UNCORRECTED PROOF

Atmospheric Environment ] (]]]]) ]]]–]]]

Ozone in the Arctic lower troposphere during winter andspring 2000 (ALERT2000)

Jan W. Bottenheima,*, Jose D. Fuentesb, David W. Tarasicka, Kurt G. Anlaufa

aMeteorological Service of Canada, 4905 Dufferin Street, Toronto, Ont., Canada M3H 5TY4bDepartment of Environmental Sciences, University of Virginia, Charlottesville, VA, USA

Received 4 June 2001; received in revised form 28 September 2001; accepted 11 January 2002

Abstract

A summary of the temporal and vertical characteristics of ozone in the Arctic boundary layer as observed during

winter and spring 2000 near Alert, Nunavut, Canada (821N, 621W) is presented. The measurements were made during

the Polar Sunrise Experiments ALERT2000. Particular attention is given to identifying chemical and atmospheric

characteristics of short-lived (o2 days) ozone depletion events that occur during dark and sunlit periods in the Arcticboundary layer. During these events the atmospheric boundary layer becomes turbulent and as a result atmospheric

layers aloft, exceeding 100m in depth, can become fully devoid of ozone. It is shown that these depletion episodes are

often associated with air masses, whose origin is primarily in Eurasia, laden with chemical species of anthropogenic

origin. Nevertheless, it is postulated that ozone depletion is largely driven by halogen chemistry, in particular involving

bromine compounds, and this occurs during the transport of air masses across the Arctic Ocean. A detailed analysis of a

period in mid April suggests that, later in the season during 24-h sunlit periods, locally occurring ozone depletion

chemistry is an important process, and we speculate that photolysis of bromoform of marine origin is the fuse that starts

a local ‘‘bromine explosion’’. A severe ozone depletion episode occurred in late April. During this prolonged episode,

lasting 14 consecutive days, the atmospheric column extending from the surface to about 1400m remained almost

completely free of ozone. We present and discuss evidence that atmospheric thermodynamics and air mass transport

history explain the dynamics of ozone depletion episodes in the high Arctic.r 2002 Published by Elsevier Science Ltd.

Keywords: Ozone; Ozone depletion; Arctic boundary layer; Photochemistry; Air masses; Atmospheric static stability

1. Introduction

Tropospheric ozone (O3) is one of the most important

atmospheric constituents. It serves as a major green-

house gas (Mickley et al., 2001), thus playing a key role

in the energy balance of the atmosphere. Furthermore,

O3 represents the main source of hydroxyl radicals (OH)

through its photolysis, which produces electronically

excited oxygen atoms that rapidly combine with water

vapor molecules to form OH. O3 reacts with nitrogen

dioxide (NO2) to produce nitrate radicals (NO3). These

radical species in concert with O3 largely determine the

oxidative capacity of the atmosphere, keeping it clean of

harmful chemical species, and their absence could exert

dramatic and unpredictable consequences. The dramatic

O3 loss in the Arctic boundary layer during the polar

sunrise has therefore been of great interest since it was

first reported in the 1980s (Bottenheim et al., 1986;

Oltmans and Komhyr, 1986). Since that time, extensive

research has demonstrated that O3 depletion in the

Arctic boundary layer is a yearly phenomenon and

widely occurs throughout the Arctic (Mickle et al., 1989;

Oltmans et al., 1989; Anlauf et al., 1994; Leaitch et al.,

1994; Solberg et al., 1996; Rasmussen et al., 1997;

Hopper et al., 1998). Surface O3 depletion has also been

observed in coastal Antarctica (Wessel et al., 1998).

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3B2v7:51cGML4:3:1 AEA : 3825 Prod:Type:COM

pp:1210ðcol:fig::1;2ÞED:MaduraiDurai

PAGN: EVK SCAN: OLIVER

*Corresponding author.

E-mail address: jan.bottenheim@cc.gc.ca (J.W. Botten-

heim).

1352-2310/02/$ - see front matter r 2002 Published by Elsevier Science Ltd.

PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 1 2 1 - 8

UNCORRECTED PROOF

In the early years of this research it became apparent

that the O3 depletion process was related to the presence

of bromine (Barrie et al., 1988). These authors

postulated that photolysis of bromoform (CHBr3) of

oceanic origin would produce bromine atoms that would

then destroy O3 in a catalytic cycle

BrþO3-BrOþO2; ðR1Þ

2BrO-2BrþO2: ðR2Þ

However, subsequent determination of the absorption

spectrum of CHBr3 demonstrated that this process

would not be able to sustain a sufficient supply of Br

atoms to the observed rate of O3 destruction (Moortgat

et al., 1993). It is now generally accepted that in the

Arctic the reactive bromine originates from activation of

sea salt bromide, Br� (Platt and Moortgat, 1999).

