ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-18-9789-2018Observations and source investigations of the boundary layer bromine monoxide (BrO) in the
Ny-Ålesund ArcticBoundary layer BrO in the Ny-Ålesund ArcticLuoYuhanyhluo@aiofm.ac.cnSiFuqisifuqi@aiofm.ac.cnZhouHaijinDouKeLiuYiLiuWenqingKey Laboratory of Environmental Optics and Technology, Anhui
Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei,
230031, ChinaNational Synchrotron Radiation Laboratory, University of Science
and Technology of China, Hefei, 230027, ChinaYuhan Luo (yhluo@aiofm.ac.cn) and Fuqi Si (sifuqi@aiofm.ac.cn)11July201818139789980113June201723August201719June201826June2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/18/9789/2018/acp-18-9789-2018.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/18/9789/2018/acp-18-9789-2018.pdf
During polar spring, the presence of reactive bromine in the polar boundary
layer is considered to be the main cause of ozone depletion and mercury
deposition. However, many uncertainties still remain regarding understanding the
mechanisms of the chemical process and source of the bromine. As Arctic
sea ice has recently been dramatically reduced, it is critical to investigate
the mechanisms using more accurate measurements with higher temporal and
spatial resolution. In this study, a typical process of enhanced bromine and
depleted ozone in the Ny-Ålesund boundary layer in late April 2015 was
observed by applying ground-based multi-axis differential optical absorption
spectroscopy (MAX-DOAS) technique. The results showed that there were bromine monoxide
(BrO) slant columns as high as 5.6 × 1014 molec cm-2 above the
Kings Bay area on 26 April. Meanwhile, the boundary layer ozone and gaseous
elemental mercury (GEM) were synchronously reduced by 85 and 90 %,
respectively. Based on the meteorology, sea ice distribution and air mass
history, the sea ice in the Kings Bay area, which emerged for only a very
short period of time when the enhanced BrO was observed, was considered to be
the major source of this bromine enhancement event. The oxidized GEM may be
directly deposited onto snow/ice and thereby influence the polar ecosystem.
Introduction
Bromine monoxide (BrO) is one of the key reactive halogen species that has
profound impacts on the atmospheric chemistry of the polar boundary layer (PBL), especially the oxidative capacity of the troposphere (Saiz-Lopez
and von Glasow, 2012). The presence of reactive bromine (in some situations
called “bromine explosion”) is considered to be the main cause of the
depletion of boundary layer ozone, known as “ozone depletion events” (ODEs)
(Platt and Hönninger, 2003). Furthermore, halogens can efficiently
oxidize gas-phase mercury, which can lead to a decrease of gaseous mercury,
known as “atmospheric mercury depletion events” (AMDEs) (Lu et al., 2001; Ariya et al.,
2002, 2004; Lindberg et al., 2002; Steffen et
al., 2008). Enhanced BrO was first detected by long-path differential
optical absorption spectroscopy (LP-DOAS) observations (Platt,
1994). Satellite measurements confirmed that the phenomenon of bromine
enhancement covered larger areas of polar regions by deriving daily global
BrO maps (Wagner et al., 2001; Richter et al., 1998; Platt and Wagner, 1998; Sihler et al., 2013). The primary source of reactive bromine has been
explained by a series of photochemical and heterogeneous reactions at the
surface of the frozen ocean during polar spring (Fan and Jacob, 1992). A
typical heterogeneous reaction model between the gaseous and condensed
phases is shown in Fig. 1. Bromine is released from ice surfaces to the
atmosphere in an autocatalytic chemical mechanism that oxidizes bromide to
reactive bromine. The reaction of HOBr is proposed to be the catalyst that
drives the recycling reaction, which is an acid-catalyzed reaction
(Simpson et al., 2007). Sea-ice (first year) surfaces, brine and frost
flowers have been considered as possible sources (Kaleschke et
al., 2004; Lehrer et al., 2004).
Comparisons of BrO mixing ratios at four main Arctic observation
sites.
