Introduction
Bromoform in the marine environment
Bromoform (CHBr3) is a brominated methane-like hydrocarbon which is a
volatile liquid at room temperature. Bromoform is naturally produced by kelp
and phytoplankton in the upper layers of the ocean . A few
anthropogenic sources are known, including water treatment, nuclear power
plants , and rice paddies ; however, these
tend to be small on a global scale . It was
estimated that globally between
2.2 × 1011–2.5 × 1012 g CHBr3 yr-1
is produced of which only
3.0 × 1010 g CHBr3 yr-1 is
anthropogenic , the rest being
from natural sources, including
1.3 × 1011 g CHBr3 yr-1 from brown
algae and
1.7–2.0 × 1011g CHBr3 yr-1 from
phytoplankton . Outgassing to the
atmosphere constitutes the largest known oceanic loss of bromoform, which is
relatively stable to chemical loss pathways (hydrolysis and nucleophilic
substitution) in seawater at ambient temperatures
. The production of bromoform in the
oceans forms an important step in the biogeochemical cycling of bromine
through the Earth system .
The production of bromoform by phytoplankton and kelp has been shown to be
stimulated through oxidative stress , and a maximum rate has been linked with the photosynthetic cycle
. However, the specific reason for bromoform production in
these organisms remains unknown .
Production by kelp is thought to be the dominant natural bromoform source to
the marine environment . Different species of kelp are
known to produce bromoform at varying rates (e.g. ).
Laboratory studies have measured significantly higher mixing ratios from
kelp, per weight, when compared to phytoplankton
. However, kelp species are
coastally constrained, while phytoplankton are able to cover hundreds of
square kilometres . A question remains
regarding the dominant contribution to the global bromoform budget.
Implications for atmospheric chemistry
The rate of outgassing to the atmosphere, the gas flux rate, is proportional to
the wind speed and the solubility of the gas
. The majority of the
outgassed bromoform remains below the tropopause, with a small amount
escaping to the stratosphere
. The photolysis of bromoform is
the dominant sink once in the atmosphere, which results in an atmospheric
lifetime of 2–3 weeks . The photolysis of
bromoform releases bromine radicals into the atmosphere. These bromine
radicals are an important catalyst in the destruction of ozone in the upper-troposphere and lower-stratospheric region .
Ozone in this region plays two key functions: in the upper-troposphere (UT)
ozone is a potent greenhouse gas, whereas in the lower stratosphere (LS) it
forms part of the ozone layer, absorbing incoming UV radiation
. In the UT, bromine radicals, released predominantly
from bromoform, are known to catalytically react with ozone. This results in
the destruction of the ozone and subsequent loss from the region
. Thus, bromine chemistry
could play a significant role in climate change through ozone depletion in
the UT .
Estimates have been made of both the amount of bromoform reaching the upper
troposphere and the magnitude of the impact this has on climate change. These
estimates are based on poorly constrained source emissions from the global
ocean . It is estimated that between 1.6 and
3.0 ppt of inorganic bromine is contributed directly from bromoform
to the lower stratosphere . The background atmospheric
bromoform mixing ratios are estimated to be 1–2 ppt. However, local
mixing ratios can be elevated above this. This typically occurs in regions
with extensive kelp beds and in areas of strong coastal upwelling (e.g.
). The skill of atmospheric chemistry models
would be greatly enhanced if there was better quantification of the source
strength of bromoform and, in turn, its impact on bromine radicals and ozone
chemistry in different regions. Such enhancement of modelling capacity would
lead to a vastly improved understanding of the roles of the source and
product gases in the UT–LS region.
Quantifying the inventories of bromoform emissions is thus critical in better
characterizing the oxidative capacity of the atmosphere. This is particularly
pertinent in the tropics, where deep convection results in a greater
percentage of bromine radicals reaching the UT–LS region
. Understanding the sources in the tropics
is therefore of great specific scientific interest .
However, there exists a paucity of measurements of bromoform in the tropics
. Existing data in this region tend to be from transient
ship cruises, which only provide a discrete snapshot at the point in
space/time that the cruise transects the area of interest. Similarly, no time
series of measurements at a fixed point currently exists for a coastal site
in southern Africa. The Cape Point Global Atmospheric Watch (GAW) monitoring station provides a point
from which to begin addressing this lack of southern African measurements.
Furthermore, the Cape Point monitoring station fills a critical Southern
Hemisphere latitudinal gap between Cape Matatula, American Samoa
(14∘ S), and Cape Grim, Tasmania (41∘ S) .
