Introduction
Air pollution due to shipping is a serious concern for coastal regions in
Europe (; ). Globally, nearly 70 % of the
exhaust emitted from ship traffic occurs within a corridor of 400 km
along the coastline . Since emissions from ships can be
transported in the atmosphere over several hundreds of kilometres, they have
the potential to diminish the air quality in coastal areas. In addition to
the primary emitted particles in the ship exhaust, secondary particles are
formed in the atmosphere by oxidation of emitted gaseous precursors –
nitrogen oxides (NOx) and sulfur dioxide (SO2) –
during the dispersion of the ship exhaust. Mainly by contributing to the
ambient levels of fine particulate matter, PM2.5 (particles with a
diameter of less than 2.5 µm), emissions from ship traffic are
responsible for a large number of premature deaths globally .
According to , the worldwide use of cleaner marine fuels with a
lower content of sulfur will strongly reduce the ship-related premature
mortality and morbidity by 34 % and 54 %, respectively. In northern
Europe, the health-related external costs from international shipping in the
Baltic Sea and North Sea are expected to decrease by 36 % between 2000
and 2020 . This reduction is mainly a consequence of the
introduction of the sulfur emission control area (SECA) for the Baltic Sea
(enforced 2005) and North Sea (enforced 2006), which stepwise reduced
sulfur content in ship fuels.
However, air emissions of NOx from ship traffic remained almost constant
throughout the last decade, and the impact of NOx will remain a concern for
health. Shipping emissions of NOx on the Baltic Sea are of similar magnitude
as the combined land-based NOx emissions from Finland and Sweden in all
emission sectors . While EU air quality legislation will lead
to a decline in land-based emissions of NOx in the future, ship emissions – without more stringent emission control measures on NOx – will rise with the
projected annual growth of maritime traffic in the Baltic Sea of about 5 %
.
As a consequence, the relative importance of shipping emissions compared
to land-based emission sources of NOx is expected to increase. A review of
model studies on ship emissions showed that NOx emissions from international
shipping on European seas could be equal to land-based emission sources in Europe
(EU-27) from 2020 onwards and confirmed that the contribution of the shipping sector to
future air pollution in Europe will increase .
The atmospheric transformation of emitted NOx from shipping is especially
relevant for the formation of ozone . Shipping emissions are
estimated to play an important role in ozone (O3) levels compared to the road
transport sector near the coastal zone in Europe . A regional
impact study by found that the contribution of shipping emissions
to surface NOx levels causes an increase in surface O3 by up to
4–6 ppbv over the eastern Atlantic and western Europe. O3 can
damage vegetation, reduce plant primary productivity and agricultural crop yields
and is also a serious concern for human health .
Ship exhaust emissions of NOx are further converted to
gaseous nitrous acid (HNO3) through atmospheric oxidation.
This conversion of nitrogen dioxide
(NO2) to HNO3 takes place at a rate of approximately 5 % h-1, causing an atmospheric lifetime of NOx of about 24 h
. HNO3 is a sticky compound, which is,
in the presence of ammonia
(NH3), converted by gas-phase–particle partitioning to particulate nitrate
(NO3-). Nitrate is removed from the atmosphere via dry and wet scavenging,
contributing to deposition of oxidized nitrogen to the sea. Atmospheric deposition of
nitrogen (N)-containing compounds play a role in the eutrophication of the coastal
marine environment e.g..
Eutrophication of the sea is caused by high
inputs of nutrients (nitrogen and phosphorus), resulting in the production of algal
blooms, followed by the accumulation of organic material which after sedimentation
results in the depletion of oxygen in the bottom water of stratified areas of the sea.
Atmospheric deposition of nitrogen accounts for approximately one-third of the total
nitrogen input to the Baltic Sea .
