Ballast water treatment is required for vessels to
prevent the introduction of potentially invasive neobiota. Some treatment
methods use chemical disinfectants which produce a variety of halogenated
compounds as disinfection by-products (DBPs). One of the most abundant DBPs from oxidative ballast water treatment is bromoform (CHBr3), for which we
find an average concentration of 894±560 nmol L-1 (226±142µg L-1) in the undiluted ballast water from
measurements and the literature. Bromoform is a relevant gas for atmospheric
chemistry and ozone depletion, especially in the tropics where entrainment
into the stratosphere is possible. The spread of DBPs in the tropics over
months to years is assessed here for the first time. With Lagrangian
trajectories based on the NEMO-ORCA12 model velocity field, we simulate DBP
spread in the sea surface and quantify the oceanic bromoform
concentration and emissions to the atmosphere from ballast water discharge at
major harbours in the tropical region of Southeast Asia. The exemplary
simulations of two important regions, Singapore and the Pearl River Delta,
reveal major transport pathways of DBPs and anthropogenic bromoform
concentrations in the sea surface. Based on our simulations, we expect DBPs
to spread into the open ocean, along the coast and through advection with
monsoon-driven currents into the North Pacific and Indian Ocean.
Furthermore, anthropogenic bromoform concentrations and emissions are
predicted to increase locally around large harbours. In the sea surface
around Singapore, we estimate an increase in bromoform concentration by 9 % compared to recent measurements. In a moderate scenario in which 70 % of
the ballast water is chemically treated, bromoform emissions to the
atmosphere can locally exceed 1000 pmol m-2 h-1 and double
climatological emissions. In the Pearl River Delta all bromoform is directly
outgassed, which leads to an additional bromine (Br) input into the
atmosphere of 495 kmol Br a-1 (∼42 t CHBr3). For
Singapore ports the additional atmospheric Br input is calculated as 312 kmol Br a-1 (∼26 t CHBr3). We estimate a global
anthropogenic Br input from ballast water into the atmosphere of up to 13 Mmol a-1. This is 0.1 % of global Br input from background bromoform
emissions and thus not relevant for stratospheric ozone depletion.
IntroductionBallast water treatment
Ballast water is necessary for ships to maintain stability and draught
during voyage and port operations. Usually, ballast water is taken up during
cargo unloading and discharged during loading operations. However, the
uptake and discharge of ballast water by commercial ships represent the main driver
of the global spread of marine invasive species, which can cause negative
impacts on ecosystems, economies and public health (Ruiz et al., 2000;
Briski et al., 2012). In September 2017, the Ballast Water Management
Convention (IMO, 2004) entered into force, aiming to minimise the survival of
organisms carried in ballast water tanks. According to the convention,
shipowners from ratified flag states have different options to manage their
ballast water, one being the on-board operation of ballast water treatment
systems (BWTSs), which are type approved by the member states. Over the next
years, more than 75 000 vessels have to instal such BWTSs in order to
control the transport of potentially harmful species (David and Gollasch,
2015).
Different BWTSs are available which can be separated into physical and
chemical oxidation methods (e.g. David and Gollasch, 2015). Physical methods
include filtration, cavitation and treatment with ultraviolet radiation.
Chemical treatment is achieved via, e.g. electrolysis, chlorination or
ozonation. Electrolysis makes use of electricity in the ballast water to
generate sodium hypochlorite as an oxidant from the chemical reaction of salt
in the seawater. During chlorination, a chemical such as sodium hypochlorite
or chlorine dioxide is added in solution to the ballast water. Ozone
treatment forms hydroxyl and oxyl radicals that react with bromide ions in
seawater to hypobromous acid or to the hypobromite ion, which act as
disinfecting agents (Werschkun et al., 2012). As of January 2019, 76 systems
had received approval, 33 of which use chemical treatment (IMO, 2019). However,
it is currently unknown which treatment methods will be applied most
on ships over the next years.
Chemical BWTSs that apply oxidative treatment have been shown to produce a
variety of so-called disinfection by-products (DBPs) including
trihalomethanes, halogenated acetic acids and bromate (Delacroix et al.,
2013; Werschkun et al., 2014; Shah et al., 2015). The generally proposed
mechanism for generating DBPs is the reaction of oxidants such as chlorine
and ozone with organic and inorganic substances, such as bromide (Br-) and
iodide (I-), in the water via the formation of hypobromous (HOBr) and
hypoiodous (HOI) acid. The nature and amount of DBPs generated in seawater
depend on many factors including the type of oxidant, the injected
concentration, the amount and composition of dissolved organic matter (DOM)
(Liu et al., 2015), and the concentrations of the specific halide ions, i.e.
salinity (Shah et al., 2015). The chlorination and ozonation of seawater, for
example, have been shown to produce bromoform (CHBr3) as one major DBP
(Jenner et al., 1997; Padhi et al., 2012; Liu et al., 2015). Bromoform
concentration is generally higher for chlorination than ozonation or other
treatment methods. Moreover, in the presence of DOM mainly organohalogens
are produced as DBPs (Shah et al., 2015), which is often the case for harbour
seawater with a high influence from land use or river runoff.
DBPs like bromoform will spread in the marine environment once the ship
discharges its ballast water. To receive approval with the Ballast Water
Management Convention, BWTSs need to include a risk assessment according to
the methodology of the Joint Group of Experts on the Scientific Aspects of
Marine Environmental Protection – Ballast Water Working Group (GESAMP-BWWG)
(IMO, 2017). The methodology tries to identify if the DBPs found in ballast
water have an ecotoxicological effect on marine life as well as human
health. The risk assessment uses a worst-case scenario in which the DBPs
discharged into the harbour are modelled to calculate their predicted
environmental concentration. A recent study by David et al. (2018) showed
that the GESAMP-BWWG methodology does not fully account for potential
environmental risks. To date, the GESAMP-BWWG methodology has not assessed
the environmental impacts of volatile DBPs on atmospheric chemistry.
