Introduction
The importance of the halogens (X = Cl, Br, I) in atmospheric chemistry and
climate became clear decades ago after observations of these substances were
made in the stratosphere and also in the troposphere (e.g., Molina and
Rowland, 1974; Farman et al., 1985; Barrie et al., 1988; Oltmans et al.,
1989; Fan and Jacob, 1992; Hausmann and Platt, 1994; Solomon, 1999). Indeed,
reactive halogen species (RHSs) are of special interest in the troposphere
for limiting the lifetime of species such ozone (O3), mercury (Hg),
dimethyl sulfide (DMS) and organic compounds; for affecting the partitioning
of NOx (NO + NO2) and HOx (OH + HO2); and, in the
case of iodine, for participating in new aerosol formation. As such, the
presence and the impact of the tropospheric halogen chemistry have been the
subject of numerous studies with focus on remote regions and on environments
under anthropogenic influence (e.g., Simpson et al., 2015 and references
therein). Particular attention has been paid by the scientific community to
the role of halogens in the polar regions. Although not unique to these
regions, it is in the polar areas where bromine becomes particularly
relevant, directing the oxidizing capacity of the atmosphere during
springtime and causing ozone- and mercury-depletion events (ODEs and AMDEs,
respectively). For details on sources, sinks and historical background, the
reader is kindly referred to the compendium works of Simpson et al. (2007, 2015), Steffen et al. (2008), and Ariya et al. (2015)
and references therein, for example.
Briefly, while the presence of reactive bromine in the global pristine
troposphere is primarily due to the photolysis and oxidation of very-short-lived bromocarbons emitted from the oceans
(e.g., Carpenter et al., 2014),
in the polar regions its dominant source is of inorganic
origin and is linked to heterogeneous chemistry. Through experimental and
modeling studies, it is known that a set of heterogeneous reactions based
on acidic substrates comprising hypobromous acid (HOBr) and bromide
(Br-) take place in sea ice, open leads, brine, frost flowers,
snowpacks or sea-salt aerosols, for example (e.g., Fan and Jacob, 1992; Vogt et al.,
1996; Platt and Lehrer, 1997; Abbatt et al., 2012; Pratt et al., 2013,
Toyota et al., 2014; Simpson et al., 2015, Thompson et al., 2015, 2017;
Custard et al., 2017; Wang and Pratt, 2017). These reactions are summarized
as
HOBraq+(H+)aq+(Br-)aq↔Br2aq+H2O
or
HOBraq+(H+)aq+(Cl-)aq↔BrClaq+H2O,BrClaq+(Br-)aq↔Br2Cl-aq,Br2Cl-aq↔Br2aq+(Cl-)aq,
yielding the possibility that molecular bromine (Br2) transforms from
the aqueous (aq) to the gas phase. When this is followed by the photolysis
of Br2 into two bromine atoms, an autocatalytic release of bromine is
triggered, resulting in an exponential buildup of reactive bromine BrOx
(Br + BrO) in the troposphere and the so-called “bromine explosion”
events (e.g., Fan and Jacob, 1992; Platt and Lehrer, 1997; Wennberg, 1999;
Simpson et al., 2015). These events were first observed in the Arctic
region by correlating the detection of filterable bromine with ODEs (e.g.,
Barrie et al., 1988) and, later on, by observing a very high amount of BrO
(tens of picomoles per mole) in the boundary layer just after the polar sunrise
(e.g., Hausmann and Platt, 1994; Tuckermann et al., 1997). Since then,
several studies have tried to determine the chemical sources, sinks and
pathways of these compounds (e.g., Simpson et al., 2007, 2015). In
particular, the main BrO source reactions involve
Br2⟶λ2Br,BrCl⟶λBr+Cl,Br+O3→BrO+O2.
In pristine environments (i.e., very low nitrogen oxide), along with
photodissociation (in polar spring JBrO ∼ 3 × 10-2 s-1; e.g., Thompson et al., 2015), the BrO sink reactions
associated with the catalytic ODEs are
BrO+BrO→2Br,→Br2,BrO+ClO→BrCl,→Br+Cl,BrO+HO2→HOBr,BrO+OH→Br+HO2,
where Reaction 10 (R10) represents the main channel for the abovementioned bromine
explosions causing ODEs (e.g., Bottenheim et al., 1986; Barrie et al., 1988;
Oltmans et al., 1989; Platt and Hönninger, 2003; Simpson al., 2007),
where the ozone loss rate is limited by the BrO itself and cross
reactions (R8)
and (R9) and estimated as (e.g, Hausmann and Platt, 1994; Le Bras and Platt,
1995; Platt and Jenssen, 1995; Platt and Lehrer, 1997)
-d[O3]dt=2kBrO+BrOBrO2+kBrO+ClOBrOClO.
Overall, the attempts from the scientific community to estimate the presence
of BrOx in the Antarctic troposphere were initiated 20 years ago with
ground-based DOAS measurements of BrO from Arrival Heights
(77.8∘ S, 166.7∘ E), observations compatible
with the presence of 30 pmol mol-1 in the first 2 km of the troposphere
(Kreher et al., 1997). Due to the complexity of performing measurements in
such a hostile and remote environment, very few ground-based scientific
works have followed that study (summarized in Table 1). In addition to the
sparse ground-based measurements, the presence of tropospheric BrO in the
Antarctic region has also been addressed through satellite observations
(e.g., Wagner and Platt, 1998; Wagner et al., 2001; Richter et al., 2002;
Theys et al., 2011), shipborne measurements (e.g., Wagner et al., 2007)
and, more recently, by airborne DOAS measurements (e.g., Hüneke et al.,
2017). In spite of the elapsed years and the efforts of the scientific
community, compared to its northern counterpart, the current
characterization of BrOx in the Antarctic troposphere is very poor
given the very scarce geographical coverage available with vertical
information. Moreover, most of the observations are campaign based in random
years and hence the time coverage is also quite limited (see Table 1). The
present work aims at improving this geographical, vertical and time coverage
by adding two Antarctic sites to those few observing BrO in the Antarctic
troposphere. These observations were made by endurable, stable and sensitive
DOAS instrumentation built specifically for long-term measurements in
hostile environments. Particularly, herein we present the observations
performed during 2015 from two stations. The measurement sites and
methodologies are introduced in Sect. 2. Section 3 puts forward the results
obtained in terms of time series of BrO along with time series of the
aerosol optical thickness, near-surface O3 and meteorology parameters.
Then it deepens into the details of the vertical information gained after
these observations and assesses the budget and distribution of inorganic
reactive bromine in the troposphere of Antarctica. Section 4 summarizes the
work.
Summary of the published ground-based observations of
tropospheric BrO made in Antarctica. Published works of tropospheric BrO
observations performed from different Antarctic stations. The time periods
of the observations, measurement technique used and maximum BrO reported
are also included. The “∼” symbols in the maximum BrO reported
correspond to estimated values. For details, please refer to the respective
publications.