ObservationBrO mixingSitesperiodsratioMethodsReferencesGreenland ice sheet(72∘ N, 38∘ W,3200 m a.s.l.)14 May–15 June 2007, 9 June–8 July 20083–5 pptLP-DOASStutz et al. (2011)Barrow, Alaska(71∘19′ N, 156∘40′ W)26 February–16 April 2009∼ 30 pptMAX-DOAS and LP-DOASFrieß et al. (2011)Alert, Nunavut (82∘32′ N, 62∘43′ W)20 April–9 May 2000∼ 30 pptMAX-DOASHönninger and Platt (2002)Ny-Ålesund, Svalbard (78.9∘ N, 11.8∘ E)20 April–27 April 1996∼ 30 pptLP-DOASTuckermann et al. (1997)
However, the actual situation is that the ODEs do not always occur
concurrently with episodes of BrO enhancement. There are only few reports of
Arctic ODEs that are assumed to have been observed primarily as a result of
local-scale chemical mechanisms (Jacobi et al., 2006; Bottenheim et al., 2009). As the photochemical reactions happen quickly and the lifetimes of
the intermediate products (e.g., the reactive bromine radicals) are quite
short, more accurate data with a higher temporal resolution are needed to
analyze the chemical process in the PBL and investigate the source of
bromine.
Chemical reactions of the BrO–ozone cycle.
The MAX-DOAS (multi-axis differential optical absorption spectrometer)
technique has the advantage of being able to clearly separate the
tropospheric and stratospheric portions of the atmospheric column and even
derive a crude vertical profile (Frieß et al., 2011). When
pointing to a direction slightly above the horizon, the spectrometer can
obtain high sensitivities for the trace gases close to the ground due to the
long light path through the trace gas layers. This technique is also an
important calibration of satellite observations, which have lower spatial
and temporal resolutions compared with ground-based measurements. In the
Arctic area, ground-based MAX-DOAS observations have been made at Barrow,
Alaska (71∘ N, 157∘ W), Alert, northern Canada
(82.5∘ N, 62.3∘ W) and Ny-Ålesund, Svalbard
(78.9∘ N, 11.8∘ E) (Table 1). Additionally, air-borne
(Neuman et al., 2010; Pöhler et al., 2013) and ship-borne
measurements (Jacobi et al., 2006; Leser et al.,
2003; Wagner et al., 2007; Bottenheim et al., 2009) are important supplements for the analysis and
modeling of bromine chemistry.
(a) Sea ice extent on April 2015 in the Arctic area (data from
http://nsidc.org/data/seaice_index/, last access: 9 July 2018). (b) Monthly mean sea ice
concentration anomalies on April 2015 compared to averages from 1979 to
2015. (c)Two meter air temperature anomalies on April 2015 compared to averages
from 1979 to 2015. (b and c data are from
http://nsidc.org/soaclast access: 9 July 2018).
However, recently, Arctic sea ice coverage has dramatically reduced,
especially in eastern Greenland and northern Europe. Influenced by the North
Atlantic warm current (NAWC), the near-surface air temperatures and
sea-surface temperatures (SST) are becoming higher in northern Europe (Fig. 2). In recent years, Kings Bay in Ny-Ålesund had ice-free open water all
year round, which is a unique characteristic compared with other areas at
the same latitude in the Arctic. Therefore, it is critical to gain a better
understanding of the possible sources of reactive bromine and the impact
of halogen activation on PBL ozone depletion and mercury deposition within a
rapidly changing Arctic. In this study, an event of enhanced bromine and
depleted ozone in Ny-Ålesund was caught in late April. The key role of
bromine was confirmed by ground-based MAX-DOAS measurements. This event
provides a rare opportunity to investigate the source of bromine and the
process of ozone depletion in this area.
Instruments and methodsInstrument setup
The MAX-DOAS measurement site is located at Yellow River station
(78∘55′30′′ N, 11∘55′20′′ E) at Ny-Ålesund on the west coast of Spitsbergen. The observation
position is shown in Fig. 3. To give a rough idea of the climate conditions,
monthly mean sea ice concentration anomalies and air temperature anomalies
during April 2015 are shown in Fig. 2. Observations were obtained from 25
April to 15 May 2015. Due to the wavelength adjustment, no data were
available during a short period from 28 to 29 April.
The MAX-DOAS field observation in Ny-Ålesund, Arctic.