Brominated very short-lived substances
Trihalomethanes generated in BWTSs such as bromoform are also formed
naturally in the oceans. Bromoform is of biological origin with both macroalgae
and microalgae as potential producers, which oxidise primary metabolic
compounds with haloperoxidases in the presence of hydrogen peroxide (e.g.
Theiler et al., 1978; Moore et al., 1996). Currently available measurements
of bromoform in seawater suggest a large spatial variability with elevated
abundances in coastal, equatorial and upwelling regions due to biological
sources (Quack and Wallace, 2003; Ziska et al., 2013; Fuhlbrügge et al.,
2016).
Bromoform is the most important carrier of organic bromine from the ocean to
the atmosphere, contributing together with dibromomethane (CH2Br2)
up to 70 % of organic bromine to the marine troposphere (Hossaini et al.,
2012). Both compounds have relatively short lifetimes of around 2 weeks
(CHBr3) and 3 months (CH2Br2) in the tropical boundary
layer of the atmosphere and thus belong to the so-called very short-lived
substances (VSLSs; Carpenter and Reimann et al., 2014). Given the highly
variable oceanic production and its short lifetime, the atmospheric
distribution of bromoform is characterised by strong variations (Quack and
Wallace, 2003).
Upon their release into the atmosphere, bromoform and other brominated VSLSs
impact atmospheric chemistry. VSLSs are quickly oxidised or photodissociated
to reactive halogen species, which participate in the depletion of
tropospheric ozone by catalytic cycles (Saiz-Lopez and von Glasow, 2012).
Furthermore, reactive halogen species reduce tropospheric NOx by the
formation of nitryl halides (XNO2, where X=Cl, Br and I) (Simpson et
al., 2015) and alter tropospheric HOx ratios towards OH (Sherwen et
al., 2016). Thereby, reactive halogens from VSLSs impact the atmospheric
lifetimes of dimethyl sulfide (DMS), many pollutants and greenhouse gases, such as methane and
mercury (Simpson et al., 2015; Saiz-Lopez and von Glasow, 2012).
In the stratosphere, VSLSs also contribute to the depletion of ozone. Due to
their short lifetime, they are mostly oxidised and subsequently removed
through tropospheric precipitation. However, in regions of deep convection,
they can be entrained into the stratosphere through rapid vertical transport
(e.g. Aschmann et al., 2009; Tegtmeier et al., 2015). Deep convective
events are most common in the tropics near the Equator where solar
irradiance is high throughout the year and the ocean is an efficient source
of bromoform and other VSLSs (Quack and Wallace, 2003). Observational (e.g.
Dorf et al., 2006) and modelling (e.g. Warwick et al. 2006; Liang et al.,
2010) studies have suggested that VSLSs provide a significant contribution
to stratospheric total bromine (Bry), with current estimates ranging
between 2 and 8 ppt (Carpenter and Reimann et al., 2014; Wales et al., 2018).
Once brominated VSLSs have reached the stratosphere, they participate in
ozone depletion at middle and high latitudes (Yang et al., 2014; Sinnhuber
and Meul, 2015).
Motivation
Recent publications have analysed the production of DBPs from oxidative
ballast water treatment and assessed its ecotoxicity (Delacroix et al.,
2013; Shah et al., 2015; Werschkun et al., 2014). These studies focussed on
risk assessments on board ships or in the near-ship environment. So far,
the focus has been on the small-scale immediate exposure of DBPs to humans and
the marine environment. The long-term effects of ballast water discharge on
regional to global scales has not been assessed so far and the atmosphere,
as a sink for volatile halocarbons, has not been considered in any existing
risk assessment of oxidative ballast water treatment. In particular,
brominated species such as bromoform are frequently produced in treated
ballast water and are known to impact atmospheric chemistry. In this study,
we provide a first analysis of how DBPs and any other passive substances
contained in treated ballast water spread over a period of months to years
around different harbours in Southeast Asia (Sect. 3). The derived spread
can serve as a proxy for assessing the environmental impact of any chemical
or biological species contained in treated or untreated ballast water. In a
second step, we derive an estimate of bromoform released from ballast
water into the marine environment and quantify its emission into the
atmosphere (Sect. 4). We further discuss the methods and data used
for this study including port statistics, DBP concentrations in ballast
water and our Lagrangian simulations (Sect. 2).
MethodsPort statistics
For a regional to global analysis, volumes of ballast water discharge for
individual ports and the typical bromoform concentration in the treated
ballast water are needed. As ships are usually not required to report
ballast water operations to harbour officials, exact numbers of localised
ballast water discharge are not available. Thus, any approach to calculate
such numbers from ship size or cargo is challenging (Seebens et al., 2013).
Here, we derive estimates of the discharge volumes by linking the annual
amount of global ballast water volume with the cargo throughput at each
port. Global annual discharge volume is estimated to range from 3 to 5 billion m3 (Tamelander et al., 2010; Endresen et al., 2004;
David, 2015). In addition to the global ballast water amount, it is known
that the discharged ballast water amounts on average to roughly 33 % of
the loaded cargo volume (David, 2015). We use the global ballast water
discharge as 33 % of the global 10 286.9 million tonnes of loaded goods
(UNCTAD, 2017) to obtain a ballast water volume of 3.4 billion m3, which
agrees well with the estimates from the studies mentioned above.
The cargo throughput is obtained from the world port ranking 2016 published
by the American Association of Port Authorities (AAPA, 2016). This statistic
includes the 100 biggest ports for two categories: containerised and
bulk cargo. Containerised cargo is given in twenty-foot
equivalent units (TEUs), while bulk cargo is given in tonnes. In order to
combine these two rankings, we generate a modified world port ranking and
calculate the percentage share of containerised and tonnage goods according
to their global ratio given by the Review of Maritime Transport (UNCTAD,
2017). The percentage share of containerised goods in 2016 amounts to 16.6 %, while the rest makes up 83.4 % of the total goods loaded (UNCTAD,
2017). According to the percentage share of their category and their
individual size in the ranking, each container and bulk port is assigned its
relative fraction under the simplified assumption that these ports account
for all of the global commercial ship trade and receive all of the global
ballast water. Since many of the harbours appear in both statistics
(containerised and bulk), both percentage values are added to give their total
cargo share, which forms our modified world port ranking encompassing 144
ports (Supplement table). We use the calculated percentages to divide global
ballast water volume among all ports to derive the estimated discharge for
each port.
