Observations of BrO and its vertical distribution during surface ozone depletion at Alert

Atmospheric Environment 36 (2002) 2481–2489

Observations of BrO and its vertical distribution during surface ozone depletion at Alert . Gerd Honninger*, U. Platt Institut fur . Umweltphysik (IUP), Universitat . Heidelberg, Im Neuenheimer Feld 229, 69120 Heidelberg, Germany Received 6 April 2001; received in revised form 23 July 2001; accepted 1 November 2001

Abstract During the ALERT2000 polar sunrise experiment at Alert, Nunavut, Canada, we performed measurements of boundary layer bromine oxide radicals (BrO) by differential optical absorption spectroscopy (DOAS) using scattered sunlight in the spectral range from 320 to 400 nm. For the first time the Multi-Axis-(MAX)-DOAS method was applied to derive vertical profile information of BrO. BrO was observed at slant column densities (SCD) of up to 1015 molecules/ cm2 during a 10-day period of complete surface ozone depletion. The largest BrO column densities were found by observing scattered sunlight from 51 above the horizon, and SCDs were decreasing with increasing elevation angles of the light-receiving telescope. For zenith scattered light the lowest absorption was recorded. Radiative transfer modelling and the calculation of air mass factors show that in most cases the bulk of the observed BrO was present in a layer of 170.5 km thickness above the surface (in the boundary layer). The inferred extent of the BrO layer agrees very well with the observed height of the ozone depletion layer (Bottenheim et al., Atmos. Environ., 2002) from ozone sonde data. Assuming that BrO layer is well-mixed, volume mixing ratios reached levels of 20–30 ppt BrO. These values are consistent with previous measurements of BrO during low ozone events in the Arctic boundary layer. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Halogen chemistry; Bromine oxide; Ozone depletion; Polar sunrise; Radicals; DOAS

1. Introduction Sudden ozone depletionsFlow ozone eventsFin the polar boundary layer after sunrise are a well known phenomenon since the 1980s (Oltmans and Komhyr, 1986; Bottenheim et al., 1986). Barrie et al. (1988) reported high filterable bromine concentrations during ozone depletion events, indicating that bromine species are involved in the destruction of ozone. In the following studies, the key role of bromine was confirmed by differential optical absorption spectroscopy (DOAS) measurements of BrO during the Polar Sunrise Experiment PSE92 (Hausmann and Platt, 1994) and

*Corresponding author. Fax: +49-6221-546-405. E-mail address: [email protected] . (G. Honninger).

ARCTOC96 (Tuckermann et al., 1997; Martinez et al., 1999). These measurements of BrO in the boundary layer were usually based on long path DOAS systems using an artificial light source and an absorption path defined by the light source/receiving telescope and a retroreflecting mirror (e.g. Platt, 1994; Hausmann and Platt, 1994). The observed column densities divided by the length of the light-path yield precise data on the average concentrations of the absorbers along the light path. While the artificial lightFDOAS approach has many advantages including high accuracy and operation independent of sunlight, disadvantages of this setup are the relatively large experimental effort and complicated logistic requirements (power, necessity to have equipment at either end of a several km long light path). Another disadvantage is the complete lack of vertical information unless several light paths are used (see Martinez et al., 1999).

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 0 4 - 8

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Satellite observations of total BrO columns derived from the GOME satellite instrument confirm the abundance of BrO covering large regions of the polar boundary layer (Wagner and Platt, 1998; Richter et al., 1998; Wagner et al., 2001). During the Polar Sunrise Experiment ALERT2000 at Alert a novel approach consisting of a combination of ground-based zenith and various off-axis DOAS measurements (MAX-DOAS) was applied for the first time to detect BrO close to the surface. The off-axis geometry was introduced by Sanders et al. (1993) to observe stratospheric OClO over Antarctica during twilight. The destruction of ozone in the polar boundary layer can be described by several reaction sequences. A central process is the BrO self-reaction, usually being the rate limiting step of the ozone destruction cycle I: Br2 þ hn-2Br;

ð1Þ

followed by 2Br þ 2O3 -2BrO þ 2O2 ;

