Snow-pile and chamber experiments during the Polar Sunrise Experiment ‘Alert 2000’: exploration...

Atmospheric Environment 36 (2002) 2707–2719

Snow-pile and chamber experiments during the Polar SunriseExperiment ‘Alert 2000’: exploration of nitrogen chemistry

Harald J. Beinea,b,*, Florent Domin!ec, William Simpsond, Richard E. Honratha,Roberto Sparapanib, Xianliang Zhoue, Martin Kingd,f

aMichigan Technological University, Houghton, MI, USAbC.N.R.-Istituto sull’Inquinamento Atmosferico, Rome, Italy

cCNRS-Laboratoire de Glaciologie et G!eophysique de l’Environnement, St. Martin d’H"eres, FrancedGeophysical Institute and University of Alaska, Fairbanks, AK, USA

eWadsworth Center, U. Albany, NY, USAfDepartment of Chemistry, King’s College London Strand, UK

Received 4 June 2001; received in revised form 8 October 2001; accepted 1 November 2001

Abstract

Snow chamber and snow-pile experiments performed during the ‘Alert 2000’ campaign show significant release of

NO, NO2, and HONO in steady ratios under the influence of irradiation. Both light and a minimal degree of heating are

required to produce this effect. We suggest diffusion and re-distribution of NO3� in the form of HNO3 as an important

step in the mechanism of active nitrogen release from the snowpack. r 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Nitrogen oxides; Arctic; Snow; Photochemical release; Diffusion of HNO3

1. Introduction

In addition to the measurements of many ambient

trace species and parameters during the Polar Sunrise

Experiment 2000 at Alert, Nunavut, several different

snow experiments were carried out. These were mainly

of two kinds; snow-piles and chambers. The experiments

were made during both the dark and the light intensive

and used either lamps or shading to effect rapid changes

in the light conditions. This paper concerns specifically

the nitrogen chemistry in snow and exchange processes

in and above dark and sunlit snow surfaces. Snow

chamber studies were developed recently to study in a

defined ‘reactor’ the recycling of nitrate, and emissions

of NOx and HONO (Honrath et al., 2000; Zhou et al.,

2001; Dibb et al., 2002) and organic species (Sumner

et al., 2002). The snow-pile studies described here are a

new approach to investigate a confined amount of snow

without the complication of wall effects inside a

chamber. Also for the first time, the snow used in all

experiments was characterized both for physical proper-

ties and chemical ion content. The aim of this study is (a)

to give an overview of a number of snow experiments to

which many researchers contributed with their measure-

ments, and (b) to specifically explore nitrogen chemistry.

We are interested in which species are released, which

are the factors that lead to the release, and what is the

relationship between the intake, reservoir and release of

nitrogen in the snow.

As discussed by Honrath et al. (2000) NO3� photolysis

in an aqueous surface phase of the snow layer may

produce both NO2 and NO2 (aq)� . The latter would

continue to react towards NO and HONO. A first aim

of the snow experiments was to quantify release rates of

these species, and their timing, to be able to test the

mechanism. Recent laboratory experiments on sub-mm

spray frozen aqueous nitrate solutions confirmed the

*Corresponding author. C.N.R.-Istituto sull’Inquinamento

Atmosferico, Via Salaria Km 29.3 CP10, 00016 Monterotondo

Scalo, Rome, Italy.

E-mail address: [email protected] (H.J. Beine).

1352-2310/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 1 2 0 - 6

production of both NO2 and NO2 (aq)� (Dubowski et al.,

2001). In these experiments NO2 was directly released

only from a o50mm surface layer, while in deeper layers

secondary reactions occurred.

The photochemical reactions of NO3� in snow so far

have been discussed in terms of chemistry. We present

evidence in this work for transport processes that occur

in the snow, and identify gas-phase HNO3 as important

link for re-distributing NO3� in the snowpack.

Although the species that contribute to the NO3�

signal in the snow are not known with certainty, there

are clear indications what these may (Bergin et al., 1995)

or may not (Dibb et al., 1998) be. In order to identify the

importance of a possible in situ source of NO3� onto the

snowpack (Ianniello et al., manuscript in preparation),

we attempt a mass balance and nitrogen balance for the

snow used in our experiments.

Finally, there are several reasons why manipulated

snow in such experiments may give different results than

natural snow in a pristine surrounding. In particular, the

experimental conditions in the snow piles turned out to

be rather extreme. The discussion of the mechanisms in

this study is an attempt to extract information about the

natural behavior without over interpreting our indivi-

dual results.

2. Experimental

The experimental methods and ambient data from the

various measurements used here are described in detail

in companion papers (Beine et al., 2002; Domin!e et al.,

2002; Simpson et al., 2002). Some of the results

pertaining to HONO were recently published separately

(Zhou et al., 2001). Briefly, NO was measured with the

Michigan Tech. University (MTU) chemiluminescence

instrument. NO2 was detected as NO following UV-

broadband photolysis (Peterson and Honrath, 1999).

For the NOx measurements in the chamber, the upper

inlet was replaced with a connection to the snow

chamber or pile. The length of the inlet tubing was kept

equal for both inlets. Thus, ambient air NOx measure-

ments were available at 0.5m inlet height, and allowed

us to monitor changes in the experiments with respect to

ambient conditions. HONO was measured following

DNPH derivatization using HPLC analysis (Zhou et al.,

1999). In addition to the chamber measurements,

ambient air was drawn for reference from 10 cm above

the snow surface. Snow sampling for both physical and

chemical properties was performed by F. Domin!e

following a strict sampling protocol (Cabanes et al.,

2002; Domin!e et al., 2002). Specific surface areas (SSA)

of snow were determined by methane absorption at 77K

(Hanot and Domin!e, 1999). Analysis of inorganic ions

from denuder and filter measurements (Ianniello et al.,

manuscript in preparation) and discrete snow samples

(Domin!e et al., 2002) was performed using the ion

chromatographic system (IC) of the Italian National

Research Council (C.N.R.-IIA). The IC analyses have a

precision better than 5% (Beine et al., 2001, and

references therein). In all cases the snow for these

experiments was handled only with special clean

equipment, and the person handling the snow wore

special protective clean-room clothing and gloves. The

individual experiments described in this work are

identified with either P (pile) in Table 1 or Q (quartz-

chamber) in Fig. 5. This is not a complete list of all snow

experiments performed during the PSE 2000. The stated

flowrates are in all cases total flow through the snow. In

the pile experiments this is the sum of all inlet flows of

the connected instruments; in the chamber experiments

an additional pump was used to hold this flow constant.

