Atmospheric Environment 38 (2004) 5399–5409

ARTICLE IN PRESS

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doi:10.1016/j.at

Soluble reactive nitrogen oxides at South Pole duringISCAT 2000$

Jack E. Dibba,*, L. Gregory Hueyb, Darlene L. Slusherb, David J. Tannerb

aClimate Change Research Center, Institute for the Study of Earth, Oceans and Space, University of New Hampshire,

Durham, NH 03824, USAbSchool of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA

Received 15 August 2002; accepted 29 January 2003

Abstract

The mist chamber/ion chromatography technique was used to measure soluble gaseous nitrite (NO2�) and nitrate

(NO3�) within the lowermost atmosphere (2 cm up to 10m above the snow) and in the firn air at South Pole during the

last 2 weeks of December 2000. Collected NO2� and NO3

� are attributed to nitrous (HONO) and nitric (HNO3) acids,

respectively. Firn air mixing ratios of HONO were more than five times higher than those just above the snow. A single

day of measurements showed HONO mixing ratios to be significantly lower at 10m than 85 cm above the snow. These

gradients suggest that HONO is produced in the snowpack and fluxes out into the overlying air. A strong and persistent

local source of HONO is required to sustain mean mixing ratios of 30 ppt against very fast loss by photolysis under the

intense 24 h sunlight at South Pole. For HNO3, measurements in the atmosphere were made with two independent

techniques. A CIMS technique provided data at 1min resolution, at a sampling height of 10m. The mist chamber/ion

chromatography technique integrated over nominally 30min intervals, with most measurements made 85 cm above the

snow. Mixing ratios of HNO3 were highest (mean 38 ppt) at 85 cm above the snow, compared to those both in the firn

air and at 10m above the snow. Comparing HNO3 mixing ratios between 85 cm and firn air suggests flux into the snow,

while the comparisons between 85 cm and 10m suggest fluxes in the opposite direction. We speculate that the maximum

just above the snow surface reflects in situ production of HNO3, supported by fluxes of NOx and OH precursors (e.g.,

HONO, HCHO, HOOH) out of the snow. Comparison of NO3� concentrations in snow immediately upwind of the

Atmospheric Research Observatory, 50m away from the building and 15 km away from the station revealed no

significant differences. This finding indicates that the very high NO mixing ratios observed during ISCAT 1998 and

2000 cannot be attributed to NO3� contamination of the snow surrounding the sampling location.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Photochemistry in snow; South Pole; Nitric acid; Nitrous acid; ISCAT 2000

to the special section of Atmospheric Environ-

2000.

ing author. Tel.: +1-603-862-3063; fax: +1-

ess: jack.dibb@unh.edu (J.E. Dibb).

e front matter r 2004 Elsevier Ltd. All rights reserve

mosenv.2003.01.001

1. Introduction

A series of discoveries in 1998 sparked intense interest

in the photochemical processing that occurs in sunlit

snow. An unexpected diurnal cycle in NOx above the

seasonal snowpack when the sun first returned to Alert

in March was the first surprise (Ridley et al., 2000). Just

a few months later, a team investigating the partitioning

of N oxides at Summit, the highest point on the

d.

ARTICLE IN PRESSJ.E. Dibb et al. / Atmospheric Environment 38 (2004) 5399–54095400

Greenland ice sheet, found NOx to constitute B30% of

NOy. Here too, there were indications that the

unexpectedly high abundance of NOx was supported

by fluxes out of the snow (Honrath et al., 1999). While

these findings from the Arctic were being presented and

discussed at the 1999 Fall meeting of the American

Geophysical Union, a third team of researchers inves-

tigating the interactions between gas and particulate S

species (ISCAT) was observing NO mixing ratios as high

as 600 ppt (e.g., 900 ppt NOx) above the snow at the

South Pole (Davis et al., 2001). In comparison, the

‘‘surprisingly high’’ mixing ratios of NOx at Alert and

Summit peaked below 100 ppt. The ISCAT team also

found that OH 10m above the snow at South Pole was

comparable to, or even higher than, the 24 h average

mixing ratios in the equatorial marine boundary layer

(one of the highest global OH production regions)

(Mauldin et al., 2001).