ð2Þ

BrO þ BrO-2Br þ O2 ;

ð3Þ

net: 2O3 -3O2 :

ð4Þ

This catalytic cycle provides an efficient destruction mechanism for ozone in the boundary layer. At levels of 20–30 ppt BrO ozone destruction rates of the order of 1–2 ppb/h result (Tuckermann et al., 1997; Lehrer, 1999). In addition to BrO the hydrogen radicals (OH+HO2=HOx) are involved in ozone destruction cycle II. Reaction 2 followed by BrO þ HO2 -HOBr þ O2 ;

ð5Þ

HOBr þ hn-Br þ OH;

ð6Þ

OH þ COðor VOCÞ-HO2 þ CO2 ðor VOC productsÞ; ð7Þ where the rate-limiting step is usually reaction 5 with k5 =4.5  1011 cm3s1. At 15 ppt BrO the net effect of the HOx cycle is comparable to that of reactions (2) and (3) (at a typical level of 1 ppt HO2). Note that the efficiency of cycle II is linearly dependent on the BrO concentration, whereas the BrO dependence of cycle I is quadratic. Thus at high BrO levels cycle I will dominate, at low BrO cycle II will. At 30 ppt BrO 66% of the ozone destruction will take place by cycle I. In addition the efficiency of cycles I and II can be enhanced by the presence of other halogen oxide species (i.e. IO, ClO) due to crossreactions (e.g. BrO+IO) or reaction (5) occurring with IO or ClO instead of BrO.

The high amounts of reactive bromine in the polar boundary layer can be explained by autocatalytic release from the sea-ice surface (especially salt crystals on freshly frozen sea ice) and/or sea salt aerosol deposited on the snowpack. Heterogeneous reactions involving sea salt surfaces are necessary for this release mechanism (Mozurkewich, 1995; Vogt et al., 1996; Platt and Lehrer, 1997). A simplified outline of the mechanism starts with HOBr being highly soluble in water (i.e. the concentrated brine on the surface of fresh sea-ice) and also in the sea salt aerosol where it can react with Br ions: ðHOBrÞg -ðHOBrÞaq ;

ð8Þ

 ðHOBrÞaq þ Hþ aq þ Braq -ðBr2 Þaq þ H2 O;

ð9Þ

ðBr2 Þaq -ðBr2 Þg :

ð10Þ

In Eqs. (8)–(10) gas phase species are indicated by ‘g’ and liquid phase species by ‘aq’. Since every Br atom entering the liquid phase has the potential to release two Br atoms to the gas phase, this results in an exponential increase of reactive gas phase bromine, the so-called bromine explosion (Platt and Lehrer, 1997). The bromine explosion mechanism has also been studied in the laboratory (Behnke et al., 1999). It was shown in several studies that the autocatalytic release mechanism is appropriate to explain the observations (Vogt et al., 1996; Tuckermann et al., 1997). In this paper we present and discuss recent results from the ALERT2000 field campaign, where boundary layer BrO was observed by a MAX-DOAS instrument at a time resolution of about 5 min. This experiment also provided detailed information on the vertical distribution of BrO near the surface.

2. Field observations 2.1. Location Alert, Nunavut, Canada is located at the northern end of Ellesmere Island in the Canadian Arctic at 82.51N, 62.31W. The DOAS instrument was setup at an ice camp (821320 N, 621430 W) on the Arctic Ocean sea ice surface just west of Williams Island in Black Cliffs Bay. The measurement site was located at 7 km distance from the Alert base camp and about 9 km north of the Global Atmospheric Watch (GAW) station. The viewing direction of the MAX-DOAS telescope (see below) was true north and the closest land shore was in a distance of 3 km perpendicular to the viewing direction. The view towards the horizon was unobstructed in this direction; the light entering the telescope was received from 51, 101, 201, 401, 601 and 901 (zenith direction) above the horizon.