Significance of Cape Point location
Here we present the first ever bromoform data set recorded at the Cape Point
GAW station (34.3∘ S 18.5∘ E,
Fig. ). This station offers a unique location from which to
measure bromoform mixing ratios in a subtropical region but is also suitable
to sample air from the South Atlantic and Southern Ocean. Wind direction and
radon concentration (222Rn) at Cape Point have been extensively
used to classify the arriving air masses .
A mixture of air sources have been recorded at Cape Point.
classify these as follows: 100 % clean marine (baseline,
222Rn < 350 mBqm-3) to 100 % continental
(with/without anthropogenic influence,
222Rn > 1500 mBqm-3) and intermediate (mixture of
baseline and continental,
800 < 222Rn < 1500 mBqm-3). The subtropical
location of Cape Point may make this region a particularly significant source
of bromoform to the atmosphere, specifically when considering the potential
impact on global ozone budgets. The region lies in close proximity to the
tropics, where deep convection is able to rapidly transport the outgassed
bromoform into the UT–LS, where bromine initiated catalytic ozone destruction
occurs. To quantify the importance of the measurements made at Cape Point to
tropical deep convection it is necessary to note how the synoptic conditions
change seasonally over South Africa. During summer, approximately 5 % of
trajectories from South Africa escape to the Atlantic (10∘ S), while
75 % of transport exits to the southeast . Ridging high-pressure systems, present during spring and autumn, increase the transport to
the tropical Atlantic to 25 % . Moreover, data recorded
here are of particular value as the size of the contribution from the Cape
Point region is to date largely untested. The Cape Point data presented here
represent the first of their kind in Africa or for the South Atlantic region
.
Location of Cape Point in relation to Cape Town. Kelp range along
the entire coast. These are dominated by Ecklonia maxima Papenfuss
south of Yzerfontein but transition to predominantly Laminaria pallida north of Yzerfontein.
Adapted from Kuyper 2014.
The Southern Ocean is largely regarded as a highly biologically active
region, especially during the spring and summer . This
region may provide a significant contribution to the global atmospheric
loading of bromoform. However, the Southern Ocean is widely under-sampled
when it comes to bromoform measurements. Although there have been sporadic
ship cruises to the Southern Ocean , no long-term work has
been done in the Atlantic sector of the Southern Ocean. The data presented
here therefore offer the first fixed-point measurements of bromoform in air
from the Atlantic sector of the Southern Ocean.
In addition to receiving baseline air from the South Atlantic and Southern
Ocean, Cape Point lies in close proximity to extensive kelp beds. The kelp
beds extend along the South African coast to the north and east of Cape
Point. A variety of remote-sensing techniques have been used to assess the
extent and composition of kelp beds in 19 predefined areas along the Cape
coast . The studies have shown that kelp beds are present
in all 19 areas ranging from a minimum of 11 ha coverage in Table Bay to a
maximum of just under 1000 ha north towards the Namibia border. The species
composition was predominantly Ecklonia maxima Papenfuss south of
Yzerfontein but transitioned to predominantly Laminaria pallida north of Yzerfontein
(Fig. ). Thus, Cape Point is an ideal location to sample the
open ocean, local tidally affected kelp beds, as well as the occasional
anthropogenic pollution event from the greater Cape Town region, based on the
seasonally varying wind direction. Addressing the paucity of data from this
region will be instrumental in solving the persistent conundrum as to the
major source of bromoform in the atmosphere.
Methods
The separation, identification, and quantification of bromoform was achieved
using a gas chromatograph (GC) with an electron capture detector (ECD)
system. This featured a custom-built thermal adsorption–desorption trap for
the pre-concentration of atmospheric samples and delivery of analytes onto
the GC column . Specific details of the sampling
method in this campaign are described below.
Sampling
The measurements of bromoform were made at the Cape Point Global Atmospheric
Watch station in the austral spring of October and November 2011. The GAW
station sits at the top of a coastal cliff (230 m a.s.l) at the end of a
peninsula south of Cape Town (Fig. ). The manual nature of the
GC system, coupled with periods of instrument downtime, resulted in a
quasi-continuous sampling pattern with a measurement frequency of
approximately 45 min to 1 h. A total of 135 discrete bromoform
measurements were made in air samples during this period.
