as a pollution tracer. The isotopic composition of nitrate (δ¹⁵N, δ¹⁸O and Δ¹⁷O) combined with photochemical calculations suggest that the occurrence of this spring peak is linked to a significantly weakened stratospheric ozone (O₃) layer. The weakened O₃ layer resulted in elevated UVB (ultraviolet-B) radiation on the snow surface, where the production of OH and NOₓ from the photolysis of their precursors was enhanced. Elevated NOₓ and OH concentrations resulted in enhanced nitrate production mainly through the NO₂ + OH formation pathway, as indicated by decreases in δ¹⁸O and Δ¹⁷O of nitrate associated with the spring peak. We further examined the nitrate concentration record from a shallow ice core covering the period from 1772 to 2006 and found 19 years with double nitrate peaks after the 1950s. Out

of these 19 years, 14 of the secondary nitrate peaks were accompanied by sulfate peaks, suggesting long-range transport of nitrate as their source. In the other 5 years, low springtime O₃ column density was observed, suggesting enhanced local production of nitrate as their source. The results suggest that, in addition to direct transport of nitrate from polluted regions, enhanced local photochemistry can also lead to a spring nitrate peak. The enhanced local photochemistry is probably associated with the interannual variability of O₃ column density in the Arctic, which leads to elevated surface UV radiation in some years. In this scenario, enhanced photochemistry caused increased local nitrate production under the condition of elevated local NOₓ abundance in the industrial era.

## 1 Introduction

Knowledge of the abundance and variability of reactive nitrogen oxides (NOₓ = NO + NO₂) is valuable because of the critical role that NOₓ plays in determining the oxidative capacity of the atmosphere. The oxidative capacity of the atmosphere is determined by the tropospheric abundance of hydrogen oxide radicals (HOₓ = OH + HO₂) and O₃ and largely controls the residence times of pollutants (e.g., CO)

<footnote n="1">¹Department of Chemistry & Biochemistry, South Dakota State University, Brookings, SD, USA</footnote>

<footnote n="2">²Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA</footnote>

<footnote n="3">³CNRS, LGGE (UMR5183), 38041 Grenoble, France</footnote>

<footnote n="4">⁴Université Grenoble Alpes, LGGE (UMR5183), 38041 Grenoble, France</footnote>

<footnote n="5">⁵Department of Earth and Space Sciences, University of Washington, Seattle, WA, USA</footnote>

<footnote>* now at: Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA</footnote>

<footnote>** now at: Program in Atmospheric and Oceanic Sciences/GFDL, Princeton University, Princeton, NJ, USA</footnote>
and greenhouse gases (e.g., CH$_{4}$). NO$_{x}$ is emitted from a variety of sources including fossil fuel combustion, biomass burning, soil emissions and lightning (Logan, 1983). In the atmosphere, NO$_{x}$ cycles rapidly between NO and NO$_{2}$ according to:

$$NO + O_3 \rightarrow NO_2 + O_2, \qquad (R1)$$

$$NO + HO_2(RO_2) \rightarrow NO_2 + OH(RO), \qquad (R2)$$

$$NO_2 + hv \xrightarrow{O_2} NO + O_3. \qquad (R3)$$

The main sink of NO$_{x}$ is thought to be atmospheric oxidation to nitrate which distributes between the gas and particulate phases. In general, the formation of nitrate involves reactions of NO$_{x}$ with OH and O$_{3}$:

$$NO_2 + OH \rightarrow HNO_3, \qquad (R4)$$

$$NO_2 + O_3 \rightarrow NO_3 + O_2, \qquad (R5)$$

$$NO_3 + DMS/HC \rightarrow HNO_3 + \text{products}, \qquad (R6)$$

$$NO_3 + NO_2 \rightarrow N_2O_5, \qquad (R7)$$

$$N_2O_5 + H_2O_{(aq)} \rightarrow 2HNO_3_{(aq)}. \qquad (R8)$$

Reactive halogen species (e.g., BrO) may also play a role in NO$_{x}$ cycling and nitrate formation, especially in the Arctic (Morin et al., 2007). R4 mainly occurs during daytime as the result of diurnal variations in OH concentrations. The conversion to nitrate via O$_{3}$ (R5–R8) is negligible during the day, since the NO$_{3}$ radical is rapidly photolyzed back to NO$_{x}$ in sunlight. Globally, oxidation of NO$_{x}$ by OH (R4) is thought to be the dominant nitrate formation pathway on an annual basis (Alexander et al., 2009). Once formed, nitrate is removed from the atmosphere mainly through wet and dry deposition into the surface. Efforts have been made to use ice core nitrate records to assess information about past changes in the global NO$_{x}$ environment (Fischer et al., 1998; Mayewski et al., 1990) and potentially about the past variations in the atmospheric oxidation capacity (Alexander et al., 2004). However, nitrate concentration in polar snow is also influenced by variations in snow accumulation rate and by post-depositional processes such as evaporation and photolytic (Röthlisberger et al., 2002) of nitrate in near-surface snow. Post-depositional processing of snowpack nitrate has hampered the interpretation of ice core nitrate records in terms of assessing past atmospheric NO$_{x}$ variability.

Examination of detailed chronological nitrate records in snowpits and ice cores may provide clues to the primary factor(s) controlling nitrate concentration and its temporal variability in snow. For example, a clear seasonal pattern in nitrate concentration may be linked to a source with an annual cycle and/or modulated by seasonally varying atmospheric conditions. Many previous studies of snow samples and ice cores from central Greenland have found that nitrate concentration reaches a maximum in summer snow and a minimum in winter snow (Davidson et al., 1989; Finkel and Langway, 1986; Whitlow et al., 1992). This seasonal variation in nitrate concentration in general is due to the active photochemical

reactions during polar summer (Yang et al., 1995), when peroxyacetyl nitrate (PAN), snowpack nitrate and other NO$_{x}$ reservoir species act as local NO$_{x}$ sources through recycling reactions (Moxim et al., 1996; Thomas et al., 2012). A few investigators (Burkhart et al., 2006; Yang et al., 1995; Whitlow et al., 1992) noticed that a single nitrate peak appears annually in summer snow/ice layer deposited prior to 1900, whereas two nitrate peaks are seen in some post-1900 annual layers. Although it seems the timing of the additional nitrate peak differs, as observed in ice cores from different locations (Burkhart et al., 2006), Yang et al. (1995) stated that at Summit the additional peak occurs in late winter/early spring, which is consistent with the dating in Burkhart et al. (2006).

The occurrence of this occasional spring nitrate peak has been hypothesized to be associated with pollution transport from the mid-latitudes (Burkhart et al., 2006; Yang et al., 1995), though this hypothesis and the occasional nature of the spring nitrate peak were not carefully examined. Anthropogenic NO$_{x}$ emissions from fossil fuel combustion have increased the concentrations of atmospheric NO$_{x}$ and other nitrogen species (e.g., HNO$_{3}$) in the northern hemisphere since 1900, especially in the last 50 years (Galloway et al., 2003). A corresponding increase in nitrate concentrations in Greenland ice cores has been documented (Mayewski et al., 1990). Direct transport of NO$_{x}$ from the mid-latitudes to the Arctic is unlikely given the short lifetime of NO$_{x}$ (1–3 days; Levy et al., 1999). Alternatively, long-range transport of nitrate and NO$_{x}$ precursors (e.g., PAN) from anthropogenic NO$_{x}$ source regions could lead to increases in snow nitrate concentrations in Greenland. However, the relative importance of nitrate and NO$_{x}$ precursors to Greenland snow nitrate is unclear. An earlier study suggested a significant contribution from PAN (e.g., Ford et al., 2002), whereas a recent study (Geng et al., 2014) suggests that transport to Greenland occurs mainly in the form of gaseous HNO$_{3}$. In addition to PAN decomposition as a potential local source of NO$_{x}$, the photolysis of nitrate in snowpack produces NO$_{x}$, which is quickly transported to the overlying atmosphere by diffusion and wind pumping (Zatko et al., 2013). This is consistent with the elevated NO$_{x}$ concentrations observed in the ice sheets during polar summers (Honrath et al., 1999). In particular, a model study (Thomas et al., 2012) suggested that at Summit, NO$_{x}$ produced from snow nitrate photolysis in summer can account for all of the observed NO$_{x}$ concentrations in the overlying atmosphere. In addition, Kramer et al. (2014) implied that, after polar sunrise, atmospheric NO$_{y}$ (odd nitrogen species other than PAN and NO$_{x}$) can release NO$_{x}$ through photochemistry. In summary, there are two ways that pollution transport can influence the springtime nitrate budget at Summit: (1) direct transport of nitrate and (2) transport of NO$_{x}$ precursors. In order to cause a spring nitrate peak, the latter requires local conditions favoring NO$_{x}$ release from its precursors and its subsequent conversion to nitrate.

The isotopic composition of nitrate can provide valuable information not available from concentration measurements.
ment alone, regarding, for example, the pathways of NOₓ
conversion to nitrate in the atmosphere (Michalski et al.,
2003). Stable isotope ratios in nitrate are expressed as δ¹⁵N,
δ¹⁸O and Δ¹⁷O, where Δ¹⁷O = δ¹⁷O – 0.52 × δ¹⁸O and
δ = R<sub>sample</sub>/R<sub>reference</sub> – 1 (with R denoting the ¹⁵N/¹⁴N,
¹⁸O/¹⁶O and ¹⁷O/¹⁶O isotope ratios). The references are N₂-
AIR and Vienna Standard Mean Ocean Water (VSMOW)
for N and O, respectively. δ¹⁵N and Δ¹⁷O of nitrate (here-
after denoted as δ¹⁵N(NO₃⁻) and Δ¹⁷O(NO₃⁻), respectively)
have been used to investigate the origin and fate of NOₓ
in the Arctic troposphere (Morin et al., 2008). In particu-
lar, Δ¹⁷O(NO₃⁻) is related to oxidizing conditions and ni-
trate formation pathways (R4 vs. R5–R8) in the atmosphere
(Alexander et al., 2004; Alexander et al., 2009; Kunasek et
al., 2008; Michalski et al., 2003; Morin et al., 2011). For
example, Michalski et al. (2003) found that nitrate formed
during nighttime (i.e., via R5-R8) has higher Δ¹⁷O than ni-
trate formed during daytime (via R4) because of the high
Δ¹⁷O of the dominant nighttime oxidant O₃ (Johnson et
al., 2000; Lyons, 2001) which is transferred to nitrate. The
Δ¹⁷O(NO₃⁻) in Greenland snow also reflects the seasonality
of nitrate production, with O₃ oxidation (R1, R5) being more
important in winter than in summer (Kunasek et al., 2008).