The MAX-DOAS instrument operated at Ny-Ålesund consists of both indoor
and outdoor conponents. The telescope receives scattered sunlight from multiple
angles and is controlled by a stepper motor to adjust elevation angles from
horizon (0∘) to zenith (90∘). The field of view of the
telescope is approximately 1∘. The scattered sunlight is
imported through the quartz fiber with a numerical aperture of 0.22 into the
indoor spectrograph (Ocean Optics MAYA pro) with a one-dimensional CCD array
(ILX511 linear array CCD) containing 2068 pixels. The wavelength range of
the spectrograph is from 290 to 420 nm; thus, enabling the analysis of
trace gases including O3, NO2, BrO, OClO, HCHO and O4. The
spectral resolution is approximately 0.5 nm (FWHM). The CCD detector is
cooled at -30 ∘C, while the whole spectrometer is thermally
stabilized at +20 ∘C using a thermal controller. A computer sets
the configuration of the system and controls the automatic measurements. The
integration time (typically ranging from 100 to 2000 ms in multiples of
100 scans) of each measurement depends on the intensity of scattered light,
which can be influenced by clouds and visibility. A standard mercury lamp is
used for spectra calibration. Calibration measurements of dark current and
offset are performed after each measurement.
The telescope is pointed towards the northeast, which covers the
Kings Bay area (Fig. 3). Kings Bay is an inlet on the west coast of
Spitsbergen, one part of the Svalbard archipelago in the Arctic Ocean. The
inlet is 26 km long and 6 to 14 km wide. The range of MAX-DOAS measurement
is an area with a radius of approximately 10 km, which covers the central
area of the fjord. The sequence of the elevation angles is 2,
3, 4, 6, 8, 10, 15, 30 and 90∘ above the horizon.
Data evaluation
The spectra, measured with the setup described above, are analyzed using the
well-established DOAS retrieving method (Platt, 1994). The
wavelength calibration is performed using the QDOAS software developed by
the Belgian Institute for Space Aeronomy (BIRA) by fitting the reference
spectrum to a high-resolution Fraunhofer spectrum (Kurucz et al., 1988).
The spectral analysis of BrO is performed at 340–359 nm, encompassing three
BrO absorption bands, which improves the accuracy of the inversion. O3
(223, 243 K) (Bogumil et al., 2003; Vandaele et al., 1998), NO2
(298, 220 K) (Vandaele et al., 1998), O4 (Hermans
et al., 2003), BrO (228 K) (Wilmouth et al., 1999), OClO (233 K)
(Kromminga et al., 2003), and ring structure (Chance and Spurr,
1997) are involved in the inversion algorithm. The O4 retrieval is
performed using the same set of cross sections as for BrO but in the
wavelength interval of 340–370 nm. The high-resolution cross sections are
convoluted with the instrument slit function, which is determined by measuring the
emission line of a mercury lamp. A fifth-order polynomial is applied to
eliminate the broad band structures in the spectra caused by Rayleigh and
Mie scattering. Furthermore, a nonlinear intensity offset is included in the
fit to account for possible instrumental stray light. A wavelength shift and
stretch of the spectra are allowed in the fit in order to compensate for
small changes in the spectral adjustment of the spectrograph.
Examples of spectral retrieval of BrO. The spectrum was recorded
under clear sky conditions at 2∘ elevation on 26 April 2015, 19:59 UTC, SZA = 86∘. (Black lines represent retrieved spectral signatures
fitted result for absorber, and red lines represent fitted cross sections).
The fit procedure yields differential slant column densities (DSCD) using
zenith sky measurements of each sequence as a Fraunhofer reference for the
analysis, which eliminates the influence of stratospheric BrO change. An
example of the fit result of BrO is shown in Fig. 4. The spectrum was
recorded on 26 April 2015 19:59 UTC (SZA = 86∘) at an elevation
angle of 2∘. The BrO DSCD is 5.10 × 1014 molec cm-2. The residual root mean square is 4.59 × 10-4,
resulting in a statistical BrO DSCD error of 1.63 × 1013 molec cm-2.