Continued investigations by the Summit team (at

Summit in summers 1999 and 2000 and also in northern

Michigan early in 1999) have demonstrated that a source

of snowpack NOx is photolysis of NO3� contained in/on

the snow grains (Honrath et al., 2000; Dibb et al., 2002).

This will likely also produce OH in/on the snow, leading

to possible production of HONO and HNO3. If these N

oxides are produced in the snowpack, their release into

the firn air and overlying atmosphere will lead to rapid

cycling between HONO, HNO3 and NOx. Elevated NO

mixing ratios should enhance OH levels in the gas phase

as well, through their impact on the HO2/OH ratio, and

photolysis of HONO will further boost both NO and

OH. We have also observed large enhancements of

HCHO, HCOOH, CH3COOH, CH3Br, CH3I, C2H4,

C3H6 and several light alkyl nitrates in the air filling

pore spaces (firn air) of the snowpack. Production of

these gases has been tentatively attributed to oxidation

of abundant, but not yet speciated, organic carbon in the

snow (Dibb and Arsenault, 2002; Jacobi et al., 2002;

Swanson et al., 2002). Fluxes out of the snow cause

enhancements of NOx, HONO, HCHO and HOOH at

least up to 1m above the snow (Honrath et al., 2002;

Jacobi et al., 2002), and probably higher. The large

enhancements of organic gases in firn air presumably

also support snow-to-air fluxes, but these have not yet

been quantified.

Photochemical modeling, constrained by observed

values of reactive gases above the snow, suggested that

OH is greatly enhanced at Summit, with photolysis of

HCHO and HONO constituting the dominant radical

sources, and photolysis of O3 contributing only a third

of total OH production (Yang et al., 2002). Chen et al.

(2001) also found that production of OH from snow-

released species such as HCHO and HONO was

sometimes required to account for the high OH mixing

ratios measured at South Pole during ISCAT 1998.

Similar production of reactive N species and organic

gases in snow, with release to the overlying atmosphere,

has been demonstrated by further work at Alert

(Sumner and Shepson, 1999; Zhou et al., 2001, and the

collection of papers in the special issue of Atmospheric

Environment on ALERT 2000 and Summit 2000,

volume 36, Issues 15/16). Investigations at the German

Neumayer station in coastal Antarctica have revealed

photochemical production of NOx in sunlit snow, and

have quantified snow-to-air fluxes (Jones et al., 2000,

2001). In addition, laboratory studies have targeted

improved understanding of photochemical processes in

and on ice (e.g., Dubowski et al., 2001, 2002).

When the ISCAT team returned to South Pole for a

second field campaign in November 2000, the focus (and

team composition) was significantly broadened (see the

ISCAT 2000 Overview, Davis et al., this issue a).

Important new efforts during ISCAT 2000 included

measurements of NOx snow/atmosphere flux and

sampling of surface and near surface snow, as well as

the firn air, using techniques developed at Summit. We

also added instrumentation to measure NO2, HNO3,

HONO and HO2NO2 along with NO (only NO was

measured in the 1998 campaign).

The primary focus of this paper is on HONO and

HNO3 in the firn air and atmosphere at South Pole. In

particular, we compare observations at South Pole to

those made at Summit using the same measurement

techniques. Because logistical requirements of several of

the ISCAT instruments required that they be operated

inside the Atmospheric Research Observatory (ARO)

building very close to the center of the station, we also

conducted a limited spatial survey in order to assess

whether station or building effects greatly modified the

composition of nearby snow (perhaps contributing to

the very high levels of NO measured in 1998).

2. Methods

Competition for space at the South Pole research

station was intense during the 2000/01 season. The

ISCAT team was generally limited to six persons on site

at any time. As a result, it was only possible to conduct

the sampling and analyses for soluble N-bearing acids

described herein for just over 2 weeks. To ensure that

these measurements would complement the full suite of

ISCAT data, our sampling began in mid-December after

all other instrumentation had been brought on line.

Most ISCAT measurements were made in or on the

ARO, but micrometeorological instruments to quantify

turbulent exchange just above the snow were mounted

on the NOAA tower B100m upwind of the building

(Davis et al., this issue b; Oncley et al., this issue).