After atmospheric nitrate is deposited in the snow, UV
photolysis will convert snow nitrate back to NOx, which
is then released into the atmosphere (Honrath et al., 1999;
Thomas et al., 2012; Zatko et al., 2013). This snow-sourced
NOx will be re-oxidized in the atmosphere into nitrate,
which is subsequently re-deposited into surface snow or
transported away. Reformation of nitrate in the condensed
phase of snow grains can also occur if the nitrate being pho-
tolyzed is trapped inside the snow grain instead of on the
surface (Meusinger et al., 2014). This is the so-called post-
depositional processing of snow nitrate, which includes the
steps of photolysis, recombination of photoproducts in the
condensed phase and in the overlying atmosphere (i.e., recy-
cling), and the re-deposition and/or export of snow-sourced
nitrate. Post-depositional processing induces isotopic effects
(Frey et al., 2009). In general, the photolysis of nitrate will
enrich δ¹⁵N and δ¹⁸O of nitrate remaining in snow, but
will not alter Δ¹⁷O as photolysis induces mass-dependent
fractionation. If the photolytic products of nitrate are re-
oxidized and re-deposited into the snow surface (i.e., no ex-
pert/no net loss), no measurable effect on δ¹⁵N(NO₃⁻) will
be observed, assuming negligible nitrogen isotopic fraction-
ation during the re-formation of nitrate. However, changes in
δ¹⁸O(NO₃⁻) and Δ¹⁷O(NO₃⁻) are expected even without net
loss of nitrate. The reformation of nitrate in the condensed
phase of snow grains will cause oxygen isotope exchange
with water (Frey et al., 2009), which lowers δ¹⁸O(NO₃⁻)
and Δ¹⁷O(NO₃⁻). In addition, the re-oxidation of the snow-
sourced NOx to nitrate in the overlying atmosphere will oc-
cur mainly through the OH formation pathway. This is be-
cause active snow nitrate photolysis requires strong radia-

tion, which is also when atmospheric nitrate is mainly formed
through the daytime reaction channel (NOₓ + OH). Recy-
cling of snow-sourced NOₓ in the atmosphere will thus likely
lower δ¹⁸O(NO₃⁻) and Δ¹⁷O(NO₃⁻), particularly if the ini-
tially deposited nitrate was formed through the O₃ oxida-
tion pathway (e.g., formed in polar winters when δ¹⁸O(NO₃⁻)
and Δ¹⁷O(NO₃⁻) values are the highest; Alexander et al.,
2009; Kunasek et al., 2008). The deposition of this re-
formed nitrate will then lower the bulk snow δ¹⁸O(NO₃⁻)
and Δ¹⁷O(NO₃⁻). At sites with snow accumulation rates
greater than 100 kg m⁻²/a, the oxygen isotope exchange in
the condensed phase is minimal (Erbland et al., 2013). This
is consistent with the discussion in Fibiger et al. (2013)
that the condensed phase process cannot explain the ob-
served relationship between δ¹⁸O(NO₃⁻) and Δ¹⁷O(NO₃⁻)
in surface snow given the high snow accumulation rate at
Summit (233 ± 38 (1σ) kg/m²/a during 2000–2006; Cole-
Dai et al., 2013). However, the Fibiger et al. (2013) anal-
ysis neglected atmospheric process, i.e., the reformation of
nitrate from snow-sourced NOₓ in the atmosphere and its
subsequent deposition in surface snow. This process alters
δ¹⁸O(NO₃⁻) and Δ¹⁷O(NO₃⁻) simultaneously and can ex-
plain the relationship between δ¹⁸O(NO₃⁻) and Δ¹⁷O(NO₃⁻)
in surface snow observed by Fibiger et al. (2013). In addition,
Fibiger et al. (2013) suggested that there is little to no post-
depositional loss of nitrate at Summit, which might be true.
However, post-depositional loss is only one (possible) step of
the post-depositional processing. Post-depositional process-
ing of snow nitrate at Summit could be very active while lit-
tle post-depositional loss occurs, given a fast recycling rate
of nitrate in the air-snow interface.

In this study, the concentrations of major ions (Na⁺, NH₄⁺,
K⁺, Mg²⁺, Ca²⁺, Cl⁻, NO₃⁻ and SO₄²⁻) and the isotopic
composition of nitrate in a snowpit from central Greenland
were measured. The concentration data were used to estab-
lish their temporal patterns and to identify any spring nitrate
peak(s). The isotopic data were used to assess the chemistry
of nitrate in any identified spring peaks and to discern the
origin of the peak. Further examinations were conducted on
a shallow ice core to determine whether the mechanism lead-
ing to the spring nitrate peak observed in the snowpit is repre-
sentative of the occasional nature of the spring nitrate peaks
observed in Greenland snow since 1900.

2 Methods

## 2.1 Snowpit and ice core sample collection

In July 2007, six snow blocks (dimensions: 0.35 m
long × 0.25 m wide × 0.35 m deep) were excavated from the
surface down to a depth of 2.1 m at Summit, Greenland
(72.5° N, 38.5° W; elevation: 3200 m). These snow blocks
were wrapped with clean polyester film with their dimen-
sions labeled and stored in a hard-shell box to avoid exter-
nal compaction during delivery. Several ice cores, includ-
ing a 79 m shallow core, were drilled approximately 100 m
from the location of the snow blocks. These snow blocks and
the ice cores were shipped frozen to the laboratory at South
Dakota State University (SDSU) and stored in a freezer at
-20 °N until analysis. At the same time, a set of snowpit
samples were also collected in the field every 5 cm from the
surface down to the depth of 2.10 m, at the same location of
the collected snow blocks. This set of snowpit samples (re-
ferred to as SP-1) were double sealed in clean plastic bags
and shipped frozen to Laboratoire de Glaciologie et Géo-
physique de l'Environnement (LGGE) for nitrate concentra-
tion and isotope analysis. All tools/containers directly touch-
ing the snowpit samples and/or the snow blocks were pre-
cleaned with 18 MΩ water before use.

The density profile of the snow blocks were also measured
in the field. In particular, a small snow cube was collected ev-
ery 5 cm from the wall of a snowpit. The weight and the vol-
ume of the snow cube were measured and then the density of
the snow was calculated. The snow density profile from the
surface to the depth of 2.1 m was included as supplemental
data.

## 2.2 Chemical and isotope analysis

At SDSU, another set of snow samples (referred to as SP-2)
was, after the removal of a surface layer of at least 1 cm in
thickness, chiseled out from the snow blocks in a vertical se-
quence and collected in clean plastic sample containers. All
tools and sample containers were pre-cleaned with 18 MΩ
water prior to use, and clean disposable plastic gloves were
always worn during sample handling. In total, 71 samples
with a depth resolution of 3 cm were obtained. These samples
were allowed to melt at room temperature and the meltwater
samples were then analyzed by ion chromatography for con-
centrations of major ions in snow (Na+, NH₄⁺, K+, Mg²⁺,
Ca²⁺, Cl⁻, NO₃⁻ and SO₄²⁻). The ion chromatography in-
strumentation and methodology are similar to that described
by Cole-Dai et al. (2006). Ice blanks were made from deion-
ized water and processed and analyzed following the same
procedure as that for the snow samples. High blank values
of Ca²⁺ (up to 5 µg L⁻¹) indicated that the snowpit samples
were probably contaminated with Ca²⁺ during sample prepa-
ration. No evidence of contamination was found for the other
ions. Therefore, the snowpit Ca²⁺ data were not used in the
interpretation in this study. The 79 m shallow core was an-
alyzed for the ionic species using the technique of continu-
ous flow analysis with ion chromatography (CFA-IC) detec-
tion (Cole-Dai et al., 2006). One advantage of the CFA-IC
technique is that it minimizes the potential contamination by
eliminating sample preparation. Replicate analysis of blanks
with CFA-IC showed no contamination of any of the ions,
including Ca²⁺.

The concentration measurements of the SP-2 samples de-
scribed above were used to estimate the volume of meltwater

needed to provide sufficient nitrate mass (500 nmoles, allow-
ing for replicate measurements) for isotopic analysis. Based
on that estimate, the snow blocks were carved vertically to
yield large samples for isotope measurement. A total of 29
clean snow samples (referred to as SB) were obtained from
the depth interval of 0.7 to 1.75 m within the snow blocks
(snow outside this depth interval had been used for other pur-
poses). The depth resolution of these samples varies from 2 to
6 cm because the concentration of nitrate is different at differ-
ent depths. These samples were melted at room temperature,
concentrated to 10 mL solutions following the method de-
scribed by Frey et al. (2009), collected in HDPE bottles and
stored frozen until isotopic analysis. Measurements of N and
O isotope ratios in nitrate of the SB samples were performed
in the stable isotope laboratory at University of Washing-
ton (UW) using the bacterial denitrifier method (Kaiser et
al., 2007). Briefly, nitrate in the samples was first converted
into N₂O gas by the bacteria *Pseudomonas aureofaciens*;
the N₂O was carried on-line by helium gas into a heated
gold tube where it was thermally decomposed to N₂ and O₂.
These were then separated by gas chromatography, and the
isotopic ratio(s) of each gas (¹⁵N/¹⁴N for N₂, ¹⁸O/¹⁶O and
¹⁷O/¹⁶O for O₂) was measured with an isotope ratio mass
spectrometer. The δ¹⁵N values were calculated with respect
to N₂-AIR and calibrated against the two international ref-
erence materials IAEA-NO-3 (δ¹⁵N = 4.7‰) and USGS34
(δ¹⁵N = -1.8‰) (Kaiser et al., 2007). The δ¹⁷O and δ¹⁸O
values were calculated with respect to VSMOW and cal-
ibrated against the two international reference materials
USGS34 (δ¹⁷O = -14.5‰, δ¹⁸O = -27.9‰) and USGS35
(δ¹⁷O = 51.3‰, δ¹⁸O = 57.5‰) (Kaiser et al., 2007). The
Δ¹⁷O values were then calculated by using the linear equa-
tion Δ¹⁷O = δ¹⁷O - 0.52 × δ¹⁸O. No replicate samples were
possible due to the limited amount of snow available in the
snow blocks, and no seasonally resolved isotopic measure-
ments of the ice core samples were preformed because of the
limited amount of ice available. The analytical uncertainty of
Δ¹⁷O(NO₃⁻) and δ¹⁸O(NO₃⁻) measured at the UW laboratory
was estimated to be 0.1‰ and 0.5‰ (1σ), respectively; that
of δ¹⁵N was 1.0‰ (1σ), based on repeated measurements
of the oxygen and nitrogen isotope ratios in the international
reference materials USGS35 and IAEA-NO-3, respectively
(Table 1 for details).

The SP-1 samples at LGGE were measured for nitrate con-
centration and isotopic composition (δ¹⁵N, δ¹⁸O and Δ¹⁷O).
A small portion of each sample was taken for nitrate con-
centration measurement using the well-established Griess
method in continuous flow analysis (Seal Analytical, method
no. 1000223-1784-09). The rest of the samples were pro-
cessed following the similar procedure described above; the
N and O isotope ratios were determined by the bacterial den-
itrifier method. Instrumentation details at the LGGE labora-
tory are similar to those described in Erbland et al. (2013).
International reference materials (USGS34, USGS35 and
IAEA-NO-3) were used for data reduction. The uncertain-
<table><caption>**Table 1.** Uncertainties of replicate measurements of reference gases and standards using the bacterial denitrifier method (Kaiser et al., 2007) at the University of Washington IsoLab.</caption><thead><tr><th></th><th></th><th>δ<sup>15</sup>N (‰)</th><th>δ<sup>17</sup>O (‰)</th><th>δ<sup>18</sup>O (‰)</th><th>Δ<sup>17</sup>O (‰)</th></tr></thead><tbody><tr><td>Zero Enrichment<sup>a</sup><br>(N = 30)</td><td></td><td>0.02</td><td>0.10</td><td>0.01</td><td>0.10</td></tr><tr><td>IAEA-NO-3<br>(vs. reference gas)<sup>b</sup></td><td>Short-term<sup>d</sup><br>(N = 6)</td><td>5.1 ± 1.0</td><td>28.9 ± 0.4</td><td>56.1 ± 0.6</td><td>-0.3 ± 0.1</td></tr><tr><td>USGS35<br>(vs. reference gas)<sup>b</sup></td><td>Short-term<br>(N = 6)</td><td>1.4 ± 1.6</td><td>63.6 ± 0.2</td><td>85.2 ± 0.4</td><td>19.3 ± 0.1</td></tr><tr><td>IAEA-NO-3 (normalized)<sup>c</sup></td><td>Long-term<sup>e</sup><br>(N > 100)</td><td>4.7 ± 0.8</td><td>12.9 ± 0.8</td><td>25.3 ± 1.3</td><td>-0.6 ± 0.4</td></tr><tr><td></td><td>Short-term<br>(N = 6)</td><td>4.7 ± 1.0</td><td>12.9 ± 0.6</td><td>25.2 ± 0.8</td><td>-0.6 ± 0.3</td></tr><tr><td>USGS35<br>(normalized)<sup>c</sup></td><td>Long-term<br>(N > 100)</td><td>2.5 ± 0.9</td><td>51.2 ± 0.9</td><td>57.1 ± 1.4</td><td>21.6 ± 0.4</td></tr><tr><td></td><td>Short-term<br>(N = 6)</td><td>1.9 ± 2.1</td><td>51.1 ± 0.3</td><td>56.8 ± 0.5</td><td>21.6 ± 0.1</td></tr></tbody></table>

ties of Δ¹⁷O(NO₃⁻), δ¹⁸O(NO₃⁻) and δ¹⁵N(NO₃⁻) measured at the LGGE laboratory were 0.3‰, 2.0‰ and 0.3‰, respectively, estimated as the reduced standard deviation of the residuals from the linear regression between the measured reference materials and their accepted values.