2.1. Snow-piles

During the dark intensive several experiments with

snow-piles were performed. A total of three snow-piles

were built at the Special Studies Trailer near Alert (SST;

82127.290N, 62131.860E). These snow-piles were homo-

geneous. They were built using one layer of snow only,

which was characterized beforehand. Both snow chem-

istry and physics were characterized before and after the

experiments for the first two piles. Two similar piles were

built near a second measurement site (FTX) with similar

snow. Here only their snow characteristics, but not the

actual experiments, are discussed.

Snow-piles were made with snow that fell on 3

February, while moderate winds were blowing. This

snow was therefore wind-accumulated in wind-sheltered

spots, where its thickness was 3–8 cm. It was overlain by

a homogeneous layer, about 5–10mm thick, fallen under

calm conditions on 7 February (see Domin!e et al., 2002,

for details). The 7 February layer was removed and only

the 3 February layer was used. This layer consisted of

sub-millimeter size columns and bullet combinations

(Domin!e et al., 2002). It was essentially uncohesive,

although a few fragile chunks were noticed during

sampling, and a few others may have formed during the

formation of the pile. Numerous IC analyses of 3

February snow samples taken from both the SST and

the FTX area showed the homogeneity of the NO3�

content of the snow within 10% (Domin!e et al., 2002).

The base of an even area of 40� 40 cm was cleaned from

hoar and 7 February snow. A Teflon filter pack without

any filter (to create some dead air space and prevent

clogging) was set in the middle of the base connected to

1/400 tubing, and covered evenly with the snow. After the

experiments the piles were taken apart to sample snow at

a number of locations and to confirm the uniformity of

the pile. For the very first pile a thermocouple was added

after experiments were already in progress. In the later

piles thermocouples were buried a few cm above the

H.J. Beine et al. / Atmospheric Environment 36 (2002) 2707–27192708

filter pack. The dimensions of the piles were radius

E30 cm, and height E35–40 cm. Thus the maximum

volume, weight, and snow surface area were 65 l, 11 kg,

and 600m2, respectively, for 3 February snow. At the

SST several instruments sampled from the same inlet

inside the snow-pile: NOx, HCHO, O3, PAN, HONO,

VOC, and Hg. The total flow rate was up to 9 slm.

Typically, air was sampled for several hours in darkness,

then a lamp was used to irradiate the pile from about

40 cm above the top.

Table 1 summarizes the times of the snow-pile

experiments. After sampling from the pile for ca. 12 h

under ambient dark conditions, a 150W Xe-arc lamp

(ILC-Cermax) with a Pyrex beaker was used to irradiate

the pile. The Pyrex beaker was added to eliminate UV-C

and attenuate some short wavelength UV-B. The

spectrum for the lamp used in these experiments with

the Pyrex beaker is compared to downwelling solar

irradiance in Fig. 1. The unfiltered lamp spectrum (not

shown) contained UV down to 250 nm and thus

resembled solar irradiance at the top of the atmosphere,

i.e. with 0 D.U. ozone. In all pile experiments a Pyrex

beaker was used in front of the lamp to filter the UV-C

part below 290 nm. We did detect significant amounts of

radiation o300 nm, which is essentially absent (o0.1%

of 400 nm radiation) in skylight at Alert. Therefore, the

pile experiments could have been influenced by photo-

chemical pathways that are not active in Alert skylight.

With the light beam hitting the pile center, the Eppley

irradiance was estimated as about one order of

magnitude brighter than noon-time sun at Alert (for

DOY 113). This very high intensity spot was then

rapidly diffused within the snowpack. Since the e-folding

depth of UV radiation in snow is only a weak function

of wavelength, the pile was illuminated with a spectrum

similar to the one shown in Fig. 1.

The sampling cycleFambient darkness, UV irradia-

tionFwas repeated once. The flow through the pile was

then turned off completely over night, and then

sampling was resumed for 1 day under ambient

conditions.

The experiences with the first pile determined much of

the strategy for the following experiments. After the

experiment 1P2Xe it was found that the top part of the pile

had melted and produced an ice/slush cone that reached

in the center of the pile. The temperature at the base was

elevated, also. To identify heat effects on the snow, the

second pile 2P was irradiated with an IR heat lamp only.

After a short period of ambient sampling the pile was

irradiated at 25% and the 100% intensity over night.

Table 1

Three piles at the SST

Codea Experiment Start times, EST (DOY)

1P Constructed from 3 Feb. snow1P1

A Ambient air 18 Feb., 15:20 (49.643)1P2

Xe Irradiation with 150W Xe-arc lampb 19 Feb., 3:05 (50.131)1P3


Xe Irradiation with 150W Xe-arc lampb 20 Feb., 15:35 (51.653)1P5


OFF All off. No flow 21 Feb., 20:35 (52.857)1P7

A Ambient air 22 Feb., 9:10 (53.385) end: 22 Feb., 11:15 (53.465)2P Built after some strong wind. The 3 Feb. layer was

windblown, harder and denser, and may have

contained other snow2P8


H25 Heat lamp 25% 22 Feb., 15:50 (53.661)2P10

H100 Heat lamp 100% 22 Feb., 19:30 (53.818)2P11

A Ambient air 23 Feb., 10:16 (54.420) end: 24 Feb., 13:48

(55.575)3P Constructed from top 2 cm of a layer of fresh

snow3P12

A Ambient air 9 Mar., 18:18 (69.762)3P13

Cold-UV Xe-arc lampb: cold UVc 9 Mar., 23:13 (69.967)3P14

A Ambient air 10 Mar., 10:17 (70.428) end: 11 Mar., 17:55

(71.747)

aThe experiments are identified by this code; number of snow-pilelocation Pnumber of experiment

experiment ; where P stands for pile. The experiments discussed

here were mostly carried out at the SST, this location is not again specified for each experiment.bA glass beaker was set in front of the Xe-arc lamp in all experiments to filter out part of the ‘hard’ UV spectrum; See Fig. 1 for a

spectrum of the lamp.cA pyrex beaker with 300ml H2O was set in front of the Xe-arc lamp to filter out the IR part of the spectrum.