G. Honninger, U. Platt / Atmospheric Environment 36 (2002) 2481–2489 .

2.2. Experimental setup of the MAX-DOAS instrument

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-6 -7

NO2

-8 -9 -10

-3

optical density [x10 ]

The DOAS system operated at Alert received scattered (Rayleigh and Mie scattering) and reflected sunlight from different elevation angles. This MAXDOAS setup combined zenith measurements (elevation angle a ¼ 901) with measurements at a ¼ 601; 401, 201, 101 and 51 above the horizon. The MAX-DOAS receiving telescope could be automatically moved to point in the above directions using a stepper motor drive. It consisted of a quartz lens ( f ¼ 100 mm, diameter=30 mm) and a (round) bundle of 25 individual quartz fibres (core diameter 100 mm) which transmits the light from the focal point of the quartz lens to the entrance slit of a 500 mm Czerny–Turner Spectrometer (Acton Research Corporation Spectra Pro 500) for spectral analysis. At the entrance slit the fibre exits form a column of 2.5 mm height. The aperture of the telescope was o11. Atmospheric absorption spectra were recorded in the ultraviolet wavelength range from 320 to 400 nm with a spectral resolution of 0.3 nm FWHM. Continuous measurements were performed after polar sunrise from 20 April to 9 May 2000 with a time resolution of 5 min for the 51 geometry and 1 h for the higher elevation angles, respectively. Besides BrO, ozone, NO2 and O4 absorptions were measured simultaneously. Also included in the following analysis are the meteorological data (temperature, wind speed and direction, opacity) and ambient ozone data provided by the GAW station, Alert.

0 -2 -4 -6 -8 -10 -12 0

O4

-2 -4

O3

-6 -8 0 -1 -2 -3 -4 -5 -6 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4

BrO

Residual

346

348

350

352

354

356

358

λ [nm]

Fig. 1. Example of the BrO DOAS analysis. Solid lines: fit result for the respective absorber, dotted lines: reference spectra from convoluted cross section spectra. The atmospheric spectrum was recorded on 29 April 2000 at W ¼ 821:

2.3. DOAS evaluation The spectra recorded in the wavelength range from 320 to 400 nm were analyzed for atmospheric trace gases by differential optical absorption spectroscopy (DOAS) (e.g. Platt, 1994). DOAS identifies and quantifies trace gases by their specific narrow-band optical absorption structures in the UV and visible spectral regions. Scattered sunlight is highly structured due to solar Fraunhofer lines. These structures are removed by subtracting a Fraunhofer reference spectrum (FRS). For FRS a noontime measurement spectrum taken at a ¼ 901 was used, where only small background trace gas absorption was present. In order to detect the BrO absorptions, reference spectra of NO2, O3, O4, a Ring spectrum (to account for the ‘Ring effect’ caused by rotational Raman scattering (Bussemer, 1993)) the FRS and the BrO reference spectrum are simultaneously fitted to the atmospheric spectrum using a nonlinear least-squares method (Stutz and Platt, 1996). A polynomial of second order is also fitted to remove broadband absorption structures due to Rayleigh and Mie scattering. An example of the fit result for the spectral range from 346 to 359 nm as used for the BrO

evaluation is shown in Fig. 1. The fit procedure yields differential slant column densities (DSCD) for the atmospheric absorbers (differential with respect to the FRS). 2.4. The Multi Axis-DOAS (MAX-DOAS) technique The MAX-DOAS setup for ground-based observation is shown in Fig. 2. The ground-based observer receives scattered sunlight using a telescope. By moving the telescope light can be received from different directions thus allowing to derive spatial information on the absorbers. For observation of zenith scattered sunlight the photons received in the telescope have travelled a relatively long path in the stratosphere and relatively short path in the troposphere (a more quantitative description is given below). The low telescope elevation angles, however, emphasise the absorption path in the lowermost atmospheric layers. In particular the sensitivity for absorbers in the boundary layer is strongly enhanced. Fig. 2 illustrates the situation for the setup at Alert, where elevation

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Fig. 2. Ground-based MAX-DOAS setup: spectra were recorded from 51, 101 and 201 above the horizon as well as from zenith direction. The arrows indicate photon paths through the atmosphere and into the telescope (O). A1–A4 mark the intersections of the main photon paths (wide arrows) for the 4 elevation angles and the upper limit of a sample trace gas layer near the surface. The absorption path in the trace gas layer (OA1, OA2, OA3 and OA4, respectively) decreases approximately according to 1=sinðaÞ: Thus the sensitivity for absorbers in the boundary layer increases strongly for the low elevation angles (Figure not to scale).