A Shimadzu GC-8A with a Perkin Elmer F-22 ECD was used to record the
bromoform concentrations. A J & W Scientific DB-624
(30 m × 320 × 1.8 µm, 5 % polarity
film) capillary column was used in the oven to achieve the separation of
samples . A 30 mL min-1 nitrogen flow was added
directly to the ECD in the form of make up gas. Helium (grade 5.0, Air
Liquide) at a constant flow rate of 5 mL min-1 was maintained through the column at the start of the each analysis. The oven was
held at 35 ∘C for 5 min following the injection of a sample.
Thereafter, the temperature was increased to 60, 90, 150, and 200 ∘C
every 5 min. The temperature in the oven was increased at
65 ∘C min-1 and held isothermally once the new temperature was
reached.
Air samples were pre-concentrated in a custom-built thermal desorption unit
(TDU, Kuyper et al., 2012). Adsorbents (Carbopac X and Carboxen 1016, 9 mg
each) held in a glass tube were cooled to -20 ∘C during the
trapping phase. The cooling of the system was achieved by a recirculating
chiller filled with glycol. To exclude air from the adsorbent trap a flow of
helium (100 mL min-1, grade 5.0) was maintained both before and after
sampling. Samples were dried using magnesium perchlorate, held in a glass
moisture trap, before being passed to the trap, as per .
Air was passed through the adsorbent trap at 100 mL min-1 for
15 min, resulting in a 1.5 L sample volume. The sampling flow rate
was checked weekly by means of a digital flow meter. An oil-free piston pump
was used to draw air through a 60 m Decabon sampling line and the
adsorbent trap. This was routed through a T-piece with the excess gas vented
to the atmosphere. A mass flow controller was used to regulate the gas flow
through the adsorbent trap. The pump was operated at 400 mL min-1, and
a needle valve on the exhaust was used to ensure sufficient pressure in the
sampling line for the mass flow controller to operate.
A built-in resistance wire heated the TDU glass tube to 400 ∘C to
desorb samples for injection. A second-stage cryo-focusing system was used at
the head of the column, with liquid nitrogen, to improve the chromatography.
The liquid nitrogen was held at the head of the column for the duration of
the primary injection. Thereafter, boiled water was used to desorb the
samples trapped at the head of the column.
Calibration
An external calibration method was used to verify the system performance. A
custom-built permeation oven was used to deliver aliquots of bromoform at
varying concentrations to the trap . A bromoform
permeation tube held at 70 ∘C (permeating at 343 ng min-1)
was flushed with nitrogen (grade 5.0, Air Liquide) at 100 mL min-1.
This gas mixture was continually passed through a 100 µL sample
loop and exhausted through a halocarbon trap. Aliquots of
100–300 µL (one to three sample loops) of the resulting permeation gas
(bromoform diluted in nitrogen) were introduced to the thermal desorption
unit from the permeation oven. Calibration samples were passed through the
drying trap as for air samples; thus, any loss would be consistent for air and
calibration methods. The calibration points were analysed using the same
temperature programme as air samples to ensure identical retention times.
These were also used for the identification of bromoform.
A complete calibration curve (Fig. ) was measured prior to the
start of the experimental period. The peak area was determined from the
repeated injection of one to three loops of diluted bromoform in nitrogen gas. Peak
areas were calculated through the trapezoid integration method and were
computed in MATLAB . The mixing ratios of the injected loops
were calculated from the number of moles of bromoform injected, as follows.
Each loop injection resulted in 0.343 ng of bromoform being loaded on
the trap, based on the calibrated rate of the permeation tube
. The number of moles of bromoform on the trap
was calculated from this mass which resulted in
1.36 × 10-12 mol being loaded on the trap, per sample
loop injection. The number of molecules of bromoform was calculated by
multiplying the number of moles by the Avogadro constant to yield the number
of bromoform molecules on the trap. The total number of molecules in a sample
was calculated by multiplying the air number density
(2.5 × 1025 molecules m-3) with the sample volume
(1.5 L). The bromoform mixing ratio of one loop was calculated as the number
of bromoform molecules of one loop as a fraction of number of molecules in a
sample multiplied by 1012 to yield parts per trillion.
Combined calibration data for the GC-ECD system for bromoform, based
on multiple loop injections and interpolation over the entire sampling
period. Peak area calculated using the trapezoid method.
A complete system calibration was performed at the start of the sampling at
Cape Point (12/13 October 2011). Thereafter, a calibration point of one to
three loops was measured approximately daily to account for system drift.