Several studies have used atmospheric chemistry-transport models (CTMs)
to investigate the composition and fluxes of atmospheric nitrogen to the
Baltic Sea basin (; ; ;
; ; ) mainly focusing on the
influence of meteorological and climatological factors and the interannual
variability of meteorological conditions. Annual atmospheric deposition of
total nitrogen to the Baltic Sea basin computed with the CTM model
EMEP MSC-W declined by 27 % between 1995 (305 ktyr-1) and
2015 (222 ktyr-1) (; data normalized
to interannual changes in meteorological conditions). While the deposition
of oxidized nitrogen decreased by 35 % during this period, reduced
nitrogen, i.e. mainly NH3 and particulate ammonium
(NH4+), decreased by only 12 % . Based on
atmospheric CTM calculations, it has been estimated that the atmospheric
deposition of N-containing compounds originating from ship exhaust, depending
on the season, can contribute to more than 50 % of the total atmospheric
deposition of nitrogen in some areas of the Baltic Sea .
Emissions from shipping are regulated globally by Annex VI “Regulations for the
Prevention of Air Pollution from Ships” to the
Marine Pollution Convention (MARPOL) of the International Maritime Organisation (IMO).
The NOx emission reduction scheme of IMO MARPOL Annex VI
is based on the tier standards as described in the NOx
technical code . Tier 1,
implemented in the year 2000, introduced emission standards for ships constructed
between 1 January 2000 and 1 January 2011 up to 10 % stricter than those that
applied for ships built before 2000. Tier 2, implemented in 2011, enforced up to
15 % stricter standards than Tier 1 for ships constructed after 1 January 2011.
Tier 1 and Tier 2 limits are worldwide and apply to all new marine diesel engines.
The third regulation stage, Tier 3, will only affect ships sailing inside the
designated nitrogen emission control areas (NECAs). A NECA for the Baltic
Sea, North Sea and English Channel will become effective in 2021. In the following, we
refer to the northern European NECA simply as the NECA. From
1 January 2021 onwards, newly built ships in the Greater North Sea and Baltic Sea have
to comply with the stringent Tier 3 regulations for NOx emissions,
which are approximately 75 % stricter than Tier 2. To fulfil the requirements of Tier 3,
ship owners have to use abatement methods such as exhaust control technologies
(catalyst converters, etc.) or use liquefied natural gas as fuel for new ships.
For the North Sea, , using a regional atmospheric CTM
system and detailed shipping emission inventories for the present-day and
future situations, estimated that, upon introduction of the NECA in
2016, levels of NO2, particulate nitrate and ozone in 2030 would
not change compared to the year 2011, because the growth in ship traffic
compensates for potential emission reductions. A delayed introduction of the
NECA by 5 years (in 2021) would cause concentration increases of
these pollutants by 10 %–15 % compared to today . The
study by assumes an increase in ship number by
1 % yr-1, an increase in transported cargo of 2.5 % yr-1
and a ship renewal rate of 2.5 % yr-1 independently of ship size. The
study considered no gains in fuel efficiency of newly built ships. Clearly,
predicted consequences of the Tier 3 NOx emission
regulation on future shipping emissions depend critically on the projected
growth of transported volume, the increase in ship number and the share of
new ships in the future fleet. In a similar study,
investigated the effect of the NECA introduced in 2016 on the air
quality in 2030, assuming a moderate increase in ship activity. According to
their future scenario, total NOx emissions in the Baltic Sea
and the North Sea will be almost unchanged in 2030 compared to 2010 if the
NECA is not implemented. However, implementation of the NECA in
2016 will lead to significantly lower NOx emissions from
ships in 2030, resulting in slight reductions in the burden on health due to
shipping . The emission study by , which
calculates the emissions separately for every ship, taking into account
expected traffic growth and fleet renewal, corroborates the strong decrease in NOx shipping emissions (by 11 % in 2020 and by
79 % in 2040) when the NECA is established in 2016.
The present study is part of the BONUS project SHEBA (Sustainable Shipping
and Environment of the Baltic Sea Region; http://www.sheba-project.eu,
last access: 6 February 2019). The main goal of the study is to investigate
the effect of the implementation of the NECA in 2021 on the air
quality in the Baltic Sea region and on the total deposition of nitrogen to
the Baltic Sea in 2040. In addition to the effect of the NECA
regulation, we also look into possible future developments which might
diminish the beneficial effect of the NECA, such as failing to achieve
increased fuel efficiency of ships.