angles of 51, 101 and 201 were combined with zenith observations. Radiative transfer model (RTM) calculations show that for zenith observation the scattering processes at 355 nm are most likely in the lower stratosphere and upper troposphere, for decreasing elevation angles the most probable scattering altitude shifts towards the ground. For the employed elevation angles most of the light received by the telescope was scattered into the telescope above the trace gas layer at the surface. Hence the effective absorption path can be approximated as the distances OAi (i ¼ 1; 2; 3; 4), which increase according to 1/sin(a) for decreasing a: From SCDs, which depend on the solar zenith angle and the viewing geometry of the receiving telescope, vertical column densities (VCD) V can be calculated using the formula S ¼ V  A; where A denotes the air mass factor (AMF). For a rough approximation, the geometric approach using the formula AE1=cosðWÞ (for scattering below the trace gas layer) or AE1=sinðaÞ (for scattering above the trace gas layer) can be used, where a is the telescope elevation angle. AMFs for this study were calculated using a Monte Carlo RTM for single Rayleigh and Mie scattering and input data like pressure, temperature and ozone profiles of the atmosphere as well as a priori profiles of the respective absorbers (Marquard et al., 2000). The numerical limit of the RTM did not allow the calculation of AMFs for telescope angles smaller than 81. The situation for an absorber like BrO which has a

stratospheric as well as a tropospheric component is illustrated in Fig. 3. For BrO being present only in the stratosphere (Panel A), the AMF depends strongly on the solar zenith angle W: The highest AMFs are calculated for large solar zenith angles. The dependence on the telescope elevation angle is relatively weak as can be seen from the small differences between the symbols. Only for high SZA a remarkable difference exists. Also very small is the dependence on the solar azimuth angle, which is represented in Fig. 3 as vertically extended dots. For BrO being present only in the boundary layer up to 1 km at a mixing ratio of 20 ppt the situation changes completely (Panel B). The dependence of the AMF on W is very weak. However, the AMF depends strongly on the telescope elevation angle a: The AMF increases with approximately a 1=sinðaÞ dependence as expected from the geometric approximation. The dependence on the solar azimuth angle is also very small in this case. Therefore, it is obvious that for an absorber in the boundary layer the observed slant column densities and the corresponding AMFs vary accordingly with a: Because the differences in the AMF for the used observation geometries are quite large, the method is very sensitive for absorbers near the surface. Since the successive measurements at the different elevation angles a are taken within a short period of time (5–15 min) there is essentially no change in W and thus in the stratospheric part of the AMF during a series of a measurements (in particular not at small W’s; where Astrat varies relatively little with W). Thus the stratospheric

G. Honninger, U. Platt / Atmospheric Environment 36 (2002) 2481–2489 .

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Fig. 3. MAX-DOAS Air Mass Factors (AMF). Left Panel (A): only stratospheric BrO present, strong dependence on W; dependence on elevation angle (different symbols) only for high W: Right Panel (B): 20 ppt BrO present in the boundary layer, weak W dependence, large differences for different elevation angles. The relatively small dependence on the solar azimuth angle is indicated as vertical scatter of the data points.

ϑ [°]

22.4

25.4

28.4

1.5

4.5

80 70

elevation angles 5° 10° 20° 90°

10

8

14

-2

BrO DSCD [10 cm ]

7.5

6

4

2

0 22.4

25.4

28.4

1.5

4.5

7.5

date April / May 2000 [GMT] Fig. 4. BrO slant column densities measured during ALERT2000: the different telescope elevation angles are shown as different symbols.