H.J. Beine et al. / Atmospheric Environment 36 (2002) 2707–2719 2709

Sampling of that pile was concluded with one day of

ambient dark conditions.

During the third pile experiment snow was irradiated

with a ‘cold’ UV lamp: a beaker filled with water was

placed before the Xe lamp. The IR part of the lamp

spectrum was filtered. Thus, a distinction between UV

and IR effects was possible from these three experi-

ments. Little attenuation from water in the ‘cold’ UV

experiments is seen in the UV. Again, the irradiation

period was preceded and followed by sampling under

ambient dark conditions.

2.2. Snow chamber experiments

During the light intensive, experiments were per-

formed using the MTU snow chamber. This quartz

chamber was 1m long, and had an inner diameter of

17.8 cm. It was filled to 94.5 cm length. The volume of

snow was about 23.5 l. This equaled about 4.7 kg H2O,

with a surface area of 234m2. Air was drawn only

through the inlet, a 1/200 Teflon port.

Similar to the snow-piles, the snow chamber was filled

with one layer of snow only, and then set vertically on

one end over night, so that the snow had a chance to

anneal, and to reduce wall-effects. The chamber was

used for four experiments with NOx: two with snow, and

two blanks with the empty chamber. The flowrates were

kept constant using an additional pump and a mass flow

controller downstream of the chamber. This was

adjusted in the range 6.2–12.75 slm, as discontinuous

samples such as for VOCs were taken, and some

instruments were not used in all experiments.

2.3. Residence times in the snow chamber

Air flow through the chamber was characterized by

the addition of NO calibration gas using a step function

standard addition. These results indicate that the snow-

filled chamber behaved only somewhat like an ideal plug

flow reactor. Little or no channeling was observed, but a

long tail in the response to the added NO was observed,

indicating the presence of ‘dead volume’: regions of the

interior of the chamber that exchanged more slowly with

the main flow. Thus, no NO reached the outlet before

80% of the chamber residence time (which was 67 s), but

it took approximately two residence times for the exit

concentration to reach its steady-state value. In contrast,

the empty snow chamber exhibited a significant amount

of mixing in the longitudinal direction: NO was detected

at the outlet after only 27% of the residence time (which

was 128 s), and it took 2.3–3.5 residence times for the

outlet concentration to reach its steady-state value.

2.4. Blank experiments with the snow chamber

In experiments in which the snow is enclosed by a

chamber, it is possible that the surfaces of the chamber

walls influence the observations (Honrath et al., 2000).

To quantify such effects, a blank experiment was

performed during the light intensive with the MTU

chamber on two days, 22 and 24 April. During the

second day, Alert plume air was sampled shortly after

the chamber was connected, and the blank test was

aborted. Maximum blank values for HONO, NO and

NO2 of ca. 6, 10 and 22 pmol/mol, respectively, were

observed. These values are significantly smaller than the

production in the snow filled chamber. The cause of this

artifact is not known, but it may be emission from the

snow surface around, or contamination of the chamber,

which may have been not fully dry during the experi-

ments.

3. Results

Results were obtained for the physical and chemical

characteristics of the snow used in the experiments and

for the air pulled through the chambers/piles from a

number of instruments. In this work we describe the

results from the snow characterization and for active

nitrogen in the gas phase.

The ‘production’ of species in the chamber/piles refers

to the difference between the measured mixing ratio

from the chamber/pile and the ambient mixing ratio

measured at the same time through the inlet at 0.5m

above the ground (10 cm for HONO). Note that

0.001

0.01

0.1

1

Rel

ativ

e ir

radi

ance

/ ar

bita

ry u

nits

(N

orm

aliz

ed a

t 400

nm)

420400380360340320300

Wavelength / nm

Fig. 1. Normalized spectrum (at 400 nm) of the 150W Xe-arc

lamp with Pyrex beaker (dashed line) that was used for the

snow-pile experiments during the dark intensive. The solid line

shows the solar spectrum at Alert, the dotted line shows the

spectrum of the lamp attenuated by water. In this setup at Alert

the Pyrex beaker was used to filter out the UV part of the

spectrum.


gradients of NO, NO2, or HONO may influence this

result, especially during the light intensive (Beine et al.,

2002; Zhou et al., 2001). The inlet for the chamber was

8 cm above the snow surface level.

ProductionðNOxÞ ¼Mixing ratio at pile=chamberðNOxÞ

�Ambient mixing ratioðNOxÞ: ð1Þ

3.1. Snow-pile results

Fig. 2 shows an overview over all snow-pile experi-

ments that were performed at the SST. It is immediately

obvious that significant amounts of NO, HONO, or

NO2 were only produced in the illumination experiments

with the Xe-arc lamp. Neither NO nor HONO showed

any significant release during the heat or the ‘cold-UV’

experiment. NO2 showed small and variable production

of 20–50 pmol/mol during all heat and cold-UV experi-

ments, whether the light was turned on or not. There

was no statistically significant difference induced by the

change in light conditions. The sampling site was

impacted by local pollution during the experiments with

the heat lamp. The ambient airmasses showed variable

NO2 mixing ratios of up to 400 pmol/mol; this would

mask a possible production. However, no such ambient

influence was detected during the illumination with the

cold-UV lamp. This result indicates that in the pile

experiments both light and heat are necessary for the

release of these chemicals. This finding was not expected

in light of previous measurements of NO2 release in

snow chamber experiments. During the SNOW99 study

Honrath et al. (2000) found significant release of NO2 in

diffuse sunlight. However, heating of the snowpack due

to absorbed sunlight was not explicitly ruled out. The

temperature in the midlatitude snow that was used in

SNOW99 was up to �21C, which in our experiments

would only be found in the center of the melt zone of the

pile. Thus, a certain minimum temperature may be

necessary for the release of NOx.