Our study focus was set on the coastal region of Southeast Asia (10∘ S–40∘ N, 95–145∘ E)
where 38 harbours from our modified world port ranking are located,
comprising 57 % of the global shipping industry
(Fig. 1). This region is to a major extent
located in the tropics, which makes it very relevant for the entrainment of
oceanic VSLS emissions into the stratosphere.
Estimated annual ballast water discharge volume (106 m3)
from each harbour in the modified world port ranking in Southeast Asia with
the names of the 26 largest ports
(Supplement table). Contours and black contour lines show climatological
ocean surface velocities (cm s-1) from NEMO-ORCA.
For our detailed analysis, we choose two different locations in the tropics
that are characterised by large harbours and different ocean dynamics. The
first area is Singapore, where the two ports of Singapore and Tanjung
Pelepas (Malaysia) are located very close to the Equator. This location in
the Maritime continent is characterised by sea surface currents of over 0.2 m s-1 in the climatological mean (Fig. 1).
The other location is the Pearl River Delta where the harbour cities
of Guangzhou, Hong Kong and Shenzhen are located. There, only weak coastal
currents can be found in the climatology.
Bromoform production from oxidative ballast water treatment
In a second step, we derive estimates of the bromoform concentration
produced during chemical ballast water treatment. Since there are many BWTSs
that use different chemical treatment methods with different water
parameters and residence times, the produced amount of bromoform can show
large variations. Here, we determine a range of possible bromoform
concentrations which can be used in our analysis to estimate the
environmental input of bromoform. For this purpose we use measurements of
chemically treated ballast water taken during shipboard tests, as well as
literature data. The formation of disinfection by-products in BWTSs is most
commonly investigated during land-based tests. In contrast, we have
conducted one of the first shipboard tests of the formation of major
halocarbons in treated ballast water. The samples were taken from the
discharge of treated ballast water for three unnamed BWTSs, two in Norway and
one in Germany, which use chlorination techniques. This allows us to obtain
a more robust estimate of the initial bromoform concentrations in the
ballast water. The bromoform measurements were carried out with a
purge-and-trap gas chromatograph–mass spectrometer (GC–SM) system with a
detection limit of around 0.1 pmol L-1. Bromoform concentrations of
244.5±163.6µg L-1 (967.6±647.4 nmol L-1)
were found in 12 ballast water samples taken in Norway from two different
BWTSs (Table 1). Bromoform concentrations of 202.0±74.0µg L-1 (799.1±292.7 nmol L-1) were
found in nine ballast water samples taken in Germany from one BWTS
(Table 1). These samples were taken on board the
vessel at three time periods during ballast water discharge in 15–20 min
intervals. The particulate organic matter in the water was 11.1 to 12.6 mg L-1.
Bromoform (CHBr3) data from samples of undiluted
ballast water given as an average and standard deviation (µg L-1;
nmol L-1). Samples 1 and 2 are measurements from shipboard tests of a chlorination BWTS.
Samples 3 and 4 are data from the IMO Marine Environmental
Protection Committee (MEPC) reports on approval for different BWTSs using
chemical treatment.
In addition, we use bromoform concentrations given in the reports of
the International Maritime Organization (IMO) Marine Environment Protection
Committee (MEPC) for Final Approval of BWTSs (https://docs.imo.org, last access: 3 July 2019).
Mean bromoform values for seawater and brackish water from the MEPC reports
on 29 BWTSs, 22 of which use chlorine as the main disinfecting agent, are also
given in Table 1.
Different chemical treatment systems show greatly varying bromoform
concentrations, as illustrated by the large standard deviations in the MEPC
data. This is due to different doses and types of oxidant, varying residence
time in the tank, and different water properties such as salinity,
temperature and the amount of DOM (Shah et al., 2015). In general, the
systems using chlorination as the main disinfecting agent generate higher
bromoform concentrations than ozonation systems. Samples of the same
treatment system (German system in Table 1), show a
smaller standard deviation. Overall, our shipboard DBP measurements are in
the range of the land-based test results published in the MEPC reports,
suggesting a similar amount of bromoform production. On average 226±142µg CHBr3 L-1 can be expected in ballast water, which
corresponds to 894±560 nmol L-1 (Table 1) with the mean values of all four data sets in good agreement. The values
shown here mainly stem from chlorination-based treatment systems. Therefore,
we will focus on chlorination-based treatment in the following.
The exact percentage of vessels that will eventually use chemical BWTSs is
unknown. Oxidative water treatment is more suited for larger vessels such as
bulk carriers or tankers, which are typically the types of ships that carry the
largest volumes of ballast water (Maritime Impact, 2017). Thus, we assume
that 70±20 % of the ballast water will be chemically treated,
producing DBPs. In order to capture the range of uncertainty resulting from
the variations of the bromoform concentrations in ballast water samples and
from the unknown share of chemical BWTSs, we set up three scenarios: LOW,
MODERATE and HIGH (Table 2). The scenarios are
assigned an initial bromoform concentration corresponding to the mean and
the mean ± 1 standard deviation, and they use
different shares (50 %, 70 % and 90 %) of chemically treated ballast water.
Based on these two assumptions, we derive different amounts of annually
discharged bromoform for the selected regions in Singapore and the Pearl River
Delta (Table 2). In these scenarios, other brominated species like
dibromomethane have been neglected because their concentrations in treated
water were usually more than 10 times lower than bromoform concentrations.