Recent observations (Foster et al., 2001) of molecular

bromine (Br2) and bromine chloride (BrCl) in the

boundary layer air and snow pack at Alert support this

mechanism. In addition to reaction (R2), other recycling

mechanisms of bromine oxide (BrO) into Br have been

proposed, in particular, the oxidation of the bromide ion

(Br�) by such compounds as (hydroxyl bromide) HOBr

on aerosols (Fan and Jacob, 1992) and the snow pack

(Tang and McConnell, 1996) which would lead to a

‘‘bromine explosion’’ (Platt and Lehrer, 1997)

BrOþHO2-HOBrg; ðR3Þ

HOBrg-HOBraq; ðR4Þ

HOBraqþBr�þHþ-Br2aqþH2O; ðR5Þ

Br2aq-Br2g: ðR6Þ

While the basic features of the O3 destruction

mechanism seem well understood, several puzzling

questions remain unresolved. Firstly, while the bromine

explosion mechanism can explain the presence of

sufficient Br atoms to rapidly destroy O3, what starts

the process in the first place remains unclear. Tang and

McConnell (1996) showed that among other possibi-

lities, photolysis of CHBr3 could provide an adequate

bromine ‘‘seed’’. Michalowski et al. (2000) suggested

that the reaction O3+Br�-BrO�+O2 might be ade-

quate. Other possibilities have also been proposed

(Finlayson-Pitts et al., 1990; McConnell et al., 1992;

Mozurkewich, 1995). Secondly, it remains to be

answered why the O3 destruction is only observed in

the spring after the polar night, but not during the fall.

Thirdly, since sea-salt bromide should be abundantly

available all over the snow-covered Arctic, and yet O3 is

not ubiquitously absent throughout the atmospheric

boundary layer, the presence of sea-salt bromide

appears to be a necessary but not a sufficient condition

for O3 depletion to take place. It appears that the most

chemically severe and spatially extensive O3 depletion

episodes occur over the Arctic Ocean. In agreement with

this assertion, detailed meteorological analyses of O3depletion events at Alert suggest that air masses of

marine origin are the most likely to show reduced O3levels (Hopper and Hart, 1994; Hopper et al., 1998).

Questions also remain concerning the vertical extent of

the atmospheric layer impacted by O3 depletion.

Previous work (Mickle et al., 1989; Anlauf et al., 1994;

Gong et al., 1997) near Alert showed the O3 depletion to

occur in an atmospheric layer extending from the

surface to 300–400m. However, O3 depletion episodes,

reported for the Norwegian Arctic (Solberg et al., 1996),

impacted atmospheric columns reaching the 2-km level.

Specific to the conditions at Alert, earlier investigations

suggested that O3 depletion was largely controlled by a

meteorological modulation effect. Changing air masses

with or without O3 appeared to largely dictate the

observed O3 temporal patterns (Bottenheim et al., 1990;

Barrie and Platt, 1997). However, in a recent study by

Boudries and Bottenheim (2000), it was argued that

locally occurring O3 depletion (meaning over the snow-

covered land surface adjacent to the Arctic Ocean)

should not be precluded.

This paper presents new O3 data obtained at Alert

during the ALERT2000 field campaign to investigate the

vertical and temporal characteristics of ozone during the

winter and spring. One goal of this research is to

demonstrate that O3 removal processes in the Arctic

boundary layer can occur at regional and local scales.

We focus on O3 sonde measurements that were made

from early February to early May 2000. These data

allow us to obtain an overall assessment of the extent of

the O3 depletions in the boundary layer at Alert, before,

during, and after the change from the 24-h dark period

to 24-h sunlight interval. We combine the O3 sonde data

set with surface O3 data and other chemical observa-

tions, as well as upper-air and surface meteorological

observations to investigate the links between atmo-

spheric thermodynamics and the chemical characteristics

of O3 depletion at Alert.

2. Measurement protocols

The O3 profile data reported here were obtained by

the addition of an electrochemical cell (ECC) ozone-

sonde package to the regular meteorological radio-

sondes flown twice a day by the weather observer

stationed at Alert. Alert is one of several stations where

the Meteorological Service of Canada (MSC) carries out

weekly soundings of the atmosphere using balloon-

borne ECC ozonesondes. At Alert, sondes have been

flown since December 1987. The routine program has

frequently been intensified during the winter months to

study O3 loss in the stratosphere (e.g. Kerr et al., 1993;

Fioletov et al., 1997). For the period of interest here,

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J.W. Bottenheim et al. / Atmospheric Environment ] (]]]]) ]]]–]]]2

AEA : 3825

UNCORRECTED PROOF

data are available for a minimum of one flight every

other day during February and most of March, and two

flights a day for the 17 April–8 May period. Sondes were

prepared according to standard procedures (Komhyr,

1986) that are known to produce excellent results with

typical accuracy in the troposphere of75% (Smit et al.,1996). One addition to the routine procedure was the

attachment of an inverted parachute to the balloon

package in order to slow down the ascent rate of the

balloon over the first few kilometers and hence increase

the data density, a procedure developed by S. Oltmans

(personal communication to J. Davies, 2000). The

resulting ascent rate was on average 4m s�1 in the lower

troposphere, and the data were archived at 10 s intervals.

All ozonesonde data have been deposited in, and are

available from the World Ozone and Ultraviolet

Radiation Data Centre, Meteorological Service of

Canada, Environment Canada (http://www.msc-

smc.ec.gc.ca/woudc/). Further details on the Canadian

ozone sounding program can be found in Tarasick et al.

(1995).

The surface O3 data presented here are from the

ongoing measurement program at the Baseline station

Alert, which forms part of the Global Atmospheric

Watch (GAW) network of the World Meteorological

Organization (WMO) (AES, 1999). These data are

obtained from two parallel ozone monitors (model 49,

Thermo Environmental Instruments Inc., Franklin,

MA), which are based on UV absorption (Anlauf et al.,

1994). These instruments are calibrated twice yearly with

a standard traceable to the O3 standard maintained by

the National Institute of Standards and Technology

(NIST) in Washington, DC. Instrument zero and spans

are routinely determined on alternate days by auto-

mated procedures in the field. The data are archived as

5-min averages.