BrO column can be regarded as an essentially constant offset to the observed Sða; WÞEStrop ðaÞ þ Sstrat : 2.5. Time series of BrO during ALERT2000 Measurements of BrO were performed on 20 days from 20 April to 9 May 2000. During 12 days BrO SCDs

clearly exceeded the stratospheric background levels. The smallest detectable deviation from the stratospheric background column is 1  1013 molecules/ cm2. The time series of BrO DSCDs and the corresponding W during the campaign are shown in Fig. 4. The first week of measurements was a period of low tropospheric BrO. The diurnal cycle of BrO DSCDs shows variations

G. Honninger, U. Platt / Atmospheric Environment 36 (2002) 2481–2489 .

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of 75  1013 molecules/cm2 and only few cases with significant difference for the different elevation angles. Only two small tropospheric BrO events occurred (e.g. 23–24 April and 25–26 April). Ambient ozone dropped somewhat but did not reach zero

50

23.4

25.4

27.4

29.4

1.5

3.5

5.5

7.5

9.5

(a)

40 30 20 10 0 11 10 9 8 7 6 5 4 3 2 1 0

33 30 27 24 21 18 15 12 9 6 3 0

periods of low clouds/snow

21.4

23.4

25.4

27.4

29.4

1.5

3.5

5.5

7.5

BrO mixing ratio [ppt]

14

-2

BrO DSCD [10 cm ]

GAW station O3 [ppb]

21.4

during that time. Probably the depletion was just starting or occurring in a layer not extending all the way down to the surface (near surface layers of zeroozone air were observed in Antarctica (Wessel et al., 1998)).

9.5

DATE [UT]

30

(b)

DOAS BrO mixing ratio [ppt]

27 24 21 18 15 12 9 6 3 0 0

5

10

15

20

25

30

35

40

GAW station O3 mixing ratio [ppb] Fig. 5. (a) Ambient ozone data from the GAW station, Alert, and corresponding DOAS BrO values measured at the Ice Camp. Only the BrO values measured at 51 elevation are shown. BrO mixing ratios are calculated assuming a homogeneous BrO layer of 1 km thickness at the surface. (b) DOAS BrO data versus ambient ozone mixing ratios. The grey circles indicate data points from the first hours of the ozone depletion event (26 April 2000). All data points from the 51 elevation measurements are shown.

G. Honninger, U. Platt / Atmospheric Environment 36 (2002) 2481–2489 .

3. Discussion 3.1. The vertical extent of the BrO layer Using measured BrO SCDs from Alert the height of the BrO layer can be derived. In Fig. 4 the different SCDs for the used elevation angles are plotted as different symbols. As a general pattern the smallest SCD is measured for zenith observation, and in most cases SCDs are increasing approximately according to 1=sinðaÞ for the low elevation angles. As a case for modelling the BrO layer 4 May 2000 is studied because BrO was elevated during the ozone depletion period and the cloud-free conditions of 4 May allow reliable modelling of the AMFs. The high sensitivity of the measurements for different thicknesses of the BrO layer can be seen in Fig. 6, where only data from 4 May 2000 is shown. The four different lines show the expected behaviour for a vertical extent of the BrO layer of 0.5, 1 and 2 km, respectively, and for an elevated layer between 1 and 2 km. A BrO layer height of 1 km at the ground is most compatible with the measurements, whereas BrO layers of 0.5 and 2 km can be considered as lower and upper limits. A BrO layer starting e.g. at 1 km altitude and extending to 2 km cannot explain the observed SCDs. There are cases, however, where the observed SCDs show a different pattern which could be explained by an elevated BrO layer. In the majority of cases, however, the comparison of modelled and measured SCDs shows that the observed BrO can be attributed to the boundary layer. 3.2. BrO during ozone depletion In Fig. 5a the high-resolution BrO time series is shown together with the ozone data set from the GAW station, located approximately 9 km south of the ice camp. BrO mixing ratios were calculated assuming a homogeneous surface layer of 1 km thickness. The anti-correlation of ozone and BrO is clearly visible over the complete measurement period.