Several future shipping emission scenarios for the year 2040 were designed. These
scenarios were based on the projected development of the economic growth and ship
traffic volume in accordance with the study by . Land-based emission
sources are assumed to follow the emission reduction due to current EU legislation.
Three cases with respect to future air quality were considered: (1) implementation of
the NECA in 2021, (2) no implementation of the NECA and (3) alternative
assumptions for the fuel efficiency of the ship fleet in combination with NECA.
A regional atmospheric CTM system using the Community Multiscale Air
Quality (CMAQ) model (; ), similar to
that used in the study by , was used to simulate the
present-day and future air quality conditions in the Baltic Sea region. The
advantage of the applied CTM system for the Baltic Sea compared to
previous studies in the same region (; ;
) is the higher spatial and temporal resolution of all
components driving the chemistry-transport calculations. The meteorological
fields, the emissions from ship traffic and the emissions from land-based
sources were considered at a grid resolution of 4×4 km2
for the innermost model domain in the nested CMAQ runs. A higher
resolution of shipping emissions, which are obtained based on ship positions
acquired from 4 min AIS (Automatic Identification System) records and
detailed ship characteristics using the Ship Traffic Emission Assessment
Model (STEAM; ; ) in
combination with the higher resolution of the chemistry-transport computation
allow for a better resolution of the individual ship's plumes. Moreover,
the high-resolution meteorology (0.025∘ grid) resolves convective
precipitation, which is expected to improve the timing and amount of
predicted rainfall, crucial for the determination of the nitrogen inputs to
the Baltic Sea.
The focus of the present study will be on the computational model results for
summer, defined as the average of the period June–August (JJA), when
assessing the changes in air quality and deposition between the future
scenarios and the present-day situation. In summer, emissions from shipping
are highest and the photochemical conversion of the ship exhaust constituents
into compounds that are readily scavenged by precipitation is faster than in
other seasons. Therefore, ship-originated oxidized nitrogen deposition to the
sea is highest during the summer . In addition, the seasonal
variation of air quality indicators and of the accumulated nitrogen
deposition to seawater is presented.
A first set of model runs was performed for the situation in the year 2012. The present
day model results on nitrogen deposition and the air quality situation is analysed.
Modelled deposition of nitrogen was evaluated in two steps: first the predicted
rainfall amount and frequency are compared to daily precipitation measurements from rain
gauge stations in Sweden, and second the wet deposition of oxidized and reduced
nitrogen is compared against measurements of the “Cooperative Programme for Monitoring
and Evaluation of the Long-range Transmission of Air Pollutants in Europe”
(EMEP) programme. Present-day model results on air quality are evaluated with
measurements from the regional background stations of the EMEP monitoring
network in the Baltic Sea region. A companion paper by presents a
more detailed comparison of the model results for the current air quality situation
with land-based observations of air pollutant concentrations in the Baltic Sea region.
The contribution of shipping emissions to the modelled concentration of air pollutants
was determined from the difference between a reference run that included all emissions
and a “Noship” run that excluded emissions from ship traffic (zero-out method).
A second set of model runs was performed to assess the effect of projected emissions
from shipping for the year 2040. Future air quality and nitrogen deposition is
analysed in order to investigate (1) the effect of establishing the NECA in
2021 compared to a future situation without NECA and (2) the effect of a lower
fuel efficiency increase than expected based on a continuation of the current trend.
Changes in the ship contribution to regulated air pollutants and to nitrogen deposition
over seawater between the present-day simulation and the future scenario simulations
are presented. Finally, recommendations with respect to the future regulations and
their possible impacts and side effects are given.