Surface meteorological data presented here are from

the routine measurement program at the GAW station

(AES, 1999) with the exception of the total global

radiation data, which were obtained from the Climate

Archives of the MSC. Further details about the

prevailing synoptic weather patters during ALERT2000

are provided elsewhere (Strong et al., 2001).

3. Results and discussion

Fig. 1 shows the temporal distribution (contours) for

the O3 vertical profile extending from the surface to

4000m. Also shown, for the same period, are the

surface-based O3 measurements from the GAW station.

In Fig. 2, for the corresponding period, we present the

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Panel A: Ozone contour plot from sonde data

Color code

nM.M-1

He

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Panel B: Surface ozone data from GAW station

01-Feb 15-Feb 29-Feb 14-Mar 28-Mar 11-Apr 25-Apr 09-May

Mo

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Fig. 1. Ozone mole fractions during winter and spring 2000 at Alert. Panel A: ozone contours for the lower 4 km of the troposphere.

Color-coding as shown next to the figure (in nmolmol�1). The contours were drawn from one sonde per two days in February and

March, and two sondes per day from 17 April to 10 May. Panel B: surface ozone (in nmolmol�1) observed at the Alert Baseline

observatory. The time axis is identical for both panels, but due to the sparse sonde data set a perfect agreement between the lowest data

point in panel A, and the surface measurements in panel B should not be expected.

J.W. Bottenheim et al. / Atmospheric Environment ] (]]]]) ]]]–]]] 3

AEA : 3825

UNCORRECTED PROOF

contours of potential temperature and the total, incom-

ing global radiation to indicate the availability of solar

radiation during the measurement period. Some features

are immediately apparent from these data.

For almost the whole period, the atmospheric layer

extending from the surface up to 1500m remained

largely statically stable as evidenced by the decline of the

potential temperature field with altitude (Fig. 2). These

strong stable atmospheric conditions were more pro-

nounced during the dark periods and resulted from the

more effective radiational cooling of the lower atmo-

sphere. The O3 signature showed complex features and

positive gradients (O3 increasing with altitude) in this

stable boundary layer, also for the periods before sunrise

as shown by the radiation data (Fig. 2). These positive

O3 gradients can be explained in terms of two key

processes. First, under strong atmospheric stable condi-

tions, very little vertical mixing occurred to transport O3from aloft to the surface. Second, increases of O3 with

altitude suggest a sink mechanism at the surface-

atmosphere interface, taking place during the dark

period. While such a profile would be expected over

most terrestrial surfaces, surface O3 loss over snow- and/

or ice-covered areas was until recently believed to be an

ineffective sink process (Landenberg and Schurath,

1999; Wesely and Hicks, 2000) and hence, if anything,

a slight negative gradient might be expected. However,

recent observations at Summit, Greenland (Peterson and

Honrath, 2001), and during ALERT2000 (Albert et al.,

2001) demand a reassessment of the assumption that O3is stable over snow. Hence, the observed gradient in the

lower boundary layer at Alert may reflect this efficient

surface loss of O3, occurring under the influence of

statically stable atmospheric conditions (see Fig. 2)

A second feature, best observed from the surface

record, but not limited to the surface, is the frequent

occurrence of pockets of air that appear to be partially

depleted of O3. During early spring, O3 levels mostly

exhibited an invariant temporal signature with average

O3 mole fractions between 30 and 45 nmolmol�1

(Fig. 1), but these nearly invariant O3 levels were

occasionally punctuated by sudden but short-lived

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Panel A: Potential temperature contour plot from sonde data

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01-Feb 15-Feb 29-Feb 14-Mar 28-Mar 11-Apr 25-Apr 09-May

Panel B: Incoming total solar radiation

W.m

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Panel A: Potential temperature contour plot from sonde data

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Fig. 2. Panel A: contours of potential temperature (in K) during winter and spring 2000 at Alert for the lower 4 km of the troposphere.

Color coding as shown next to the figure (in K). The contours were drawn from one sonde per two days in February and March, and

two sondes per day from 17 April to 10 May. Panel B: total incoming global radiation (in kWm�2) measured at the climate station

Alert. As in Fig. 1 the time axis is identical for both panels.

J.W. Bottenheim et al. / Atmospheric Environment ] (]]]]) ]]]–]]]4

AEA : 3825

UNCORRECTED PROOF

declines in the O3 signature. At Alert, it has been known

for some time that sudden surface O3 declines in the

magnitude of 10–20 nmolmol�1 could be observed as

early as mid to late February when no direct sunlight is

present (Barrie and Bottenheim, 1991). As an example,

Fig. 3 shows a case when little or no O3 variation is

expected a priori due to the absence of sunlight until

early March. The sudden and substantial O3 decrease

(Fig. 3) coincided with an increases in reactive nitrogen

(NOy), black carbon (BC), and carbon monoxide (CO),

thus providing evidence to support the notion that the

episode was the result of a polluted air mass transported

to the region from long distances. Worthy et al. (1994)

have shown that until about April, transport of air

masses that bear such an anthropogenic signature is

generally fast, and originates in Eurasia. One explana-

tion for the partial depletion could therefore be a

memory effect of O3 titration by NO at the source.