Since DSCDs are dependent on the light path, wavelength and observation
geometry, DSCDs are then converted to vertical column density (VCD) by
dividing by the differential air mass factor (DAMF), which is the averaged
light path enhancement for solar light traveling through the atmosphere
compared to a straight vertical path.
We perform the radiative transfer modeling (RTM) simulations using SCIATRAN
software (Rozanov et al., 2005) to obtain the modeled DAMF using
five different assumed BrO profiles with evenly distributed air masses: (a) 0–0.5 km; (b) 0–1 km; (c) 0–2 km; (d) 0.5–1 km; and (e) 1–2 km (Fig. 5a). The models
are run under clear sky conditions with no aerosol input. Remarkable
differences exist for different input profiles. For the BrO layers of 0–0.5, 0–1 and 0–2 km, the DAMFs all increase with decreasing elevation
angles. However, for the BrO layers of 0.5–1 and 1–2 km, the dependence
on the telescope elevation angle is weaker, especially at small elevation
angles.
Modeled DAMF (a) and BrO DSCD (b) using a radiative transfer modeling
simulation. DAMF are the differences of air mass factor for low elevation angles and zenith
direction. The models are performed assuming clear sky conditions with no
aerosol. In (b), the tropospheric BrO VCD is 5 × 1013 molec cm-2. The measured BrO DSCDs during the event are also shown
(solid dots). The color codes of the measured BrO DSCDs, which are also
shown in (b) (solid dots), are put into a one-to-one correspondence with the
dots in (c)
The modeled BrO DSCDs for different input BrO profiles are shown in Fig. 5b.
The input BrO VCD is 5 × 1013 molec cm-2. The measured
BrO DSCDs from 26 April 20:00 UTC to 27 April 04:00 UTC are also plotted
(Fig. 5c). Since the inaccuracy of modeled BrO becomes larger at lower
elevation angles, elevation angles of ≥ 8∘ should receive more
attention. From Fig. 5b, we can obviously see that the measured BrO DSCDs
are best reproduced by the model for layer 0–1 km before midnight. This
suggests that the BrO layer between 0 and 1 km can be considered as the most
likely distribution. Therefore, BrO volume mixing ratios (VMR) are calculated
assuming a homogeneous BrO layer with a thickness of 1 km at the surface.
Complementary data
Ny-Ålesund is a science community hosting over fifteen permanent
research stations. Atmospheric measurements have been measured continuously
at Zeppelin station, Ny-Ålesund since 1990. Located on Zeppelin
Mountain, with an altitude of 474 m a.s.l., it is a background
atmosphere observatory operated by the Norwegian Polar Institute (NPI) and
the Norwegian Institute for Air Research (NILU), which are part of the
Global Atmosphere Watch (GAW) framework. At the Zeppelin station, surface
ozone was measured by UV photometry, and gaseous mercury in the air was
measured using a Tekran mercury detector. Hourly surface ozone and gaseous
mercury data are downloaded from the EBAS database
(Tørseth et al., 2012).
Meteorology data, including temperature, air pressure, relative humidity,
wind direction and velocity, and global radiation data are recorded by the
AWIPEV atmospheric observatory in Ny-Ålesund. According to the
radiosonde records of temperature, humidity and wind speed from AWIPEV, the
height of the troposphere is approximately 8000 m and the height of the
boundary layer is approximately 1200 m at Ny-Ålesund.
A webcam on the 474 m Zeppelin Mountain records the sea ice change of Kings
Bay and the cloud situation of Ny-Ålesund
(https://data.npolar.no/_file/zeppelin/camera/, last access: 9 July 2018).
To roughly estimate the BrO distribution, BrO maps of the Northern
Hemisphere by GOME-2 products are downloaded from
http://www.iup.uni-bremen.de/doas/scia_data_browser.htm (last access: 9 July 2018). Stations overpass BrO vertical column densities for MetOp-A
(GOME-2A) and MetOp-B (GOME-2B) in the Ny-Ålesund, Arctic are downloaded
from https://avdc.gsfc.nasa.gov/index.php?site=580525926&id=97 (last access: 9 July 2018).
Time series of BrO DSCDs at 2∘, surface ozone, solar zenith angle and
meteorology data during the measurement.