After the initial calibration the daily calibration points were coerced into
a regular matrix, with 8 h time steps. This resulted in three calibration
points per day. Gaps between calibration points were interpolated using a
three-point running mean. An overall r2 of 0.82 between the peak area and
mixing ratio was achieved using this system during the sampling period
(Fig. ). An analysis of peak area of repeated two-loop
injections indicated a system precision of 22.2 % based on the relative
standard deviation (RSD), including the running mean estimates. Following an
analysis of the calibration curve a limit of detection of 0.21 ppt
was determined for this system. An interquartile range (IQR) method was used
to search for and remove outlying data within the data set
. The IQR is the difference between the 25 and 75 %
quartiles. This is then multiplied by 1.5 and either added to or subtracted
from the 75 or 25 % quartile, respectively. Any value greater than the
upper bound was removed, whereas the lower bound was below the limit of
detection (LOD). The upper bound was calculated to be
71.4 ppt.
Ancillary measurements: Cape Point, Global Atmospheric Watch
The GAW station at Cape Point is operated by the
South African Weather Service. In addition to the standard meteorological
parameters, numerous climate relevant gases are quantitatively measured here,
including CO2, CH4, CO, radon (222Rn), and
O3 .
Air samples were drawn in at the top of a 30 m high sampling mast. A
continuous-flow system was used in the laboratory to exclude the accumulation
of any contamination. Sequential cold trapping at -5 and -40 ∘C
along the flow path was used to dry air samples prior to measurement. A
30 min mean was applied to all data to standardize different sampling
periods.
The ozone measurements were made on a Thermo Electron 49C analyser. These
analysers are based on the UV absorption technique and calibrated every two
months. Daily zero and span measurements were used to assess the long-term
stability of the detectors. A Trace Analytical RGA3 was used to measure
atmospheric CO mixing ratios. The detector uses a reduction of mercuric oxide
(HgO) to determine the concentration of CO (Brunke et al., 2004). A
measurement was made every 15 min with a calibration occurring every
2 h. Radon (222Rn) measurements were made in an
Australian Nuclear Science and Technology Organisation (ANSTO)-build, two-stage α-decay system which detects the collected
radon daughter products . A sample
was measured half-hourly and calibrated monthly.
Ancillary measurements: NOAA HYSPLIT model, marine boundary layer height, and diurnal cycle
NOAA Hysplit model
The HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT:
http://www.ready.noaa.gov/) model was used in addition to the chemical
tracers to examine the source of air masses being sampled .
These trajectories were generated using the NCEP reanalysis data as the
meteorological data in R. The back trajectories were calculated for 96 h
prior to bromoform measurement. The trajectories were merged with the
bromoform data set to allow for integrated analysis using the
“openair” package . This package comprises a range
of statistical tools to examine back trajectories, in order to identify
source regions or contributions.
The potential source contribution function (PSCF) in R openair
calculates the probability that a source exists at a specific location
. The PSCF is calculated by the ratio of
trajectories with elevated concentrations to the number of times those
trajectories pass through a specific point, defined as grid cells. A value
for each grid cell is calculated. These grid cell values can be the same when
the sample concentrations are either marginally above or greatly elevated
from a defined criterion, e.g. the mean .
Consequently, the difference between strong and moderate sources can be
difficult to distinguish. The concentration-weighted trajectory (CWT) method
can be used to potentially identify source areas through the calculation of
concentration fields. The mean of the concentration for each grid cell was
calculated as follows:
ln(C‾ij)=1∑k=1Nτijk∑k=1Nln(ck)τijk,
where i and j are the grid indices, k is the index of the
trajectory, N is the total number of trajectories used, ck is the
pollution concentration of trajectory k upon arrival, and τijk
is the residence time of trajectory k in grid (i, j)
. High concentrations at the measurement site would, on average, be caused by grid cells with elevated values of
C‾ij, thus indicating possible source regions. The CWT back
trajectory calculation was performed on the entire data set of bromoform
measurements at Cape Point. As a first approximation of the offshore sources
of bromoform to Cape Point a CWT model
analysis of the back trajectories was performed.
Marine boundary layer (MBL) height
Twice daily radiosondes were released from Cape Town international airport at
local midnight and noon. The airport lies approximately 60 km
northeast of Cape Point. The height of the MBL was determined by the surface
and elevated temperature inversion methods from the radiosonde data
. The calculated boundary layer height at the
airport was used as a proxy for the marine boundary layer at Cape Point.
Tidal height
The tidal height for Cape Town was obtained from the South African
Hydrographic Office (SAHO). Tidal gauges are used to measure the height in
the harbours around South Africa. Due to periodic instrument failures of some
of the gauges around Cape Town during the bromoform sampling period, tidal
height estimates were used to interpolate over any gaps. The height is given
in metres above a SAHO locally defined chart datum, and therefore, the lower
the value the lower the tide.