Fig. 3 shows the timeseries of NO, NO2, and HONO

during the Xe-arc lamp experiments. All three species

rise in a similar, exponential fashion. NO drops

immediately to the background level as the Xe-arc lamp

is turned off, while both NO2 and HONO levels decay

more slowly. HONO and NO2 drop exponentially with a

lifetime of ca. 30min and 75–130min, respectively. All

three species show some memory effect; when the light is

turned on the second time, an initial higher mixing ratio

is immediately reached. NO2 is known to be adsorbed to

ice surfaces, while NO is not (Ammann et al., 2000). The

adsorbed NO2, however, may facilitate the production

of HONO or NO.

The relative ratios of the released nitrogen species

changed over the time of the irradiation significantly

(Fig. 4). When the pile was first exposed to UV light

only NO2 was released. Over the next 5 h the NO2

2 4 6 8 10 12 14

0

50

100

150

200

250

300

NO

pro

duced in p

ile [p

mol/m

ol]

2 4 6 8 10 12 14

0

100

200

300

400

500

NO

2 p

rod

uce

d in

pile

[p

mo

l/m

ol]

2 4 6 8 10 12 14

0

20

40

60

80

100

120

HO

NO

pro

duce

d in

pile

[p

mol/m

ol]

1 A

2 X

e3

A

4 X

e5

A6

Off

7 A

8 A

9 H

25

1

0 H

10

0

11

A

12

A1

3 c

old

-UV

14

A

1P 2P 3P

Name of Experiment

1 A

2 X

e3

A4

Xe

5 A

6 O

ff7

A8

A9

H2

5

10

H1

00

1

1 A

12

A1

3 c

old

-UV

14

A

1P 2P 3P

Name of Experiment

1 A

2 X

e3

A4

Xe

5 A

6 O

ff7

A8

A9

H2

5

10

H1

00

1

1 A

12

A1

3 c

old

-UV

14

A

1P 2P 3P

Name of Experiment

Fig. 2. (a) NO, (b) NO2, and (c) HONO produced in the snow-pile experiments. The figures show box-and-whisker plots for all snow-

pile experiments (see Table 1 for ‘name of experiment’). For all 3 species significant release from the piles occurred only during

irradiation with the Xe-arc lamp. In the box plot the center vertical line marks the median of the sample. The length of each box shows

the range within which the central 50% of the values fall, with the box edges (hinges) at the first and third quartile. The whiskers show

the range of values that fall within the inner fences. Values between the inner and outer fences are plotted with asterisks. Values outside

the outer fence are plotted with circles (Systat 9, 1999).


fraction dropped to 50%. HONO, with some delay,

jumped to ca. 30% and increased slowly, while NO

increased steadily during that time. A minimum NO2

fraction was seen in both experiments; this minimum

was used to scale the x-axis (time) in Fig. 4. In the

second pile experiment time =0 is the starting point and

reflects the memory effect of NO2 in the pile. A steady

ratio of 3:6:2 for NO:NO2:HONO was reached after a

further 6 h. The initial release of NO2 only may reflect

the primary process in the snow, while secondary

processes; heating, transport and further photolysis lead

to the increasing production of the other species. This

was recently discussed quantitatively in laboratory

experiments on thin spray frozen aqueous nitrate

solutions (Dubowski et al., 2001): NO3 (aq)� was found

to uniformly photolyze in a liquidlike surface layer to a

depth of 400 mm to NO2 and NO2 (aq)� . Only NO2 formed

in the uppermost 50mm was able to escape, while the

remainder reacted to NO and HONO. In these experi-

ments the production of NO2 was found to increase

significantly with temperature.

In a noon-time solar spectrum for Alert (DOY 108),

the ratio of J(HNO3)/J(NO3 (aq)� ) at 243K was about 2.9.

Wavelengths below 315 nm favor HNO3 photolysis,

while longer wavelengths favor the photolysis of NO3�.

NO3 (aq)� is typically present in concentrations orders of

magnitudes larger than HNO3 in the interstitial air, so

that a small fraction of NO3 (aq)� is necessary at the

surface for this to represent a much more important

precursor for NO2 than HNO3 (Abbatt, 1997). Using

the Xe-arc lamp, however, the J(HNO3)/J(NO3 (aq)� )

ratio increased up to 15. Together with the higher lamp

intensity, this may have favored the photochemistry of

gaseous HNO3 in the interstitial air, compared to

natural sunlight. The time delay to reach stable ratios

in Fig. 4 was thus an indication of the occurrence of a

probably small fraction of gaseous HNO3 in the snow-

pile, which diffused out from the snow crystals. This

50 51 52 53

Day of Year 2000

0

100

200

300

400

500

NO

2 p

roduce

d [p

mol/m

ol]

50 51 52 53

Day of Year 2000

0

20

40

60

80

100

120

HO

NO

pro

duced [p

mol/m

ol]

Xe-arc lampambient

50 51 52 53

Day of Year 2000

0

50

100

150

200

250

300

NO

pro

duce

d [p

mol/m

ol]

Fig. 3. NO, NO2 and HONO produced during illumination with the Xe-arc lamp.

-5 0 5 10 15Time of the Experiment [hours]

20

40

60

80

100

% o

f to

tal N

rel

ease

HONONO2NO

released N species

Fig. 4. Ratio of the released N species (%) during the two UV irradiation experiments (combined dataset). The x-axis shows time (h).