Similar observations and arguments have recently been

made concerning surface O3 data from Barrow, Alaska

(Oltmans et al., 2001). However, Fig. 3 also indicates

that the O3 depletion is well matched with a concurrent

loss in gaseous Hg and extensive loss of Hg is not

observed in anthropogenically polluted air. Schroeder

et al. (1998) proposed that the halogen chemistry, which

is usually held responsible for O3 depletion, concomi-

tantly causes Hg depletion. Hence, it seems more likely

that most O3 depletions early in the year are also the

result of bromine-induced reactions. One possible

scenario that emerges from the data in Fig. 3 is as

follows: Air polluted masses emanating in Eurasia are

rapidly transported to Alert. Before reaching the high

Arctic and ultimately Alert, these air masses traverse

Arctic Ocean regions, north of Siberia where sunlight is

available as well as sea salt to produce conditions

conducive to O3 and Hg depletion. We note that the

event described in Fig. 3 coincided with an increase in

molecular Br2, which led Spicer et al. (2001) to speculate

that local Br-driven chemistry cannot be excluded as

well. Sumner et al. (2001) present a similar case study

from observations made in March 1998 and derive

implications for the atmospheric formaldehyde abun-

dance in the Arctic.

The O3 declines discussed earlier were not just

confined to the lower layers of the atmosphere. An

important case is shown in more detail in Fig. 4,

illustrating a sequence of events during 12–15 March

2000. Commencing on 12 March, O3 levels monotoni-

cally decreased from the surface up to an altitude of

300m, whence they remained constant up to 500m, and

finally and as expected they increased with altitude. For

the 15 March sounding, all the O3 was removed from the

layer between 200 and 450m, followed by rapid

increases with altitude. The O3 levels were recovered

by the next day (16 March) following the episode

(Fig. 4). These O3 vertical variations were strongly

modulated by atmospheric thermodynamics and dy-

namics. For the days when substantial O3 removal took

place, there was evidence of a shallow (B400–500m)mixed layer (Fig. 4) with an average potential tempera-

ture of 248K. This shallow atmospheric layer exhibited

strong wind shear and complex atmospheric motions

(see wind speed, Fig. 4), in which strong winds

(>10m s�1) were observed in the mixed layer and in

the entrainment zone. These dynamic features observed

during 12–15 March would promote an effective link

between the atmospheric layers aloft with the surface.

Therefore, if the air layers aloft remained devoid of O3as the air masses traveled throughout the Arctic then it is

reasonable to expect that the surface O3 declines can be

partly explained by this linking mechanism identified in

Fig. 4. This ‘‘atmospheric layering’’ could retain much

of the original chemical composition and thus remain

unperturbed for prolonged periods of time. We note that

while anthropogenically modified air masses cannot be

excluded in explaining the O3 depleted air, the masses

traveling over the Arctic Ocean can be lifted up and be

decoupled from the surface air. As these air masses

travel over the ocean, their O3 abundance is reduced due

to the bromine-driven chemistry.

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Fig. 3. Surface observations during the period 29 February–1

March 2000 at Alert.

J.W. Bottenheim et al. / Atmospheric Environment ] (]]]]) ]]]–]]] 5

AEA : 3825

UNCORRECTED PROOF

During the period from mid-April to mid-May, the

surface O3 record shows several episodes when the O3mol fraction drops to levels of 5 nmolmol�1 or less.

Short-time O3 depletions of this nature were observed

around 4–6, 9–10, 15,16 and 20 April. In some cases

there were periods of increased wind speed (up to about

5m s�1) during the episode (4–6, 10 April), while the

local wind direction might be construed as suggesting

direct transport from the Arctic Ocean. In general,

however, there appeared to be little correlation between

the wind data and the pattern of O3 depletion. The

duration of these O3 depletion episodes varied, but in

general they lasted at least a day.

We discuss the period 16–22 April in detail. Fig. 5

shows the details of the surface measurements taken

during this time. It can be seen that the O3 mol fraction

never reached the level of about 40 nmolmol�1 as would

be expected for O3 in pristine boundary layer air at this

time of the year (Oltmans and Komhyr, 1986).

Furthermore, during 19–20 April, O3 levels rapidly

changed from about 20 nmolmol�1 to minimum values

of B2 nmolmol�1. Throughout the whole period, localwind speed remained below 2m s�1. Wind direction data

suggested a southwest to westerly origin of the air mass,

which is not directly from the Arctic Ocean. However,

we note that previous analyses of surface wind

information indicate that local wind direction does not

yield reliable information of the true origin of an air

mass (Hopper et al., 1998) due to the dominance of calm

conditions. The history of back trajectories during this

period indicated a mostly stagnated air mass, which

slowly moved from the northwest-northeast to the

measurement. Following the onset of the O3 depletion

event, the wind flows were from the south and southwest

site (for further details see Strong et al., 2001).

Between 16 and 20 April, O3 varied from 20 to

30 nmolmol�1. Calm conditions dominated in the lower

atmosphere, suggesting little advection of different air

masses to the site. The vertical profile data (Fig. 2)

support this picture: potential temperature data showed

a strong, statically stable boundary layer. It is interesting

to speculate on the conditions that created an air mass

with a nearly constant, partially depleted O3 mole

fraction under calm conditions. A large increase in O3would clearly be ascribed to slow air mass advection,

given the low levels of NOx (B10 pmolmol�1) and

hence limited possibility for O3 production. An abrupt

decrease would have suggested local, active Br-driven

depletion chemistry at play. It cannot be excluded that

the O3 mole fraction was simply a reflection of pre-

mixing of air masses with different amounts of O3.

However, concurrent experiments during this period

showed that photochemistry in the local snowpack was

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Date

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0

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8W dir W spO3

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.M _ 1 ),

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Fig. 5. Surface observations during 16–22 April 2000 at Alert.