Diurnal cycle
A mean diurnal cycle was calculated from the full range of Cape Point
measurements using the “timeVariation” function of openair
. The data were sorted into 24 hourly bins. The time of the
sampling was used to assign an hourly bin to each measurement. The mean and
95 % confidence interval of the mean in each bin were then calculated.
Results and discussion
The bromoform mixing ratios at Cape Point were measured to be in the range
4.4–64.6 ppt with a mean of 24.8 ± 14.8 ppt
(Fig. ). Bromoform was typically in the range of
4–20 ppt, but on several occasions elevated mixing ratios were
encountered that could last for several hours (Fig. ). The
range of variability observed at Cape Point is comparable to previously
published work, specifically with reference to coastal sites
(Table ).
Time series plot of bromoform and meteorological measurements at
Cape Point during October/November 2011. Tide height given in metres above
SAHO datum, radon in mBqm-3, wind direction in degrees, wind speed
in ms-1, and radiation in Wm-2.
The measurements were made in a variety of air masses ranging from clean
marine to continental air. This suggests that a number of sources may have
impacted on the bromoform mixing ratios at Cape Point. Nearly 57 % of
bromoform measurements recorded here were below the mean. This indicates
that the mean value is skewed by a few elevated bromoform mixing ratios. When
examined over the whole data set, the bromoform mixing ratios showed only weak
correlations with the measured meteorological and physical measurements (r2<0.4).
Selected comparison measurements of bromoform in air samples above
coastal, upwelling, open-ocean, and lower marine boundary layer regions. Tidal
height is given in metres above the standard datum.
CHBr3 (ppt)
Location
Date
Latitude
min
max
mean
Reference
Region
New Hampshire
Jun–Aug 2002–2004
43.1∘ N
0.2
37.9
5.3–6.3
Coastal
Hateruma Island
Dec 2007–Nov 2008
24∘ N
0.5
7
0.91–1.28
Coastal
Mauritanian upwelling
Mar–Apr 2005
16–21∘ N
0.1
0.6
0.2
Upwelling
Cape Verde
May–Jun 2007
16.8∘ N
2.0
43.7
4.3–13.5
Coastal
R/V Sonne
July 2014
2–16∘ N
0.79
5.07
2.08
Open ocean
R/A Falcon
July 2014
2–16∘ N
0.99
3.78
1.90
MABL WASP
Atlantic Ocean
Oct–Nov 2002
10∘ N
0.5
27.2
–
Open ocean
SHIVA
Nov–Dec 2011
0–8∘ N
1.23
3.35
1.81
MABL WASP
Borneo
Apr–Jul 2008
4.70∘ N
2–5
60
–
Coastal
Strait of Malacca
Jun–Jul 2013
2–6∘ N
1.85
5.25
3.69
Coastal
Sulu–Sulawesi
Jun–Jul 2013
2–6∘ N
1.07
2.61
1.60
Coastal
Christmas Island
Jan 2003
1.98∘ N
1.1
31.4
5.6–23.8
Coastal
San Cristobol Island
Feb–Mar 2002, 2003
0.92∘ S
4.2
43.6
14.2
Coastal
Peruvian upwelling
Dec 2012
5–16∘ S
1.5
5.9
2.9
Upwelling
Indian Ocean
Jul–Aug 2014
2–30∘ S
0.68
2.97
1.2
Open ocean
Cape Point
Oct–Nov 2011
34.5∘ S
4.4
64.6
24.8
This study
Coastal
Cape Grim
2003
40.7∘ S
1.3
6.4
2.9
Coastal
Coastal South America
Dec 2007–Jan 2008
55∘ S
1.8
11
7.4
Coastal
Antarctic coast
Dec 2007–Jan 2008
65∘ S
2.1
4.9
3.2
Coastal
Antarctic Ocean
Dec 2007–Jan 2008
65–67∘ S
1.9
3.9
2.3
Open ocean
Link to tidal cycle
The full tidal spectrum was captured at Cape Point during the bromoform
sampling period, including two neap tides and a spring tide. A maximum tidal
range of approximately 2 m was observed during the spring tide. This range
decreased to a maximum of 1 m during the neap tides
(Fig. ). Exposure of kelp (which as discussed is present in
abundance at Cape Point) to the atmosphere at low tide has been linked with
an increase in atmospheric bromoform mixing ratios; for example, a site at
which this has been observed is Mace Head on the west coast of Ireland
. An increase in the oxidative stress on the kelp
initiated by solar radiation is thought to drive this correlation
. However, the measured bromoform mixing
ratios reported here do not correlate well with the tidal pattern. While the
maximum tidal range in the vertical at Cape Point is comparable to that at
Mace Head, the horizontal extent is much smaller. This may explain the lack
of local correlation. Consequently, during low tide at Cape Point, only the
tops of the kelp fronds become exposed to the atmosphere. This is common
around the coast of South Africa. Nonetheless, the elevated bromoform events
with the highest mixing ratios appear to mostly occur shortly after low tide
(Fig. ). It is therefore likely that the extensive local
kelp beds are an important source of the bromoform observed at the
station.