Using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT)
model via the NASA ARL READY website (http://ready.arl.noaa.gov/HYSPLIT_traj.php, last access: 9 July 2018)
(Rolph et al., 2017; Stein et al., 2015), back trajectory analyses were carried out
to determine the history of air masses. Back trajectories of 72 h were
driven by meteorological fields from the NCEP Global Data Assimilation
System (GDAS) model output.
Results
The time series of BrO DSCDs at 2∘, surface ozone concentrations,
solar zenith angle (SZA), air pressure, air temperature, relative humidity,
wind velocity and wind direction from 25 April to 15 May are presented in
Fig. 6. Starting from late afternoon on 26 April, BrO DSCDs clearly exceeded
the background levels and peaked at 5.6 × 1014 molec cm-2.
In the same period, surface ozone sharply decreased from ∼ 80 ppb to several ppb and did not recover to normal values until 29 April.
During this period, the wind velocity frequently changed between 1 and 7 m s-1,
with unstable wind directions and mixing heights. Over a period of one week,
elevated BrO levels went back down to the detection limit (by 4 May) under a
stable boundary layer. From the 4 to 5 May, partial ozone (not near the zero
level) was depleted in the absence of BrO.
BrO DSCDs of different elevation angles during the enhancement period.
The time series of BrO DSCDs from 26 April 14:00 UTC to 28 April 12:00 UTC
at every elevation angle (2, 3, 4, 6, 8, 10, 15 and 30∘) are plotted in Fig. 7. Results of different elevation angles were
obviously distributed during the BrO enhancement period. However, the
differences in the BrO DSCDs ≤ 4∘ are very small (upright plot
in Fig. 7), indicating that the highest value of BrO is probably not above
the surface. To better understand the vertical distribution of reactive
bromine at the Arctic boundary layer, a comparison between the measured BrO
DSCDs from the MAX-DOAS measurements with the modeled ones from the SCIATRAN
model is performed (Fig. 5). The measured BrO DSCDs best match the model for
the 0–1 km layer during the enhancement, which means that the BrO
enhancement event was a regional rather than an in situ process.
(a) Sunshine duration; (b) solar zenith angle; (c) BrO DSCDs from MAX-DOAS at elevation
angle of 2∘; (d) BrO VMR (ppt); (e) surface ozone (ug m-3); and
(f) gaseous mercury (ng m-3) from 25 April at noon to 28 April at noon 2015. BrO
mixing ratios are calculated assuming a homogeneous BrO layer of 0–1 km.
The sunshine duration, SZA, BrO DSCDs from the MAX-DOAS at a 2∘ elevation angle, BrO volume mixing ratio, surface ozone and gaseous mercury
data from 26 to 28 April are plotted in Fig. 8. The BrO VMRs were calculated
assuming a 0–1 km layer of the BrO profile. The highest BrO VMR is
approximately 15 pptv during the ODE. Ozone, as well as gaseous mercury,
dropped extremely fast right after the enhancement of BrO. However, there
seems to be insufficient reactive bromine present locally in the boundary
layer since the ozone slowly increases just four hours later (on 26 April at
21:00 UTC). Afterwards, both ozone and mercury have a slow recovery with a
fluctuation on 27 April. A tiny increase of BrO occurs around 27 April at 20:00 UTC.
This could be explained by the fact that Br / BrO photochemical
reactions are taking places where there is enough ozone to react. When ozone
drops to the lower limit of the reaction, the reaction of
Br +O3→ BrO +O2 would stop (i.e., the situation observed on the night of 26
April). When ozone recovers to a certain level, the reaction starts again.
Back trajectory model of air masses arriving at Ny-Ålesund
ending on 27 April at 18:00 UTC at 10 (a) and 500 m a.s.l. (b). Every 6 h a new
trajectory starts, and each trajectory runs for 72 h.
Back trajectory model of air masses arriving at Ny-Ålesund
ending on 26 April at 18:00 UTC and 500 m a.s.l. (c). Every 6 h a new
trajectory starts.
Discussion
In this research, high concentrations of tropospheric BrO have been detected
using the ground-based MAX-DOAS technique. A BrO column as high as
5.6 × 1014 molec cm-2 was detected above Kings Bay,
Ny-Ålesund. The retrieval shows that the enhancement occurred
accompanied by severe ozone depletion and mercury deposition.