# 3 Results

## 3.1 Dating and identifying spring nitrate peak(s)

The concentration data from the SP-1 and SP-2 samples and the isotopic data from the SP-1 and SB samples were plotted as a function of depth in Fig. 1 (data are available in Supplement). The concentration profiles of nitrate from SP-1 and SP-2 are nearly identical and each displays five peaks approximately at the same depths (labeled 1, 2, 3, 3' and 4 in Fig. 1d). We used the sodium (Na⁺) peak as a winter snow layer indicator and the peak of the Cl⁻/Na⁺ ratio as a summer snow layer indicator (Whitlow et al., 1992). Sodium concentration in Greenland snow peaks in winter/late winter due to winter Arctic aerosols (with high sea-salt content) and/or late winter cyclonic storms; Cl⁻/Na⁺ peaks in summer due likely to long-range transport of aged sea-salt aerosols (Whitlow et al., 1992), which are depleted in Na⁺ because of the preferential removal of Na⁺ in aerosols relative to gaseous HCl following sea salt-acid displacement (Legrand and Delmas, 1988). Since the snowpit and snow blocks were excavated in July 2007, the partial peak of Cl⁻/Na⁺ at the surface identifies the summer of 2007. The

other Cl⁻/Na⁺ peaks represent the summer of 2006, 2005 and 2004, respectively (Fig. 1e). The nitrate peaks 1, 2, 3 and 4 are in almost identical layers as the Cl⁻/Na⁺ peaks, providing independent evidence that they can be considered summer peaks. Nitrate peak 3' is between a winter Na⁺ peak and a summer Cl⁻/Na⁺ peak, clearly identifying it as a spring peak. This dating by Cl⁻/Na⁺ and Na⁺ peaks (referred to here as method A) is consistent with the results of weekly recorded snow accumulation data determined by stake height measurements at Summit (data are available at ftp://ftp.summitcamp.org/pub/data/GEOSummit/Bales_ UCM/BambooForest/BambooForestAccumulationLog.xls) (referred to here as method B). The approximate months of snowfall at depths of 0, 0.5, 1.0, 1.5 and 2.0 m identified by method B were indicated with vertical dashed lines in Fig. 1. According to method B, snow in the layer of the peak 3' fell in February 2005, suggesting it is an early spring peak; this is consistent with the results of previous studies indicating that the additional annual nitrate peak occurs in late winter/early spring (Burkhart et al., 2006; Yang et al., 1995). Some small discrepancies exist between the two dating methods. For example, the month of snowfall at the surface was identified as May 2007 using method B instead of July. This discrepancy is due to negligible snowfall (around 2 cm) from May to July 2007. The month of snowfall at the depth of 2 m was identified as March 2004 using method B, suggesting that peak 4 is also a spring peak. However, according to the snow accumulation data, there was only about 3 cm of snow accumulation from March to June 2004 (Supplement

<footnote><sup>a</sup> Continuous flow measurements of reference gases (O₂ and N₂) relative to themselves; values are one standard deviations (1σ).</footnote>

<footnote><sup>b</sup> Refer to raw values not corrected for any isotopic effects during the analytical procedure; reported values are means ± RMSD (root-mean-square deviation).</footnote>

<footnote><sup>c</sup> Refer to corrected values using the least squares linear regression curve between the measured and accepted values of reference materials; reported values are means ± 1σ (standard deviation).</footnote>

<footnote><sup>d</sup> Refer to the time period (2 days) when samples in this study were measured.</footnote>

<footnote><sup>e</sup> Refer to the period of instrument running since October 2009 to present.</footnote>
<figure><img src="image_4.png" /><figcaption>**Figure 1.** δ¹⁸O(NO₃⁻) (a), δ¹⁵N(NO₃⁻) (b), Δ¹⁷O(NO₃⁻) (c) and concentrations of NO₃⁻ (d), Na⁺ (e), SO₄²⁻ (f) and Mg²⁺ (g) in the snow samples (data are 3-point running averages). SP-1 data were plotted in gray with plus signs; SP-2 and SB data were plotted in black with circle and star signs, respectively. The pink and green stars represent samples in SB used to calculate the winter and 2005 spring mean Δ¹⁷O(NO₃⁻), respectively; the red and blue plus signs represent samples in SP-1 used to calculate the winter and 2005 spring mean Δ¹⁷O(NO₃⁻), respectively. The vertical dashed line indicates the approximate months of snowfall at depths of 0, 0.5, 1.0, 1.5 and 2.0 m by method B as described in the text.</figcaption></figure>

Fig. 1). Negligible snowfall during this time resulted in nitrate produced in summer dry-depositing into the prior spring snow layers, making it appear as if the peak occurs in spring using method B. In addition, the nitrate profiles in SP-1 and SP-2 are slightly different (i.e., nitrate peaks are not exactly at the same depths), which is likely due to the spatial variability of snow accumulation rates in the field due to snow drifting. But the overall temporal patterns of the nitrate profiles in SP-1 and SP-2 are similar, suggesting

there was negligible disturbance of the stratigraphy of the snow blocks during delivery and storage.

The record of nitrate concentrations from the shallow ice core is shown in Fig. 2 (data are available in Supplement). The annual layer dating of this core was performed by counting the annual spring peak of calcium (Cole-Dai et al., 2013). From this ice core, we found there are 19 years with two nitrate peaks in the period of 1960 to 2006, obtained by subtracting total calcium peaks from total nitrate peaks in this period (Fig. 2). We do not attempt to specifically identify the
<figure><img src="image_5.png" /><figcaption>**Figure 2.** (a) Nitrate concentration record from the shallow ice core; (b–d) show seasonal peaks of $NO_3^-$, $Na^+$ and $Ca^{2+}$, respectively, in two sections of the ice core (data shown in **b**, **c** and **d** are 3-point running averages). Vertical dashed lines in (a) indicate the years of snowfall at relevant depths.</figcaption></figure>

<table><caption>**Table 2.** Frequency (number of years per decade) of years with double nitrate peaks in each decade from 1960 to 2006 in the shallow ice core.</caption><thead><tr><th>Decade</th><th>1960s</th><th>1970s</th><th>1980s</th><th>1990s</th><th>2000–2006</th></tr></thead><tbody><tr><th>Frequency</th><td>2</td><td>5</td><td>3</td><td>6</td><td>3</td></tr></tbody></table>

seasons of these peaks due to the lack of a reliable summer layer indicator in the firn core. The $Cl^-/Na^+$ ratio is influenced by atmospheric acidity (details in Supplement). In the preindustrial period, atmospheric acidity peaks in summer due to high summer production of non-sea-salt sulfate and nitrate (Whitlow et al., 1992). This leads to an enhancement in excess chloride in summer through acid-sea-salt displacement (Legrand and Delmas, 1988). Given the low $Na^+$ concentrations in summer, the $Cl^-/Na^+$ peak falls almost precisely in summer in the preindustrial period, making it a good summer layer indicator. However, in much of the industrial era, the annual $Cl^-/Na^+$ peak shifts toward the major annual non-sea-salt sulfate peak (usually in winter; see Fig. S2, top panel), inhibiting its ability to serve as a summer layer indicator. This makes it difficult to unambiguously identify the secondary nitrate peak in the years with double nitrate

peaks as occurring in a specific season (e.g., spring), though we note that previous studies have been able to identify the secondary nitrate peak as occurring in the spring by using summer layer indicators (e.g., $H_2O_2$ and $\delta^{18}O(H_2O)$ independent of major ion concentrations (Burkhart et al., 2006; Whitlow et al., 1992; Yang et al., 1995). We note that in the snowpit, sulfate concentrations are as low as they were prior to 1900 (Geng et al., 2014), minimizing its impact on the timing of the $Cl^-/Na^+$ peak. The frequency of years with double nitrate peaks in each decade is listed in Table 2. Prior to 1960 only a single nitrate peak is observed in each annual layer, consistent with the finding by Finkel and Langway (1986) that the spring nitrate peak started appearing after the 1950s; however, others (Burkhart et al., 2006; Yang et al., 1995) stated in general that elevated spring nitrate concentrations are seen in snow layers after around 1900.

## 3.2 $\delta^{18}O(NO_3^-)$, $\delta^{15}N(NO_3^-)$ and $\Delta^{17}O(NO_3^-)$ variations

The $\delta^{18}O(NO_3^-)$, $\delta^{15}N(NO_3^-)$ and $\Delta^{17}O(NO_3^-)$ data from the SP-1 and SB samples are shown in Fig. 1a, b and c, respectively. The depth range of the SP-1 samples (0 to 2.10 m) corresponds to the period of the 2007 summer to 2004 summer, while the depth range of the SB samples (0.7 to 1.75 m) cor-
responds to the time period of the 2006 spring/summer to the 2004/2005 winter. In general, all three isotopic signatures of nitrate display large seasonal variations. $\delta^{18}\text{O}(\text{NO}_3^-)$ is low in summer and high in winter, while $\delta^{15}\text{N}(\text{NO}_3^-)$ is high in spring/early summer and low in winter, consistent with previous measurements of Summit snow samples from Hastings et al. (2004). In the spring of 2005 (the time period corresponding to the depth of the spring nitrate peak), the means ($\pm 1\sigma$) of $\delta^{15}\text{N}(\text{NO}_3^-)$ from the SP-1 and SB samples were (5.3 $\pm$ 3.3)‰ and (6.4 $\pm$ 2.1)‰, respectively, much higher than winter values (–9.2 $\pm$ 3.3)‰ from the SP-1 samples and (–7.4 $\pm$ 4.4)‰ from the SB samples. A local minimum in $\delta^{18}\text{O}(\text{NO}_3^-)$ was also observed from both the SP-1 and the SB samples at the depth of the 2005 spring nitrate peak (Fig. 1a).