For 1P2Xe time was set =0 at DOY 50.302 after an initial period of large changes that lasted ca. 5 h. For 1P4

Xe no such initial period was

observed, and the start of the illumination at DOY 51.677 was set as time =0.


HNO3 was important for transport and redistribution

within the pile.

With a flowrate of about 7.5–9 lmin�1 through the

pile, the median hourly sum of released nitrogen from

these three species is 43.1 and 193.6 ngNh�1, for the two

experiments, respectively. The total amount of nitrogen

released as NOx or HONO from pile 1P in these two

experiments was 0.47 and 3.57mgN. The temperature in

the snow was measured during the second irradiation,

when a melt hole had already been drilled by the lamp.

While the surface snowpack temperature was –371C, the

temperature measured about 1 cm away from the melt

hole was –91C to –101C. Temperatures near the side of

the pile were –301C, to –341C. For comparison, the

temperature during the cold-UV experiment did not

increase over –301C in the pile. After those measure-

ments, the wind started to pick up and blew away most

of the pile over the following night. The pumping inlet

remained safely covered, and the snow remaining

around it was definitely the snow that was originally

placed on it. That snow was sampled for anions only

(Table 2).

3.2. Mass and nitrogen balance in snow during the pile

experiments

Tables 2 and 3 summarize the changes in the snow

chemistry during the pile and chamber experiments,

respectively. In the snow-piles close to the air inlet, Cl�,

NO3�, and SO4

2� were increased significantly. The gain of

Cl� together with a loss of Na+ excludes an enrichment

of sea-salt aerosols due to flow through the pile. An

enrichment of crustal material (CaSO4) is possible.

At the SST pile 1P, a decrease in cations is observed,

which has not been observed in snow in this way, and is

unexpected, since mineral particles do not dissolve. It is

conceivable that particles could be removed from

homogeneous snow through sublimation. A sub-micro

size particle is thus set free and could be carried in the

airflow through snow particles, which are on the order

of 0.2mm in size. It was for example shown that depth

hoar is less enriched in ions than other snow layers

because it undergoes strong sublimation and re-crystal-

lization (Domin!e et al., 2002). In this way aerosols fall

out and the snow can lose particles.

For the nitrogen balance of the snow-piles we would

expect the release of NOx and HONO to be consistent

with the uptake of NO3� from particles and the total

concentration of NO3� in the pile. Table 2 shows that

during the irradiation experiment the concentration of

NO3� increased in the pile. This result seems somewhat

counter-intuitive, as a decrease is expected due to the

release of NOx and HONO. The increase over the entire

experiment in snow-NO3� was on the order of 80 ng/g. If

no water/snow from the pile were lost, this would

translate into an increase of ca. 220 mgN. This apparent

Table 2

Snow characteristics during the dark intensive pile experiments. All mixing ratios in ppbw (ng/g)

Date Sample Density

(g cm�3)

SSA

(m2g�1)

Cl� NO2� Br� NO3

� SO42� Na+ NH4

+ K+ Mg2+ Ca2+

9 Feb. 3 Feb. snowa 0.16 0.069b 494.3 8.8 13.2 278.3 271.2

10 Feb. 3 Feb. snow 0.16 0.067 549.3 7.9 159.3 142.7 150.7 12.9 18.5 55.0 137.5

15 Feb. 3 Feb. snow 0.16 0.057 636.9 0.0 2.5 43.0 188.3 197.3 5.4 11.6 41.5 89.5

20 Feb. 3 Feb. snow 0.16 0.046 468.8 61.8 232.4 210.5 9.0 14.2 47.6 123.7

18 Feb. Before FTX1 PA

1 c,d 459.6 6.5 9.0 114.4 228.9 162.3 9.0 12.8 46.4 129.9

19 Feb. After FTX1 PA

1 , inlet 511.6 7.8 12.8 215.4 348.1 157.8 15.1 17.9 55.0 180.6

19 Feb. After FTX1 PA

1 , side 429.0 10.2 188.6 257.0 139.6 10.5 10.7 43.5 129.1

22 Feb. After 1P7A 0.043 525.7 9.1 157.5 306.0

Difference FTX1 PA

1 inlet 52.1 1.2 3.8 101.0 119.2 �4.5 6.1 5.1 8.6 50.7

Difference FTX1 PA

1 side �30.6 �6.5 1.2 74.2 28.1 �22.7 1.4 �2.1 �2.8 �0.8

Difference 1P7A

e 66.1 �6.5 0.1 43.1 77.1 �162.3 �9.0 �12.8 �46.4 �129.9

Difference 1P7A

f �0.004g 56.9 9.1 95.7 73.7 �210.5 �9.0 �14.2 �47.6 �123.7

aFrom ‘Site A’ (Domin!e et al., this issue).bThis SSA declined linearly with time in undisturbed snow. From 8 samples measured until 20 Feb. a linear regression of

SSA=0.1523–0.00207�DOY (R2 ¼ 0:81; p ¼ 0:0022) emerged.cSome of the experiments shown in Table 1 were duplicated near the FTX with similar snow, flow rates and other experimental

conditions. The samples shown here were taken from these piles at the FTX. Domin!e et al. (2002) have shown the spatial uniformity of

the snow around the sampling sites.dTriplicate samples were taken in the middle of the pile near the inlet and from the side of the pile. Before airflow or irradiation was

started, these measurements gave identical results.eComparing to before FTX

1 PA1 data.

fComparing to 3 Feb. snow at ‘site A’ sampled on 20 Feb.gThis is identical to the expected value for ‘aging’ of undisturbed snow; see (b) above.


‘enrichment’ is 2 orders of magnitude larger than the N

loss to the atmosphere.

A possible loss path for water is sublimation into the

airflow. The flowrate through the pile during the first

experiment 1P varied between 7.5 and 9 slm. The total

flow through this pile was 37.5m3. The maximum

amount of sublimation can be estimated using the

saturation vapor pressure of water over ice (e.g. Marti

and Mauersberger, 1993) and assuming that the incom-

ing air flow is dry. Even if we assume that all the air

flows through the melt-zone at the maximum tempera-

ture of 01C, the total amount of water lost from the pile

is only on the order of 180 g. This amounts only to 1.5%

of the total mass of the pile and is thus negligible for the

enrichment in NO3�.