Publication
Station
Period of the measurements
Measurement technique
BrO vmr
reported
(maximum pmol mol-1)
Kreher et al. (1997)
Arrival Heights
3 months in 1995 (autumn
Zenith sky DOAS
∼ 30
(77.8∘ S, 166.7∘ E)
and spring)
Frieß et al. (2004)
Neumayer
17 days during spring 1999 and
Zenith sky DOAS
∼ 13
(70.6∘ S, 8.2∘ W)
17 days during spring 2000
Schofield et al. (2006)
Arrival Heights
1 month and 22 days during
Zenith sky and Direct
∼ 13
(77.8∘ S, 166.7∘ E)
spring 2002
Sun DOAS
Saiz-Lopez et al. (2007a)
Halley
12 months, February 2004–
LP-DOAS
20
(75.6∘ S, 26.5∘ W)
February 2005 (i.e., summer,
autumn, winter and spring)
Buys et al. (2013)
Halley
38 days during spring 2007
CIMS
13
(75.6∘ S, 26.5∘ W)
Grilli et al. (2013)
Dumont d'Urville
4 days during summer
CEAS
< 2
(66.7∘ S, 140∘ E)
2011/2012
Roscoe et al. (2014)
Halley
2 months and 4 days during
MAX-DOAS
∼ 25
(75.6∘ S, 26.5∘ W)
spring 2007
Frey et al. (2015)
Dome-C
1 month during summer
MAX-DOAS
∼ 2–3
(75.1∘ S, 123.3∘ E)
2011/2012
This work
Marambio
4.5 months in 2015
MAX-DOAS
26.0
(64.2∘ S, 56.6∘ W)
(spring, summer and
part of autumn)
This work
Belgrano
4.5 months in 2015
MAX-DOAS
8.1
(77.9∘ S, 34.6∘ W)
(spring, summer and
part of autumn)
Observations from two Antarctic stations
During 2015, ground-based spectroscopic measurements were performed from two
Antarctic research stations: Marambio and Belgrano II. Details of the
measurement sites and methods are provided below, along with ancillary
observations.
Sea ice concentration surrounding the two Antarctic stations. The
figure shows the sea ice concentration in Antarctica at the end of the
austral summer (left) and at mid-winter (right) of 2015. The sea ice maps
are downloaded from https://seaice.uni-bremen.de/databrowser/ (last access: 17 November 2017) (Spreen et
al., 2008). The two Antarctic stations of Marambio and Belgrano are marked
in yellow in both figures. Note the variability in the sea ice mainly near
Marambio.
Measurement sites
In 2010, in collaboration with the National Antarctic Direction of Argentina/Argentinian Antarctic Institute (DNA/IAA), the National Institute
for Aerospace Technology (INTA) deployed a MAX-DOAS
(Multi-axis Differential Optical Absorption Spectroscopy; e.g., Platt and
Stutz, 2008) instrument at the research base of Belgrano II
(77∘52′ S, 34∘37′ W; 256 m a.s.l.), at the
southern end of the Weddell Sea (from now on referred to as “Belgrano”).
Later on, in 2015, similar instrumentation was installed at the site of
Marambio (64∘13′ S, 56∘37′ W; 198 m a.s.l.), located on Seymour Island (a small island just east of James Ross
Island), on the northern tip of the Antarctic Peninsula. Since then,
MAX-DOAS observations have been maintained remotely. In 2016, both DOAS stations
were accepted as part of the NDACC (Network for the Detection of Atmospheric
Composition Change, http://www.ndsc.ncep.noaa.gov/, last access: 11 December 2017), aiming at long-term
atmospheric observations (e.g., De Mazière et al., 2018). Note that,
given their location around the Weddell Sea, long-term trace gas
observations from these Antarctic sites provide a great opportunity for
investigating the troposphere–sea ice interactions in two different
scenarios: a station surrounded by seasonal sea ice (Marambio) and another
where the perennial (edged) sea ice dominates (Belgrano). Figure 1 shows the
locations at which INTA has instrumentation deployed in Antarctica and it shows the sea
ice concentration (Spreen et al., 2008) surrounding these stations by the
end of the austral summer and by mid-winter of the year 2015, which
was the first year that both instruments operated simultaneously.
Measuring method
The spectral measurement technique used for the observations presented in
this work was MAX-DOAS, gathering UV/VIS scattered skylight in the sunlit
atmosphere. Through this technique, tropospheric vertical profiles of
aerosol extinction coefficients (AECs) and BrO volume mixing ratios (vmr's) can
be inferred. Specific details of the instruments' deployment and the spectral
analysis and inversion scheme are provided in the following.
MAX-DOAS instruments
Although on a few occasions tropospheric BrO has been measured in remote
regions with in situ techniques (e.g., chemical ionization mass
spectrometry; Liao et al., 2011), the operational activities in remote and
hostile environments render the DOAS (differential optical absorption
spectroscopy) technique a very suitable approach given its sensitivity,
versatility and instrumental endurance (e.g., Platt and Stutz, 2008). Either
with active setups (e.g., long-path DOAS) or with passive ones (e.g.,
zenith DOAS, MAX-DOAS, satellite observations), the DOAS technique has been
used broadly to research the troposphere in remote environments (e.g.,
Wagner and Platt, 1998; Bobrowsky et al., 2003; Wagner et al., 2007;
Saiz-Lopez et al., 2007a, b; Puentedura et al., 2012; Prados-Roman et al.,
2015; Peterson et al., 2017). In particular, the MAX-DOAS instrumental
setup referred to in this work consists of a telescope scanning the
atmosphere at different elevation angles, inferring with it
vertically resolved information of the status of the atmosphere regarding
aerosols and trace gases (e.g., Hönninger et al., 2004; Wagner et al.,
2004). This is indeed an advantage that the MAX-DOAS configuration offers
over the standard setup of the long-path DOAS, for example, and also over in situ
instruments (e.g., chemical ionization mass spectrometry, CIMS), whose
information is commonly limited to the instrument's altitude. Also, the
MAX-DOAS specific ability to characterize the low troposphere overcomes the
often limited sensitivity of the satellite observations to the planetary
boundary layer.
Details of the MAX-DOAS instruments installed in Antarctica. The
NEVA II instrument is located at the Belgrano research station while the NEVA III
instrument is placed at Marambio.
Belgrano (NEVA II)
Marambio (NEVA III)
Spectrometer
TRIAX 180
MicroHR
CCD
Hamamatsu S7031-1008
Spectral resolution (nm)
0.6
0.5
Azimuth viewing angle (∘)
62
116
Elevation angles (∘)
2, 3, 5, 10, 15, 30, 60, 90
1, 2, 3, 5, 10, 20, 30, 90
The hardware and software of the MAX-DOAS instruments referred to in this
work were developed by INTA. For decades, the group has been investigating
the atmosphere from different sites of the world using the DOAS
technique, particularly from polar regions (e.g., Gil et al., 1996, 2008; Yela et al., 2017). The two MAX-DOAS instruments referred to in
this study consist of an outdoor unit with a temperate pointing system
developed and built at INTA (Fig. 2), comprising a stepper motor and a
telescope with an 8 cm focal length fused silica lens yielding a field of
view of 1∘. The sunlight is focused in a quartz fibre
bundle, which is directed into the indoor unit comprising a
temperature-stabilized Czerny–Turner monochromator and a CCD camera fully
developed by INTA based on a Hamamatsu S7031-1008 sensor, kept at -40 ∘C ± 0.05 ∘C with a temperature
control developed and built at INTA. Both instruments operate in off-axis
mode scanning the atmosphere from the horizon to the zenith every 15 min
while the solar zenith angle (SZA) is lower than 85∘. For
a SZA higher than 85∘, the telescopes are fixed at zenith
position and during the polar night no measurements are performed (i.e.,
April–August at Marambio and March–September at Belgrano). Further details
of the MAX-DOAS instruments installed at Antarctica are provided in Table 2.