O3 (nM.M-1)10 20 30 40

Hei

gh

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)

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Fig. 4. Details of vertical profile data (lowest 1 km) for 12–16 March 2000 at Alert.

J.W. Bottenheim et al. / Atmospheric Environment ] (]]]]) ]]]–]]]6

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UNCORRECTED PROOF

active and produced HOx radical precursors such as

HCHO and HONO that were observed in the air mass

from which the surface O3 measurements were made

(Sumner et al., 2001; Zhou et al., 2001). Hence, active

chemistry must have been taking place. We postulate

that the air was in a steady-state condition where the O3mole fraction was determined by slow advection of O3-

rich air (trajectory data suggest an origin to the

southwest as mentioned above, and such air is known

to be free tropospheric air descending to the surface due

to katabatic flow from the mountain ranges to the

southwest of the Alert site), offset by a combination of

ozone loss in the surface layer due to deposition to the

snow pack (Albert et al., 2001) and local BrOx-driven

depletion chemistry.

On 20 April, a more severe O3 depletion event

occurred in which the O3 mole fraction dropped to

below 5 nmolmol�1 (Fig. 5). The vertical profile data

showed the same feature that was observed by Boudries

and Bottenheim (2000) in their 1998 case study, namely

the development of a very shallow, thermally stable

boundary layer of less than 400m depth on the evening

of 19 April when the depletion starts (Fig. 6). Successive

vertical profiles of relative humidity showed only one

profile (19 April, 24Z) with substantially higher relative

humidity suggesting the transport of marine air to the

measurement site. In the lower atmosphere (o400m),the northwesterly wind speeds (Fig. 6) for the 18–21

April period were light (o3m s�1) but the possibilitythat for a short period air containing much less O3 than

during the previous days was slowly advected to the site

cannot be conclusively discarded. No precipitation was

experienced during this period, and the radiation data

did not show a decrease in radiation intensity. Hence, an

increase in the surface area due to falling ice crystals on

which Br activation might have occurred during the 19–

20 April period can be excluded as a reason for increased

local O3 depletion efficiency. Another hypothesis is that

this air mass was enriched in CHBr3. It has been

observed before that short-term O3 depletions correlate

negatively with CHBr3 and this has been ascribed to the

marine origin of the air masses in which these

observations were made (Yokouchi et al., 1994). Slow

photolysis of CHBr3 could have been the initial source

of Br atoms required to initiate the Br explosion

(reactions (R3)–(R6) (Tang and McConnell, 1996). No

CHBr3 data were collected during ALERT2000 to allow

a test of this hypothesis. We note, however, that the

available hydrocarbon data do show the expected

variations from reactive halogen chemistry (Cl or Br

atom; Bottenheim, unpublished), and we conclude that

the depletion during this period was indeed driven by

locally occurring BrOx chemistry.

On 27 April, a long O3 depletion episode started and

continued beyond the end of the ALERT2000 study.

This episode exhibited different characteristics com-

pared to the events analyzed and discussed above.

Anthropogenic pollution tracers did not increase, but a

concomitant decrease in gaseous Hg was observed,

suggesting that Br chemistry was involved. This

particular O3 depletion episode started with little

indication from the local meteorology that abrupt

changes in atmospheric conditions were going to take

place. However, several hours after the onset of the

depletion the local wind speed abruptly and substan-

tially increased. Concurrently, the decrease in total

radiation as shown in Fig. 2 suggested the presence of

snow and ice crystals in the boundary layer, drastically

increasing the surface area on which Br-driven O3depletion chemistry might take place. This rapidly

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111O3 (nM.M-1)

10 20 30 40

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Wsp (m s-1)

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Fig. 6. Details of vertical profile data (lowest 1 km) for 18–21 April 2000 at Alert.

J.W. Bottenheim et al. / Atmospheric Environment ] (]]]]) ]]]–]]] 7

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UNCORRECTED PROOF

moving air mass brought air from the northwest Arctic

Ocean to the experimental sites. Highly variable air

motions persisted at the study sites for the two days

following the onset of the O3 depletion episode. The

vertical profile data showed that the atmospheric layer

rapidly became impacted by almost complete O3removal extending from the surface to an altitude of

1400m. Before the O3 depletion event, this atmospheric

layer remained under the influence of strong static

stability regimes. Following the onset of the episode, this

atmospheric layer experienced adiabatic conditions

followed by strong stability regimes (Fig. 7). This

episode lasted a period of more than two weeks with

little to no O3 being observed at the surface (Fig. 1). We

consider this O3 depletion episode to be a typical case of

an O3–devoid air mass which was advected to the region,

exhibiting similar traits as the ones often observed at Ny(Alesund (Solberg et al., 1996). The O3 data obtained at

the Ice Camp on towers (Steffen et al., 2001) and at the

GAW site suggested that during the two days when wind

speed was high (>5m s�1) the whole Alert region

including the plateau right up to the mountain range

beyond the experimental sites (ca. 40 km from the coast)

became totally devoid of O3. After the passage of the

fast-moving air masses, the lower 1500m of the atmo-

sphere came under the influence of strong statically

stable conditions which effectively inhibited vertical

mixing and transport of O3 from aloft to the surface

(Fig. 7). Supporting evidence for this pattern can be seen

in the vertical profile data of Fig. 1: the depth of the

depleted layer decreased with time, but air with an O3mole fraction normal for this time of year

(B40 nmolmol�1 and up) never reached the surface

until after the termination of the O3 sonde program on 8

May.