Because of concerns that our equipment might impact

both radiation measurements (by NOAA) and the

ISCAT micrometeorological sensors if installed on the

ARTICLE IN PRESSJ.E. Dibb et al. / Atmospheric Environment 38 (2004) 5399–5409 5401

tower, we initially set up two mist chamber samplers

half-way between the tower and ARO, hoping to merge

gradient measurements of HONO and HNO3 with the

turbulent flux data. For the last half of the campaign,

both samplers were moved to a location immediately

upwind of ARO so that firn air could be sampled from

the same snow drift that other ISCAT investigators were

able to access through long inlets extending from the

second floor of the building (Davis et al., this issue a).

2.1. Gas sampling and analysis

We quantified HNO3 and HONO by operating two

mist chamber samplers simultaneously for 8–12 h every

day from 13 to 27 December (except for the 21st and

24th). Pairs of samples were collected over nominal

30min intervals. Details of this sampling technique, and

sample analysis by ion chromatography within 30min of

collection, have been provided in Dibb et al. (1994, 1998,

2002) and Dibb and Arsenault (2002). Briefly, the

samplers create a dense mist of ultrapure water, which

scavenges soluble gases from the sampled air stream.

Extensive laboratory, ground-based, and airborne test-

ing have established that HNO3 is quantitatively

collected and dissociates to NO3� in the sample. Further,

there are no known positive or negative interferants. At

Summit (in fact, in all samples we have ever collected

above sunlit snow) mist chamber samples are invariably

found to also contain NO2� (Dibb et al., 2002). We have

attributed the observed NO2� to the presence of HONO

in the sampled air, and have conducted laboratory tests

confirming that the technique does collect HONO with

high efficiency. So far we have not conducted the testing

required to assess whether other soluble N oxides could

contribute to the signal, or whether there may be

negative interferants in some air masses. Pernitric acid

(HO2NO2) is one candidate interferant which has been

observed above sunlit snow both at Alert (X. Zhou,

unpublished data) and at South Pole during the ISCAT

2000 study (Slusher et al., 2002). There was no strong

correlation between the measurements of HO2NO2 and

soluble NO2� at South Pole. Furthermore, we find that

soluble NO2� and NO increased when the temperature at

22m exceeded that at 2m (implying a very shallow

surface mixed layer collecting emissions from the snow,

Davis et al., 2001), while HO2NO2 showed little relation

to this stability indicator. In this paper, we will present

the soluble NO2� results as HONO mixing ratios. Chen

et al. (this issue) point out that the HONO levels we

report lead to significant over prediction of OH and

NOx in their photochemical box model. Model un-

certainties still remain, suggesting that the observation/

prediction discrepancy may be reduced by model

improvement, but it would also be valuable to compare

the mist chamber/ion chromatography technique to

other, more specific, measurements of HONO. The mist

chamber technique also quantitatively samples HCOOH

and CH3COOH; these results are discussed in Dibb and

Arsenault (2002).

During the summer at South Pole katabatic down-

slope winds prevail. The ARO is located upwind from

the rest of the station, and defines the vertex of a clean-

air sector (10–120o) in which all travel is strictly

controlled. The solar elevation is nearly constant

through the course of the day, though the ARO building

casts a sizeable shadow which can suppress photochem-

istry (Mauldin et al., 2001). Sampling was largely

restricted to the half-day when the sun was on the

upwind side of ARO. One of the samplers was fitted

with a Teflon prefilter attached directly to the sampler.

This was mounted 85 cm above the snow surface and

sampled ambient air from this height throughout the

experiment. The second sampler was equipped with a

2m length of heated 0.95 cm o.d. PFA tubing and then a

Teflon prefilter. The heated inlet allowed this sampler to

pull air from any height between 1m above and 1m

below the snow surface. On the first 2 days of sampling,

and usually for one or two sample pairs on the other

days, this inlet was suspended at B85 cm above the

snow in order to assess agreement between the two

samplers when they were both sampling the ‘‘same’’ air.

For 7 of the sampling days the heated inlet was used to

sample firn air from various depths (details of several of

these experiments are presented below). On the other 4

days, attempts were made to measure gradients in the air

above the snow; on 3 days the heated inlet was placed

2 cm above the snow, on the final day of sampling this

mist chamber was moved to the second floor of ARO

and the inlet was positioned adjacent to the CIMS inlet

B10m above the snow.