INTA's MAX-DOAS instruments mounted at the two Antarctic stations.
The outdoor unit of the MAX-DOAS instrument installed at Belgrano is shown in
(a) while the one at Marambio is shown in (b). By scanning
the atmosphere at different elevation angles (yellow lines), vertical
information of aerosols and trace gases can be retrieved.
Spectral analysis and vertical profile inversion
The spectral analysis of the DOAS observations shown in this work was
performed with INTA's software LANA (e.g., Gil et al., 2008; Peters et
al., 2017). The retrieval of BrO was centered in the 335–358 nm spectral
range, including the absorption cross sections of BrO (Fleischmann et al.,
2004), O4 (Thalman and Volkamer, 2013), CH2O (Meller and Moortgat,
2000), OClO (Kromminga et al., 2003), NO2 (Vandaele et al., 1998),
O3 (Bogumil et al., 2003) and of a pseudo-Ring spectra (Chance and
Spurr, 1997), along with a fifth-degree closure term and constant
intensity offset. In order to decrease possible instrumental instabilities
and to minimize the influence of stratospheric trace gases in the retrieval,
the zenith spectrum from each scan was used as a reference. Moreover, only
data gathered in off-axis mode with a SZA < 75∘ were
used in this work.
Similarly, the O4 differential slant column densities (dSCDs) were also
retrieved (337–370 nm spectral window) in order to invert the vertical
profile of the AEC and therefore to
characterize the scattering properties of the atmosphere and the light path
of the photons reaching the detector. The reliability of the aerosol
vertical information retrieved by MAX-DOAS observations has already been
demonstrated under different visibility conditions (e.g., Frieß et al.,
2016). This retrieval is based on the concept that the concentration O4
is known and stable in the atmosphere. Hence, a variation in the O4
dSCDs is usually related to a change of the optical path, generally due to
the presence of aerosols (e.g., Hönninger et al., 2004; Wagner et al.,
2004). The inverted AEC vertical profile was then used as input for the
linear inversion of the vertical profile of the BrO vmr.
In this work, the inversion of the vertical profiles of the AEC and the BrO
concentration was based on the optimal estimation method (e.g., Rodgers,
2000). The radiative transfer model (RTM) used was LIDORT (Spurr, 2008) and
the inversion scheme was BePRO (BIRA, Clémer et al., 2010). The
procedure consisted of a two-step approach. First, the AECs were retrieved
from the observed O4 dSCDs through an iterative nonlinear process
(e.g., Hendrick et al., 2014; Córdoba-Jabonero et al., 2016). The
inferred AEC was then used to invert the targeted BrO profiles.
The RTM input parameters characterizing the profile retrievals were
carefully chosen for polar conditions and always bearing in mind that the
aim is to gain long-term observations in the hostile conditions of
Antarctica. For the measurements performed from the Marambio station, the
pressure (P), temperature (T), O3 and NO2 vertical profiles were
obtained from the standard atmosphere for subarctic latitudes (Anderson et al., 1986).
For the observations made from Belgrano, the considered P, T and O3
profiles were obtained from monthly averaged available ozonesonde records
(from 1999 to 2006, e.g., Parrondo et al., 2014), while the NO2 profile
was taken from the same standard atmosphere. The modeled atmosphere was
stratified into layers of 100 m from 0 to 4 km altitude, layers of 1 km from
4 to 6 km (wider grid related to less sensibility at those altitudes) and
layers of the same width of those of the standard atmosphere above this
altitude. The retrieved profiles were obtained up to an altitude of 6 km. In
the inversion scheme, the diagonal elements of the measurement uncertainty
covariance matrix were the square of the dSCD error after the DOAS fit
(1σ). The statistics of the DOAS fit are included in the
Supplement. The diagonal elements of the a priori covariance
matrix in the inversion were calculated as 100 % of the a priori profile
for BrO and based on Clémer et al. (2010) for the aerosol extinction
retrieval. Note that, given the large variability in the visibility
conditions at Antarctica, the true aerosol profile can strongly differ from
the a priori profile. In order to allow the extinction profile retrieval to
capture these variations, we followed the method described in Clémer et al. (2010). In this method, the diagonal element of Sa closest to the
surface (i.e., Sa (1,1)) is set equal to the square of a scaling
factor (β) times the maximum partial aerosol optical depth (AOD) of the extinction profile
obtained in the precedent iteration. In this study, β has been set to 1.
The other diagonal elements decrease linearly with altitude down to 20 %
of Sa (1,1). Note that, despite not being a statistically Bayesian
method, it allows the profiles corresponding to large AOD to differ
significantly from the a priori AOD profile, while the profiles with smaller AOD present
lower variations from the a priori AOD profile. The non-diagonal elements were
calculated following a Gaussian distribution with a correlation length of
100 m for aerosols and 300 m for trace gases (e.g., Hendrick
et al., 2004). Therefore, the error of the retrieved profiles provided in
this work contains the measurement error (experimental dSCD error) and the
smoothing error of the retrieval. The last takes into account that the
retrieval is an estimate of the true profile smoothed by the averaging
kernel functions.
Given that the measurements in Antarctic stations are frequently affected by
blowing snow, the aerosol optical properties were obtained using
Henyey–Greenstein phase functions for the single-scattering albedo SSA = 0.999982 and asymmetry parameter g= 0.89, corresponding to typical values
of clean ice crystal (e.g., Frieß et al., 2011). After several tests
considering typical snow albedos (between 0.8 and 0.9), the surface albedo
was set to 0.8. Note that this value is consistent with observations of the
sea ice albedo performed in the UVA spectral range in the Arctic region
(e.g., Ehrlich, 2009). In order to avoid unrealistic values, an upper limit
for AOD was set to 0.5, therefore neglecting all
the observations made in such complicated conditions from the light
scattering point of view. Also, only retrievals with degrees of freedom
higher than 1 were taken into consideration. The degrees of freedom of the
data set presented in this work are summarized in the Supplement
(Table S1).
After the AEC vertical profile was estimated at 360 nm by means of the
measured O4 dSCDs, the aimed BrO vmr vertical profiles were retrieved
at 338 nm using the calculated AEC as input of the RTM. In order to properly
include the inferred AEC in the retrieval of the BrO vmr profiles, they were
calculated at the corresponding wavelength using an Ångström parameter of
2.2 (e.g., Hegg et al., 2010). An exponential decreasing profile
corresponding to AOD = 0.02 was chosen as the a priori AEC vertical profile for the AEC vertical
profile. The scale height of the a priori AEC was set to 0.5 km for
Belgrano and to 2 km for Marambio since these values provided the lower
differences between observed and modeled O4 dSCD for each station. The
a priori BrO vertical density corresponded to an exponentially decreasing
profile with a scale height of 1 km and a surface value of ∼ 1.5 pmol mol-1.