Air mass characterization
Radon (222Rn) and CO have been extensively used as tracers for
continental and anthropogenic contamination, respectively, in air mass
characterization including at Cape Point . The measurements
of radon (222Rn) and carbon monoxide (CO), which were generally
extremely low, show short elevated periods in the observations
(Fig. ). This indicates that majority of the bromoform
measurements made at Cape Point were under clean marine conditions. Of the
1535 half-hourly measurements that make up the meteorological data observed
at Cape Point during October/November, 68 % were of clean marine origin.
The bromoform mixing ratios in this clean air displayed a mean
23.5 ppt and ranged between 5.12 and 64.6 ppt
(Table ). The variations in 222Rn and CO
concentrations occurred concurrently and mostly when the wind was from a
northwesterly direction, which suggests a continental origin and therefore
anthropogenic contributions to the chemical composition of the air masses.
The continental contaminated air made up 9 % of the total measurements,
with intermediate air masses accounting for 7.5 % of the measurements.
Comparison of bromoform mixing ratios from different air mass
sources, sorted by radon concentration.
Clean marine
Intermediate
Continental
222Rn mBqm-3 (number)
< 350 (1028)
800–1500 (115)
> 1500 (45)
Mean CHBr3 ppt (number of samples)
23.5 (91)
24.3 (12)
NA
Range CHBr3 (ppt)
5.1–64.6
4.4–46.7
NA
The bromoform mixing ratios in intermediate air samples showed a similar mean
to that of clean marine air with a mean of 24.3 ppt
(Table ). The introduction of intermediate or continental air
at Cape Point potentially allows for the determination of the scale of the
anthropogenic contributions in general for this region. Since the intrusion
of intermediate air occurs predominantly in winter, a longer time series
could test the relative contributions more extensively. In the case presented
here, we are not able to conclusively separate anthropogenic and biogenic
sources, due to the limited, single-species data set. However, given the
small difference in means, the data suggest that an anthropogenic
contribution is not significant. The extensive kelp beds present to the north
of Cape Town further complicate the matter. An expanded suite of sampled
compounds would assist in the separation of sources through the examination
of related compounds such as the ratio to CH2Br2. It has been well
documented that the contribution of anthropogenically produced bromoform is
generally smaller than from natural processes on a global scale
. While, on a local scale anthropogenic source can
dominate , during this sampling period the dominant
contribution of bromoform was from the clean marine air masses and, therefore,
from biogenic sources (Table ).
Atmospheric bromoform measurements from Mace Head, Ireland, show periods of
elevated mixing ratios (Dickon Young, personal communication, 2017). Analysis
of these elevated mixing ratios at Mace Head suggests that the local marshes
may be the most likely source. However, the reason why the marshes should be
a source of bromoform remains unclear at this time. Although not surrounded
by marshes, Cape Point is enclosed by natural vegetation called fynbos. It is
possible that the fynbos releases bromoform into the local atmosphere. This
would be particularly pertinent with air masses arriving from the north. A
small study has previously examined the bromoform emissions from fynbos when
burnt . The measured mixing ratios in this study showed a
high degree of variability with a mean of 33.9 ppt and standard
deviation of 40 ppt. The limited scope and high variability meant
that no firm conclusions could be drawn regarding the release of bromoform
from the fynbos .
Meteorology
Wind speed has a complicated relationship with observed bromoform mixing
ratios in marine air. The processes of bromoform sea–air flux and atmospheric
dilution, both proportional to wind speed, oppose each other in their effect
on the atmospheric concentration of bromoform at a given location. At low
wind speeds there is a low dilution and bromoform flux into the atmosphere.