The CIMS instruments, and the specific techniques

used to measure HNO3 and HO2NO2 with it at South

Pole, are described in detail elsewhere (Leibrock and

Huey, 2000; Slusher et al., 2001). For most of the period

from 18 to 28 December the CIMS instrument sampled

air through a short inlet penetrating the wall in a second

floor laboratory in ARO. In this configuration, the

system measured HNO3, HO2NO2 and SO2 in ambient

air 10m above the snow. For 3 days (20–22 December)

the CIMS instrument was fitted with a long inlet in order

to measure NO2 in firn air. During these periods CIMS

data are not available at the 10m height. In this paper,

we are primarily interested in comparing simultaneous

measurements of HNO3 at 85 cm and at 10m above the

snow (made by mist chamber and CIMS techniques,

respectively).

2.2. Snow sampling and analysis

On 10 of the 13 days when mist chamber sampling

was conducted, surface snow was collected B3m

upwind of the gas phase samplers. Three adjacent

ARTICLE IN PRESSJ.E. Dibb et al. / Atmospheric Environment 38 (2004) 5399–54095402

replicates of the dominant surface layer were collected

into precleaned polyethylene bottles using lexan scra-

pers. Care was taken to collect a known area from only

the uppermost stratigraphic layer (see Dibb et al., 1994,

1998 for more details and rationale). All surface snow

samples collected at the site half way to the tower were

from an aged layer that was 1 cm thick. On 21 December

a snowstorm deposited a new surface layer that was

B0.7 cm deep. This layer was sampled directly upwind

of ARO on the last 5 days of the investigation. Both of

these surface layers were subjected to minor drifting, and

the addition of frequent diamond dust, in the intervals

between sample collection. In addition, three shallow

snowpits were sampled at 3 cm depth resolution. Two of

these were dug at the locations of the mist chamber

sampling to characterize the snow from which firn air

was sampled (one at the first location between ARO and

the tower, and the second in the drift right at the base of

ARO). The third pit was sampled at a location known as

E-30, 15 km from the station along grid 290�. All

samples were kept frozen during shipment back to our

laboratory in New Hampshire. They were melted in

small groups which were immediately analyzed by ion

chromatography. Concentrations of eight major ions

were determined but we report only on NO3� in this

Fig. 1. Concentrations of NO3� in surface and near surface snow near

adjacent replicates collected from the dominant surface layer adjacen

deviation of the replicates on each sampling day. For the first 5 days s

trail between ARO and the NOAA tower, on the last 5 days sample

shows the depth profile of NO3� in the snow where firn air samplin

resolution on 27 December.

paper. All analyses were completed within 1 month after

the samples arrived in New Hampshire.

3. Results

3.1. Nitrate in snow

The average NO3� concentrations in surface snow

were nearly constant (in the range 11.1–11.4 nmol g�1)

for the first 4 days, increased to 12.7 nmol g�1 on 19

December and jumped to a higher near constant level

(16.3–18 nmol g�1) for the final 5 days of sampling

(Fig. 1). The sampling location was changed between the

19th and the 22nd, but, based on the shallow pits

discussed below, we feel the increase reflects the new

snow that accumulated on the 21st rather than any effect

of proximity to ARO. These concentrations of NO3� are

relatively high for a remote location, though not

unexpected. For comparison, average concentrations

during a 6-week long season at Summit in 1999 (17

June–3 August) and a month long Summit campaign in

2000 (6 June–6 July) were 6.3 and 4.4 nmol NO3� g�1,

respectively (Honrath et al., 2002). It should be noted

that maximum concentrations at Summit reached 25

ARO during ISCAT 2000. Upper panel plots the mean of three

t to the mist chamber samplers. Error bars depict the standard

ampling was conducted just inside the clean-air sector along the

s were collected immediately upwind of ARO. The lower panel

g was conducted next to ARO. The pit was sampled at 3 cm

ARTICLE IN PRESSJ.E. Dibb et al. / Atmospheric Environment 38 (2004) 5399–5409 5403

and 8 nmol g�1 in the two different seasons, and we

could select 2-week long intervals from the 1999 Summit

season where average NO3� concentrations were nearly

twice those at South Pole during ISCAT, or where they

were less than half as high. The NO3� concentrations in

surface snow at South Pole during the summer should be

considered generally comparable to those at Summit

during the same season.