Chemistry-transport modelling
CMAQ model description
Regional chemistry transport model simulations with the Community Multiscale
Air Quality (CMAQ) model v5.0.1 (; ;
) were performed to assess the effect of emissions
from ship traffic on the present-day and future air qualities of the Baltic Sea
region. The CMAQ model computes the air concentration and deposition
fluxes of atmospheric gases and aerosols as a consequence of emission,
transport and chemical transformation. The atmospheric chemistry of reactive
species is treated by the Carbon Bond V mechanism , with
updated toluene chemistry and chlorine radical chemistry
(mechanism cb05tucl; ).
The aerosol scheme AERO5 is used for the formation of secondary inorganic
aerosol (SIA). Aerosol growth and nucleation is simulated by three log-normal
distributed modes, each represented by three moments .
The Aitken and accumulation modes represent PM2.5
and the coarse mode represents
particulate matter with diameter >2.5 µm (PMcoarse). The
instantaneous gas-phase–aerosol equilibrium partitioning of sulfuric acid
(H2SO4), HNO3,
hydrochloric acid (HCl) and NH3
on the fine-particle modes is solved with the ISORROPIA v1.7 mechanism
.
Dynamic mass transfer is simulated for the coarse particle mode because
large particles often do not reach equilibrium with the gas phase for typical
atmospheric timescales . For the coarse mode, semi-volatile
inorganic species are allowed to condense and evaporate, while H2SO4 does
not evaporate again from the coarse mode. Because of the dynamic mass transfer to
coarse particles it is possible to use CMAQ for the simulation of chloride
(Cl-) replacement by NO3- in mixed marine and urban air masses
, which could be an important aerosol process in the Baltic Sea
region.
Sea salt emissions were calculated in line by the parameterization of
, as described in . Sea salt surf zone emissions
were deactivated because of considerable overestimations in some coastal
regions . The formation of secondary organic aerosol
(SOA) from isoprene, monoterpenes, sesquiterpenes, benzene, toluene,
xylene and alkanes (; ) is included.
SOA formation pathways include the traditional two-product
representation, reaction of volatile organic compounds (VOCs) to give
non-volatile products, oxidative ageing of primary organic aerosol,
acid-catalysed enhancement of SOA mass, oligomerization reactions and
in-cloud aqueous-phase oxidation.
Three types of clouds are modelled in CMAQ: subgrid convective precipitation
clouds, subgrid non-precipitating clouds and grid-resolved clouds.
CMAQ simulates the aqueous-phase chemistry in all cloud types.
For the two types of subgrid clouds, the cloud module in CMAQ vertically
redistributes pollutants and calculates in-cloud and precipitation scavenging.
Since the meteorological model provides information about the grid-resolved clouds,
CMAQ subsequently does not apply further cloud dynamics for this cloud type.
Subgrid clouds are only simulated in CMAQ when the meteorological
driver uses a convective cloud parameterization.
Hence subgrid clouds are treated by CMAQ on the coarser outer-resolution grids
(16 and 64 km) but not on
the 4×4 km2 model domain because the convective clouds are
resolved for the fine-grid resolution by the meteorological model.
Wet deposition of gases and particles is computed by the resolved cloud model of
CMAQ, which estimates how much certain vertical model layers contributed to the
precipitation. The precipitation flux for each model layer is computed as a function of
the non-convective precipitation rate, the sum of hydrometeors (rain, snow and
graupel) and the layer thickness (see for details).
Dry deposition is determined as the product of the atmospheric concentration and the
deposition velocity. The dry-deposition velocity is modelled in CMAQ using the
resistance analogy, where resistances are defined along pathways from the atmosphere to
the surface, which act in parallel or in series. Details on the deposition pathways in
CMAQ can be found in . The deposition velocity for particles
is calculated based on the aerosol size distribution, as well as meteorological and
land-use information. For large particles, the dry-deposition transfer is by turbulent
air motion and by direct gravitational sedimentation. The dry-deposition algorithm for
particles includes an impaction term in the coarse mode and the accumulation mode.