Variations in the usage of the different treatment methods will lead to
variations of anthropogenic bromine release. We include these variations by
applying the three scenarios in which we include a best- and a worst-case
scenario, LOW and HIGH, respectively.
Scenarios for the simulation of ballast water (BW) spread with
different initial bromoform concentration and annual bromoform amount for
two regions in Southeast Asia, Singapore and the Pearl River Delta (PRD).
ScenarioCHBr3 concentrationPercentage of vessels usingCHBr3 Singapore CHBr3 PRD in BW (µg L-1)chemical BW treatment (%)(106 g a-1)(kmol a-1)(106 g a-1)(kmol a-1)MODERATE226703011943170LOW84508321145HIGH368906325090356Lagrangian simulations
In contrast to earlier studies which focussed on the local effect of DBPs
from ballast water (e.g. David et al., 2018), we investigate the long-term,
large-scale influence of DBPs in the ocean and atmosphere. Therefore, we
need regional to global ocean velocity and surface wind data, which can be
obtained from high-resolution ocean general circulation models (OGCMs). We
simulate the spread of treated ballast water and the DBPs contained within
by applying a Lagrangian trajectory integration scheme to the 3-D velocity
output from an eddy-resolving OGCM. The model output stems from a hindcast
experiment with the ORCA0083 model configuration based on the NEMO-ORCA code
version 3.6 (Madec, 2008). The ORCA0083 configuration from the European
DRAKKAR consortium (The DRAKKAR Group, 2007) has a horizontal resolution of
1/12∘ and 75 vertical levels, with 46 levels in the upper
1000 m and spacing increasing with depth (see also Marzocchi et al., 2015;
Durgadoo et al., 2017). Atmospheric forcing comes from the DFS5.2 data set
(Dussin et al., 2016) and varies on a range of scales, from synoptic to interannual
and longer. The experiment ORCA0083-N06 used in this study was run by the
National Oceanography Centre, Southampton, UK. Model output is given at a
temporal resolution of 5 d for the time period 1963 to 2012.
To simulate the spread of DBPs in the surface ocean, the ARIANE software
was used (Blanke et al., 1999). ARIANE performs offline trajectory
calculations by passively advecting virtual particles along analytically
computed 3-D streamlines. This method has been developed and extensively used
for analysing mean large-scale spreading of water masses or minor species
from a known source over different time periods (e.g. Durgadoo et al., 2017;
van Sebille et al., 2015; Rühs et al., 2019). In our study, the DBPs from
ballast water discharge are approximated as particles that are passively
advected with the simulated flow. The streamline calculations are purely
advective and no diffusivity is applied. For both regions of interest, 10
individual simulations are conducted starting each year in January from 2001
to 2010. The 10 different simulations are used to obtain more robust
ensemble results and avoid extremes from internal variability. In each
simulation, particles are continuously released close to the port site at
every model output time step (once every 5 d), which represents a
continuous ballast water discharge at this location. Subsequently, the
particle advection is simulated for 2 years. For the purpose of
calculating seasonal and annual means and to allow for an initial
accumulation period, only months 12 to 23 (December of one year to November of the following year) are
analysed for each simulation. Additionally, all particles older than
11 months are not considered in the analysis so that the total particle number is
constant at each time step.
The experiments were run for the Pearl River Delta region and the
Singapore region. The Pearl River Delta region comprises three major ports,
Hong Kong, Guangzhou and Shenzhen, for which we derive a total annual
ballast water discharge volume of 271 million m3 (8 % of
the global ballast water discharge) from the modified world port ranking
(Supplement table). The Singapore region comprises the ports of Singapore
and Tanjung Pelepas, with a ballast water amount of 190 million m3 (5.6 %) each year. The discharge location where
particles are released has been chosen in the vicinity of the harbours at
approximately 8 to 40 km off the coast, as the model resolution does not
allow for the capture of small-scale coastal structures such as harbours. Our method
ensures minimal influence of the land boundaries on the initialisation of
the simulation. We assume that the DBPs are transported from the inner
harbour into the adjacent coastal areas where our model simulations are
initialised. For many ports this is reasonable since rivers and tidal
flushing cause a steady turnover of coastal waters with the ocean.
For the analysis of the experiments, we distinguish (1) the passive spread of
DBPs without any environmental sinks (hereafter PASSIVE) and (2) the spread
of bromoform as a major volatile DBP accounting for atmospheric fluxes and
oceanic sinks (hereafter FLUX). For the PASSIVE analysis, we consider the
full history of simulated particle positions, which is equivalent to assuming
no particles getting lost through sinks in the ocean or emission into the
atmosphere. The resulting distribution shows where DBPs in ballast water or
assumingly dimensionless and immotile species can be transported through
ocean currents within 1 year.
For the FLUX analysis, each particle is given an initial mass of bromoform
based on the ballast water volume of the harbour and the produced bromoform
according to the three scenarios MODERATE, HIGH and LOW
(Table 2). Moreover, different sinks of bromoform,
such as constant exchange at the air–sea interface and chemical loss rates,
are taken into account.
We calculate the bromoform air–sea exchange based on the flux
parameterisation from Nightingale et al. (2000) for all particles that reach
the mixed layer at a certain time step. The mixed layer depth (MLD) is
defined as the ocean layer in which the vertical density gradient does not
exceed 0.02 kg m-3 referenced to the 10 m depth. According to ORCA, the
annual mean MLD is less than 20 m deep within our research area. Since the
results are given at a 5 d temporal resolution and the MLD is
relatively shallow, it is reasonable to assume that the whole mixed layer is
in contact with the atmosphere at least once during each time step. Treated
ballast water provides an additional source of bromoform to the environment,
adding to the natural bromoform occurring in the ocean and atmosphere. Given
the additive nature of the ocean and atmospheric terms in the air–sea flux
parameterisation, it is possible to calculate the flux of the anthropogenic
and natural bromoform portions separately. For our simulations, we only
consider bromoform from ballast water treatment and apply the air–sea flux
parameterisation to the anthropogenic bromoform in water and air. We have
conducted sensitivity tests with an atmospheric transport model which shows
that outgassed anthropogenic bromoform is quickly advected from the sea
surface to other areas and different heights. Therefore, we can assume that
anthropogenic bromoform in the atmosphere is always zero at the ocean
surface in the region of interest. The air–sea exchange is linearly
proportional to the gas transfer velocity of bromoform, which depends on
surface wind velocities and sea surface temperature and salinity. Surface
wind velocities are taken from the NEMO-ORCA forcing data set DFS5.2
(Dussin et al., 2016).