The depth profile of NO3� in the drift in front of ARO

indicates a rapid decrease with depth (Fig. 1), as has

been noted in previous investigations (e.g., Whitlow

et al., 1992; Dibb and Whitlow, 1996). The gradient very

near the surface is particularly steep. The 6.5 nmol g�1

concentration in the uppermost (0–3 cm) pit sample

(Fig. 1 lower panel) represents the average of

B18 nmol g�1 in the 0.7 cm thick surface layer (Fig. 1

upper panel) andB5.2 nmol g�1 in the lowermost 2.3 cm

(based on measured densities of 0.11 and 0.28 g cm�3 in

the top 0.7 and 3.0 cm, respectively). The rapid decrease

of NO3� with depth in South Pole snow results in average

concentrations in the top 30–50 cm that are B12those at

Summit (see also Whitlow et al., 1992).

Depth profiles of NO3� in the other pits sampled in

2000 were similar to that in Fig. 1, decreasing rapidly

from surficial (0–3 cm) maxima between 7 and

8 nmol g�1 (not shown). To facilitate comparison, we

have simply averaged the NO3� concentration in the top

45 cm in all three pits (Table 1). The pit closest to ARO

was about 30% higher than the pit between the building

and the tower, but B5% lower than the E-30 pit 15 km

away. This would suggest that the small enhancement at

the foot of ARO compared to 50m further upwind is

not related to the building itself. Expanding the

comparison to three pits sampled 10 km from the station

in 1994, and a single pit sampled in 1988 40 km away

(along the edge of the clean-air sector) provides no clear

indication of NO3� enhancement right at South Pole

station compared to the surrounding region. Similar

ranges in average concentrations were found in both

years when two pits were sampled, and the value in the

1988 pit (which would be least likely to suffer any

impacts from the station) falls within these ranges.

Table 1

Average concentrations of NO3� (nmol g�1) in the top 45 cm of

seven snowpits sampled near South Pole from 1988 to 2000

ARO

Drifta Towerb E-30c ’94-1d ’94-2d ’94-3d ’88e

2.48 1.93 2.57 2.16 2.44 1.79 1.96

a Immediately upwind of ARO.bHalf way between ARO and NOAA tower.cFifteen kilometer from South Pole station.dFrom Dibb and Whitlow (1996).eFrom Whitlow et al. (1992).

3.2. Nitric and nitrous acids in air

The mixing ratio of HNO3 85 cm above the snow

ranged from 6 to 124 ppt (average 38, median 35)

(Fig. 2). At 10m above the snow the range was o5–

68 ppt (average 22, median 22) (Slusher et al., 2002). It

should be noted that these summary statistics represent

slightly different sampling intervals for the two techni-

ques. We compare simultaneous measurements made

over 8 days below. Nitrous acid ranged 5–71 ppt in the

samples collected at the 85 cm height (average 30,

median 33). In comparison, average mixing ratios of

HNO3 just above the snow at Summit in the 1999 and

2000 summer seasons were about 12those during the last

2 weeks of December at South Pole (17 and 20 ppt,

respectively). The mean HONO mixing ratio at Summit

was only 7 ppt in 1999, but reached 15 ppt in 2000. Zhou

et al. (2001) reported HONO mixing ratios 5m above

the snow at Alert in April that showed significant diel

variation, peaking near 10 ppt around noon and

dropping to near 6 ppt at night. Limited sampling just

20 cm above the snow at Alert revealed a larger

amplitude of variation, with the maximum near noon

reaching B40 ppt. At Summit the median HONO

mixing ratios were lower than the means (4 and 12 ppt,

respectively), while the reverse was true at South Pole.

The variability we have observed at Summit, from year

to year, and even within any season, indicates that

extreme caution should be taken when interpreting

results based on just 2 weeks of sampling at South Pole.

Given that caveat, the ISCAT 2000 sampling suggests

that HONO mixing ratios may be sustained at

significantly higher levels above the snow at South Pole

compared to Summit.

Despite their similar distributions, the mixing ratios of

HNO3 and HONO were not very well correlated 85 cm

above the snow at South Pole (r2 ¼ 0:14; n ¼ 179). A

notable exception is provided by the sharp increases in

HNO3 and HONO on 15 December, followed by low

mixing ratios of both gases the next day (Fig. 2).

Similarly, both acids tended to be enhanced 18–20

December, but overall it appears that the processes

controlling HNO3 and HONO above South Pole snow

are not tightly linked.