4. Summary and conclusions

The vertical and temporal O3 distribution presented in

this study illustrates complex dynamics of O3 patterns in

the Arctic boundary layer at Alert. The results obtained

as part of the ALERT2000 field campaign showed that

the Arctic boundary layer is always partly depleted of

O3. The recent understanding of the effective O3chemical sink in the snowpack may partly explain this

feature. In addition, frequently more severe O3 depletion

events of short duration were observed. As other

investigations previously reported, these short-lived

episodes can occur as early as February when the Sun

does not rise above the horizon. The magnitude of O3depletion increases with season as mole fractions do not

reach the 5 nmolmol�1 level until April when sufficient

actinic irradiance levels exist to drive local photochem-

istry in both snow pack and overlying atmosphere.

Early spring episodes are associated with fast-moving

air masses whose origins are mostly in southern regions

such as the Eurasian continent. These air masses exhibit

elevated concentrations of anthropogenic compounds

that might suggest titration of O3 by NO in the source

regions. However, the concomitant depletion of gaseous

Hg with O3 provides evidence that Br chemistry may

play a significant role in the depletion process. Rapid

transport, followed by strong air mass stagnation in the

stably stratified lower troposphere, provides the ideal

conditions to keep such air masses coherent. Such a

scenario would also suggest that pockets of O3-depleted

air should be observed above the surface, and these have

indeed been identified.

The more severe episodes later in the season are

postulated to be due to a balancing of several factors.

Prolonged O3 depletion episodes are most likely due to

similar advection of air masses that have been exposed

to depleting conditions for much longer periods of time

and more active chemistry (24 h sunlight of much greater

intensity than earlier in the season). However, there are

numerous periods of moderate to severe O3 depletion

that are difficult to understand in this light since there is

no clear indication of rapid transport (wind speeds less

than 2m s�1 and no clear indication of change in

weather patterns). We postulate that these episodes

represent a steady-state condition where slow advection

of O3-rich air is concurrently depleted in O3 due to

surface loss and local Br-atom chemistry. This steady

state can be shifted to short periods of almost total O3depletion if the advected air has a more marine

signature. Even if this is the case, it is not clear what

specific feature of such air masses enhances the observed

O3 behavior (Hopper and Hart, 1994). We speculate that

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April 20, 23Z

O3 (nM.M-1)0 10 20 30 40 50

Hei

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t (m

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500

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1000

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1750

Θ (K)

245 255 265 275

April 27, 23Z

O3 (nM.M-1)0 10 20 30 40 50

Θ (K)

245 255 265 275

ΘO3

ΘO3

Fig. 7. Comparison of ozone and potential temperature during

two ozone depletions at Alert.

J.W. Bottenheim et al. / Atmospheric Environment ] (]]]]) ]]]–]]]8

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UNCORRECTED PROOF

the presence of CHBr3 may be the answer. While not

measured in 2000, it is known that an increase in CHBr3is normally observed during O3 depletions and its

photolysis might produce sufficient Br atoms to start

the bromine explosion mechanism. Interestingly, if this

mechanism is in fact active, then it may explain why O3depletions are only occurring in the spring and not in the

fall: the seasonal pattern of CHBr3 shows a maximum in

the winter (Cicerone et al., 1988).

5. Uncited References

Boudries, 2001; Grannas, 2001.

Acknowledgements

The authors wish to thank Paul Shepson (Purdue

University) for many stimulating discussions, John

McIver and John Kivisto (MSC-Alert) for their excellent

job in handling the ozone sonde flights, Jonathan Davies

for quality assurance and quality control of the sonde

data, Sandy Steffen (Hg), Doug Worthy (CO), Sangeeta

Sharma (black carbon), Lori Leeder (GAW station

meteorology), and Harold Beine (NOy) for providing us

with data used in Figs. 3 and 5, and the personnel at

CFS Alert for general logistical support. Above all, we

thank Alan Gallant without whose tireless efforts

ALERT2000 would never have taken place. JDF

acknowledges support from the National Science

Foundation (Grant No. OPP-0000173) to participate

in the ALERT2000 project.

References

AES, 1999. Canadian Baseline Program, Summary of Progress

to 1998. Atmospheric Environment Service, Toronto,

Environment Canada.

Albert, M., Shepson, P., Bottenheim, J., 2001. Atmospheric

Environment, this issue.

Anlauf, K.G., Mickle, R.E., Trivett, N.B.A., 1994. Measure-

ment of ozone during Polar Sunrise Experiment 1992.

Journal of Geophysical Research 99D (12), 25345–25354.

Barrie, L.A., Bottenheim, J.W., 1991. Sulphur and nitrogen

pollution in the Arctic atmosphere. In: Sturges, W.T. (Ed.),

Pollution of the Arctic Atmosphere. Elsevier Press, Am-

sterdam, pp. 155–184.

Barrie, L.A., Platt, U., 1997. Arctic tropospheric chemistry: an

overview. Tellus 49B, 450–454.

Barrie, L.A., Bottenheim, J.W., Rasmussen, R.A., Schnell,

R.C., Crutzen, P.J., 1988. Ozone destruction and photo-

chemical reactions at polar sunrise in the lower Arctic

troposphere. Nature 334, 138–141.

Bottenheim, J.W., Barrie, L.A., Atlas, E., Heidt, L.E., Niki, H.,

Rasmussen, R.A., Shepson, P.B., 1990. Depletion of lower

tropospheric ozone during Arctic spring: the polar sunrise

experiment 1988. Journal of Geophysical Research 95, 101–

127.