Total NO3� concentrations in atmospheric particles

showed a median value of 90.5 ng/m3 during the dark

intensive, and 135.5 ng/m3 during the light intensive

(Ianniello et al., in preparation). The maximum total

source of NO3� from particles into the snow-pile during

the dark campaign can thus be estimated as 0.76 mgN.

This gain of N through particulate NO3� is about 1/5 of

the loss due to emission as NOx and HONO during the

snow-pile experiments. The gains through NO3� deposi-

tion as well as the losses through the release of NOx and

HONO are three orders of magnitude smaller than the

total amount of nitrogen present in the snow-pile. In a

11 kg snow-pile the total amount of nitrogen was 0.15–

0.7mgN. The NO3� ‘enrichment’ in the pile was thus the

largest contribution to the N-balance.

3.3. Mobility of NO3�

An almost two-fold increase in NO3� was observed

after the irradiation experiments. We demonstrated that

atmospheric exchange cannot explain this increase, and

are therefore led to the conclusion that redistribution

and migration of nitrate took place. We suggest here

that the strong thermal gradient induced in the pile by

heating released NO3� from the snow in the form of

HNO3. Adjacent snow that has remained cold took up

this released HNO3. The ionic balance of the snow

shows that it was acidic. Thus, the main cation

interacting with NO3� is H+. Under those conditions,

NO3� will diffuse fast in ice (Thibert and Domin!e, 1998).

At –351C and around 01C the diffusion coefficient of

NO3� in ice with H+ as the counter cation is about

10�11 cm2 s�1, and 3� 10�10 cm2 s�1, respectively. The

snow crystals used in the snow-piles were found to be

hollow columns and bullets (Domin!e et al., 2002) with

walls 30–50 mm thick.

The time for diffusion out from the snow crystals,

t ¼ x2=D; is a maximum of 7 days at –351C, and 6 h at

01C. Clearly, NO3� had the time to diffuse out of heated

snow crystals and some of the HNO3 released may have

diffused into unheated crystals. HNO3 is about seven

times more soluble in ice (where it dissolves as H+ and

NO3�) at –351C than at 01C (Thibert and Domin!e, 1998),

so that most of the NO3� present in the snow may have

been released by heating. We cannot at this point

evaluate quantitatively the exchanges of NO3� between

different parts of the snow-pile, but the data presented

here show that our hypothesis of exchange of NO3�

between heated and unheated snow is consistent with the

known behavior of HNO3 in ice. The pile 1P was mostly

eroded by the wind after the experiments, and the

samples shown in Table 2 were taken from the zone

immediately surrounding the inlet. This would be the

cooler zone where ions dissolve. This zone would be

expected to show higher concentrations of NO3�.

The high mobility of NO3� may also give a hint to the

possible mechanism that releases NOx and HONO. With

the diffusion distance x ¼ ðDtÞ1=2 of NO3� on the same

order of magnitude as the wall thickness of the hollow

column ice crystals, NO3� can thus diffuse out of the

snow crystals within the initial 5 h (Fig. 4) (xE25mm at

01C within 5 h) and would be likely present in the form

of HNO3 in the gas phase. The high HNO3 photolysis of

the lamp, compared to natural sunlight, thus made this

process visible.

As NO3� photolyzes in the ice phase, the product NO2

may diffuse out of the ice, as observed by Dubowski

Table 3

Snow characteristics during the light intensive chamber experiments. All mixing ratios in ppbw (ng/g)

Date Sample Cl� NO2� Br� NO3

� SO42� Na+ NH4

+ K+ Mg2+ Ca2+

14 April Before 1Q1 195.3 9.1 11.0 242.3 414.9 64.8 12.2 7.7 31.6 37.6

17 April After 1Q1 475.2 8.9 10.3 267.9 484.7 163.6 17.4 19.3 56.7 110.3

Difference 279.8 �0.2 �0.8 25.6 69.8 98.7 5.3 11.6 25.1 72.7

17 April Before 2Q2; outleta top 488.9 9.8 7.2 325.8 455.6 154.1 16.8 14.7 60.9 255.8

21 April After 2Q2; outlet top 448.3 9.1 8.7 327.3 471.0

21 April After 2Q2; mid-outlet 482.5 9.3 8.1 341.7 484.7

Difference outlet top �40.6 �0.7 1.5 1.5 15.4

Difference outlet middle �6.4 �0.5 0.9 15.8 29.0

aThe outlet of the snow chamber is where the sampling inlets of the instruments are connected.


et al. (2000). In their laboratory experiment, films of

KNO3 solutions at pH 6 were used. With K+ as counter

cation, the diffusion rate of NO3� would be too slow to

allow it to diffuse out from the ice crystals. In the case of

natural acidic snow, the high mobility of NO3�, and its

release to the gas phase as HNO3, allows photolysis in

the gas phase, or possibly on the ice surface. Considering

the extensive remobilization evidence here, gas phase

photolysis is indeed a strong possibility. The process

that we propose could not have been observed by

Dubowski et al. (2000) whose experimental conditions

were different.

3.4. Snow chamber results

Fig. 5 shows results of the chamber experiments 1Q

and 2Q. Zhou et al. (2001) already showed the HONO

timeseries. Interruptions in the timeseries may be due to

any of three reasons: (a) The sampling flow was

interrupted from the chamber once in both experiments.

These times can be seen from row 4, which shows the

flow through the chamber. (b) During the experiment,2Q local pollution with high NO and NOy mixing ratios

was sampled during three short episodes. These data

were removed from the figures. (c) Sunlight was shaded

either with Al foil (indicated in the timeseries with +) or

black plastic foil (indicated with � ). The shading

experiment with Al foil during 2Q coincided with a

local pollution event; the data are inconclusive. Shading

of the snow chamber stopped the NO production

immediately. The production of NO2 and HONO

declined more slowly, similar to the snow-pile results.