Model nests used in the simulations with CMAQ and for the
spatial maps of model results: (a) computational grid for northern
Europe with 16×16 km2 resolution (CD16, green) and
the high-resolution grids of 4×4 km2 for southern Baltic
Sea (CD04a, dark red) and northern Baltic Sea (CD04b, dark
blue). (b) Exemplary structure of spatial maps spanning from
latitude 53.30∘ N (southern border)
to 65.80∘ N (northern border) and
longitude 9.85∘ E (western border) to
30.95∘ E (eastern border). Green
shaded area is the high-resolution area which shows output from regional
model runs with a grid resolution of 4×4 km2. Dark red
outline marks the extent of the southern part of the Baltic Sea region and
dark blue outline marks the extent of the northern part of the Baltic Sea
region, for which model output from two high-resolution nests were used. For
the overlap area, the arithmetic mean of results from both nests was used. In
the post-processing of model results, the native Lambert conformal projection
of CMAQ output was transformed to a regular lat–long grid; therefore
the two outlined areas do not fill complete rectangles. The entire domain
shown in panel (b) was interpolated to a uniform resolution of
0.05∘ in the post-processing. White areas of the map are covered by
the output from the model nest with 16×16 km2
resolution.
In the resistance method it is assumed that the surface concentration of the chemical
species is zero. However, NH3 can be both emitted from and deposited to
surfaces depending on its atmospheric concentration. This bidirectional nature of the
air–surface exchange can modify the atmospheric transport and environmental impact of
ammonia. Bidirectional fluxes of NH3 over marine surfaces have been
documented in a review by . In fact, inclusion of the
bidirectional air–water exchange in a CTM resulted in lower overall dry deposition
of NH3 to coastal waters . However, until now, the
parameterization of the bidirectional flux has not been evaluated to a large extent
for marine waters. Although the bidirectional flux of NH3 is implemented in
CMAQ v5.0.1, the option was not used in this study. Because we are mainly
interested in the differences in total nitrogen deposition due to changes in emission
alone, the outcome of this study will be less affected by the sensitivity of the
modelled nitrogen deposition to bidirectional fluxes of ammonia.
Set-up of the model
Nested simulations with CMAQ were performed on a horizontal resolution of
4×4 km2 to simulate the current and future air quality situation
for the entire Baltic Sea region. The model was set up on a
64×64 km2 grid for the whole of Europe, subsequently on an intermediate
nested 16×16 km2 grid for northern Europe, and finally on two
nested 4×4 km2 grids, one for the southern Baltic Sea (Baltic
major) and one for the northern Baltic Sea (including Bothnian Bay and Gulf of
Finland). The nesting is visualized in Fig. a and the geographic
details of the high-resolution domain is shown in Fig. b. The vertical
dimension of the model extends up to 100 hPa in a sigma hybrid pressure
coordinate system with 30 layers. Twenty of these layers are below approximately
2 km; the lowest layer extends to ca. 36 m above ground. A spin-up
period of 1 month (December 2011) was used for the initialization of the model runs,
which is sufficiently long to prevent initial conditions having an effect on the simulated
atmospheric concentrations of the investigated period (year 2012).
Meteorological fields
The meteorological fields that drive the CTM were simulated with the
COSMO-CLM, version 5.0, for the year 2012 , using the ERA-Interim reanalysis and spectral nudging technique to force the model. COSMO
itself is the operational weather forecast model applied and further developed by a
consortium of national weather services, whereas COSMO-CLM stands for the climate
mode used and developed by the limited-area modelling community (clm-community;
).
The meteorological runs were performed first on a 0.11×0.11∘
rotated lat–long grid using 40 vertical layers up to 22 km for the whole of Europe.
The output was used as the forcing of a high-resolution nested meteorology run on a
0.025×0.025∘ grid; 50 vertical levels were used for this
simulation for the Baltic Sea region. The convection permitting configuration is used
on the high-resolution grid, e.g. only shallow convection is based on the Tiedtke scheme,
resolving convective precipitation clouds. The meteorological fields were processed
afterwards using a modified version of CMAQ's Meteorology-Chemistry Interface Processor (MCIP; )
to match the extension, resolution and projection of the CMAQ nested grids.
Based on the temperature anomalies and precipitation anomalies for the decade
2004–2014 for Baltic Proper, the year 2012 was chosen as the meteorological
reference year for the CTM simulations.