Ancillary data
As a consequence of its logistically complicated location and very harsh
weather conditions, there are very scarce observational data describing the
atmosphere at Antarctica. In order to interpret the bromine
results, in
addition to the spectra gathered by the MAX-DOAS measurements,
near-surface O3 vmr's measured at both stations were also compiled. At
both sites, the surface O3 was measured with ozone analyzers (Thermo
Environmental Instrument, Thermo Fisher Scientific, model 49; i.e., TEI49).
The operation principle of this in situ instrumentation consists of the
attenuation of an ultraviolet light beam (254 nm) by an air sample
containing ozone and has a manufactured sensitivity and limit of detection
of 1 nmol mol-1. In the case of the Marambio station, which contributes to
the GAW network (Global Atmosphere Watch, WMO World Data Centre for
Greenhouse Gases, WDCGG), the measurements of surface O3 were carried
out by the National Meteorological Service of Argentina (SMN) and can be
retrieved from the WMO WDCGG
(https://ds.data.jma.go.jp/gmd/wdcgg/, last access: 18 September 2017). These data from the TEI49 from
Marambio are compared every year against the regional standard (WMO,
RCC-BsAs, TEI49PS). At the research site of Belgrano, the year-round surface
O3 has been measured by INTA since February 2007 (e.g., Jones et al., 2013).
At this site, the inlet of the analyzer, protected from rain, snow and dust,
is placed 0.85 m above the roof of the base in the cleanest area of the
station, free of pollution from the research site.
Additionally, the weather information was obtained from the observations
performed by the SMN at Marambio (WMO station 89055) and by INTA at
Belgrano. In the case of Marambio, the weather station is installed in the
so-called scientific pavilion of the Marambio Antarctic station together
with an automated met station (AMS), which measures the temperature,
humidity, precipitation, wind speed and direction in addition to the atmospheric
pressure. The data acquisition system is carried out through a
Campbell Scientific CR100 datalogger. In the case of Belgrano, the weather parameters
are gathered by a Vaisala weather station installed at the site in 2009. In
this case, the weather station is installed on the roof of the base, on a
210 cm mast and it provides wind speed and direction, atmospheric pressure,
temperature, and relative humidity.
Results and discussions
This section is divided into three main parts. First, it presents the time
series of the DOAS measurements and the ancillary observations performed
during 2015, offering an overview of the information gathered within the
frame of this study. Then, the details and discussion of the retrieved BrO
and AEC vertical distributions at the two sites are provided. Finally, the
activation of bromine in the Antarctic troposphere during 2015 is
investigated along with the reactivity of the Antarctic troposphere with
regard to inorganic reactive bromine.
Time series
This section presents the observations gathered during 2015 at each station.
It first shows the DOAS measurements in terms of AOD
and BrO vertical column densities (VCDs). Later on, it shows the results of
the ancillary observations (weather parameters and surface ozone).
DOAS observations: BrO VCD2km and AOD2km
Herein, we present the AOD and the BrO VCD measured with the DOAS technique
during the sunlit period of 2015. This period lasted for about 8.5 months at
Marambio and 7.5 months at Belgrano (Figs. 3 and 4). Due to instrumental
issues, there were missing data at the beginning of the year at both
stations. Note, however, that the polar sunrise and therefore the peak season of the bromine activation was well covered at both
sites (e.g., Simpson
et al., 2007).
Time series of the aerosol optical depth (AOD) in the first 2 km
of the troposphere as observed at Belgrano (a) and at
Marambio (b) during 2015. The horizontal scale indicates the time of the year while the
left vertical scale shows the AOD. The scale on the right shows the hours of
light at each station (shown in yellow in the plots). Note that the same scales
apply to both figures. Time periods without MAX-DOAS observations (i.e.,
instrumental issues or SZA > 75∘) are indicated with
shaded areas.
As mentioned in Sect. 2.2.2, the sensitivity of the MAX-DOAS observations
decreases with altitude. Hence, aiming also at comparing both stations, here
we refer to the AOD and BrO VCD inferred in the first 2 km of the
troposphere at each site (i.e., AOD2km and VCD2km, respectively).
Figure 3 shows the AOD2km retrieved at both stations (mean relative
error of 40 %). Observations indicate that the aerosol optical thickness
of the low troposphere at Belgrano was generally higher than at Marambio. At
Belgrano, 62 % of the AOD2km was lower than 0.05 (12 % between
0.05 and 0.1) compared to 90 % of the AOD2km at Marambio that was
below 0.05. In addition to this geographical dependence of the AOD, Fig. 3
also suggests that the period with higher aerosol thickness lasted longer in
the southernmost station of Belgrano. As seen in the figure, while high AODs
were observed at Belgrano during most of the sunlit period, the AOD at
Marambio intensified from September until December. The geographical
variability in the aerosol load within Antarctica has already been reported
(e.g., Savoie et al., 1993; Minikin et al., 1998), although further studies
on the distribution of aerosols are needed in order to understand the
interannual variability in the different sources (e.g., Giordano et al.,
2017). Further insights into the aerosol properties within the Antarctic
troposphere during 2015 will be provided in a following work
(Gómez-Martín et al., 2018).
Time series of the BrO vertical column density (VCD) in the first
2 km as observed at Belgrano (a) and at Marambio (b) during 2015. The
horizontal scale indicates the time of the year while the left vertical
scale shows the BrO VCD. The scale on the right shows the hours of light at
each station (shown in yellow in the plots). Note that the same scales apply to
both figures. Time periods without MAX-DOAS observations (i.e., instrumental
issues or SZA > 75∘) are indicated with shaded areas.
Regarding the BrO, the pseudo-vertical column densities (VCD2km)
are calculated by integrating the BrO concentration obtained within the
first 2 km of the troposphere. Results, shown in Fig. 4, indicate that BrO
was present in the sunlit Antarctic troposphere at both stations. The median
BrO VCD2km values at both sites were quite similar (∼ 0.5 × 1013 molec cm-2, with a mean relative
error of 10 %) and 75 % of the observations at both stations fell below a similar
value (0.8 × 1013 molec cm-2). Also, at both sites, the
maximum BrO VCD2km values were observed after the polar sunrise (i.e.,
SZA < 75∘ herein) and the BrO levels were undetectable
just before the polar sunset (i.e., SZA > 75∘ herein)
and immediately after the polar sunrise. However, the magnitude of and
variability in the BrO VCD2km maximums direct the difference between
both stations, with an absolute maximum BrO VCD2km observed at Marambio
that is 3.2× higher than at Belgrano. As can be observed in Fig. 4, it is
also worth noticing the clear photolytic activation of BrO at Marambio
during austral spring, with levels an order of magnitude higher than the
median BrO VCD2km values at the station. Insights into the vertical
distribution of BrO and aerosol extinction in the Antarctic troposphere are
provided later on in Sect. 3.2.
Meteorological parameters (temperature T and snowfall)
observed during 2015 at both stations. Data are provided by the World
Meteorological Organization (WMO), the Argentinian Meteorological Centre and
INTA's meteorological station (Belgrano). The snowfall is based on surface
synoptic observations.