The observed seasonality of $\Delta^{17}\text{O}(\text{NO}_3^-)$ (Fig. 1c) is consistent with the expectation of high $\Delta^{17}\text{O}(\text{NO}_3^-)$ during winter (polar night) due to the dominance of the $\text{O}_3$ oxidation pathway of $\text{NO}_x$ (R5–R8). The magnitude of the seasonality is also consistent with the observations of $\Delta^{17}\text{O}(\text{NO}_3^-)$ at Summit by Kunasek et al. (2008). The seasonality of $\Delta^{17}\text{O}(\text{NO}_3^-)$ is regulated by the shift of the dominant nitrate formation pathway via OH oxidation (R4) in polar summer/day to that via $\text{O}_3$ (R5-R8) in polar winter/night (low summer values and high winter values). Modeling by Kunasek et al. (2008) showed that, in early spring, $\Delta^{17}\text{O}$ of locally produced nitrate at Summit should be close to that in winter snow, as oxidation by OH (R4) is very limited at this time due to the lack of sunlight. In other words, $\Delta^{17}\text{O}(\text{NO}_3^-)$ values at Summit in winter and early spring should be similar. However, in the early spring of 2005 the $\Delta^{17}\text{O}(\text{NO}_3^-)$ values display apparent declines from the prior winter values (Fig. 1c). To test whether the declines are significant, we calculate the mean $\Delta^{17}\text{O}(\text{NO}_3^-)$ value in the early spring of 2005 and compare it to the mean of measured winter $\Delta^{17}\text{O}(\text{NO}_3^-)$ values. The winter mean $\Delta^{17}\text{O}(\text{NO}_3^-)$ was calculated from the 2004/2005 to 2006/2007 winters from the SP-1 samples and from the 2004/2005 to 2005/2006 winters for the SB samples (samples used to calculate the winter and 2005 early spring means are marked in Fig. 1c). The mean winter value was (32.9 $\pm$ 1.1)‰ from the SP-1 samples and (32.4 $\pm$ 0.6)‰ from the SB samples. In contrast, the mean $\Delta^{17}\text{O}(\text{NO}_3^-)$ value in the early spring of 2005 was (31.1 $\pm$ 0.6)‰ from SP-1 and (30.8 $\pm$ 0.8)‰ from SB, approximately 1.7‰ lower than the winter means (statistically significant based on a one-tailed t test: for SP-1 samples, $t = 3.434$, DOF = 24, $P = 0.001$; for SB samples, $t = 4.637$, DOF = 17, $P \le 0.0005$). A previous study measuring $\Delta^{17}\text{O}(\text{NO}_3^-)$ in Summit snow by Kunasek et al. (2008) also noted significantly low $\Delta^{17}\text{O}(\text{NO}_3^-)$ values (around 26‰) in the spring of 2005, compared to around 33‰ in the prior winter of $\Delta^{17}\text{O}(\text{NO}_3^-)$.

## 4 Discussion

The low $\Delta^{17}\text{O}(\text{NO}_3^-)$ values in the spring of 2005 suggest an increase in OH oxidation in the formation of nitrate associated with the peak. This is qualitatively consistent with the observed decrease in $\delta^{18}\text{O}(\text{NO}_3^-)$ from a previous study (Hastings et al., 2004), as increases in the relative importance of OH oxidation will also result in decreases in $\delta^{18}\text{O}(\text{NO}_3^-)$. The extra nitrate deposited during the spring of 2005 was estimated to be 5.7 nmol $\times$ cm$^{-2}$, which was calculated by subtracting the flux of nitrate (9.4 nmol $\times$ cm$^{-2}$) in the spring of 2006 from the nitrate flux in the spring of 2005 (15.1 nmol $\times$ cm$^{-2}$). The 2006 spring was a normal spring without nitrate peak. This suggests a 60% increase in nitrate deposited in the spring of 2005 compared to the spring of 2006. The additional nitrate in the spring of 2005 could either come from enhanced transport of nitrate produced elsewhere or enhanced local nitrate production involving $\text{NO}_x$ precursors. In the discussion to follow, we consider separately the possibility of enhanced transport and enhanced local production of nitrate in the Arctic being responsible for the spring 2005 nitrate peak. We further examine whether the explanation for the case of spring 2005 is representative of the occasional nature of the additional nitrate peaks observed in the firn core.

### 4.1 Enhanced transport of nitrate

#### 4.1.1 Stratospheric denitrification

Stratospheric denitrification refers to the sedimentation process of polar stratospheric clouds containing nitric acid trihydrate. Research on nitrate in Antarctic snow (Mulvaney and Wolff, 1993) suggested that stratospheric denitrification associated with the winter polar vortex could result in a late winter/early spring nitrate peak. Although denitrification occurs less frequently and less extensively in the Arctic than in Antarctica due to the warmer winter and the weaker and less persistent Arctic vortex (Waugh and Randel, 1999), significant denitrification has been observed in Arctic for some exceptionally cold winters, including the winter of 2004/2005 (Mann et al., 2003; Jin et al., 2006; Kleinbohl et al., 2005). Could stratospheric denitrification in the winter of 2004/2005 have resulted in the early spring nitrate peak?

Stratospheric nitrate is expected to possess high $\Delta^{17}\text{O}$ since it is mainly formed via $\text{O}_3$ oxidation (McCabe et al., 2007; Savarino et al., 2007), and stratospheric $\Delta^{17}\text{O}(\text{O}_3)$ is up to 5‰ higher than that in the troposphere (Liang et al., 2006; Lyons, 2001; Mauersberger et al., 2001). In addition, stratospheric nitrate can also be formed via $\text{ClO}$ oxidation (McCabe et al., 2007). Nitrate formed via $\text{ClO}$ oxidation also possesses high $\Delta^{17}\text{O}$ (McCabe et al., 2007; Savarino et al., 2007) due to the internal non-statistical distribution of isotopes in $\text{O}_3$ that transfers its terminal oxygen atoms to $\text{ClO}$ (Bhattacharya et al., 2008).
If the additional nitrate in the 2005 spring snow originated from stratospheric denitrification, the Δ¹⁷O(NO₃⁻) would be expected to be anomalously high, not low as was observed. It is possible that the Δ¹⁷O value of nitrate deposited into the snow surface is reduced by post-depositional processing, because the photolytic recycling of snowpack nitrate (nitrate photolysis followed by re-formation and re-deposition of nitrate) can reduce the Δ¹⁷O signature of nitrate finally preserved in snow (Erbland et al., 2013). If all of the nitrate produced in the stratosphere undergoes photolytic recycling, the stratospheric Δ¹⁷O signature would be completely erased (i.e., Δ¹⁷O of the nitrate would be the same as that of nitrate produced in the tropospheric boundary layer). Given the high snow accumulation rate at Summit (the degree of snowpack nitrate photolysis decreases with increasing snow accumulation rate; Röthlisberger et al., 2002), it is unlikely that all additional nitrate underwent photolytic recycling. Consequently, Δ¹⁷O of the preserved nitrate originally produced in the stratosphere should be no lower than that of the tropospheric nitrate deposited in a typical spring. Therefore, the observed Δ¹⁷O(NO₃⁻) suggests stratospheric denitrification cannot account for the spring nitrate peak.

## 4.1.2 Transport from mid-latitudes

Previous studies (Burkhart et al., 2006; Yang et al., 1995) hypothesized that the spring nitrate peak results from pollution transport in the industrial era, though the occasional nature of the peak was not explicitly discussed. In case of strong meridional transport of nitrate, a spring nitrate peak may be expected. In addition, the Δ¹⁷O of nitrate formed in the mid-latitudes is likely lower than that of nitrate formed in the Arctic troposphere due to the latitudinal gradient in Δ¹⁷O(NO₃⁻), resulting from the latitudinal gradient in O₃/HOₓ ratio (Alexander et al., 2009). Thus, enhanced long-range transport of mid-latitude nitrate to Greenland during the spring could elevate nitrate concentrations in snow with relatively low Δ¹⁷O(NO₃⁻), consistent with the observations. Direct transport of nitrate from the mid-latitudes to Summit is also consistent the conclusions in Kunasek et al. (2008) and Geng et al. (2014).

Pollution from the northern mid-latitudes is transported to the Arctic by poleward meridional circulation, which is strong when the North Atlantic Oscillation (NAO) is in its positive phase (Eckhardt et al., 2003). However, the NAO index data in the 2004/2005 winter and early spring of 2005 are not strong compared with other years during the period of 1997 to 2009 (Osborn, 2011), suggesting that meridional transport was not particularly strong in the spring of 2005 compared to other springs. However, frequent pollution transport events occurred even in years with a negative NAO phase (Kramer et al., 2014). To examine the possibility of pollution transport as the direct cause of the 2005 spring nitrate peak, we examined the profiles of SO₄²⁻ and Mg²⁺ from the snowpit. Enhanced transport from the mid-latitudes

will also elevate concentrations of other species derived from anthropogenic and continental sources, such as SO₄²⁻ and Mg²⁺; SO₄²⁻ in particular has a very similar atmospheric lifetime (4 to 6 days) as nitrate (3 to 7 days) (Park et al., 2004; Pye et al., 2009). However, the data from the snowpit show that neither SO₄²⁻ nor Mg²⁺ concentrations were elevated (Fig. 1f and g) in the spring of 2005 when the nitrate peak was present. Therefore, it is unlikely that the 2005 spring nitrate peak was caused by enhanced long-range transport of nitrate from the mid-latitudes during that time period.

In addition to pollution from industrial areas, biomass burning (e.g., wild fire) plumes from boreal North America could also potentially increase snow nitrate concentrations in Greenland. However, the global fire emission data (Giglio et al., 2013) suggest that fire emissions in boreal North America (or in temperate North America and Europe) were not strong in the spring of 2005 compared to other years in the period of 1997 to 2011. In addition, chemical species transported from fire events will likely cause episodic, sharp spikes, such as the NH₄⁺ spikes frequently observed in Greenland ice cores (Savarino and Legrand, 1998). The duration (> 1 month) of the 2005 spring nitrate peak argues against an episodic biomass-burning source.

## 4.2 Enhanced local production

### 4.2.1 PAN decomposition

Previous work has found that PAN is the dominant form of reactive nitrogen in the Arctic troposphere during winter and spring (Beine and Krognes, 2000; Bottenheim et al., 1993; Honrath and Jaffe, 1992; Jacobi et al., 1999), and its decomposition yields NOₓ that is subsequently converted to nitrate. If the decomposition rate of PAN was enhanced in the spring of 2005, more NOₓ would be produced, resulting in an increase in local nitrate production. The possibility of PAN decomposition in spring leading to the spring nitrate peak has been mentioned in Yang et al. (1995) but not explicitly examined. Here we provide a detailed examination of the possible role of PAN decomposition in the spring 2005 nitrate peak.

PAN decomposes to NOₓ either thermally or via photolysis (Talukdar et al., 1995). At temperatures above 255 K thermal decomposition dominates, while photolysis becomes more important at lower temperatures (Talukdar et al., 1995). The typical spring (February and March) air temperature at Summit is about 240 K (242 K in 2005 and an average of 236 K in the springs of 2006 to 2008; data are from http://www.summitcamp.org/resources/files). At these low temperatures, the NOₓ release from PAN is dominated by photolysis at wavelengths between 290 and 345 nm (Talukdar et al., 1995; Flowers et al., 2005).

The stratospheric O₃ layer filters out most of the UVB (λ = 290 – 320 nm) portion of the solar spectrum. A weakened stratospheric O₃ layer will allow more UVB penetration into the troposphere. In Fig. 3, the spring (average of Febru-
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Summit springtime O₃ column density (February and March averages) and poleward eddy heat flux in the lower stratosphere (40–80° N, January and February averages) from 1979 to 2006. The solid blue line is the average of springtime O₃ column density from 1979 to 2006 (390 ± 50 (1σ) DU), and the dashed blue line is the linear least-squared regression of the O₃ column density from 1979 to 2006. The symbols marked as red and green indicate years with double nitrate peaks, while the green symbols indicate one of the double nitrate peaks is accompanied with a sulfate peak.</figcaption></figure>

ary and March values) O₃ column density at Summit for the years 1979–2006 are shown (data are from the NASA Total Ozone Mapping Spectrometer program). The 2005 spring level (290 DU) was significantly lower than the average of spring values from 1979 to 2006 (390 ± 50 (1σ) DU), suggesting a relatively high UVB flux to the surface in the spring of 2005.