1/sin 1

2

3

4

5

6

7

8

9

10

11

elevation angle < 8˚ not supported by model

2.5

model for layer 0-0.5km 2.0

model for layer 0-1km

-2

SCD BrO [10 cm ]

1.5

14

Starting 26 April in the late morning hours BrO DSCDs rose to very high values of more than 9  1014 molecules/cm2. During that time there was strong wind from the north causing drifting snow. The next day BrO DSCDs reached a maximum of more than 1  1015 molecules/cm2. Over a period of 10 days elevated BrO levels were present, during the same time surface ozone levels were below the detection limit most of the time (see Fig. 5a). Only on 6 May, when BrO had decreased to stratospheric background levels, ozone at the surface was recovering to 20–30 ppb. There were still dips in the ambient ozone time series coinciding with elevated BrO levels.

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model for layer 0-2km

1.0

model for layer 1-2km

0.5

measured

0.0

90

20

10 8 Elevation Angle α [˚]

5

Fig. 6. Measured and modelled SCDs for the used elevation angles. SCDs are modelled for four different layers. Only data from a cloud-free day (4 May 2000) was used for this case study, the tropospheric BrO VCD was 2  1013 molecules/cm2.

Cloudy periods are hashed to indicate that the radiative transfer might be strongly influenced by cloud layers due to multiple reflection and by blowing and drifting snow at the surface due to multiple scattering. This fact could have led to enhancedabsorption paths in the lowest atmospheric layer and thus to an overestimate of the calculated BrO mixing ratio. However, taking the uncertainty of the mixing layer height (B50%) into account, BrO levels could also have been underestimated in some cases. The lack of information on the exact shape of the BrO profile constitutes another possible source of uncertainty. The fact that enhanced BrO levels were still observed when ozone was already completely depleted at the surface could be explained by Br/BrO reactions taking place not at the surface but rather in parts of the boundary layer where ozone is still present. BrO can be produced by reaction 2 destroying ozone that is mixed in from the free troposphere. In Fig. 5b DOAS BrO mixing ratios are plotted versus ozone mixing ratios. The anti-correlation of both species is obvious and agrees very well with previous studies (Hausmann and Platt, 1994; Tuckermann et al., 1997; Martinez et al., 1999; Lehrer, 1999). In general high levels of BrO (up to 30 ppt) coincide with nearly complete or complete ozone depletion at the surface whereas during periods of background ozone levels no BrO is detectable in the boundary layer. This behaviour is only the case for ‘old’ air masses, where catalytic ozone destruction by Br/BrO was already taking place for some days. The grey data points in Fig. 5b indicate the very beginning (26 April) of the long ozone depletion

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period when the catalytic ozone destruction was just starting and both species, BrO and ozone, could be observed. These results show again the importance of BrO for the understanding of Arctic ozone chemistry and stress the importance of vertical profile measurements of all involved species. The results of these measurements in the Arctic could also contribute to the understanding of bromine chemistry in the marine boundary layer on a global scale.

4. Conclusions High levels of BrO radicals (around 30 ppt) have been found, correlated to ozone depletion in the boundary layer. This result is in very good agreement with previous measurements by Long Path DOAS during ozone depletion events in the Arctic. In most cases the largest fraction of the BrO total column was located in a surface layer of approximately 1 km thickness. This agrees very well with the height of the ozone depletion layer determined by Bottenheim et al. (2002). Enhanced BrO columns could still be found when ozone was already depleted at the surface, probably due to ozone advection from aloft. It was shown that MAX-DOAS allows the determination of relatively precise BrO concentration levels and layer heights with simple instrumentation. The operation of stand alone long term measurements is also possible with the same set-up. The AMF modelling without taking into account clouds and multiple scattering is only valid for cloudfree conditions. However, the MAX-DOAS technique yields qualitatively correct values at high time resolution also during periods of blowing/drifting snow when other optical instruments like Long Path DOAS cannot operate well due to low visibility. It is planned to improve our instrument to allow simultaneous measurements at different elevation angles a rather than the sequential (though in a short period of time) observations employed in this study. This further development should allow the determination of several parameters of the vertical distribution function. In addition observations of O4 and improvements of the model itself will allow more precise radiation transport calculations. Finally we plan to utilize the MAX-DOAS technique for many other trace gases (e.g. ozone, NO2).

Acknowledgements We like to thank MSC/Environment Canada for the logistical support during this study. We also thank the Global Atmospheric Watch station Alert for providing meteorological and ambient ozone data.

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