4. Discussion

At Summit, HNO3 constitutes a nearly negligible

fraction of NOy, unlike most regions in the remote

troposphere (Dibb et al., 1998; Ford et al., 2002).

Gradient measurements of HNO3 above the snow

suggest that sometimes there are upward fluxes (Dibb

et al., 1998; Honrath et al., 2002), a finding that is

completely at odds with the paradigm that deposition of

HNO3 and aerosol-associated NO3� constitutes the final

ARTICLE IN PRESS

Fig. 2. Mixing ratios of HNO3 and HONO 85 cm above the snow at South Pole during ISCAT 2000.

J.E. Dibb et al. / Atmospheric Environment 38 (2004) 5399–54095404

step in the cycling of tropospheric N oxides. At the same

time, NO3� is the dominant ion in snow, especially in the

summer. This leads to two overarching open questions

(that are inter-related): (1) why is HNO3/NOy so low

just above snow? and (2) what is the source of NO3� in

the snow if it is not HNO3? Despite several intensive

sampling seasons, definitive answers to these questions

are still lacking. For HONO, the fundamental question

is inverse to question 1 for HNO3. Why are the HONO

mixing ratios so high, given its very short lifetime

against photolysis (o10min between 5:00 and 19:00 on

clear days (though longer at night and when cloudy) in

mid-summer at Summit (Yang et al., 2002))? In this case,

we have clearly established that HONO is produced in

the upper layers of the sunlit snowpack, leading to very

high mixing ratios in firn air which support snow-to-air

fluxes (Dibb et al., 2002; Honrath et al., 2002).

The ISCAT 2000 measurements suggest that these

same questions are valid at South Pole. Although no

measurements of NOy or peroxyacyl nitrates were made,

HNO3 generally accounts for a small fraction of the sum

of NO, NO2, HNO3, HONO, and HO2NO2, hence must

be a small fraction of total NOy (Huey et al., this issue;

Davis et al., this issue b). The lifetime of HONO against

photolysis is similar or even shorter (constant at

B10min, Chen et al., this issue) at South Pole than

Summit, yet the mixing ratios are comparable or higher

above the snow at South Pole. Measurements of

gradients in ambient air above the snow, and mixing

ratios in firn air, were conducted at South Pole to

determine whether flux out of the snow could support

the high HONO mixing ratios, and to see if deposition

to the snow might be helping to maintain relatively low

HNO3 mixing ratios.

4.1. Firn air

We measured profiles of gas phase HNO3 and HONO

down to 35 cm into the snow pack on two different days

during ISCAT 2000. (See Dibb et al. (2002) and Dibb

and Arsenault (2002) for detailed descriptions of the

sampling probe, protocol, and discussion of sampling

artifacts that prevent interpreting such profiles as true in

situ firn air depth profiles.) One key artifact is that the

samples are drawn from an appreciable volume of the

snowpack, and likely include ambient air quickly sucked

through the snow to the depth of the probe inlet. On 16

December just one profile was measured, but on the

23rd the sampling profile was repeated. All three profiles

were quite similar, showing HNO3 mixing ratios in firn

ARTICLE IN PRESSJ.E. Dibb et al. / Atmospheric Environment 38 (2004) 5399–5409 5405

air to be roughly half those in air 85 cm above the snow

(Fig. 3). True mixing ratios in the firn air are likely even

lower if dilution with ambient air could be accurately

accounted for. Higher mixing ratios of HNO3 in air

above the snow compared to firn air would be consistent

with losses by deposition to the snow. In contrast,

measured HONO mixing ratios in firn air ranged from 5

to 12 times higher than those in ambient air, increasing

with depth (Fig. 3). The increasing trend with depth may

largely reflect greater dilution of very elevated mixing

ratios in shallow firn air by cleaner ambient air when the

probe is closer to the surface. Regardless of the absolute

mixing ratios and depth profile in firn air, the enhance-

ments are large and should support snow-to-air fluxes of

HONO.

4.2. Gradients in ambient air

Attempts to measure gradients between 2 and 85 cm

above the snow on 3 different days were inconclusive.

Some differences were observed, but they were rarely

Fig. 3. Mixing ratios of HNO3 and HONO in the firn air at South Pol

the time that firn air was sampled are plotted for comparison. As disc

85 cm above the snow (denoted by the boxes) could be quantified on t

Plotting the ambient mixing ratios at any height between 2 and 85 cm

larger than the average agreement obtained for 37 pairs

of samples collected with both inlets at 85 cm above the

snow (B30% for HNO3 and 12% for HONO).