The photolysis of PAN in the spring of 2005 could have been enhanced due to the elevated UVB radiation caused by stratospheric O₃ loss, leading to increased local NOₓ abundance and subsequently resulting in the unusual spring nitrate peak in snow. The increase in PAN photolysis resulting from the decreased O₃ column density (290 DU) is estimated with the UCAR tropospheric ultraviolet and visible (TUV) radiation model (available at http://cprm.acd.ucar.edu/Models/TUV/). Calculations with the TUV model showed a 24% increase in the photolysis rate constant of PAN ($j = (9.0 \pm 5.4) \times 10^{-8} \text{ s}^{-1}$ vs. $(7.3 \pm 4.3) \times 10^{-8} \text{ s}^{-1}$; both are daytime averages) in the spring of 2005 compared to that in a typical spring (column O₃ density of 390 DU). This would lead to an increase of only $1.6 \times 10^5 \text{ cm}^{-3}$ in NOₓ number concentration, which is, on average, 0.04% of the observed springtime NOₓ concentration of $(3.9 \pm 3.1) \times 10^8 \text{ cm}^{-3}$ in the Arctic (Stroud et al., 2003). The above calculation assumes steady state of PAN with number concentration of $(2.3 \pm 0.7) \times 10^9 \text{ cm}^{-3}$ (Stroud et al., 2003) and $[NO_2] = (([PAN] \times j_{PAN})/k)^{1/2}$, where $[NO_2]$ and [PAN] represent the number concentrations of NO₂ and PAN, respectively; $j_{PAN}$ ($\text{s}^{-1}$) is the photolysis rate constant of PAN calculated from the TUV model; and $k$ ($\text{cm}^3 \times \text{s}^{-1}$) is the reaction rate constant of PAN formation at $T = 240$ K and $P =$

650 hPa (Summit springtime condition), calculated based on the equation from Atkinson et al. (2006). The additional production of NOₓ from enhanced PAN photolysis is negligible compared to the observed 60% enhancement in nitrate deposition flux $(5.7 \text{ nmol} \times \text{cm}^{-2})$ during the spring of 2005. This is consistent with the model prediction by Stroud et al. (2003) that, during spring, PAN is a net sink of NOₓ rather than a source. A similar conclusion was also reached by Singh et al. (1992), who found that the PAN reservoir is not a significant source of NOₓ until summer.

### 4.2.2 Snowpack NOₓ emissions

The photolysis of nitrate in the snowpack at snow depths of up to 20 cm or more (Zatko et al., 2013; France et al., 2011; Grannas et al., 2007) emits NOₓ to the overlying atmosphere (Honrath et al., 1999; Frey et al., 2009), serving as a local NOₓ source. NOₓ originating from the photolysis of nitrate in the snowpack can be re-oxidized to nitrate in the atmosphere and re-deposited into the surface. A recent model study (Thomas et al., 2012) suggested that at Summit the photolysis of snowpack nitrate alone can sustain observed NO concentrations in the local atmospheric boundary layer. If the emission of NOₓ from snowpack were enhanced, local atmospheric nitrate production would also be expected to be elevated. The reduction in O₃ column density in the spring of 2005 led to more UV radiation at the surface, which in turn may significantly enhance the photolysis of snow nitrate. Consequently, the oxidation of NOₓ released from the snowpack by enhanced photolysis of snowpack nitrate at depths, and subsequent re-deposition of the nitrate into the surface in spring, would result in elevated nitrate concentration in surface snow during spring.

In order to estimate the impact of the reduced overhead O₃ column density in the spring of 2005 on local NOₓ concentration via the photolysis of snowpack nitrate, we used the TUV model to calculate the surface actinic flux at Summit in the spring of 2005 (290 DU) vs. normal springtime with average O₃ column density (390 DU, which was close to the value of 380 DU in the spring of 2006). The parameterization from Zatko et al. (2013) was then used to calculate the photolysis frequency ($\text{s}^{-1}$) of snowpack nitrate at wavelengths from 298 to 345 nm (peak wavelength for nitrate photolysis is around 320 nm; Chu and Anastasio, 2003). This calculation suggested a 30% enhancement of the snowpack nitrate photolysis frequency in the spring of 2005 relative to 2006, and thus a similar enhancement of NOₓ emission from the snowpack due to the reduced overhead O₃ column density alone. This likely contributed to the enhanced local nitrate production in the spring of 2005. However, the relative importance of snowpack nitrate photolysis to local NOₓ abundance in springtime is unknown, making it difficult to quantitatively assess the contribution from snowpack emissions to the observed spring nitrate peak. If snowpack nitrate photolysis nitrate photolysis is the dominant source of boundary
layer NOx at Summit as suggested by Thomas et al. (2012), then this could account for up to half of the additional nitrate in the observed 2005 spring nitrate peak.

The relatively high δ¹⁵N(NO₃⁻) values in the spring of 2005 (SP-1: (5.3 ± 3.3)%; SP-2: (6.4 ± 2.1)%o) indicate some post-depositional loss of snowpack nitrate at that time. Since post-depositional loss is driven by photolysis (Berhanu et al., 2014; Frey et al., 2009), the observed high δ¹⁵N(NO₃⁻) values suggest active snow nitrate photolysis during spring 2005. This is qualitatively consistent with the observations at Summit by Hastings et al. (2004) that δ¹⁵N(NO₃⁻) in surface snow is significantly higher in daytime, when snow nitrate photolysis is active, compared to during the nighttime.

### 4.2.3 Enhanced conversion of NOx to nitrate

Field observations suggest that at present there is probably sufficient NOx at Summit to ensure local nitrate production even in the spring. The number concentration of NOx in the Arctic mid-troposphere (58–85° N, 3–6 km) during spring of 2002 was reported to be (3.9 ± 3.1) × 10⁸ cm⁻³ (Stroud et al., 2003), which is similar to the summer NOx level at Summit (Jacobi et al., 2004). In addition, Kramer et al. (2014) reported that the February NOx concentration at Summit is (1.6 ± 3.2) × 10⁸ cm⁻³, which overlaps with summer values due to the large uncertainty. Kramer et al. (2014) also suggested that pollution transport during polar winters leads to accumulation of NOy as well as NOx in the Arctic and implied that part of the NOy will be converted to NOx after polar sunrise through photolysis. Under sufficient local NOy and NOx abundances in the industrial era, an increase in nitrate concentration would be expected if the oxidation rate of NOx to HNO3 were also enhanced. The NOx oxidation rate can be enhanced by increased oxidant (O3, OH, BrO) levels. Both O3 and BrO have high Δ¹⁷O values (Lyons, 2001; Morin et al., 2007), thus an increase in either O3 or BrO concentrations would result in additional nitrate with high Δ¹⁷O. The relatively low Δ¹⁷O(NO₃⁻) in the spring 2005 snow (Fig. 1c) suggests that increased tropospheric O3 and/or BrO concentrations are unlikely to be the direct cause of enhanced nitrate production.

On the other hand, increased oxidation of NO₂ by OH would produce additional nitrate with low values of Δ¹⁷O as was observed. In general, the concentration of tropospheric OH is dependent on concentrations of tropospheric O₃ and water vapor and the available UVB radiation through following reactions:

$$O_3 + h\nu \rightarrow O_2 + O(^1D) \quad (290 \text{ nm} < \lambda < 320 \text{ nm}), \quad (R9)$$

$$O(^1D) + H_2O \rightarrow 2OH. \quad (R10)$$

Either a substantial increase in UVB in the troposphere or an increase in the water vapor content at elevated atmospheric temperatures can increase OH production and therefore enhance the conversion of NOx to HNO3 via OH oxidation (R4). In addition, two other important oxidants involved in

NO–NO₂ cycling (R2), HO₂ and RO₂, are formed by reactions of carbon monoxide (CO) and hydrocarbons (RH) with OH:

$$CO + OH \xrightarrow{O_2} CO_2 + HO_2, \quad (R11)$$

$$RH + OH \xrightarrow{O_2} H_2O + RO_2. \quad (R12)$$

Increased OH will enhance NO–NO₂ cycling via HO₂ and RO₂ and reduce the relative importance of O₃ in NOx cycling, which also leads to lowered Δ¹⁷O in nitrate.

Using temperature and relative humidity data at Summit from the Greenland Climate Network data (http://cires.colorado.edu/science/groups/steffen/gcnet/order/admin/station.php), we estimate that the spring (February and March average) atmospheric water vapor mixing ratio in 2005 was 40% higher than that in 2006, due to high temperatures in the 2005 spring (−30°N) relative to the 2006 spring (−34°N). Assuming this high water vapor in spring 2005 will result in a maximum increase in OH production of 40%, the increase in water vapor is not enough to account for the 60% enhancement in the nitrate deposition flux in the spring of 2005. In fact, as discussed later, OH must be increased by 200% to explain the full magnitude of the spring nitrate peak. Although the relatively high temperature in the spring of 2005 may have contributed around 20% (40% increase vs. the required 200% increase) to the enhanced local nitrate production, it was likely not the dominant factor producing the spring nitrate peak in 2005.

To determine the effect of reduced column O₃ density on OH production at the surface, we used the TUV model to estimate the OH production rate (R9 and R10) at Summit due to enhanced photolysis of boundary layer O₃. Our calculation showed that the OH production rate due to this mechanism alone was increased by 90% in the spring of 2005, resulting from the decrease in O₃ column density from 390 to 290 DU. Assuming that the production of HNO₃ via OH oxidation was increased by the same factor, enhanced local OH production could easily account for the 60% enhancement of nitrate flux in the spring of 2005, if all of the additional nitrate was formed via OH oxidation (R4). During a typical Arctic spring, O₃ is expected to be the major oxidant in the oxidation of NOx to HNO₃ (Alexander et al., 2009), so it is unlikely that R4 is solely responsible for the local oxidation of NOx to HNO₃. Using a global chemical transport model (GEOS-Chem), Alexander et al. (2009) estimated that about 30% of total nitrate in a normal spring (average in February and March) is produced via OH oxidation at Summit. With this more realistic value, a 200% increase in the local OH production rate was needed to account for the additional nitrate flux in the spring of 2005. Therefore, the enhanced local production rate of OH due to solely the enhanced photolysis of tropospheric O₃ in the spring of 2005 can explain about 50% of the observed spring nitrate peak.

In addition to production via O₃ photolysis (R9 and R10), OH can be formed by the photolysis of snowpack-emitted
CH₂O, H₂O₂ and HONO (Dassau et al., 2002; Hutterli et al., 2001; Zhou et al., 2001; Sjostedt et al., 2007). Some model studies suggested that at Summit, photolysis of snow-sourced CH₂O, H₂O₂ and HONO contributes to OH 2-3 times more than that of O₃ photolysis (e.g., Yang et al., 2002), while the others seem to have different results (e.g., Chen et al., 2007). Although the photolysis of these species is more sensitive to UVA radiation (Grannas et al., 2007), the production/emission of HONO and CH₂O from the snowpack is influenced by snow nitrate photolysis (Thomas et al., 2012), which is sensitive to UVB radiation. In addition, a recent model study by Thomas et al. (2012) suggested that at Summit, snow-sourced NOₓ, via shifting the local OH/HO₂ ratio in favor of OH, led to as much OH production as ozone photolysis; snowpack bromine chemistry also contributed to 10-18% of local OH budget. These processes are also sensitive to UVB radiation. These model studies suggested that snow photochemistry is more important than ozone photolysis for OH production at Summit (Thomas et al., 2012; Yang et al., 2002). It is thus conceivable that under the condition of reduced O₃ column density in the spring of 2005, the OH production rate could be increased by 200% from the combined contribution of enhanced snow photochemistry and tropospheric O₃ photolysis.