The possible sources of the reactive bromine are newly formed sea ice and
frost flowers, which can provide highly concentrated saline surfaces, and
also sea salt aerosol. The transport of air masses that already contain
elevated BrO or precursors and depleted ozone, is another possible source of
enhanced BrO. Therefore, we investigated the history of the air masses
arriving at the measurement site using backward trajectories. Furthermore,
the sea ice distribution and satellite BrO maps (Fig. 10) also provide
important information.
Map of tropospheric BrO of the Northern Hemisphere by GOME-2
products from 24 April to 27 April. (cited from
http://www.iup.uni-bremen.de/doas/scia_data_browser.htm, last access: 9 July 2018).
This enhancement event represented a good opportunity to investigate the
source of the BrO and its impact on the environment of the Arctic boundary
layer. These issues are discussed in detail in the following sections based
on the air mass history, sea ice distribution and ozone loss and mercury
deposition data.
History of air masses
To define the details of the air mass origin, 72 h backward trajectories at
altitudes of 10 and 500 m a.s.l. ending on 27 April at 18:00 UTC were
calculated every 6 h (Fig. 9a). This calculation shows that air masses
at both altitudes have a discontinuous origin. We then calculated the air
mass backward trajectory ending on 26 April at 18:00 UTC for every hour (Fig. 9b). This calculation shows that the air mass has a different origin
before/after 26 April 15:00 UTC. The wind direction changed to the north
and had a higher velocity. After this, the air mass had a relatively stable
origin from a height of 1000 m. More trajectory calculations from 22
to 30 April are shown in Supplement Figs. S1 and S2 for purposes of
comparisons. From the GOME-2 BrO VCD maps from 24 to 27 April (Fig. 10), we found that enhanced BrO was observed in the east of Greenland (red
box), far north of Siberia (blue circle) and east of Spitsbergen (black box)
during the period of interest and in the days before. The BrO maps from other
days (20 April to 13 May 2015) are shown in Supplement Fig. S3.
Combining the GOME-2 BrO maps and the trajectory calculations, the source of
air masses can be discussed in detail. First, trajectory calculations showed
that transport from the east coast of Greenland and east coast of
Spitsbergen is not possible. Thus, transport from these areas of enhanced
BrO can most likely be ruled out. Second, trajectories also showed that
after 26 April 16:00 UTC, transport from the north occurred, which means
the high BrO in the blue circle might have influenced this event. However,
(a) the altitude of the air mass reaches up to 1000 m; (b) there is no
enhancement along the path; and (c) the time scale is unreasonable. The BrO
enhancement we found by the ground-based MAX-DOAS, as well as the ozone loss,
only lasted for several hours. However, the high level of BrO in the blue
circle area lasted for more than one day. Additionally, the transport of air
masses may be the reason that the BrO concentrations were slow to return to
normal values until 3 May.
Sea ice in Kings Bay, Ny-Ålesund on 26 April at 21:00 UTC, 2015
(at Ny-Ålesund Dock, photograph by Yuhan Luo).
Time series of chemical and meteorological changes during the BrO
enhancement event; blue triangles represent the existence of sea ice in
Kings Bay.
Sea ice distribution
The observations of sea ice concentration from the AMSR-E and Zeppelin
webcams indicated that the water at Kings Bay was an ice-free area during the
measurement period. However, large amounts of sea ice appeared at Kings Bay
on 26 April (Fig. 11), pushed from the bay entrance by both wind and tidal
forces, which is an unusual phenomenon in the fjord. The shape of sea ice
comprised broken ice pieces with irregular borders. An ice–sea-water mixture
filled in the gaps between sea ice. From the shape of the ice in Fig. 11,
the sea ice did not look like newly formed because of its irregular
pieces and corrugated edges. Therefore, we consider that the sea ice was
formed before floating into the bay and transformed into the ice–water mixture
after coming across a sharp temperature decrease.