Shading with the black foil increased the temperature in

the chamber (last row), while shading with Al foil

decreased it.

Up to 50 pmol/mol NO were produced in both

experiments. On noon on DOY 111 the NO production

is higher, following contamination of the chamber by a

local pollution event. NO2 and HONO production were

higher during the second experiment 2Q, due possibly to

either higher J-rates or higher NO3� concentrations in

the snow used (Table 3). The NO2 production reached

noon-time maxima of 150 and 200 pmol/mol, and the

HONO production reached 60 and 90 pmol/mol, respec-

tively. The measured temperatures in the chamber

behaved significantly different during the two experi-

ments, most likely this is due to the placement of the

sensor. During 1Q the sensor was placed towards the

middle of the snow volume, while during 2Q it was

placed close to the walls of the chamber. The

temperature profile shows that the snow pack is heated

by solar irradiation.

The ratio of NOx:HONO in the produced flux is

invariant throughout the experiments, the partitioning

of NO:NO2 correlates with J(NO2). The NOx:HONO

ratio was 3:1 in the chamber experiments. The NOx

fraction is lower than in the pile experiments, where the

ratio was 4.5:1.

3.5. Mass and nitrogen balance in the chamber

Much smaller enrichment of NO3� and SO4

2� was

found during the chamber experiments during the light

intensive (Table 3) compared to the pile experiments.

The initial concentrations were higher than in the dark,

and the changes an order of magnitude smaller. These

differences can be related to heating effects; the

chambers stayed close to ambient temperatures, and

were not exposed to significant heating in the same way

the piles were.

During experiments 1Q and 2Q a total flow of 31.0 and

33.8m3, respectively, was pumped through the chamber.

Using both the measured temperature in the chamber

and the ambient temperature to calculate the saturation

vapor pressure of H2O over ice, the maximum total

sublimation into the dry air stream was estimated as 6.3–

6.9 g for 1Q and 7.7–8.4 g for 2Q. This, as in the case of

the snow-piles, is three orders of magnitude smaller than

the total weight of snow in the chamber, and is too small

to have a significant effect.

During 1Q a significant enrichment in Cl�, and sea-

salt and crustal cations took place (Table 3). After 2Q

cations were not measured, the observed anion differ-

ences may not be significant. Airflow, irradiation and

heating effects are much more uniform in the snow

chambers than in the piles. Extreme enrichment and

diffusion effects as with the piles were therefore not

expected in the chamber experiments. In addition, only a

few places within the snow volume were sampled. The

redistribution of dissolved trace species may have led to

some spatial variability within the snow volume, the

samples shown in Table 3 may not be sufficient to

discuss these effects in detail.

The median total NO3� concentration in aerosol

particles during the light campaign was 135.5 ng/m3,

HNO3 showed a median value of 26 ng/m3 (Ianniello

et al., manuscript in preparation). The possible intake

into the snow chambers is thus on the order of 1.2 mgNfrom this particle NO3

�. During the snow chamber

experiments 1Q and 2Q the release of nitrogen as NOx or

HONO amounted to 5.29 and 5.22 mgN, respectively.

Similar to the snow-piles, the NO3� gain amounts to ca.

1/4 of this loss.

In summary neither the pile nor the chamber

experiments show a balance of N present before and

after the experiments with the incoming NO3� and the

out-flowing NOx and HONO. In the case of the snow-

piles a redistribution of NO3� is believed to have taken

place, due to heating and diffusion effects in the pile.

Sublimation in either the piles or the chambers was

estimated as being too small to have a significant effect

on the mass balance. The nitrogen influx from particle


1Q

2Q

0

10

20

30

40

50

60

70

NO

pro

du

ce

d [p

mo

l/m

ol]

0

10

20

30

40

50

60

70

NO

pro

duced [p

mol/m

ol]

0

50

100

150

200

250

NO

2 p

rod

uce

d

[pm

ol/

mo

l]

0

50

100

150

200

250

NO

2 p

roduced [p

mol/m

ol]

0

20

40

60

80

HO

NO

pro

duced [p

mol/m

ol]

0

20

40

60

80

HO

NO

pro

duced [p

mol/m

ol]

0

2

4

6

8

10

12

14

Flo

wra

te [

slm

]

0

2

4

6

8

10

12

14

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

J(N

O2

) [

10

-2 s

-1]

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

J(N

O2

) [10

-2 s

-1]

106.5 107.0 107.5 108.0 108.5

Day of Year 2000

-35

-30

-25

-20

-15

-10

Cham

ber

Tem

p. [°

C]

109.5 110.0 110.5 111.0 111.5

Day of Year 2000

-35

-30

-25

-20

-15

-10

Ch

am

be

r Te

mp

. [°C]

Flo

wra

te [s

lm]

Fig. 5. Snow Chamber experiment 1Q; DOY 106–108 (left column), and 2Q; DOY 109–111 (right column). Shown are the production

of NO, NO2, and HONO, the flowrates through the chamber, J(NO2), and the temperature in the chamber. The maximum blank

values were 10, 22, and 6 pmol/mol, respectively, for NO, NO2, and HONO.


NO3� amounted in all experiments to roughly 1/5�1/4 of

the emitted nitrogen (as NOx and HONO).

4. Nitrogen chemistry in the snow

Neither the pile nor the chamber experiments

produced the NOx:HONO flux ratios of E1:1 that were

observed in ambient air at Alert (Beine et al., 2002).

Since the ambient snow/air system operates on different

time scales, and secondary reactions may change the

original ratios, this was not expected. While it is still

unclear what the sources or possible forms of NO3� in

the snow are, there seems to be agreement that this NO3�

is the origin of the release of the various active nitrogen

species (Dibb et al., 1998; Honrath et al., 2000).

However, in addition to the discussion by Honrath

et al. (2000), we see evidence that this NO3� may occur in

the form of mobile interstitial gas-phase HNO3.