Bottenheim, J.W., Gallant, A.G., Brice, K.A., 1986. Measure-

ments of NOy species and O3 at 821 N latitude. Geophysical

Research Letters 13, 113–116.

Boudries, H., Bottenheim, J.W., 2000. Cl and Br atom

concentrations during a surface boundary layer ozone

depletion event in the Canadian high Arctic. Geophysical

Research Letters 27 (4), 517–520.

Boudries, H., et al., 2001. Distribution and trends of

oxygenated hydrocarbons in the high Arctic derived from

measurements in the atmospheric boundary layer and

interstitial snow air during he ALERT2000 field campaign.

Atmospheric Environment, this issue.

Cicerone, R.J., Heidt, L.E., Pollock, W.H., 1988. Measure-

ments of atmospheric methyl bromide (CH3Br) and bromo-

form (CHBr3). Journal of Geophysical Research 93, 3745–

3749.

Fan, S.-M., Jacob, D.J., 1992. Surface ozone depletion in the

Arctic spring sustained by bromine reactions on aerosols.

Nature 359, 522–524.

Finlayson-Pitts, B.J., Livingstone, F.E., Berko, H.N., 1990.

Ozone destruction and bromine photochemistry at ground

level in the arctic spring. Nature 343, 622–625.

Fioletov, V.E., Kerr, J.B., Wardle, D.I., Davies, J., Hare, E.W.,

McElroy, C.T., Tarasick, D.W., 1997. Long-term decline of

ozone over the Canadian Arctic to early 1997 from ground-

based and balloon sonde measurements. Geophysical

Research Letters (24), 2705–2708.

Foster, K.L., Plastridge, R.A., Bottenheim, J.W., Shepson,

P.B., Finlayson-Pitts, B.J., Spicer, C.W., 2001. The role of

Br2 and BrCl in surface ozone destruction at polar sunrise.

Science 291, 471–474.

Gong, S.L., Walmsley, J.L., Barrie, L.A., Hopper, J.F., 1997.

Mechanisms for surface ozone depletion and recovery

during polar sunrise. Atmospheric Environment 31 (7),

969–981.

Grannas, A.M., et al., 2001. Carbonyl compounds and surface

photochemistry in the Arctic marine boundary layer.

Atmospheric Environment, this issue.

Hopper, J.F., Hart, W., 1994. Meteorological aspects of the

1992 Polar Sunrise Experiment. Journal of Geophysical

Research 99D, 25315–25328.

Hopper, J.F., Barrie, L.A., Silis, A., Gallant, A.J., Hart, W.,

Dryfhout, H., 1998. Ozone and meteorology during the

1994 Polar Sunrise Experiment. Journal of Geophysical

Research 103D (1), 1481–1492.

Kerr, J.B., Wardle, D.I., Tarasick, D.W., 1993. Record low

ozone values over Canada in early 1993. Geophysical

Research Letters (20), 1979–1982.

Komhyr, W.D., 1986. Operations Handbook—Ozone Mea-

surements to 40-km Altitude with Model 4A Electrochemi-

cal Concentration Cell (ECC) Ozonesondes (Used with

1680-MHz Radiosondes). Technical Memorandum ERL

ARL-149, National Oceanic and Atmospheric Administra-

tion, Boulder, CO.

Landenberg, S., Schurath, U., 1999. Ozone destruction on ice.

Geophysical Research Letters 26, 1695–1698.

Leaitch, W.R., Barrie, L.A., Bottenheim, J.W., Li, S.M.,

Shepson, P.B., Muthuramu, P.B., Yokouchi, Y., 1994.

ARTICLE IN PRESS

1

3

5

7

9

11

13

15

17

19

21

23

25

27

29

31

33

35

37

39

41

43

45

47

49

51

53

55

57

59

61

63

65

67

69

71

73

75

77

79

81

83

85

87

89

91

93

95

97

99

101

103

105

107

109

111

J.W. Bottenheim et al. / Atmospheric Environment ] (]]]]) ]]]–]]] 9

AEA : 3825

UNCORRECTED PROOF

Airborne observations related to ozone depletion at polar

sunrise. Journal of Geophysical Research 99D, 25449–

25517.

McConnell, J.C., Henderson, G.S., Barrie, L.A., Bottenheim,

J.W., Niki, H., Langford, C.H., Templeton, E.M.J., 1992.

Photochemical bromine production implicated in Arctic

boundary-layer ozone depletion. Nature 355, 150–152.

Michalowski, B.A., Francisco, J.S., Li, S.M., Barrie, L.A.,

Bottenheim, J.W., Shepson, P.B., 2000. A computer model

study of multiphase chemistry in the Arctic boundary layer

during polar sunrise. Journal of Geophysical Research 105,

15131–15145.

Mickle, R.E., Bottenheim, J.W., Leaitch, W.R., Evans, W.,

1989. Boundary layer ozone depletion during AGASP-II.

Atmospheric Environment 23 (11), 2443–2449.

Mickley, L.J., Murti, P.P., Jacob, D.J., Logan, J.A., Koch,

D.M., Rind, D., 2001. Radiative forcing from tropospheric

ozone calculated with a unified chemistry-climate model.

Journal of Geophysical Research 104, 30153–30172.

Moortgat, G.K., Meller, R., Schneider, W., 1993. Temperature

dependence (256–296K) of the absorption cross sections of

bromoform in the wavelength range 285–360nm. In: Niki,

H., Becker, K.H. (Eds.), The Tropospheric Chemistry of

Ozone in the Polar Regions. Springer, New York, pp. 335–

369.