Thus we propose that the tropospheric UVB level, elevated due to a weakened stratospheric ozone layer in the spring of 2005, likely led to additional local nitrate production in sufficient quantities to account for the spring nitrate peak. The additional nitrate was generated primarily through increased tropospheric OH production from the enhanced photolysis of tropospheric O₃ and snowpack photochemistry involving NOₓ, halogens, HONO, CH₂O and H₂O₂. Increased NOₓ abundance from the photolysis of snowpack nitrate and/or other NOₓ precursors that accumulated during the polar winter also contribute (Kramer et al., 2014).

## 4.3 Further examination with additional snow and ice core data

Hastings et al. (2004) reported seasonal variations in concentrations of δ¹⁸O and δ¹⁵N of snow nitrate at Summit covering the years 2000 and 2001. Qualitatively, δ¹⁸O(NO₃⁻) is similar to Δ¹⁷O(NO₃⁻), because the δ¹⁸O(NO₃⁻) value is also determined by the relative importance of O₃ vs. OH oxidation (higher δ¹⁸O(NO₃⁻) values indicate increased O₃ oxidation and lower δ¹⁸O(NO₃⁻) values indicate increased HOₓ oxidation) (Hastings et al., 2004). Consequently, δ¹⁸O(NO₃⁻) is high in winter snow and low in summer snow (as shown in Fig. 1a and in Hastings et al., 2004). The data shown in Hastings et al. (2004) indicated a spring nitrate peak in 2000. The spring nitrate concentration peak in 2000 in Hastings et al. (2004) had δ¹⁸O(NO₃⁻) values of (69.8 ± 2.1)‰, similar to those of (70.5 ± 2.4)‰ during the summer of 2000. In contrast, in spring 2001 when no nitrate concentration peak was observed, δ¹⁸O(NO₃⁻) was (77.5 ± 2.4)‰, which is sim-

## L. Geng et al.: The spring nitrate peak in Greenland snow

ilar to the δ¹⁸O(NO₃⁻) values of (77.4 ± 1.9)‰ observed during the prior winter and higher than those of (69.8 ± 2.1)‰, observed during the subsequent summer. In addition, the mean δ¹⁵N(NO₃⁻) in the spring of 2000 of (5.9 ± 6.2)‰ from Hastings et al. (2004) is similar to that of (6.4 ± 2.1)‰ observed in the spring of 2005 and is higher than that of (−1.4 ± 3.0)‰, observed in Hastings et al. (2004) during the spring of 2001. In summary, the isotopic features of nitrate associated with the spring peaks observed in 2000 (Hastings et al., 2004) and in 2005 are similar, each suggesting enhanced local photochemistry as a contributor to the observed spring nitrate peaks. This is consistent with the low O₃ column density in the spring of 2000 (337 DU, Fig. 3) and 2005 (294 DU, Fig. 3). Therefore, the results of Hastings et al. (2004) support the explanation that the appearance of the spring nitrate peak in 2000 is caused by a weakened stratospheric ozone layer, consistent with the observations in the spring of 2005.

We further examined the 19 years with double nitrate peaks found in the firn core. In order to assess the effect of long-range transport of nitrate on the occurrence of the double nitrate peaks, we compared the nitrate and sulfate records. This comparison indicates that in most (14 out of 19) years, the secondary (earlier and smaller) peak is accompanied by a sulfate peak. This suggests that direct transport of nitrate from polluted regions is likely responsible for the occurrence of the double nitrate peaks in these years. However, there are 5 years, including 2000 and 2005, in which the secondary peak does not appear with a rise in sulfate concentration. To determine if enhanced local photochemistry could be responsible for the occurrence of the double nitrate peaks in these years, we examined O₃ column density data from 1979 to 2006, the time period when global O₃ data are available from satellite observations. The year-to-year variability of polar stratospheric O₃ is largely controlled by the Brewer-Dobson circulation (BDC) through direct transport and indirect coupling between dynamics and chemistry (e.g., Holton et al., 1995; Randel et al., 2002; Shepherd, 2008; Weber et al., 2011). Halogen-catalyzed chemical destruction leads to a decreasing trend in column O₃ density since 1980 (WMO, 2007) and causes sudden drops in O₃ column density in years when the winter temperatures are anomalously low (e.g., the winter of 2004/2005; Jin et al., 2006; Kleinbohl et al., 2005). The strength of the BDC is related to the wave force, which is represented by extratropical poleward eddy heat flux in the lower stratosphere (Fusco and Salby, 1999; Weber et al., 2011). From the NCEP/NCAR reanalysis data (http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.pressure.html), we retrieved the average eddy heat flux in the lower stratosphere at 40-80°N in January and February (due to the transport lag, January and February eddy heat flux determines the February and March polar stratospheric O₃ abundance) from 1979 to 2006 (Fig. 3). Reanalysis data before 1979 are also available, but are less reliable due to lack of constraints by satellite obser-
vations. Therefore, in this study we only use the data after
1979.

As shown in Fig. 3, in the period from 1979 to 2006 there
are 12 years with double nitrate peaks and 5 are years with
O₃ column density well below the average for that period.
In other years one of the double peaks is accompanied by a
sulfate peak. The results from the firn core, combined with
that from the snowpit, thus suggest that there are two mech-
anisms leading to occurrence of double nitrate peaks: (1) di-
rect transport of nitrate from polluted regions and (2) en-
hanced local photochemical production of nitrate. The sec-
ond mechanism requires local sources of NOₓ. Possible local
NOₓ sources at Summit include PAN decomposition and the
photolysis of snowpack nitrate, as well as photolysis of other
winter-accumulated NOᵧ species in the atmosphere (Kramer
et al., 2014). For example, a model study (Stroud et al., 2003)
suggested that HNO₄ is a source of NOₓ in remote regions
through recycling reactions, but its importance is unclear due
to a lack of field observations. These NOₓ precursors accu-
mulate in the polar winter through a combination of pollution
transport from the mid-latitudes and lack of sunlight and con-
tribute to springtime NOₓ abundance through photochemical
decomposition after polar sunrise (Kramer et al., 2014). This
is consistent with the observation that there are no double
nitrate peaks observed prior to the 1950s because dramatic
increases in anthropogenic emissions of NOₓ did not occur
until 1950 (Lamarque et al., 2010).

5 Conclusions

Spring nitrate peaks have been observed in Greenland ice core records beginning 1900 and were hypothesized to originate from pollution transport from the mid-latitudes in the industrial era (Burkhart et al., 2006; Whitlow et al., 1992; Yang et al., 1995). However, the specific mechanism leading to the additional peak is unclear, as it could be from direct transport of nitrate or transport of NOₓ precursors followed by enhanced local photochemistry. Here we report observations of nitrate concentrations and isotopes in a snowpit from Summit, Greenland. After identifying a spring nitrate peak in 2005, we performed a case study regarding its origin. The isotopic composition of nitrate in the snowpit, combined with photochemical calculations, suggests the presence of the 2005 spring nitrate peak was caused by enhanced local nitrate production instead of long-range pollution transport. An analysis of the possible causes suggests that this enhancement was primarily due to increased OH concentration, mainly resulting from enhanced photolysis of OH precursors (O₃, HONO, H₂O₂, etc.) in the troposphere caused by elevated UVB radiation at the surface. Elevated UVB radiation at the surface was the result of a significant reduction of the stratospheric ozone layer in the Arctic in spring 2005. Increased local NOₓ concentrations due to enhanced snowpack nitrate photolysis may also contribute up to half of the addi-

tional nitrate in the spring peak if snowpack nitrate photoly-
sis is the dominant local NOₓ source. Either the increase in
OH concentration or the increase in local NOₓ concentration
relies on enhanced photochemical reactions initiated by the
reduced O₃ column density. This mechanism is supported by
the appearance of a spring nitrate peak in 2000 (Hastings et
al., 2004), when a reduced O₃ column density also occurred.

Further examination on a firn core indicates that in most
years, direct transport of nitrate from polluted regions is
likely responsible for the additional nitrate peak. This, com-
bined with the snowpit results, suggests that there are two
mechanisms for the occurrence of the additional annual ni-
trate peak in the industrial era: (1) direct transport of nitrate
from polluted regions and (2) enhanced local photochemi-
cal production of nitrate. We note that both of these mech-
anisms are related to pollution transport, as the additional
nitrate from either direct transport or enhanced local photo-
chemistry requires enhanced nitrogen sources from anthro-
pogenic emissions.

The Supplement related to this article is available online
at doi:10.5194/acp-14-13361-2014-supplement.

Acknowledgements. Total column O_3 data are from http://jwocky.gsfc.nasa.gov/eptoms/ep.html by Earth Probe Total Ozone Mapping Spectrometer, NASA. We thank Ice Drilling Design and Operations (formerly Ice Coring and Drilling Services) at the University of Wisconsin for field assistance in drilling the Summit ice cores. A. L. Lanciki and D. G. Ferris of South Dakota State University performed the continuous analysis of the shallow ice core. We also thank J. Bautista of the University of Washington for assistance in isotopic measurements and acknowledge the support from the Quaternary Research Center at University of Washington. This work is financially supported by the NSF (awards 0612461 and 0839066 to J. Cole-Dai, PLR 0944537 and PLR 1106317 to B. Alexander, and 0538049 and 0454803 to E. J. Steig). JS and JE have been supported by a grant from Labex OSUG@2020 (Investissements d'avenir – ANR10 LABX56). The French Polar Institute IPEV is acknowledged for its financial and logistical support to JS (programme 457). LEFE-IMAGO, a scientific program of the Institut national des sciences de l'Univers (INSU/CNRS), has also provided partial funding to JS. The authors thank three anonymous reviewers and the editor for all comments and suggestions.

Edited by: J. Kaiser,
References

Alexander, B., Savarino, J., Kreutz, K. J., and Thiemens, M. H.: Impact of preindustrial biomass-burning emissions on the oxidation pathways of tropospheric sulfur and nitrogen, J. Geophys. Res., 109, D08303, doi:10.1029/2003jd004218, 2004.

Alexander, B., Hastings, M. G., Allman, D. J., Dachs, J., Thornton, J. A., and Kunasek, S. A.: Quantifying atmospheric nitrate formation pathways based on a global model of the oxygen isotopic composition (Δ¹⁷O) of atmospheric nitrate, Atmos. Chem. Phys., 9, 5043–5056, doi:10.5194/acp-9-5043-2009, 2009.

Atkinson, R., Baulch, D. L., Cox, R. A., Crowley, J. N., Hampson, R. F., Hynes, R. G., Jenkin, M. E., Rossi, M. J., and Troe, J.: Evaluated kinetic and photochemical data for atmospheric chemistry: Volume II – gas phase reactions of organic species, Atmos. Chem. Phys., 6, 3625–4055, doi:10.5194/acp-6-3625-2006, 2006.

Beine, H. J., and Krognes, T.: The seasonal cycle of peroxyacetyl nitrate (PAN) in the European Arctic, Atmos. Environ., 34, 933–940, doi:10.1016/S1352-2310(99)00288-5, 2000.

Berhanu, T. A., Meusinger, C., Erbland, J., Jost, R., Bhattacharya, S. K., Johnson, M. S., and Savarino, J.: Laboratory study of nitrate photolysis in Antarctic snow. II. Isotopic effects and wavelength dependence, J. Chem. Phys., 140, 244306, doi:10.1063/1.4882899, 2014.

Bhattacharya, S. K., Pandey, A., and Savarino, J.: Determination of intramolecular isotope distribution of ozone by oxidation reaction with silver metal, J. Geophys. Res., 113, D03303, doi:10.1029/2006jd008309, 2008.