However, when HNO3 was measured at 85 cm with the

mist chamber technique and at 10m by CIMS gradients

were consistently observed. Most of the time mixing

ratios at 85 cm were at least twice as high as those at

10m, and for most of the first 6 days of comparison the

absolute difference exceeded 10 ppt (Fig. 4). If these

gradients are real, they would be consistent with a flux

out of the snow, yet the comparison between firn air and

ambient mixing ratios suggested the opposite (Fig. 3).

To rule out the possibility that an offset between the

two techniques caused the apparent HNO3 gradient, we

ran one of the mist chamber samplers adjacent to the

CIMS inlet 10m above the snow on the final day of

ISCAT sampling. Except for one 30min interval, the

techniques agreed within 75 ppt (average just o1 ppt)

at mixing ratios that rarely exceeded 20 ppt (Fig. 5). We

conclude that the gradient observed between 85 cm and

10m during the 8 days of comparison is real, but it is not

e. Average mixing ratios measured 85 cm above the snow during

ussed in the text, no differences in mixing ratios between 2 and

he 3 days such ambient gradient measurements were conducted.

would suggest even larger gradients.

ARTICLE IN PRESS

Fig. 4. Comparison of HNO3 mixing ratios measured at 85 cm (by the mist chamber/ion chromatography technique) and at 10m (by

CIMS). The plot presents all times during ISCAT 2000 when both measurements were made.

J.E. Dibb et al. / Atmospheric Environment 38 (2004) 5399–54095406

due to emission of HNO3 from the snowpack. Rather, it

is likely that snow-to-air fluxes of NO2 and HONO (plus

other OH sources) (as have been quantified at Summit)

lead to enhanced production of HNO3 just above the

snow. Chen et al. (2001) found that production of HNO3

and HO2NO2 were the major sinks for NOx and HOx

during ISCAT 1998. The fate of these acidic N oxides is

not well constrained, but redeposition (followed by

photolysis, redeposition, photolysis, etc.) seems possible,

sustaining NOx production at high levels. Gas phase

production of HNO3 just above the snow probably also

accounts for the intervals when gradient measurements

suggest snow-to-air fluxes of HNO3 at Summit (Dibb

et al., 1998; Honrath et al., 2002).

We operated the second sampler at 85 cm above the

snow through 27 December, providing a single day of

HONO gradients between this height and 10m (Fig. 6).

Mixing ratios were significantly higher closer to the

snow for 11 out of 12 sample pairs. This sense of

gradient is consistent with snow-to-air flux, as was

suggested by the extremely elevated mixing ratios in firn

air (Fig. 3).

4.3. Comparing South Pole to Summit

It is clear that NO is often greatly enhanced at South

Pole relative to Summit (Davis et al., 2001, this issue b;

Yang et al., 2002). The comparisons outlined in the

results section suggest that the same may be true for

HNO3 and HONO, but to a lesser degree. It is possible

to speculate about the causes for these differences.

Elevated NO3� in the surface layer of the snow may

enhance production of NOx and subsequent cycling in

the N oxide family (though we note that any enhance-

ment of NO3� at South Pole relative to Summit is limited

to very shallow depths). The CIMS HO2NO2 measure-

ments suggest that deposition of this species to the snow

at South Pole may provide an additional photolysis

target yielding NOx; that would possibly enhance

HONO production as well (Slusher et al., 2002).

Although HO2NO2 has not yet been measured at

Summit, the warmer temperatures and lower NOx

mixing ratios suggest it is probably less abundant there

than at South Pole. It is likely that enhanced UV flux

(due to reduced O3 column and the lack of diel variation

ARTICLE IN PRESS

Fig. 5. Comparison between the two techniques measuring HNO3 when the inlets were next to each other 10m above the snow.

Fig. 6. Comparison of HONO mixing ratios at 85 cm and 10m above the snow at South Pole.

J.E. Dibb et al. / Atmospheric Environment 38 (2004) 5399–5409 5407

in sun elevation at South Pole) accelerates photochem-

istry as well. Much of the variation in NO at South Pole

appears to be linked to boundary layer dynamics (high

mixing ratios resulting from reduced vertical mixing in

shallow boundary layers) (Davis et al., this issue b).