Station
Mean T
Maximum
Minimum
Coldest
Mean T
Days with
(∘C)
T (∘C)
T (∘C)
month
in coldest
snowfall
month (∘C)
(%)
Marambio
-8.2
17.4
-29.9
September
-16.8
60.8
Belgrano
-13.7
3.0
-43.9
August
-19.7
47.7
Wind rose at the Belgrano (a) and Marambio (b) stations (2015).
The vertical scale indicates the frequency count. Belgrano data are
gathered from INTA's weather station and those at Marambio are provided by
the WMO and the Argentinian Weather Service. The color code in both plots
refers to different wind speed regimes: low wind conditions (< 6 m s-1, in green), medium wind conditions (6–12 m s-1, in grey) and
blowing snow conditions (> 12 m s-1, e.g., Jones et al.,
2009, in black). The statistics of each regime at each station during 2015
are indicated in parentheses.
Ancillary observations: meteorological parameters and surface
O3
Aiming at contextualizing both research stations, this section briefly
presents some weather parameters and the near-surface ozone characterizing
the sites of Marambio and Belgrano during 2015.
Regarding the weather information at each station, the mean observed values
of meteorological parameters such as temperature (T) and precipitation are
provided in Table 3. Overall, observations indicate that Belgrano station
sits at a dryer and colder location where, in 2015, temperatures dropped
below -40 ∘C. Regarding the wind measurements, the wind rose
of the 2015 measurements at each station is shown in Fig. 5 for low, medium
and high wind speeds. Note that the information reported for 2015 at
Marambio is consistent with the recent publication of Asmi et al. (2018)
referring to the 2013–2015 period. The 2015 observations at both stations
indicate that, although the higher gusts of wind were quite similar at both
stations (∼ 34 m s-1), the median wind speed at Marambio
(7.2 m s-1) was in general 50 % higher than at Belgrano. Concerning
the wind direction observed at each station during 2015 (Fig. 5), while the
air masses arriving at Marambio had no clear dominant direction, those
arriving at Belgrano usually came from the south-southwest. Hence, based on
the wind rose (Fig. 5), during 2015 Marambio was mostly influenced by air
masses coming along the west edge of the Weddell Sea but also from the
surrounding Scotia and Amundsen seas, while the air masses from continental
Antarctica dominated the observations performed at Belgrano station.
The 2015 near-surface ozone observations at the Belgrano (a)
and Marambio (b) research stations. Note the same vertical scale in
both plots. Both data sets were gathered by in situ O3 observations made
by INTA (at Belgrano) and by the Argentinian Meteorological Service (at
Marambio). The periods with MAX-DOAS data (SZA < 75∘) at
each station are contained within the green boxes.
As for the near-surface O3, the annual variation at both stations
showed the seasonal pattern expected at high-latitude stations (Antarctic
and sub-Antarctic regions, Fig. 6) and is typical of remote, low NOx
environments with O3 being accumulated during winter (maximum) and
destroyed (minimum) during summer. Also, the observed amplitude of the
surface O3 annual cycle at both stations was also characteristic of an
Antarctic station (e.g., Helmig et al., 2007; Legrand et al., 2016). While in
2015 the median values of surface O3 were quite similar at both
locations (23 nmol mol-1 at Marambio and 24 nmol mol-1 at
Belgrano), the maximum values reported at Marambio (36.8 nmol mol-1)
were about a 10 % higher than those observed at the Belgrano station.
Regarding the absolute minimum surface O3 detected, 2015 observations
at Marambio indicate ozone depletion events (ODEs) with measurements very
close to or below instrumental detection limit (1 nmol mol-1), while the
minimum surface O3 detected at Belgrano was not lower than 6 nmol mol-1. This suggests that, compared to Belgrano, Marambio is either
a more photochemically active region or a region more exposed to ozone-depleted air masses. Noteworthy is also the high variability in surface
ozone observed at Marambio during the polar sunrise compared to
observations at the Belgrano station. This behavior is characteristic of a
coastal Antarctic station as reported by Helmig et al. (2007), for example.
BrO vmr (a, b) and AEC (c, d) observed during
2015 in the troposphere of Belgrano (a, c) and Marambio (b, d). The
vertical scales show the altitude and are forced to be the same for the sake of
comparison. The horizontal scales indicate the periods of the measurements,
which depended on the station. The color code of the upper figures
corresponds to the BrO vmr and is forced to be the same for the sake of
comparison. The same applies to the color code of the lower figures, which
indicate the AEC at each station. The BrO vmr higher than 5 pmol mol-1
and AEC higher than 0.5 km-1 are shown in dark red, while values below
the
detection limit are shown in black. Time periods with no observations (SZA > 75∘) are indicated with white areas and those with
data below quality filters are shown with grey areas. The vertical grid of the
retrieval is indicated with the small ticks in the vertical axis.
Vertical profiles of BrO in the Antarctic troposphere
The time series of BrO vmr retrieved during 2015 in the lowest kilometers of
the troposphere of Marambio and Belgrano are provided in Fig. 7, along with
the AEC. For clarity, Fig. 8 shows, in more detail, some examples of the time
evolution of BrO as measured at Belgrano and at Marambio and in Fig. 9 the
sea ice conditions during those days. The wind speed on those selected days
was below 10 m s-1. As can be seen in Fig. 8, the maximum of BrO was
located close to the surface, although its specific altitude depended on the
day, always located below 1 km of altitude. Some days the peak of BrO was
located just above the surface (e.g., 11 November at Belgrano or
25 September at Marambio in Fig. 8; both days with winds ≤ 6 m s-1), while in others that BrO maximum was slightly elevated, suggesting
heterogeneous reactions aloft (e.g., 29 October at Belgrano with wind
speed ≤ 2 m s-1 or 28 November at Marambio with wind speed
6.5–9 m s-1; in Fig. 8). Worth noticing is also the time and seasonal
variability in the occurrence of the maximum of BrO vmr. As shown in Fig. 8,
on 29 October at Belgrano, some days the BrO vmr followed the
diurnal evolution with a noon maximum predicted by model studies (e.g.,
Saiz-Lopez et al., 2008) and observations (e.g., Buys et al., 2013) in which
the BrO formation is linked to the solar irradiance and the photolysis of
bromine sources (e.g., Br2, BrCl). Based on the calm wind conditions
observed during that particular day (29 October), those sources
are most probably not far from the station of Belgrano (Fig. 9). Conversely, on 28 November at Marambio, for example (Fig. 8), on other
days BrO was present in the low troposphere with a double maximum (morning
and evening), characteristic of a late spring behavior and more related to
bromine being recycled through HOBr, for example (e.g., von Glasow et al., 2002;
Pöhler et al., 2010; Liao et al., 2012; Buys et al., 2013). The slightly
higher southeasterly winds during those observations on 28
November at Marambio point towards bromine sources in the Weddell Sea
(Fig. 9). This shown time and altitude dependence of the BrO distribution in
the troposphere reinforces the benefits of the sort of instrumentation
employed in this work, which offers vertically resolved information and is
able to perform long-term observations.
Examples of the daily and vertical evolution of BrO at
Belgrano (a, c) and at Marambio (b, d). The horizontal scales indicate the
time of the day while the vertical scales show the altitude (small ticks
indicate the vertical grid of the retrieval). The color code corresponds to
the BrO vmr with values below detection limit shown in black. Observations
below quality filters are indicated by the grey dashed area. Note the
different vmr scales of each day.