Bottenheim, J. W., Barrie, L. A., and Atlas, E.: The Partitioning of Nitrogen-Oxides in the Lower Arctic Troposphere during Spring 1988, J. Atmos. Chem., 17, 15–27, doi:10.1007/BF00699111, 1993.

Burkhart, J. F., Bales, R. C., McConnell, J. R., and Hutterli, M. A.: Influence of North Atlantic Oscillation on anthropogenic transport recorded in northwest Greenland ice cores, J. Geophys. Res., 111, D22309, doi:10.1029/2005jd006771, 2006.

Chen, G., Huey, L. G., Crawford, J. H., Olson, J. R., Hutterli, M. A., Sjostedt, S., Tanner, D., Dibb, J., Lefer, B., Blake, N., Davis, D., and Stohl, A.: An assessment of the polar HOₓ photochemical budget based on 2003 Summit Greenland field observations, Atmos. Environ., 41, 7806–7820, doi:10.1016/j.atmosenv.2007.06.014, 2007.

Chu, L., and Anastasio, C.: Quantum yields of hydroxyl radical and nitrogen dioxide from the photolysis of nitrate on ice, J. Phys. Chem. A, 107, 9594–9602, doi:10.1021/Jp0349132, 2003.

Cole-Dai, J., Budner, D. M., and Ferris, D. G.: High speed, high resolution, and continuous chemical analysis of ice cores using a melter and ion chromatography, Environ. Sci. Technol., 40, 6764–6769, doi:10.1021/Es061188a, 2006.

Cole-Dai, J., Ferris, D. G., Lanciki, A. L., Savarino, J., Thiemens, M. H., and McConnell, J. R.: Two likely stratospheric volcanic eruptions in the 1450s C.E. found in a bipolar, subannually dated 800 year ice core record, J. Geophys. Res., 118, 7459–7466, doi:10.1002/jgrd.50587, 2013.

Dassau, T. M., Sumner, A. L., Koeniger, S. L., Shepson, P. B., Yang, J., Honrath, R. E., Cullen, N. J., Steffen, K., Jacobi, H. W., Frey, M., and Bales, R. C.: Investigation of the role of the snowpack on atmospheric formaldehyde chemistry at Summit, Greenland, J. Geophys. Res., 107, 4394, doi:10.1029/2002jd002182, 2002.

Davidson, C. I., Harrington, J. R., Stephenson, M. J., Small, M. J., Boscoe, F. P., and Gandley, R. E.: Seasonal variations in sulfate, nitrate and chloride in the Greenland ice sheet: relation to atmospheric concentrations, Atmos. Environ., 23, 2483–2493, doi:10.1016/0004-6981(89)90259-X, 1989.

Eckhardt, S., Stohl, A., Beirle, S., Spichtinger, N., James, P., Forster, C., Junker, C., Wagner, T., Platt, U., and Jennings, S. G.: The North Atlantic Oscillation controls air pollution transport to the Arctic, Atmos. Chem. Phys., 3, 1769–1778, doi:10.5194/acp-3-1769-2003, 2003.

Erbland, J., Vicars, W. C., Savarino, J., Morin, S., Frey, M. M., Frosini, D., Vince, E., and Martins, J. M. F.: Air-snow transfer of nitrate on the East Antarctic Plateau - Part 1: Isotopic evidence for a photolytically driven dynamic equilibrium, Atmos. Chem. Phys., 13, 6403–6419, doi:10.5194/acp-13-6403-2013, 2013.

Fibiger, D. L., Hastings, M. G., Dibb, J. E., and Huey, L. G.: The preservation of atmospheric nitrate in snow at Summit, Greenland, Geophys. Res. Lett., 40, 3484–3489, doi:10.1002/grl.50659, 2013.

Finkel, R. C. and Langway, C. C.: Changes in Precipitation chemistry at Dye 3, Greenland, J. Geophys. Res., 91, 9849–9855, doi:10.1029/JD091iD09p09849, 1986.

Fischer, H., Wagenbach, D., and Kipfstuhl, J.: Sulfate and nitrate firn concentrations on the Greenland ice sheet 2. Temporal anthropogenic deposition changes, J. Geophys. Res., 103, 21935–21942, doi:10.1029/98JD01886, 1998.

Flowers, B. A., Angerhofer, M. E., Simpson, W. R., Nakayama, T., and Matsumi, Y.: Nitrate radical quantum yield from peroxyacetyl nitrate photolysis, J. Phys. Chem. A, 109, 2552–2558, doi:10.1021/Jp045529n, 2005.

Ford, K. M., Campbell, B. M., Shepson, P. B., Bertman, S. B., Honrath, R. E., Peterson, M., and Dibb, J. E.: Studies of Peroxyacetyl nitrate (PAN) and its interaction with the snowpack at Summit, Greenland, J. Geophys. Res., 107, 4102, doi:10.1029/2001jd000547, 2002.

France, J. L., King, M. D., Frey, M. M., Erbland, J., Picard, G., Preunkert, S., MacArthur, A., and Savarino, J.: Snow optical properties at Dome C (Concordia), Antarctica: implications for snow emissions and snow chemistry of reactive nitrogen, Atmos. Chem. Phys., 11, 9787–9801, doi:10.5194/acp-11-9787-2011, 2011.

Frey, M. M., Savarino, J., Morin, S., Erbland, J., and Martins, J. M. F.: Photolysis imprint in the nitrate stable isotope signal in snow and atmosphere of East Antarctica and implications for reactive nitrogen cycling, Atmos. Chem. Phys., 9, 8681–8696, doi:10.5194/acp-9-8681-2009, 2009.

Fusco, A. C., and Salby, M. L.: Interannual variations of total ozone and their relationship to variations of planetary wave activity, J. Climate, 12, 1619–1629, doi:10.1175/1520-0442(1999)012<1619:IVOTOA>2.0.CO;2, 1999.

Galloway, J. N., Aber, J. D., Erisman, J. W., Seitzinger, S. P., Howarth, R. W., Cowling, E. B., and Cosby, B. J.: The nitrogen cascade, Bioscience, 53, 341–356, 2003.

Geng, L., Alexander, B., Cole-Dai, J., Steig, E. J., Savarino, J., Sofen, E. D., and Schauer, A. J.: Nitrogen isotopes in ice core nitrate linked to anthropogenic atmospheric acidity change, P. Natl. Acad. Sci., 111, 5808–5812, doi:10.1073/pnas.1319441111, 2014.
Giglio, L., Randerson, J. T., and van der Werf, G. R.: Analysis of daily, monthly, and annual burned area using the fourth-generation global fire emissions database (GFED4), J. Geophys. Res., 118, 317–328, doi:10.1002/jgrg.20042, 2013.

Grannas, A. M., Jones, A. E., Dibb, J., Ammann, M., Anastasio, C., Beine, H. J., Bergin, M., Bottenheim, J., Boxe, C. S., Carver, G., Chen, G., Crawford, J. H., Domine, F., Frey, M. M., Guzman, M. I., Heard, D. E., Helmig, D., Hoffmann, M. R., Honrath, R. E., Huey, L. G., Hutterli, M., Jacobi, H. W., Klan, P., Lefer, B., McConnell, J., Plane, J., Sander, R., Savarino, J., Shepson, P. B., Simpson, W. R., Sodeau, J. R., von Glasow, R., Weller, R., Wolff, E. W., and Zhu, T.: An overview of snow photochemistry: evidence, mechanisms and impacts, Atmos. Chem. Phys., 7, 4329–4373, doi:10.5194/acp-7-4329-2007, 2007.

Hastings, M. G., Steig, E. J., and Sigman, D. M.: Seasonal variations in N and O isotopes of nitrate in snow at Summit, Greenland: Implications for the study of nitrate in snow and ice cores, J. Geophys. Res., 109, D20306, doi:10.1029/2004jd004991, 2004.

Holton, J. R., Haynes, P. H., Mcintyre, M. E., Douglass, A. R., Rood, R. B., and Pfister, L.: Stratosphere-Troposphere Exchange, Rev. Geophys., 33, 403–439, doi:10.1029/95rg02097, 1995.

Honrath, R. E., and Jaffe, D. A.: The Seasonal Cycle of Nitrogen-Oxides in the Arctic Troposphere at Barrow, Alaska, J. Geophys. Res., 97, 20615–20630, 1992.

Honrath, R. E., Peterson, M. C., Guo, S., Dibb, J. E., Shepson, P. B., and Campbell, B.: Evidence of NOₓ production within or upon ice particles in the Greenland snowpack, Geophys. Res. Lett., 26, 695–698, doi:10.1029/1999GL900077, 1999.

Hutterli, M. A., McConnell, J. R., Stewart, R. W., Jacobi, H. W., and Bales, R. C.: Impact of temperature-driven cycling of hydrogen peroxide (H₂O₂) between air and snow on the planetary boundary layer, J. Geophys. Res., 106, 15395–15404, doi:10.1029/2001JD900102, 2001.

Jacobi, H. W., Weller, R., Bluszcz, T., and Schrems, O.: Latitudinal distribution of peroxyacetyl nitrate (PAN) over the Atlantic Ocean, J. Geophys. Res., 104, 26901–26912, doi:10.1029/1999JD900462, 1999.

Jacobi, H. W., Bales, R. C., Honrath, R. E., Peterson, M. C., Dibb, J. E., Swanson, A. L., and Albert, M. R.: Reactive trace gases measured in the interstitial air of surface snow at Summit, Greenland, Atmos. Environ., 38, 1687–1697, doi:10.1016/j.atmosenv.2004.01.004, 2004.

Jin, J. J., Semeniuk, K., Manney, G. L., Jonsson, A. I., Beagley, S. R., McConnell, J. C., Rinsland, C. P., Boone, C. D., Walker, K. A., and Bernath, P. F.: Denitrification in the Arctic winter 2004/2005: Observations from ACE-FTS, Geophys. Res. Lett., 33, L19814, doi:10.1029/2006gl027687, 2006.

Johnson, D. G., Jucks, K. W., Traub, W. A., and Chance, K. V.: Isotopic composition of stratospheric ozone, J. Geophys. Res., 105, 9025–9031, doi:10.1029/1999JD901167, 2000.

Kaiser, J., Hastings, M. G., Houlton, B. Z., Rockmann, T., and Sigman, D. M.: Triple oxygen isotope analysis of nitrate using the denitrifier method and thermal decomposition of N₂O, Anal. Chem., 79, 599–607, doi:10.1021/Ac061022s, 2007.

Kleinbohl, A., Bremer, H., Kullmann, H., Kuttippurath, J., Browell, E. V., Canty, T., Salawitch, R. J., Toon, G. C., and Notholt, J.: Denitrification in the Arctic mid-winter 2004/2005 observed by airborne submillimeter radiometry, Geophys. Res. Lett., 32, L19811, doi:10.1029/2005gl023408, 2005.

Kramer, L. J., Helmig, D., Burkhart, J. F., Stohl, A., Oltmans, S., and Honrath, R. E.: Seasonal variability of atmospheric nitrogen oxides and non-methane hydrocarbons at the GEOSummit station, Greenland, Atmos. Chem. Phys. Discuss., 14, 13817–13867, doi:10.5194/acpd-14-13817-2014, 2014.

Kunasek, S. A., Alexander, B., Steig, E. J., Hastings, M. G., Gleason, D. J., and Jarvis, J. C.: Measurements and modeling of Δ¹⁷O of nitrate in snowpits from Summit, Greenland, J. Geophys. Res., 113, D24302, doi:10.1029/2008jd010103, 2008.