Changes in NO3� concentrations will be caused by

transport of HNO3, but the fraction of NO3� that will be

in the form of HNO3 is negligible.

The pile experiments show clearly that light alone is

not sufficient to produce significant amounts of any of

the three observed nitrogen species; some heating of the

snow is necessary. The term ‘heating’, however, may

lead to misunderstandings: We hypothesize that the

snowpack must have a minimum temperature that

allows NO3� to diffuse out of the snow crystals within

a certain time. This minimum temperature is a function

foremost of the snow type (Table 4). In the case of the

snow chamber experiments heating of the surface snow

by solar insolation was apparently sufficient.

Hence we postulate that NO3� is set free by diffusion.

We cannot estimate the amount of surface NO3� versus

that in the snow bulk. Abbatt (1997) discussed the

possibility that most of the NO3� is adsorbed on the

surface, however, he was investigating a simple ice-

HNO3 system whereas the presence of both cations such

as Ca2+, Na+, or Mg2+, and acids such as SO42� and

Cl� may decrease the affinity of NO3� for the surface.

This point certainly may require additional research.

Consistent with previous studies on this subject we see a

photochemical process. The adsorbed NO3� can be

directly photolyzed, as described by Honrath et al.

(2000). The photolysis will result in a surface depletion

of NO3� and will lead to diffusion of nitrate from the

bulk of the snow volume to the surface at a rate

determined by D. As shown above, gas-phase HNO3 is

the most likely form of nitrate for this transport.

Heating will speed up diffusion and facilitate the

replenishment to the surface. Additionally, HNO3 may

photolyze directly.

Honrath et al. (2000) and Dubowski et al. (2001) have

identified the primary emitted species as NO2, our

results are consistent with this finding. The results from

the first 5 h of the snow-pile experiments show that NO2

is emitted first. The production of NO and HONO may

thus proceed in a secondary step through NO2�. We

observed additionally photolysis of NO2, but little

HONO photolysis on the time scales present in our

experiments.

The snow characteristics and, incidentally, the weight

of 1m2 active surface snow at Alert (4.7 kg) (Beine et al.,

2002) was equal to the filling of the snow chamber. The

noon-time production of NOx and HONO from the

chamber was 0.18mgN in a flow of around 600 l/h. This

compares to the production of ca. 1mgN/h from a

similar amount of similar snow in a natural environment

without the forced flow through a chamber. The ratio of

NOx:HONO in the chamber effluent was X3:1, in

natural snow 1:1. These findings indicate that the

residence time of the chemicals in the interstitial air in

the snow is important, more so for HONO than for

NOx. In the snow chamber experiments air spent only

ca. 2min in contact with snow. Thus, the system cannot

reach for example the NO2 (aq)� –HONO equilibrium (Li,

1994). Furthermore, incoming atmospheric NO3� does

not immediately come in contact with the reactive

surface zone in the snow chambers, unlike deposition to

a natural snow surface (Beine et al., 2002). In this case

especially transport, diffusion and equilibrium processes

seem to be important in maximizing the production of

active nitrogen species from NO3� in the snow.

Table 4

Threshold temperature for HNO3 diffusion within t ¼ 3 days

Snow type Diffusion length (mm)

(=smallest dimension/2)

D value needed

(¼ x2=t)

T for D value (1C)

Fresh columns and bullets 25 2.4� 10�11 �31

Aged columns and bullets 50 9.6� 10�11 �17

Rimed dendrites 15 8.7� 10�12 �40

Depth hoar 100 3.9� 10�10 0

Hard windpacked 120 5.6� 10�10 Impossiblea

Soft windpacked 250 2.4� 10�09 Impossiblea

aThe temperature for this diffusion coefficient would be >01C.


The results obtained here may also be useful to

understand the NO3� signal in ice cores. This signal is

known to decrease with time (Legrand and Delmas,

1986), as surface snow is always more concentrated than

deeper snow. For the moment, only physical processes

such as degassing and release during metamorphism

have been invoked to explain this process. We suggest

here that photolysis of NO3� may be an additional

process that could contribute to the observed decrease.

5. Conclusion

In this work we have presented evidence that NO3� in

acidic snow is mobilized and re-distributed in the form

of HNO3 by diffusion above a certain threshold

temperature. In the snow-pile experiments a direct

photolysis of gas-phase HNO3 was observable; this

process may contribute to nitrate photolysis in natural

snow as well. Transport in the form of HNO3 will re-

distribute NO3� to the snow surface, where it can be

more efficiently photolyzed. Because of shortened

residence times in the snow, the photolysis products in

the pile and chamber effluents showed higher NOx:

HONO ratios than in natural snow.

This mechanism may be important for the interpreta-

tion of processes in ice cores or Antarctic snow: at

temperatures below the threshold photolysis will not be

effective to reactivate NO3� as NOx. Likewise, the

formation of HNO3 will not take place in alkaline

snow. Ice core records (Thibert and Domin!e, 1998, and

references therein) show that during glacial ages, when

the ice was alkaline and NO3� tied up with heavy cations,

the seasonality in the NO3� signal was preserved,

suggesting that photolysis did not take place. On the

contrary, NO3� disappeared from acidic ice in inter-

glacials, suggesting that diffusion was necessary for the

redistribution of NO3�, so that photolysis of either

HNO3 or surface NO3� could take place.

Understanding the mechanism of NO3� redistribution

and reactivation makes it possible to parameterize this

process in models. Diffusion coefficients are known,

snow temperatures and crystal sizes need to be

measured. The remaining problem for such calculations

would be the location of NO3�; the distribution between

bulk and surface.

Acknowledgements

We would like to thank the participants of the ‘Alert

2000’ experiment for initial discussions on the idea of

snow-piles, and their support; especially P. ‘SnowBla-

ster’ Shepson, for carrying out the irradiation experi-

ments at the piles using his lamps. Funding for this

project was received from the National Science Founda-

tion, Office of Polar Programs and the European

Commission (EVK2-1999-00029 ‘NICE’).

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