Similar processes are also important on the Greenland

ice sheet (Dibb et al., 1992; Dibb and Jaffrezo, 1993;

Honrath et al., 2002), but it may be that vertical mixing

is generally weaker or less frequent at South Pole. Davis

et al. (this issue b) also suggest that the very long upwind

fetch over nearly flat snow contributes to the enhance-

ments of NOx at South Pole.

The timings of the three field seasons compared in the

previous section suggest a hypothesis that would argue

for seasonal impacts confounding any differences due to

location. The ISCAT 2000 campaign was largely

conducted in the month before summer solstice, Summit

2000 was nearly centered on solstice, while most of the

sampling at Summit in 1999 occurred after the solstice.

Average mixing ratios of HONO and HNO3 above the

snow were highest at South Pole and lowest in the 1999

Summit season, perhaps indicating that there is a

photolabile form of NO3� in the snow that can be

depleted as the integrated UV flux increases over time

through the summer. (Such photobleaching might also

impact any other chromophores in the snow.) It is likely

that several, or even most, of these hypotheses help to

explain the apparent differences between Summit and

South Pole. We do not feel that the present data sets

from either location provide a strong base from which

one could attempt to quantify the relative importance

any of these processes plays in N oxide cycling above

sunlit snow. Additional sampling throughout the sunlit

season, as well as controlled laboratory experimenta-

tion, will be needed to understand these unexpectedly

complicated systems.

5. Conclusions

The ISCAT 2000 measurements confirm that similar

photochemical and physical processes in sunlit snow

ARTICLE IN PRESSJ.E. Dibb et al. / Atmospheric Environment 38 (2004) 5399–54095408

release reactive N oxides into the lowermost atmosphere

at both Summit, Greenland and South Pole. Quantita-

tive comparisons may be premature, but much higher

peak NO mixing ratios at South Pole suggest that

chemical processing in snow may be accelerated there. It

appears that HONO is also enhanced at South Pole

compared to Summit, but the very short ISCAT 2000

record probably does not afford a valid basis for

comparison. Perhaps the main factors leading to

larger enhancements in mixing ratios of N oxides

above the snow at South Pole are dynamical (weaker

vertical mixing and longer upwind fetch over active

sunlit snow). Determining whether there are differences

in the degree of photochemical processing, and per-

haps even in the dominant chemical mechanisms,

operating in sunlit snow on the two polar ice sheets will

require much additional experimentation in the field and

laboratory.

The concentrations of NO3� in surface and near

surface snow around ARO and the South Pole sta-

tion were relatively uniform. If, as we suspect, the

source of reactive N oxides in the firn air and lower-

most atmosphere is dominated by photolysis of snow

NO3�, these results indicate that the very high NO

mixing ratios measured in 1998 and 2000 are not an

artifact caused by sampling from the ARO building.

Comparing snow NO3� concentrations right at ARO in

2000 to those up to 40 km upwind in 1988 reveals no

trend over the past 13 years, and no evidence that

station activities have enhanced the levels above regional

values.

Mixing ratios of HONO at South Pole decreased

dramatically between firn air and 85 cm above the

snow, and were even lower at 10m on the single day

that measurements were made simultaneously at 85 cm

and 10m. This gradient is consistent with production in

the snow pack supporting snow-to-air fluxes of HONO,

as have been measured at Summit. Nitric acid mixing

ratios were found to be higher 85 cm above the snow

than they were either in the firn air or 10m above

the snow. The enhancement at 85 cm relative to firn

air suggests deposition to the snow, while the compa-

rison between 85 cm and 10m indicates an upward

flux. We hypothesize that the enhancement of HNO3

just above the snow is not due to snow-to-air flux of

HNO3. Rather, fluxes of NOx and HONO (and perhaps

other OH sources) out of the snow lead to enhanced

mixing ratios of NO2 and OH, hence production of

HNO3. This source may also operate at Summit,

accounting for the occasional intervals when gradient

measurements have indicated a snow-to-air flux of

HNO3 (Dibb et al., 1998; Honrath et al., 2002). It

should be noted that significant in situ production of

HNO3 above sunlit snow compounds our difficulty in

explaining why HNO3/NOy is so low at Summit and

South Pole.

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