Lamarque, J. F., Bond, T. C., Eyring, V., Granier, C., Heil, A., Klimont, Z., Lee, D., Liousse, C., Mieville, A., Owen, B., Schultz, M. G., Shindell, D., Smith, S. J., Stehfest, E., Van Aardenne, J., Cooper, O. R., Kainuma, M., Mahowald, N., McConnell, J. R., Naik, V., Riahi, K., and van Vuuren, D. P.: Historical (1850–2000) gridded anthropogenic and biomass burning emissions of reactive gases and aerosols: methodology and application, Atmos. Chem. Phys., 10, 7017–7039, doi:10.5194/acp-10-7017-2010, 2010.

Legrand, M. R. and Delmas, R. J.: Formation of HCl in the Antarctic atmosphere, J. Geophys. Res., 93, 7153–7168, doi:10.1029/JD093iD06p07153, 1988.

Levy, H., Moxim, W. J., Klonecki, A. A., and Kasibhatla, P. S.: Simulated tropospheric NOₓ: Its evaluation, global distribution and individual source contributions, J. Geophys. Res., 104, 26279–26306, doi:10.1029/1999JD900442, 1999.

Liang, M. C., Irion, F. W., Weibel, J. D., Miller, C. E., Blake, G. A., and Yung, Y. L.: Isotopic composition of stratospheric ozone, J. Geophys. Res., 111, D02302, doi:10.1029/2005jd006342, 2006.

Logan, J. A.: Nitrogen oxides in the troposphere: Global and regional budgets, J. Geophys. Res., 88, 10785–10807, doi:10.1029/JC088iC15p10785, 1983.

Lyons, J. R.: Transfer of mass-independent fractionation in ozone to other oxygen-containing radicals in the atmosphere, Geophys. Res. Lett., 28, 3231–3234, doi:10.1029/2000GL012791, 2001.

Mann, G. W., Davies, S., Carslaw, K. S., and Chipperfield, M. P.: Factors controlling Arctic denitrification in cold winters of the 1990s, Atmos. Chem. Phys., 3, 403–416, doi:10.5194/acp-3-403-2003, 2003.

Mauersberger, K., Lammerzahl, P., and Krankowsky, D.: Stratospheric ozone isotope enrichments-revisited, Geophys. Res. Lett., 28, 3155–3158, doi:10.1029/2001GL013439, 2001.

Mayewski, P. A., Lyons, W. B., Spencer, M. J., Twickler, M. S., Buck, C. F., and Whitlow, S.: An ice-core record of atmospheric response to anthropogenic sulphate and nitrate, Nature, 346, 554–556, doi:10.1038/346554a0, 1990.

McCabe, J. R., Thiemens, M. H., and Savarino, J.: A record of ozone variability in South Pole Antarctic snow: Role of nitrate oxygen isotopes, J. Geophys. Res., 112, D12303, doi:10.1029/2006jd007822, 2007.

Meusinger, C., Berhanu, T. A., Erbland, J., Savarino, J., and Johnson, M. S.: Laboratory study of nitrate photolysis in Antarctic snow. I. Observed quantum yield, domain of photolysis, and secondary chemistry, The J. Chem. Phys., 140, 244305, doi:10.1063/1.4882898, 2014.

Michalski, G., Scott, Z., Kabiling, M., and Thiemens, M. H.: First measurements and modeling of Δ¹⁷O in atmospheric nitrate, Geophys. Res. Lett., 30, 1870, doi:10.1029/2003gl017015, 2003.

Morin, S., Savarino, J., Bekki, S., Gong, S., and Bottenheim, J. W.: Signature of Arctic surface ozone depletion events in the isotope
anomaly (Δ¹⁷O) of atmospheric nitrate, Atmos. Chem. Phys., 7, 1451–1469, doi:10.5194/acp-7-1451-2007, 2007.

Morin, S., Savarino, J., Frey, M. M., Yan, N., Bekki, S., Bottenheim, J. W., and Martins, J. M. F.: Tracing the Origin and Fate of NOₓ in the Arctic Atmosphere Using Stable Isotopes in Nitrate, Science, 322, 730–732, doi:10.1126/science.1161910, 2008.

Morin, S., Sander, R., and Savarino, J.: Simulation of the diurnal variations of the oxygen isotope anomaly (Δ17O) of reactive atmospheric species, Atmos. Chem. Phys., 11, 3653–3671, doi:10.5194/acp-11-3653-2011, 2011.

Moxim, W. J., Levy, H., and Kasibhatla, P. S.: Simulated global tropospheric PAN: Its transport and impact on NOₓ, J. Geophys. Res., 101, 12621–12638, doi:10.1029/96JD00338, 1996.

Mulvaney, R., and Wolff, E. W.: Evidence for Winter Spring Denitrification of the Stratosphere in the Nitrate Record of Antarctic Firn Cores, J. Geophys. Res., 98, 5213–5220, doi:10.1029/92JD02966, 1993.

Osborn, T. J.: Winter 2009/2010 temperatures and a record-breaking North Atlantic Oscillation index, Weather, 66, 19–21, doi:10.1002/Wea.660, 2011.

Park, R. J., Jacob, D. J., Field, B. D., Yantosca, R. M., and Chin, M.: Natural and transboundary pollution influences on sulfate-nitrate-ammonium aerosols in the United States: Implications for policy, J. Geophys. Res., 109, D15204, doi:10.1029/2003jd004473, 2004.

Pye, H. O. T., Liao, H., Wu, S., Mickley, L. J., Jacob, D. J., Henze, D. K., and Seinfeld, J. H.: Effect of changes in climate and emissions on future sulfate-nitrate-ammonium aerosol levels in the United States, J. Geophys. Res., 114, D01205, doi:10.1029/2008jd010701, 2009.

Randel, W. J., Wu, F., and Stolarski, R.: Changes in column ozone correlated with the stratospheric EP flux, J. Meteorol. Soc. Jpn, 80, 849–862, doi:10.2151/Jmsj.80.849, 2002.

Röthlisberger, R., Hutterli, M. A., Wolff, E. W., Mulvaney, R., Fischer, H., Bigler, M., Goto-Azuma, K., Hansson, M. E., Ruth, U., Siggaard-Andersen, M. L., and Steffensen, J. P.: Nitrate in Greenland and Antarctic ice cores: a detailed description of post-depositional processes, Ann. Glaciol., 35, 209–216, doi:10.3189/172756402781817220, 2002.

Savarino, J., and Legrand, M.: High northern latitude forest fires and vegetation emissions over the last millennium inferred from the chemistry of a central Greenland ice core, J. Geophys. Res., 103, 8267–8279, doi:10.1029/97JD03748, 1998.

Savarino, J., Kaiser, J., Morin, S., Sigman, D. M., and Thiemens, M. H.: Nitrogen and oxygen isotopic constraints on the origin of atmospheric nitrate in coastal Antarctica, Atmos. Chem. Phys., 7, 1925–1945, doi:10.5194/acpd-6-8817-2006, 2007.

Shepherd, T. G.: Dynamics, stratospheric ozone, and climate change, Atmos Ocean, 46, 117–138, doi:10.3137/Ao.460106, 2008.

Singh, H. B., Ohara, D., Herlth, D., Bradshaw, J. D., Sandholm, S. T., Gregory, G. L., Sachse, G. W., Blake, D. R., Crutzen, P. J., and Kanakidou, M. A.: Atmospheric Measurements of Peroxyacetyl Nitrate and Other Organic Nitrates at High-Latitudes: Possible Sources and Sinks, J. Geophys. Res., 97, 16511–16522, doi:10.1029/91JD00889, 1992.

Sjostedt, S. J., Huey, L. G., Tanner, D. J., Peischl, J., Chen, G., Dibb, J. E., Lefer, B., Hutterli, M. A., Beyersdorf, A. J., Blake, N. J., Blake, D. R., Sueper, D., Ryerson, T., Burkhart, J., and Stohl,

A.: Observations of hydroxyl and the sum of peroxy radicals at Summit, Greenland during summer 2003, Atmos. Environ., 41, 5122–5137, doi:10.1016/j.atmosenv.2006.06.065, 2007.

Stroud, C., Madronich, S., Atlas, E., Ridley, B., Flocke, F., Weinheimer, A., Talbot, B., Fried, A., Wert, B., Shetter, R., Lefer, B., Coffey, M., Heikes, B., and Blake, D.: Photochemistry in the arctic free troposphere: NOₓ budget and the role of odd nitrogen reservoir recycling, Atmos. Environ., 37, 3351–3364, doi:10.1016/S1352-2310(03)00353-4, 2003.

Talukdar, R. K., Burkholder, J. B., Schmoltner, A. M., Roberts, J. M., Wilson, R. R., and Ravishankara, A. R.: Investigation of the Loss Processes for Peroxyacetyl Nitrate in the Atmosphere - UV Photolysis and Reaction with OH, J. Geophys. Res., 100, 14163–14173, doi:10.1029/95JD00545, 1995.

Thomas, J. L., Dibb, J. E., Huey, L. G., Liao, J., Tanner, D., Lefer, B., von Glasow, R., and Stutz, J.: Modeling chemistry in and above snow at Summit, Greenland - Part 2: Impact of snow-pack chemistry on the oxidation capacity of the boundary layer, Atmos. Chem. Phys., 12, 6537–6554, doi:10.5194/acp-12-6537-2012, 2012.

Waugh, D. W., and Randel, W. J.: Climatology of arctic and antarctic polar vortices using elliptical diagnostics, J. Atmos. Sci., 56, 1594–1613, 1999.

Weber, M., Dikty, S., Burrows, J. P., Garny, H., Dameris, M., Kubin, A., Abalichin, J., and Langematz, U.: The Brewer-Dobson circulation and total ozone from seasonal to decadal time scales, Atmos. Chem. Phys., 11, 11221–11235, doi:10.5194/acp-11-11221-2011, 2011.

Whitlow, S., Mayewski, P. A., and Dibb, J. E.: A Comparison of Major Chemical-Species Seasonal Concentration and Accumulation at the South-Pole and Summit, Greenland, Atmos. Environ., 26, 2045–2054, doi:10.1016/0960-1686(92)90089-4, 1992.

WMO: Scientific assessment of ozone depletion: 2006, Geneva50, 572, 2007.

Yang, J., Honrath, R. E., Peterson, M. C., Dibb, J. E., Sumner, A. L., Shepson, P. B., Frey, M., Jacobi, H. W., Swanson, A., and Blake, N.: Impacts of snowpack emissions on deduced levels of OH and peroxy radicals at Summit, Greenland, Atmos. Environ., 36, 2523–2534, doi:10.1016/S1352-2310(02)00128-0, 2002.

Yang, Q. Z., Mayewski, P. A., Whitlow, S., Twickler, M., Morrison, M., Talbot, R., Dibb, J., and Linder, E.: Global Perspective of Nitrate Flux in Ice Cores, J. Geophys. Res., 100, 5113–5121, doi:10.1029/94JD03115, 1995.

Zatko, M. C., Grenfell, T. C., Alexander, B., Doherty, S. J., Thomas, J. L., and Yang, X.: The influence of snow grain size and impurities on the vertical profiles of actinic flux and associated NOₓ emissions on the Antarctic and Greenland ice sheets, Atmos. Chem. Phys., 13, 3547–3567, doi:10.5194/acp-13-3547-2013, 2013.

Zhou, X. L., Beine, H. J., Honrath, R. E., Fuentes, J. D., Simpson, W., Shepson, P. B., and Bottenheim, J. W.: Snowpack photochemical production of HONO: a major source of OH in the Arctic boundary layer in springtime, Geophys. Res. Lett., 28, 4087–4090, doi:10.1029/2001GL013531, 2001.