Mozurkewich, M., 1995. Mechanism for the release of halogens

from sea-salt particles by free radical reactions. Journal of

Geophysical Research 100, 14199–14207.

Oltmans, S.J., Komhyr, W.D.S, 1986. urface ozone distribu-

tions and variations from 1973–1984. Measurements at the

NOAA geophysical monitoring for climate change baseline

observatories. Journal of Geophysical Research 91, 5229–

5236.

Oltmans, S.J., et al., 1989. Seasonal surface ozone and filterable

bromine relationship in the high Arctic. Atmospheric

Environment 23 (11), 2431–2441.

Oltmans, S., et al., 2001. Evidence of O3 titration during BRW

winter dark periods. In: R.C. Schnell, D.B. King, R.M.

Rosson (Eds.), Climate Monitoring and Diagnostics La-

boratory Summary Report 25. US Department of Com-

merce, National Oceanic and Atmospheric Administration,

Boulder, CO, p. 86.

Peterson, M.C., Honrath, R.E., 2001. Observations of rapid

photochemical destruction of ozone in snowpack interstitial

air. Geophysical Research Letters 28, 511–514.

Platt, U., Lehrer, E., 1997. ARCTOC Tropospheric Ozone

Chemistry (ARCTOC). Final Report to the EU, Universit.at

Heidelberg.

Platt, U., Moortgat, G.K., 1999. Heterogeneous and homo-

genous chemistry of reactive halogen compounds in the

lower troposphere. Journal of Atmospheric Chemistry 34,

1–8.

Rasmussen, A., Kiilsholm, S., Sorensen, J.H., Mikkelsen, H.B.,

1997. Analysis of tropospheric ozone measurements in

Greenland. Tellus 49B, 510–521.

Schroeder, W.H., Anlauf, K.G., Barrie, L.A., Lu, J.Y., Steffen,

A., Schneeberger, D., Berg, T., 1998. Arctic springtime

depletion of mercury. Nature 394, 331–332.

Smit, H.G.J., et al., 1996. JOSIE: The 1996 WMO international

intercomparison of ozonesondes under quasi-flight condi-

tions in the environmental chamber at J .ulich. Atmospheric

Ozone: Quadrennial O3 Symp., lAquila, Italy.

Solberg, S., Schmidtbauer, N., Semb, A., Stordal, F., 1996.

Boundary-layer ozone depletion as seen in the Norwegian

Arctic in spring. Journal of Atmospheric Chemistry 23, 301–

332.

Spicer, C.W., Plastridge, R.A., Foster, K.L., Finlayson-Pitts,

B.J., Bottenheim, J.W., Grannas, A.M., Shepson, P.B.,

2001. Molecular halogens before and during ozone deple-

tion events in the Arctic at Polar Sunrise: concentrations

and sources. Atmospheric Environment, this issue.

Steffen, A., Schroeder, W., Bottenheim, J., Narayan, J.,

Fuentes, J.D., 2001. Atmospheric mercury concentrations:

measurements and profiles near snow and ice surfaces in the

Canadian Arctic during ALERT2000. Atmospheric Envir-

onment, this issue.

Strong, J.D., Fuentes, R.E.Davis, Bottenheim, J.W., 2001.

Atmospheric thermodynamics and circulation patterns

during the ALERT2000 field campaign. Atmospheric

Environment, this issue.

Sumner, A.L., et al., 2001. Atmospheric chemistry of for-

maldehyde in the Arctic troposphere at polar sunrise, and

the influence of the snowpack. Atmospheric Environment,

this issue.

Tang, T., McConnell, J.C., 1996. Autocatalytic release of

bromine from Arctic snowpack during Polar Sunrise.

Geophysical Research Letters 23, 2633–2636.

Tarasick, D.W., Kerr, J.B., Wardle, D.I., Bellefleur, J.J.,

Davies, J., 1995. Tropospheric ozone trends over Canada:

1980–1993. Geophysical Research Letters (22), 409–412.

Wesely, M.L., Hicks, B.B., 2000. A review of the current status

of knowledge on dry deposition. Atmospheric Environment

34, 2261–2282.

Wessel, S., Aoki, S., Winkler, P., Weller, R., Herber, A.,

Germandt, H., Schrems, O., 1998. Tropospheric ozone

depletion in polar regions: a domparison of observations in

the arctic and Antarctic. Tellus 50B, 34–50.

Worthy, D.E.J., Trivett, N.B.A., Hopper, J.F., Bottenheim,

J.W., Levin, I., 1994. Analysis of long-range transport

events at Alert, Northwest Territories, during the Polar

Sunrise Experiment. Journal of Geophysical Research 99D,

25329–25344.

Yokouchi, Y., Akimoto, H., Barrie, L.A., Bottenheim, J.W.,

Anlauf, K., Jobson, B.T., 1994. Serial gas chromatographic/

mass spectrometric measurements of some volatile organic

compounds in the Arctic atmosphere during the 1992 Polar

Sunrise Experiment. Journal of Geophysical Research 99,

25379–25390.

Zhou, X., Beine, H.J., Honrath, R.E., Fuentes, J.D., Simpson,

W., Shepson, P.B., Bottenheim, J. W., 2001. Snowpack

photochemical production of HONO: a major source of OH

in the Arctic boundary layer in spring time. Geophysical

Research Letters, in press.

ARTICLE IN PRESS

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65

67

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83

85

87

89

91

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99

101

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