Sea ice conditions around the measurement sites during the
exemplary days shown in Fig. 8. The maps of the sea ice concentration are
downloaded from https://seaice.uni-bremen.de/databrowser/ (last access: 18 May 2018) (Spreen et al.,
2008) and are sorted based on the date. The stations of Marambio and
Belgrano are marked in yellow in the figures. As can be seen in the figures,
in barely 1 month (25 September–29 October) the sea ice
surrounding Marambio underwent strong transformation, going from
medium–highly concentrated sea ice after winter (upper left figure) with
barely permanent open waters to pretty much open ocean (all
the sea ice disappearing beyond 50∘ W). During the timeframe of that sea ice
transformation, BrO VCD2km peaked at Marambio (Fig. 4). Also, note how the
edge of the sea ice near Belgrano transforms towards summer (e.g., lower
right).
Figure 10 shows a summary of the BrO vmr vertical profiles observed at each
station during the sunlit period of 2015. The median BrO vmr in the lowest
layers of the troposphere (< 0.5 km) was similar at both stations
(∼ 1.6 pmol mol-1 above the surface) with 75 % of the
BrO data below 2.5 pmol mol-1. However, as shown in Fig. 7, the maximum
BrO values observed after the polar sunrise (i.e., SZA < 75∘) at Marambio (26.0 ± 0.4 pmol mol-1) were over
3-fold of those observed after the sun rose at Belgrano (see also Fig. 4). This maximum BrO was detected during austral spring at ∼ 200 m
of altitude, with a magnitude dependent on the station. This slightly
elevated peak of BrO (e.g., Fig. 10), also mentioned above, has already
been foreseen by studies accounting for the vertical gradient of the acidity
of the aerosols and/or the effect of convection (e.g., von Glasow et al.,
2002; Wagner et al., 2007). Also note that, as shown in Fig. 7, while at
Belgrano the maximum BrO observed during October–November (8.1 ± 0.6 pmol mol-1) quadrupled its mean value measured during the rest of the
sunlit period, the BrO values observed at Marambio just after the sunrise
were over 15× higher than the BrO mean values at that station. All this
suggests that the halogen reactivity at Marambio is considerably stronger
than at Belgrano (see also Sect. 3.3). The BrO vmr ranges reported herein
(Fig. 10) are comparable to previous tropospheric Arctic studies (e.g.,
Tuckermann et al., 1997; Hönninger and Platt, 2002; Prados-Roman et al.,
2011; Liao et al., 2012; Peterson et al., 2017; Simpson et al., 2017) and
consistent with the few existing Antarctic measurements (e.g., Table 1). By
adding the BrO measurements provided in the frame of this work to the few
previous ground-based observations performed at Antarctica from different
sites, Fig. 11 depicts an updated map of the maximum values of BrO observed
in the lower troposphere of Antarctica, pointing once more to its heterogeneity
with regard to reactive bromine load. Section 3.3 offers a closer look at
this heterogeneity.
Vertical profile of the BrO volume mixing ratio in the Antarctic
troposphere (2015). The observations performed from Belgrano station are
shown in (a) while those performed from Marambio are given in (b). The median BrO values retrieved are indicated in thick blue
lines while the shaded blue areas mark the variability range of BrO vmr
throughout the sunlit period (SZA < 75∘). The a priori
BrO profiles (and error) used in the inversion are shown in dark grey. The
vertical grid of the retrieval (100 m) is indicated with the small ticks on
the vertical axis. Note the same scales in both plots.
Maximum values of BrO vmr reported in the low troposphere of
Antarctica as measured with ground-based observations. The different sites
where BrO has been reported in the low troposphere are indicated with a
colored dot. The color code of each dot (station) refers to the maximum BrO
vmr reported in literature (Arrival Heights: Kreher et al., 1997; Neumayer:
Frieß et al., 2004; Dumont d'Urville: Grilli et al., 2013; Halley:
Roscoe et al., 2014; Dome-C: Frey et al., 2015; Marambio and Belgrano: this
work). Note that only the present study provides contemporary observations
from different sites. Further details are provided on Table 1.
Overall, the observations presented in this study indicate that the vertical
profile of BrO in the Antarctic troposphere descended with altitude (Fig. 10). Note that, in this work, the detection limit of BrO is regarded as the
threshold value above which the inferred BrO is significantly higher than
the noise of the inversion. In this case, it is defined as double the
inversion error (Sect. 2.2.2) and corresponds to a mean value of 1 pmol mol-1. In previous studies of the Antarctic troposphere (e.g.,
Frieß et al., 2004; Roscoe et al., 2014), the presence of uplifted
reactive bromine was suggested. While the work of Frieß et al. (2004)
remarked on the presence of uplifted reactive bromine (> 4 km
altitude) due to advection processes, in the work of Roscoe et al. (2014),
the authors were able to differentiate between two types of BrO vertical
profiles: those with only near-surface BrO (centered at 200 m) and those
with a double peak (centered at 2 km and near surface). In the work presented here, no uplifted layers (> 2 km) of BrO were
detected,
although, given the limited vertical information content of the MAX-DOAS
observations (Supplement), double-peak types of BrO profiles
cannot be ruled out. Although the definition of the height of the boundary
layer over ice and snow surfaces (e.g., Anderson and Neff, 2008) is out of
the scope of this work, previous studies place the top of the boundary layer
in Antarctica between 100 m and 2 km, depending on the boundary layer
parameterization, station and time of year (e.g., King et al., 2006;
Nygård et al., 2013). Nygård et al. (2013) marked the height of the
boundary layer at Marambio between 1 and 1.4 km for the non-winter periods
of our interest, and the ozonesondes performed at Belgrano set that height
between 1 and 2.5 km at that site (e.g., Parrondo et al., 2014). However,
the altitude threshold between the boundary layer and the free troposphere
cannot be assessed at the time resolution of our observations. Additional
investigations at the two sites would be needed to confirm whether BrO
reaches the free troposphere and, given the case, to assess the budget of
BrO in the Antarctic free troposphere. Note that previous work on polar
environments set BrO below 1.5 and 2 pmol mol-1 in the Arctic and
Antarctic free troposphere, respectively (e.g., Frieß et al., 2011;
Prados-Roman et al., 2011; Peterson et al., 2017; Hüneke et al., 2017).
In addition to the BrO knowledge gained after this work,
the information related to the vertical AEC is also noteworthy (Fig. 7, lower panels),
sustaining the particularity of the surroundings at each station. In
addition to the aforementioned different aerosol optical thickness at both
stations (Sect. 3.1.1), there is also a noticeable difference regarding the
seasonality and altitude of the maximum AEC at the two sites. While at
Marambio the peak of the AEC appeared close to the surface with a clear
maximum extinction observed in November, the observations performed from
Belgrano suggest that, at this site, the height of the aerosol layer was
much more variable than at Marambio, manifesting once more the relevance of
vertically resolved observations within the Antarctic troposphere. As
mentioned before, the work of Gómez-Martín et al. (2018) will address these issues.