Oceanic sinks are also taken into account, although they are negligible on the timescales considered in this study. These include degradation through halide
substitution and hydrolysis with a half-life of 4.37 years (Hense and Quack,
2009), as well as remineralisation with a half-life of 5.72 years (Hense and Quack,
2009).
The particle density distribution is calculated on a 1∘×1∘ horizontal grid over the upper 20 m of the ocean (further
mentioned as the “surface”). The distribution is given as a percentage per grid
box of the total particle number (PASSIVE) and as bromoform concentration
(pmol L-1; FLUX). Statistical values are calculated over three grid
boxes with the highest concentration around the discharge location. Analyses
on seasonal to interannual timescales were conducted by averaging and
concatenating the simulations from the model years 2001 to 2010. For
the calculation of time series, we use the smoothed 2-week (15 d) running
mean of the concentration and emission rates from the three grid boxes
around the discharge location for the three scenarios MODERATE, LOW and
HIGH. Wind speed values from these boxes are also smoothed with a 15 d
running mean in order to better show the seasonal to annual variations.
The global atmospheric input of bromoform from ballast water emissions can
be estimated by multiplying the initial concentrations with the global
ballast water volume for each scenario, taking into account different
percentages of chemical treatment systems (Table 2). The global atmospheric bromine input from the source-gas bromoform is
derived by multiplying the global annual emissions with the number of bromine (Br)
atoms.
Annual mean surface (20 m) spread of DBPs from discharge in the Pearl
River Delta relative to the total number of particles released. Contours
show the area of the percentage of particles (30 %, 50 %, 70 % and
90 %) characterised by the highest density.
Surface spread of DBPs – PASSIVE
Figure 2 shows the relative particle density
distribution of DBPs averaged over 10 years released from the Pearl River
Delta. We estimate the contour lines of the percentage of DBPs (30 %,
50 %, 70 % and 90 %) that are characterised by the highest particle
density. The distribution shows that 90 % of DBPs spread past Japan
and the Korean Peninsula, with the Kuroshio into the North Pacific, and
southwards into the South China Sea towards the Philippines within
1 year. On average, 30 % of the DBPs with the highest density will stay
southward of the Pearl River Delta along the coast and are now distributed
in the Gulf of Tonkin west of the island of Hainan. There, the highest relative
particle density distribution reaches up to 3 % locally with respect to
total DBP discharge.
Annual mean surface (20 m) spread of DBPs from discharge in
Singapore relative to the total number of particles released. Contours show
the area of the percentage of particles (30 %, 50 %, 70 % and 90 %) characterised by the highest density.
For the Singapore harbour region, the relative DBP distribution averaged
over the years 2001–2010 is shown in Fig. 3. As
for the Pearl River Delta, most of the DBPs stay in the close vicinity of
the coastlines, with the highest relative density distribution of 4 %. On an
annual mean basis, the 30 % of DBPs that are characterised by the highest
particle density have been transported northwestward and accumulate in
the Strait of Malacca, in close contact with the coastlines of the
Malay Peninsula and the island of Sumatra. DBPs within the 50 %–70 %
distribution expand mostly into the Indian Ocean towards Sri Lanka, but a
small fraction is advected into the South China Sea between Borneo and
Vietnam and even into the Java Sea. The main driver for the mean state of
DBP transport from Singapore is the Indonesian throughflow, generally
directed westward through the different passages of the Indonesian
Archipelago (Gordon, 2001).
For the Pearl River Delta and Singapore, the areas of the 90 % of DBPs with
the highest particle density expand over 5.0 and 8.6 million km2,
respectively, illustrating the large possible spread of longer-lived DBPs in
ballast water. The size of the area and dominant direction of expansion are
subject to variability on different timescales.
We investigate the interannual variations in the spread of DBPs by analysing
the area extent of the 30 %, 50 %, 70 % and 90 % of particles with the highest
density for the time period 2001–2010 (Fig. 4).
The largest variations are found for the annual mean distribution of the 90 %
area which expands over 6.6–10.2 million km2 for the Pearl River Delta
region depending on the surface velocity strength in the area. The extent of the
30 % and 50 % regions varies less on interannual timescales. Our results
show that half of the longer-lived organisms and chemicals in ballast water
can be expected to be spread over a relatively constant area of 0.5–1 million km2 around the harbour, while the other 50 % is transported
into a much larger region (up to 10.2 million km2) that fluctuates
depending on interannual variations of ocean surface transport.
Annual mean area extent of DBP spread in the (a) Pearl River Delta and
(b) Singapore based on 30 %, 50 %, 70 % and 90 % of the
particles characterised by the highest density.
Since a lot of the volatile DBPs will be emitted into the atmosphere and
other short-lived non-volatile DBPs degrade in the ocean on relatively short
timescales of weeks to months, the seasonal timescales are also of interest
when evaluating the main pathways of DBP distribution. Depending on the season
of discharge, the dominant atmospheric winds and oceanic currents can vary
substantially in strength and direction in the region considered. We
calculate seasonal anomalies of the particle density distribution for the
time period 2001–2010 by subtracting the annual mean climatology from the
seasonal mean climatologies.