The chemical and meteorological information from the start of 26 April to
noon on 28 April are shown together in Fig. 12. When ozone depletion/BrO
enhancement occurs, the air temperature continuously decreases, and the
relative humidity drops from 80 to less than 65 %, with the wind
direction switching from northwest to east. The concurrent changes in the
chemical and meteorological variables demonstrate that changes in observed
chemistry are evident because of changes in transport, albeit on a small
scale.
It is also worth noting that the time periods when the sea ice existed and
the time BrO started to become enhanced (and ozone was depleted) were
not exactly the same. Figures 8 and 12 indicated that the ozone loss
started from 26 April 14:00 UTC. As described above, the sea ice existed
in the fjord after 26 April 20:00 UTC. This observation makes it reasonable
to synchronize variations in BrO and ozone, as well as the 0–1 km distribution.
Therefore, this BrO enhancement event is more likely a regional process
mainly influenced by the local environment. The sea ice is not totally fresh
ice, but the low air and water temperatures during this period may have
caused the formation of the brine ice mixture, which is the source of the
bromine radicals. The surface ozone concentrations increased along with the
melting of sea ice, which indicated that the life spans of the BrO radicals
are very short. The reactive bromine radicals gradually transformed to
soluble bromide (e.g., HOBr), which explained the sink of bromine (Fan and
Jacob, 1992).
Mercury deposition
The deposition of gaseous mercury occurred concurrently with tropospheric
ozone depletion, as well as the enhancement of BrO (Fig. 8), which
indicates that the oxidation of GEM by reactive halogen species (Br atoms
and BrO radicals) is the key process of mercury depletion.
The GEM decreases from ∼ 2 ng m-3 to lower than 0.3 ng m-3 during the BrO enhancement event. The oxidized GEM may be directly
deposited onto snow/ice or associated with particles in the air that can
subsequently be deposited onto snow and ice surfaces, which threatens
polar ecosystems and human health.
Conclusions
The typical process of enhanced bromine and depleted ozone in the
Ny-Ålesund boundary layer was observed using ground-based MAX-DOAS
techniques in late April 2015. BrO DSCDs as high as 5.6 × 1014 molec cm-2 were detected on 26–27 April. Meanwhile, severe ozone
depletion and mercury deposition occurred under a BrO VMR of 15 pptv. The
model showed enhanced BrO distributed at 0–1 km above the sea surface. By
analyzing the air mass history and sea ice conditions, this BrO enhancement
event was found more likely to be a regional process, driven by changes in
sea ice and transport on a local scale.
Measurements and retrieval of BrO column densities over
Ny-Ålesund, the Arctic, in 2015 used in this paper can be provided upon request to
Yuhan Luo from AIOFM, CAS (yhluo@aiofm.ac.cn).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-18-9789-2018-supplement.
YuL and FS conceived and planned the experiments. YuL
carried out the experiments. YuL, HZ and KD designed the
computational framework and model of the experiments and carried out the
simulations. YuL, FS, HZ, KD and YiL contributed to
the interpretation of the results. YuL took the lead in writing the
paper. WL supervised the project. All authors provided critical
feedback and helped shape the research, analysis and paper.
The authors declare that they have no conflict of
interest.
Acknowledgements
We appreciate the valuable comments from three anonymous referees. This
research was financially supported by the National Natural Science
Foundation of China (Project numbers 41676184, 41306199 and U1407135). We
gratefully thank the Chinese Antarctic and Arctic Administration and
teammates from the 2015 Chinese Arctic Expedition. We are also grateful to
Ping Wang from KNMI and Yang Wang from MPIC for providing advice
about the BrO VMR calculation. We kindly acknowledge the AWIPEV Atmospheric
Observatory in Ny-Ålesund, the Norwegian Polar Institute (NPI) and the
Norwegian Institute for Air Research (NILU) for the complementary data.
Caroline Fayt, Thomas Danckaert and Michel van Roozendael from BIRA are
gratefully acknowledged for providing the QDOAS analysis software.
Meteorological, surface ozone and gaseous mercury data are provided by the
EBAS database. We gratefully acknowledge the NOAA Air Resources Laboratory (ARL)
for providing the HYSPLIT transport model and READY website
(http://www.ready.noaa.gov) used in this publication.
Edited by: Anna Jones
Reviewed by: three anonymous referees
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