Rates (k) of reactions provided
in Sect. 1 and employed in Sect. 3.3. The temperature used for the
calculations was T= 262 K, similar to the mean temperature observed
during 2015 at each station (Table 3).
Reaction
Rate constant
Reference
(cm3 molec-1 s-1)
Br + O3
8.02 × 10-13
Sander et al. (2006)
BrO + BrO
3.54 × 10-12
Sander et al. (2006)
BrO + ClO
7.83 × 10-12
Atkinson et al. (2007)
BrO + HO2
3.03 × 10-11
Atkinson et al. (2007)
BrO + OH
4.67 × 10-11
Atkinson et al. (2007)
BrOx in Antarctica
In order to investigate the hinted heterogeneity of the Antarctic lower
troposphere regarding reactive bromine (BrOx = Br + BrO), in this
section the budget of bromine [Br] is estimated considering the steady state of
BrO in a pristine atmosphere with virtually no NO (e.g., Zeng at al., 2006),
a concentration of ClO of 1.7 × 108 molec cm-3 (typical
of Arctic conditions; e.g., Halfacre et al., 2014), and concentrations of
HO2 and OH of 2.2 × 107 and
3.9 × 105 molec cm-3, respectively (mean values observed
at the Antarctic station of Halley; e.g., Bloss et al., 2007). Hence, [Br]
can be estimated from the observed BrO and O3 concentrations as (e.g.,
Hausmann and Platt, 1994; Le Bras and Platt, 1995; Zeng et al., 2006;
Stephens et al., 2012)
Br=BrO×2kBrO+BrOBrO+kBrO+ClOClO+kBrO+HO2HO2+kBrO+OHOH+JBrOkBr+O3[O3],
where J represents the rate of photolysis (JBrO= 3.10-2 s-1
for noontime in the polar spring; e.g., Thompson et al., 2015) and k the
different reaction rates (Table 4). Since the measurements of O3 were
performed near the surface, accordingly only the BrO retrieved in the lowest
atmospheric grid (i.e., 100 m) is considered for the calculation of the Br
and BrOx budgets at each site. The possible influence of horizontal
advection and blowing snow is limited in the data set by applying an upper
limit for the wind speed of 6 m s-1. Note that previous studies
pointed to
8 m s-1 as the wind threshold for blowing snow (e.g., Jones et al.,
2009) and others indicated that the steady-state approximation is valid for
wind speeds lower than the 6 m s-1 threshold considered here (e.g.,
Liao et al., 2012).
Reactive bromine in the low troposphere of Belgrano (a) and
Marambio (b) under different O3 regimes. The vertical scale depicts
the BrOx (Br + BrO) at each station and the color code refers to the
collocated observed O3 vmr. Note that the vertical scale and the color
code apply to both figures. Only observations performed under low wind
conditions (< 6 m s-1) and a SZA < 75∘ are
included.
Based on Eq. (2) and observed near-surface BrO and O3 under low wind
conditions (time stamp of the DOAS measurements), Fig. 12 shows the 2015
seasonal evolution of the BrOx budget at each station and Fig. 13
presents the BrO–Br–BrOx statistical analysis in the form of box charts
at each site (only data for SZA < 75∘ are considered).
Since the kinetic calculations used herein are based on observations
performed under low wind conditions, these budgets may be considered to be
representative of the surroundings of each station. Figure 12 indicates
that, in agreement with Peterson et al. (2015), the presence of reactive
bromine at both stations does not only correspond to advected bromine-enriched
air masses or blowing snow. As expected from previous polar studies (e.g.,
Simpson et al., 2007, and references therein) and shown in Fig. 12, the
maximum bromine-related reactivity of the troposphere at both stations takes
place just after the photolysis is triggered with the polar sunrise. As
shown in Fig. 12, this maximum reactivity does occur at a medium O3
regime at both stations (10–25 nmol mol-1). The study of the
BrO–Br–BrOx data (Fig. 13) indicates that, during the sunlit period of
2015, the mean budget of BrOx at Belgrano (2.0 pmol mol-1) was
∼ 17 % higher than at Marambio. However, just after sunrise
(i.e., SZA < 75∘), the BrOx budget (and hence the
oxidizing capacity) at Marambio triplicated the one at Belgrano (e.g., Fig. 12). Estimated values for atomic bromine radicals present in the lowermost
troposphere during the sunlit period of 2015 were up to 1.4 pmol mol-1
at Belgrano and up to 3.4 pmol mol-1 at Marambio (Fig. 13). These
ranges are in line with previous model studies for Antarctic latitudes
(e.g., von Glasow et al., 2004; Saiz-Lopez et al., 2008) and in the lower
limit of Arctic model studies (e.g., Thompson et al., 2017).
Statistical analysis of the reactive bromine and its partitioning
estimated at Belgrano (a) and at Marambio (b) during the sunlit
period of 2015. The vertical scale, which is the same in both plots,
indicates the range of mixing ratios of BrO, Br and BrOx at both
stations (SZA < 75∘). The legend applies to both figures,
where the whiskers display the range of the maximum and minimum vmr, the
boxes in dark yellow provide the vmr ranges of 25–75 % of the data and
dashed lines depict the median vmr.
Overall, these estimations indicate that the BrOx partitioning was
clearly driven by BrO at both sites, indicating that ozone in general was
not fully depleted as confirmed by the observations (Sect. 3.1.2). The
evolution of the ratio Br to BrO after the polar sunrise (SZA < 75∘) is shown in Fig. 14 for each site. The initial Br : BrO
after dawn was ∼ 0.05 at both stations. Throughout the polar
spring, during ODEs, that ratio rose over 4-fold at both sites. The
baseline of the Br-to-BrO ratio during the sunlit period could be
approximated by an exponential growth with a time constant of about 10 days
at Belgrano and 17 days at Marambio (blue line in Fig. 14). Towards summer,
that baseline increased up to 0.17 at Belgrano and to 0.10 at Marambio. In
the simplified scheme suggested by Eq. (2) and discussed in this section,
this Br : BrO increase could be explained by the overall summer decrease in
surface O3 compared to springtime (Fig. 6). Additional investigations
on the variability and geographical distribution of the bromine source gases
throughout the year are suggested to address the bromine pathways in the
Antarctic troposphere and their consequences. Bearing in mind this
simplified scheme, based on Eq. (1) and the same fixed concentration of ClO
as for Eq. (2) (i.e., to typical values in the Arctic environment), the
bromine-mediated ozone loss rate can be assessed at each research site for
the different BrO and O3 regimes observed at low wind speed. Similar
median BrO values measured during 2015 at both stations (1.6 pmol mol-1) yield a similar ozone loss rate of
0.4 nmol mol-1 day-1 at both sites. During the more active bromine season of October–November
at Belgrano (e.g., Fig. 12), this rate speeds up to 2.9 nmol mol-1 day-1. During September at Marambio (peak bromine season at that
station), the bromine-mediated ozone loss occurs at a much faster rate of
between 0.7 and 17.4 nmol mol-1 day-1 (i.e., up to 6× faster
than at Belgrano). Former works have estimated that the bromine-driven ozone
loss in the polar atmosphere represents 44 % of the total O3 chemical
loss (e.g., Liao et al., 2012; Thompson et al., 2017). Therefore, in the
sites referred to in this work the shortest (i.e., at highest BrOx and
low wind speed) ozone chemical lifetime τO3 expected is 2.6 days
at Belgrano and 0.7 days at Marambio. Note that the estimations provided
herein are limited by the information content inherent in the in situ
technique measuring near-surface ozone (i.e., information at the exact
instrument's location) compared to the DOAS data, which integrate
information several kilometers away from the instrument (depending on
scattering conditions) in the horizontal field of view and also in the
vertical field of view (see retrieved averaging kernels in the Supplement).