Seasonal anomalies of the main pathways of ballast water spread from the
Pearl River Delta region show a clear reversal of main spread from boreal
winter (DJF) to summer (JJA) (Fig. 5). Surface
currents in the South China Sea are wind-driven and seasonally affected by
the northwest Pacific monsoon (Shaw and Chao, 1994). During DJF, the main
pathway is towards the southwest, with an accumulation of DBPs west of Hainan
and positive anomalies up to 9 %. There is a clear separation of
these positive anomalies south of the Pearl River Delta and negative anomalies
north of this region. Furthermore, the area of the 90 % DBP distribution
is located in a narrower band towards the coast during DJF. This anomaly
pattern reverses in JJA. More DBPs are transported northward, while there is
less advection to the south. However, the northeast winter monsoon prevails
much longer in the Pearl River Delta than the southwest summer monsoon,
which explains why in the annual mean the largest part of DBPs is advected
southward. During boreal spring (MAM) and autumn (SON) anomalies are less
pronounced. In MAM, the anomalies are mostly positive around the discharge
location, which means more DBP accumulation along the coast and slower
transport than in the annual mean due to weaker currents. The opposite
happens in SON with negative anomalies around the discharge location,
indicating that the fastest transport occurs during SON.
Anomaly of seasonal DBP spread compared to climatology (2001–2010)
at the surface (20 m) for discharge in the Pearl River Delta. The black contour line
shows the area of the 90 % of particles characterised by the highest
density in the seasonal mean.
A similar seasonality in DBP spread can be seen from discharge in the
Singapore region (Fig. 6). Here, close to the
Equator, the monsoon winds seasonally reverse from northwesterly winds in
JJA to southeasterly winds in DJF. As a result, more DBPs are transported
towards the northwest through the Strait of Malacca into the Indian Ocean in
DJF, and Singapore ports show a negative anomaly. As expected from the
reversed winds in JJA, fewer DBPs are advected towards the west and more
towards the east so that the DBPs can reach the Pacific Ocean. During SON,
the strongest positive anomalies can be found in the southern Strait of Malacca.
Then winds transition and become very weak, and thus DBPs cannot be
transported quickly and accumulate near the discharge location of Singapore.
The lowest anomalies are found in MAM, with a slightly enhanced accumulation of
DBPs north of Malaysia.
Anomaly of seasonal DBP spread compared to climatology (2001–2010)
at the surface (20 m) for discharge in Singapore. The black contour line shows the
area of the 90 % of particles characterised by the highest density in the
seasonal mean.
Concentration and emission of bromoform – FLUX
The oceanic distribution of bromoform from ballast water treatment and its
emissions into the marine boundary layer are estimated from the FLUX
analysis based on the simulated velocity fields from 2006 and the
corresponding Lagrangian experiments. As shown in Sect. 3, the interannual
transport variability is small and therefore a 1-year simulation is
sufficient to derive the representative emission estimates. Bromoform as a
volatile gas can be outgassed into the marine atmospheric boundary layer, as
long as it stays at the ocean–atmosphere interface. In the FLUX analysis, we
calculate the bromoform outgassing rate for all particles within the mixed
layer at every time step. We also calculate the bromoform surface concentration
in the upper 20 m and the sea-to-air flux averaged over 1 year for the
three scenarios MODERATE, HIGH and LOW. For comparison we calculate the
bromoform concentrations that would prevail without outgassing into the
atmosphere from the PASSIVE analysis.
We find that surface concentrations from the FLUX analysis are largely
reduced compared to PASSIVE. In the Pearl River Delta region, bromoform only
remains in the box around the discharge location due to the new input of
ballast water at every time step (Fig. 7). Thus,
the majority of released bromoform is instantly outgassed into the
atmosphere, resulting in a relatively constant concentration of 10 pmol L-1 in the MODERATE scenario around the discharge location, ranging
from 22 (HIGH) to 3 pmol L-1 (LOW)
(Table 3).
Surface bromoform concentration in the Pearl River Delta for the
MODERATE scenario averaged over 1 year. (a) PASSIVE analysis without loss
rates. (b) FLUX analysis with outgassing.
Average values for the FLUX experiment in Singapore and the Pearl River
Delta region for different scenarios. Values are calculated as the sum of
three grid boxes around the discharge location.
ScenarioSingapore Pearl River Delta ConcentrationEmissionTotal Br fluxConcentrationEmissionTotal Br flux(pmol L-1)(pmol m-2 h-1)(kmol a-1)(pmol L-1)(pmol m-2 h-1)(kmol a-1)MODERATE11928306101687511LOW3246813448136HIGH2319436412235321070
Also, bromoform concentrations from Singapore ballast water stay much more
centred around the discharge location when compared to the PASSIVE analysis
without outgassing (Fig. 8). Small concentrations
of 1 to 2 pmol L-1 can still be found in the Strait of Malacca. Average
bromoform concentrations around Singapore add up to 11 pmol L-1 in the
MODERATE scenario, ranging from 23 (HIGH) to 3 pmol L-1
(LOW). Measurements of bromoform in this region showed elevated surface
concentrations of up to 130 pmol L-1 (Fuhlbrügge et al., 2016), most
likely due to the combination of strong natural and already existing
anthropogenic coastal sources. The additional bromoform input expected
from ballast water discharge in Singapore would thus lead to a 9 % (18.5 %; 2.4 %) increase.
Surface bromoform concentration in Singapore for the MODERATE
scenario averaged over 1 year. (a) PASSIVE analysis without loss rates.
(b) FLUX analysis with outgassing.
Evaluation of the time series shows that wind velocities are enhanced in the
Pearl River Delta region with a strong seasonal cycle
(Fig. 9). Such strong winds cause high exchange
velocities, which in turn lead to high emission rates. The bromoform emission
rate stays constant at 1690 pmol m2 h-1 because everything
discharged into the ocean is instantly outgassed into the atmosphere.