As the wind speed increases so do the rates of dilution and gas flux. The
wind speed observed at Cape Point over this sampling period was dominated by
lower wind speeds (< 10 m s-1). The full range extended from calm
(< 5 ms-1) to occasionally gale force
(> 20 ms-1). The elevated wind speeds were associated with
transient cold fronts that influence the Cape in winter and spring
. The bromoform mixing ratios at Cape Point show a varied
response to the observed wind speed; on some occasions at high wind speeds
the mixing ratio was also elevated whereas at other times it was not. The
lack of direct correlation at this site may be evidence of the complexity and
interaction of these processes as described above.
In a coastal upwelling environment it has been shown that the height of the
marine boundary layer (MBL) can play a significant role in the observed
bromoform mixing ratio. For example, found that a
lower marine boundary layer height acted to concentrate bromoform mixing
ratios recently released from the ocean surface. Although a direct
relationship between bromoform mixing ratios and MBL height was observed at
Cape Point, it is possible that MBL height played a role in the measurements
observed. As the MBL height is elevated the rate of atmospheric dilution
increases. This would result in lower measured bromoform mixing ratios.
Conversely as the MBL decreases, so the volume of atmosphere into which gases
are diluted decreases, resulting in a concentrating effect and increase in
measured concentration. The lack of a direct observed relationship could be a
result of Cape Point sitting approximately 60 km from Cape Town
international airport where the radiosondes were released. However, the
effect of changes in the MBL height may be reflected in the variability of
the bromoform measurements.
Solar radiation and diurnal cycle
During the sampling period the solar radiation at Cape Point daily reached a
level of 600–1000 Wm-2 (Fig. ). While there
was no direct correlation between solar radiation and bromoform observed, the
highest mixing ratios occurred when the solar radiation was typically above
800 Wm-2.
The mean Cape Point diurnal cycle of bromoform mixing ratios displayed an
increase through the morning from an estimated overnight low of
22.2 ppt, based on the first measurements of the day, to a mean
maximum of 33.3 ppt at 11:00 (Fig. ). Thereafter, the
mixing ratios decreased through the afternoon. A second maximum in the mean
mixing ratios was observed in the early evening. This secondary maximum
reached a mean mixing ratio of 34.0 ppt. There were no measurements
taken between midnight and 05:00, and the first morning measurements were
taken prior to local sunrise. It is assumed that these measurements, taken
before sunrise, were representative of the nighttime conditions.
Mean diurnal cycle, calculated from all measurements binned by hour.
The black lines above and below signify the 95 % confidence interval.
This pattern in the diurnal mean bromoform mixing ratio measurements at Cape
Point is similar to that observed in previously published literature
. It has been hypothesized
that the increase in concentrations observed in the morning is as a result
of sunrise. The onset of solar radiation stimulates photochemistry leading to
oxidative stress in the kelp cells and the release of bromoform
whereas it would appear that,
through this mechanism, the maxima of bromoform mixing ratios and solar
radiation should coincide . As discussed above,
changes in the height MBL act to concentrate or dilute the mixing ratio of
samples in the lower atmosphere. The atmosphere into which gases can mix
increases through the morning as the MBL height rises, thus causing a
dilution of trace gas in the atmosphere . This is most
likely reflected in the decrease in bromoform mixing ratios at about noon and
the stabilization through the afternoon. A small contribution from the
photolysis of bromoform may be present; however, this would be neither
detectable nor significant. A decrease in the rate of production in the
afternoon or the arrival of air masses from alternate sources might explain
the decrease in the late afternoon. A decreasing MBL height in the late
afternoon or early evening would act to concentrate any locally released
bromoform. The literature suggests that bromoform production may also be
related to respiration . During respiration,
it is theorized that through the haloperoxidase enzyme reactions, excess
intracellular hydrogen peroxide (H2O2) is removed and bromoform
formed . Therefore, production into the evening is possible
and with a lowered MBL height the measured bromoform might be large. The
evening maximum in mixing ratios is, therefore, expected and consistent with
previously studies in Gran Canaria and the Southern Ocean
.
Back trajectory analysis
Given the relatively long atmospheric lifetime of bromoform (3 weeks), there
could be sources offshore that contribute to the observed measurements at
Cape Point . While not excluding the local source, the
contribution of offshore sources was investigated using the openair CWT calculations of back trajectories
associated with bromoform mixing ratios. The CWT model suggests a large fetch
and variability of source region for air masses arriving at Cape Point,
mainly to the southwest (Fig. a). This large fetch included large
areas of low bromoform entrainment, as would be expected from most of the
open ocean, and trajectories of high entrainment. The model output suggests a
number of trajectories with elevated mixing ratios. There are both offshore
and inshore trajectories (Fig. a and b). There appears to be region
to the southwest of Cape Town that generates elevated mixing ratios.