Further studies would be needed to confirm these numbers, including
investigations on different sources and sinks of bromine radicals in the
Antarctic environment, which herein are based on ozone depletion through
(only) the BrO–BrO and BrO–ClO channels, dominant however in the polar
spring (e.g., Simpson et al., 2007).
Variability in the ratio of Br to BrO after the polar sunrise at
Belgrano (a) and Marambio (b). The left axes refer to the Br-to-BrO
ratio (same scales on both plots) with the estimated ratios shown in black
and the fitted baseline in blue. The right axes on both plots refer to the
hours of light at each station. The horizontal scales indicate the time
period. Note that only data observed under low wind conditions and with a SZA < 75∘ are considered.
All these kinetic approximations are historically based on conclusions from
numerical models and laboratory and campaigned-based observations obtained
in the polar regions (mainly the Arctic; e.g., Simpson et al., 2007).
Nevertheless, the year-round erratic behavior of the wind speed in
Antarctica at each station makes the verification of these
estimated (low wind) τO3 with observations complicated. However, the
exemplary days provided in Fig. 8 with higher BrO at each station (upper
figures) may serve the purpose (low wind speeds). For instance, based on the
ozone observations (Fig. 6), the rate of O3 depletion measured at
Marambio (25 September) was 4.1 nmol mol-1 h-1 and at
Belgrano (29 October) it was 0.58 nmol mol-1 h-1.
Therefore, as suggested by the above related theoretical calculations, the
destruction of surface O3 during the bromine peak season was indeed
much faster at Marambio (7× faster than at Belgrano). Considering the
mean O3 vmr observed at each station on those days, the observed τO3 at Belgrano was 1.3 days while at Marambio it was 10-fold
shorter. Note that, as shown in Fig. 8 (upper figures), on those specific
days the BrO load at Marambio was also over an order of magnitude higher
than at Belgrano. Comparing these observed τO3 values with the τO3 estimated above from kinetics,
the measurements show shorter τO3 at both stations (50 % shorter at Belgrano and 18 % shorter
at Marambio). Despite the low statistical meaning of this sort of “case
study” exercise, the resemblance of the observed and calculated τO3 at Marambio suggests that the assumptions made at Marambio's
surroundings (e.g., the Br–Cl channel dominates the ozone depletion) is close
to reality, which seems not to be the case for Belgrano's surroundings. This
reinforces, once more, the need for further long-term investigations for a
better understanding of all the processes and key parameters involved in the
halogens' pathways in the Antarctic troposphere.
Summary and outlook
As a result of its remoteness and a more complex logistics compared to the
Arctic region, the characterization of the Antarctic troposphere with regard
to halogen compounds is still very scarce. Based on contemporary
ground-based observations performed at two Antarctic sites during 2015 (new
sites as far as tropospheric BrO observations are concerned), this study
reports on the presence and vertical distribution of reactive inorganic
bromine in the low troposphere at the two sites and discusses the
geographical distribution of BrOx. Prior to this study, throughout
Antarctica only five sites had reported ground-based observations of BrO in
the low troposphere. With the appropriate instrumental setup at the
research stations of Belgrano and Marambio, INTA has expanded this net
considerably. Moreover, to the authors' knowledge, this is the first study
in which these bromine observations are reported simultaneously from two
Antarctic stations, making it possible to gain an insight into the geographical
distribution of reactive bromine in the Antarctic troposphere. Additionally,
through the 2015 MAX-DOAS measurements performed at the two sites, this work
presents vertically resolved observations of BrO at two different Antarctic
stations with a dedicated inversion scheme for inferring the vertical
distribution of BrO throughout the Antarctic troposphere. Furthermore, the
aerosol extinction and the surface ozone at the two sites are also provided.
Overall, results show the expected seasonal and daytime variation in BrO
related to the photolytic activation of reactive bromine triggered by the
polar sunrise at the two sites. However, as referred to above and unlike some
former studies, during the sunlit period of 2015 no elevated plumes of BrO
were detected above 2 km. Also, this study reports on the positive detection
of BrO in the low troposphere (< 2 km) of Antarctica even under low
wind conditions, suggesting that the presence of this trace gas is not only
related to horizontal advection but also to surface emissions and/or
vertical mixing. As for the vertical and geographical distribution of BrO in
the lower layers of the troposphere, observations indicate a slightly
elevated BrO peak at 200 m at both stations, with a maximum value measured
at Marambio considerably higher than the observed value at Belgrano (26 pmol mol-1 vs. 8 pmol mol-1, respectively).
In general, the observations and assessments presented in this work reveal a
remarkable geographical heterogeneity of the Antarctic low troposphere with
regard to the budget of reactive bromine. Beyond blowing snow, the inferred
3-fold enhancement of BrOx at Marambio compared to Belgrano after
the polar sunrise also denotes a geographical heterogeneity on the bromine
sources. Marambio sits on a region surrounded by open waters and seasonal
sea ice while the dominant sea ice near Belgrano is perennial (Fig. 9).
Since bromine explosions are linked to heterogeneous reactions related to sea ice, open leads and snow surfaces, for example, the type of sea ice and its
seasonal evolution around each station may be a good starting point to
tackle the bromine sources riddle and to investigate how climate change may
affect the budget of BrOx in the troposphere of Antarctica. Moreover,
the geographical distribution of BrOx and its partitioning addressed
in this work also suggest that the reactivity of the troposphere at
Marambio is particularly enhanced compared to other Antarctic sites (“hot
spot”). Since the presence of BrOx in the polar atmosphere represents
a sink for elemental mercury, this study also reveals the tip of the
Antarctic Peninsula (Marambio) as a region for potentially enhanced mercury
deposition (bioaccumulation) worth looking into. Also, dedicated
investigations combining models and collocated observations of
halogenated substances (not only BrO), organic compounds, DMS, NOx,
HOx, particles and sea ice properties at different stations, for example, could
assist a thorough study of the bromine sources and pathways in Antarctica,
their geographical distribution and their projections under a changing
environment.
In addition to the bromine-related information gained from this work, this study also
emphasizes the benefits of deploying quality instrumentation in pristine and
remote locations able to provide not only surface but also vertically
resolved information. It also shows the scientific benefits of maintaining
long-term observations despite the efforts related to sustaining research
activities in such a hostile environment. The data provided by the two
ground-based instruments presented herein may, for instance, assist the
satellite retrievals to distinguish between tropospheric and stratospheric
BrO signal and hence facilitate a more accurate assessment of
stratospheric BrO and ozone trends, for example. Additionally, they could also serve
chemistry–climate models for constraining the chemistry behind processes
specifically related to polar regions, areas where global models are often
weak (particularly in Antarctica).