Therefore, both oceanic concentrations and emissions into the atmosphere
stay relatively constant throughout the time period, independent of the wind
variations. The bromoform emissions in the Pearl River Delta region range
between 440 and 3530 pmol m2 h-1 for the three different
scenarios (Table 3). This flux is much larger than
in the Singapore region where the average range is 250 to 1940 pmol m2 h-1. Around Singapore, concentrations and emissions underlie a strong
seasonality driven by the wind speed. Two times a year in summer and winter,
wind velocities increase and cause bromoform emissions to increase as well.
At the same time, oceanic bromoform concentrations around the discharge
location drop due to the increased emissions and faster oceanic transport
(Fig. 9). For weaker winds, the bromoform
response is reversed with lower emissions and higher oceanic concentrations.
Time series of the 2-week running mean of wind speed (black),
bromoform surface concentration (orange) and emissions (blue) for (a) Singapore and (b) Pearl River Delta. Solid lines show values from the
MODERATE scenario. Shaded areas show the range between the HIGH and LOW
scenarios for both bromoform concentration and emissions.
Adding up the air–sea flux rate of bromoform over 1 year, we derive the
annual air–sea flux of bromine (Br) resulting from ballast water treatment.
In Singapore, the total Br flux ranges from 80 to 640 kmol (7 to 55 t) Br a-1 from LOW to HIGH, which corresponds to an outgassing of roughly 85 % of the original 8 to 63 t Br produced as bromoform in ballast water
(Table 3). The remaining 15 % is transported from the port site into the
open ocean where it is either eventually outgassed into the atmosphere or
transported into the deeper ocean. In the Pearl River Delta region, the flux
ranges from 136 to 1070 kmol (12 to 92 t) Br a-1, which corresponds to
an outgassing of 100 % of the Br produced from ballast water treatment.
(a) Modelled bromoform emission rates updated from Ziska et al. (2013). (b) Same as (a) with additional anthropogenic emission rates
calculated as 90 % of total bromoform release from ballast water
treatment at each harbour. Black dots indicate the location of all harbours from
the modified world port ranking in Southeast Asia.
Given that 85 % of Singapore and 100 % of Pearl River Delta ballast
water bromoform is directly outgassed into the atmosphere, we expect that on
average 90 % of the anthropogenic bromoform is quickly outgassed after
ballast water discharge in the Southeast Asia region. Based on this
assumption, we estimate the anthropogenic outgassing rate of bromoform at
each of the 38 harbours in the modified world port ranking in Southeast Asia
according to its calculated ballast water volume for a MODERATE scenario.
These emission rates are calculated on a 1∘×1∘
horizontal grid box closest to the harbour so that the values can be
compared to emission maps reconstructed from observations after Ziska et al. (2013) (Fig. 10). Note that anthropogenic
emissions are always positive (from ocean to atmosphere) since they were
calculated with zero concentration in the atmosphere. The climatological
emissions can have negative (atmosphere to ocean) fluxes whereby the
reconstructed atmosphere has higher concentrations than the sea surface.
Thus, adding anthropogenic emissions to the climatology can theoretically
reduce emissions. However, the climatology from Ziska et al. (2013) shows
that bromoform emissions in coastal areas are generally characterised by
high positive emissions with 500 to 1000 pmol m2 h-1 wherein
macroalgae act as efficient bromoform producers (Quack and Wallace, 2003)
(Fig. 10a). When we add the estimated
anthropogenic bromoform emissions from ballast water to the climatological
emissions, many of the grid boxes clearly show a strong increase in
emissions, sometimes more than doubling the emission rates of bromoform
(Fig. 10b). This is especially
visible at very big harbours such as Shanghai, Singapore and the Pearl River
Delta region where the new emission rates exceed 1000 pmol m2 h-1.
These regions appear as local hot spots of anthropogenic bromoform
emissions. Most of these areas are characterised by heavy industry and other
anthropogenic activities, resulting in strong emissions of greenhouse gases
like methane and ozone. The expected additional source of bromoform to the
atmospheric environment can perturb the oxidising capacity and thus the
atmospheric lifetime of greenhouse gases and other pollutants (Saiz-Lopez
and von Glasow, 2012). The atmospheric chemistry around large ports is
highly sensitive to additional emissions of volatile DBPs from treated
ballast water.
Discussion and conclusion
We investigate a new source of halogenated disinfection by-products to the
ocean and atmosphere from the release of chemically treated ballast water.
Over the next years, more than 75 000 ships have to instal a ballast water
treatment system to prevent the continued spread of harmful invasive species
(David and Gollasch, 2015). As a side effect, halogenated DBPs at high
concentrations will be produced in ballast water and released into coastal
waters (Werschkun et al., 2012; Delacroix et al., 2013). In particular,
bromoform shows concentrations in undiluted ballast water up to 1 million
times higher than in the natural environment.
Our simulations of the DBP spread from the Singapore and Pearl River Delta
harbours in the PASSIVE analysis show that within 1 year about half of the
DBPs discharged with the ballast water spreads fast in the surface ocean,
while the other half accumulates close to coastal areas around the discharge
location with a relative abundance of 3 % to 4 % of DBPs per 1∘×1∘ grid box. The currents determining the DBP spread in Southeast
Asia are seasonally influenced by monsoon winds. In Singapore, the main
driver of DBP transport throughout the year is the westward Indonesian
throughflow, and most of the DBPs spread into the Strait of Malacca and the
Indian Ocean. For the Pearl River Delta region, the majority of DBPs is
transported southwestward during the northeast monsoon period in boreal
winter and northeastward during the southwest monsoon period in boreal
summer. Thus, non-volatile DBPs can either spread over large areas at the sea
surface or accumulate in specific regions, such as the accumulation of
DBPs from the Pearl River Delta in the Gulf of Tonkin. While interannual
variations of the DBP spread are relatively small, the seasonal cycle in
transport patterns leads to enhanced coastal accumulations depending on
the region and time of year.