Furthermore, the area directly to the south of Cape Point exhibits the
highest C‾ij values and appears to be centred over the Agulhas
retroflection region .
The Agulhas retroflection region is an area in which the highly productive
cold waters of the southern Benguela meet the warmer water from the Agulhas
current. This combination of productive waters (with potentially high
CHBr3) and warmer sea surface temperatures driving higher rates of
sea–air fluxes, could represent ideal conditions for observing higher
atmospheric bromoform mixing ratios.
Smoothed CWT calculation of bromoform based on Hysplit back
trajectories. (a) Zoomed to show full extent of 96 h back
trajectories. (b) Zoomed to focus on Cape Point. Units in colour bar
reflect bromoform mixing ratios measured at Cape Point.
The region to the southwest of South Africa extends from the coast to
45∘ S and appears to contain numerous trajectories of elevated
bromoform mixing ratios. It is possible that certainly the outer areas of
this region are warm core rings that have been shed off the Agulhas current.
The elevated area at the coast of South Africa occurs over Cape Columbine
(33∘ S) into St Helena Bay (Fig. b). This area appears to
be centred over St Helena Bay, an area known for strong coastal upwelling
. Given the limited nature of this data set we
cannot draw any firm conclusions regarding the offshore source of bromoform
to Cape Point. However, this is still an interesting aspect of the region
that will be monitored carefully in future work.
Conclusions
The data presented here represent the first fixed-point quantitative
atmospheric bromoform measurements at the Cape Point Global Atmospheric Watch
Station but also the first such data set in southern Africa. The 135 discrete
measurements made over the course of October/November 2011 exhibited a mean
bromoform mixing ratio of 24.8 ± 14.8 ppt. The maximum
bromoform mixing ratio reported here (64.6 ppt) was consistent with
past studies, for example that reported in Cape Verde (43.7 ppt,
) or New Hampshire (47.4 ppt,
). However, it should be noted that the random errors in
these measurements are quite large, with a precision of 22.2 %. The scale
of these uncertainties is due to the manual nature of the system, trapping to
injection, and oven temperature profile adjustments. Although the uncertainty
associated with the data presented here is large, we feel that the data are
still interesting as a first approximation of the range of values found in
this region. Given the uncertainty, the data should be treated with a degree
of caution.
The majority of measurements (68 %) were made in clean marine air
(222Rn), implying that for these measurements the bromoform being
sampled was entirely biogenic. From the data presented here it appears that
the most likely source of this bromoform is production from local kelp.
Most of the periods in which bromoform concentrations were elevated for a
prolonged time occurred around low tide, where kelp are exposed and most
likely to produce bromoform as a response to oxidative stress. However,
occasional intrusions of anthropogenically modified air may have contributed
to the bromoform loading at Cape Point.
In a similar manner to the marshes surrounding Mace Head, it is possible that
the fynbos vegetation at Cape Point may be a local source of bromoform to the
north. The fynbos as a local source remains speculative at this stage but
will be examined going forward.
The mean diurnal pattern appears to exhibit a similar pattern to, and fall
within the range of, previously published reports. An increase in the mixing
ratio was observed through the morning, returning to low concentrations
throughout the rest of the day. A second maximum in the mean mixing ratios
was observed in the early evening. Changes in the MBL height through the day
are the most likely source of variation in bromoform mixing ratios in the
diurnal cycle at Cape Point.
Back trajectory analysis using the CWT model from the openair package provides compelling evidence to suggest an offshore biogenic source.
The main region of the source appears to be centred on the Agulhas current
retroflection area. A second region of elevated bromoform mixing ratios
appears to exist as a transect line extending from St Helena Bay southwest
off South Africa. These will be monitored carefully going forward.
Given the relatively high concentrations reported, these data indicate that
this under-sampled region, may be particularly significant in terms of
bromoform sources to the atmosphere. Further work needs to be done to
categorize the source strength and halocarbon release from the local kelp
sources. Additional measurements, both in time, space, and halocarbon species,
will be required to attain a greater understanding of specific local
processes governing the variability in bromoform in this region. It is thus
clear that future measurements of bromoform mixing ratios at Cape Point would
make an important contribution to the field. Work is currently underway to
develop a more extensive halocarbon data set at Cape Point using updated
equipment and calibration protocols.