Based on our simulations in the FLUX analysis, we expect brominated VSLS
concentrations and emissions to increase locally in regions with high
industrial activity. Anthropogenic bromoform can locally add up to 23 pmol L-1 (0.006 µg L-1) in the HIGH scenario around the port
sites. Our simulations assume that DBPs are transported out of the harbour,
thereby providing a lower boundary for environmental concentrations.
Other studies like David et al. (2018), which use a port-based model approach
to calculate the predicted environmental concentration, estimate higher
bromoform values (e.g. 0.3 µg L-1) due to the smaller areas
considered and the missing air–sea exchange. Once ballast water treatment
has been established globally, in situ measurements will be necessary to
confirm if existing model-based results provide realistic estimates.
Our simulations reveal that bromoform emissions to the atmosphere can exceed
1000 pmol m-2 h-1 for the MODERATE scenario. This is caused by
moderate to high wind speeds above 10 m s-1, which occur especially in
the Pearl River Delta. In this region the transfer velocity is sufficiently
high so that all anthropogenic bromoform within the mixed layer is instantly
outgassed into the atmosphere. Anthropogenic bromoform from ballast water
discharge does not accumulate in the ocean but is rather an immediate
additional input of Br to the atmosphere. This new source can locally double
the climatological bromoform flux calculated from Ziska et al. (2013)
around big harbours like Singapore, Shanghai and in the Pearl River Delta.
Here, the bromoform emissions to the atmosphere are substantially larger
than natural fluxes. For the HIGH scenario, emissions of up to 3500 pmol m-2 h-1 can occur, which is in the range of the highest natural
emissions found in global shelf waters but does not exceed reported
maximum values of 4450 pmol m-2 h-1 (Quack and Wallace, 2003). The
area of Southeast Asia shown in Figure 10 gives a
total Br input of 80 Mmol yr-1 for the Ziska et al. (2013) climatology, while
anthropogenic bromoform leads to an additional 3 Mmol Br yr-1.
Over the next decades, the impact of brominated VSLSs on climate and ozone
depletion will increase due to changes in atmospheric transport and
chemistry (Hossaini et al., 2015; Tegtmeier et al., 2015; Fernandez et al.,
2017). Increasing VSLS production from anthropogenic activities needs to be
investigated and monitored in order to quantify its input to atmospheric
bromine. Measurements of VSLSs in coastal areas and ports can reveal the
local impact of anthropogenic emissions. For some harbours, they can be even
higher than our simulations suggest because our initialisation assumes a
dilution of DBPs in the coastal ocean approximately 8 to 40 km off the
coast. Moreover, anthropogenic sources like ballast water are always subject
to economic fluctuations and trading policies and are thus likely to
increase in the future. The choice of simulation scenarios covers a broad
range of possible cases, and an increase of ballast water discharge according
to current economic growth of ∼2 % per year will not
change the main results.
On a global scale the bromine emissions from ballast water treatment reach
up to 13 Mmol Br a-1 in the HIGH scenario. Compared to current
estimates of background emissions of 2 to 10 Gmol Br a-1 (see Ziska et
al., 2013, and references therein) the anthropogenic bromine input is rather
small, amounting to 0.1 %. Thus, we do not expect an impact of
anthropogenic VSLSs from ballast water treatment on global atmospheric
chemistry or the stratospheric ozone layer.
Ballast water, however, is not the only anthropogenic source of DBPs to the
coastal oceans. DBPs are also produced through the oxidation of drinking
water, wastewater, seawater in desalination plants and cooling water in
power plants (e.g. Jenner et al., 1997; Werschkun et al., 2012). In contrast
to drinking water for which by-products are strictly regulated (Richardson et
al., 2007), the chemical treatment of seawater or brackish water containing high
levels of inorganic bromine is not monitored regularly, although it can lead
to much higher levels of brominated DBPs. Thus, it is of interest to
investigate in future studies the combined effect of anthropogenic VSLSs from
all types of oxidative water treatment on the environment.
Data availability
Data from the ARIANE simulations are available upon request from the
corresponding author.
The supplement related to this article is available online at: https://doi.org/10.5194/os-15-891-2019-supplement.
Author contributions
JM wrote the paper, performed the simulations and created the output.
ST developed the research question and guided the research process. BQ
helped in the formulation of the research question and analysed water
samples. SG and MD provided water samples. AB and JVD provided the model
data; SR and JVD set up the ARIANE environment and helped with the
simulation. All authors took part in the process of paper
preparation.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “Shipping and the Environment – From Regional to Global Perspectives (ACP/OS inter-journal SI)”. It is a result of the conference Shipping and the Environment – From Regional to Global Perspectives, Gothenburg, Sweden, 23–24 October 2017.
Acknowledgements
We thank Stephanie Delacroix for providing ship-based samples of ballast
water. The authors would also like to thank Yue Jia for simulations of the
atmospheric bromoform mixing ratios. Furthermore, we wish to thank Bruno Blanke and Nicolas Grima for realising and providing the Lagrangian software
ARIANE. The OGCM and trajectory simulations were performed in the High-Performance Computing Centre at the Christian-Albrechts-Universität zu
Kiel. The OGCM model data used for this study were kindly provided through
collaboration within the DRAKKAR framework by the National Oceanographic
Centre, Southampton, UK. We especially thank Andrew C. Coward, Adrian L. New and
colleagues for making the data available. This study was carried out within
the Emmy-Noether group AVeSH (A new threat to the stratospheric ozone layer
from Anthropogenic Very Short-lived Halocarbons) funded by the Deutsche
Forschungsgemeinschaft (DFG, German Research Foundation) – TE 1134/1. Siren Rühs
received funding from the Cluster of Excellence 80 “The Future Ocean”
within the framework of the Excellence Initiative by the Deutsche
Forschungsgemeinschaft (DFG) on behalf of the German federal and state
governments (grant CP1412). Finally, we thank the editor and two anonymous
reviewers for their helpful comments to improve the paper.
Financial support
This research has been supported the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – (grant no. TE 1134/1).
Review statement
This paper was edited by David Turner and reviewed by two anonymous referees.
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