Year 2012 anomalies for 2 m temperature
(±2 ∘C) and total precipitation (±25 mm)
were closely aligned with the decadal average of the 2004–2014 period. The
meteorological year 2012 was also used in CTM calculations of the future air
quality situation to avoid complication of the interpretation of changes
between the present-day and the future. Hence, future changes in the air quality are solely due to
changed land-based and shipping emissions.
Boundary conditions
The initial conditions for the simulation and the lateral boundary conditions for the
64×64 km2 outer European domain (CD64) are taken from
APTA global reanalysis and were provided by the Finnish
Meteorological Institute (FMI). The global boundary conditions results have been
interpolated in time and space to provide hourly boundary conditions for the outer
domain. Boundary conditions for the nested intermediate grid and the two inner grids
were calculated on an hourly basis from the output of the next-outer grid. For the model
simulations with no shipping emissions, the full model chain was run again with all
emissions except for those from ship traffic in all the CMAQ grids.
Land-based emissions
Hourly gridded emissions of NOx, sulfur oxides
(SOx=SO2+SO3), carbon monoxide (CO),
NH3, PM2.5, PMcoarse and non-methane volatile
organic compounds (NMVOCs) were calculated for the year 2012 using the
comprehensive European emission model SMOKE-EU, which is an adaptation
of the US-EPA SMOKE (Sparse Matrix Operator Kernel Emissions) model
. NMVOC emissions were speciated according to the
carbon bond mechanism (cb5) (; ) of PM2.5
emissions according to the AERO5 aerosol mechanism. The
SMOKE-EU emission data are based on reported annual total emissions
from the European point source emission register (EPER), the official
EMEP emission inventory and the EDGAR HTAP v2 database
(; ; ). SMOKE-EU
distinguishes 10 major source sectors (including a number of subsector
definitions) according to the Selected Nomenclature for sources of Air
Pollution (SNAP) of the European Environmental Agency (EEA)
(Table ). For all point sources explicit plume rise
calculations based on real-world stack information were performed
.
The annual total emissions were temporally and spatially redistributed
individually for each emission sector and grid cell. Emissions of residential
heating were redistributed using the heating demand calculated from daily
average temperatures . Emissions from agricultural activity and
animal husbandry were disaggregated according to a fertilizer and plant
growth model and meteorological parameters . Finally, biogenic
emissions were calculated offline with the biogenic Emission Inventory
System BEIS version 3.4 (; ). The
SMOKE-EU emission datasets were calculated on a 5×5 km2 grid for the whole of Europe and were subsequently
interpolated to the respective CMAQ model grids.
Overview of SMOKE-EU source sectors. International shipping refers
to shipping outside North and Baltic seas.
SNAP
Description
Source
Inventory
type
1
Energy and heat production
point
EPER
2
Residential combustion
area
EMEP
3
Industrial combustion
point
EPER
4
Manufacturing processes
point
EPER
5
Refineries
point
EPER
6
Product use
area
EMEP
7
On road emissions
line
EMEP
8.1
Off road emissions
area
EMEP
8.2
Inland shipping
line
EMEP
8.3
Aviation
area
EMEP
8.4
International shipping
area
EMEP
9
Waste incineration
point
EPER
10.1
Agriculture
area
EDGAR
10.2
Animal husbandry
area
EDGAR
Shipping emissions and scenario description
Ship emission inventory for the Baltic Sea and North Sea
Shipping emissions for the Baltic Sea and North Sea with high spatial and
temporal resolution for this study were obtained from STEAM
(; ). STEAM combines the
AIS-based information and the detailed technical knowledge of the
world fleet with principles of naval architecture. This input information is
used to predict the resistance of vessels in water and the instantaneous
engine power of the main and auxiliary engines on a minute-by-minute basis
for each vessel that has sent AIS messages. The model predicts as
output both the instantaneous fuel consumption and the emissions of selected
pollutants. The dynamic modelling of shipping emissions also includes, for
example, the emission control areas and regulations, emission abatement equipment
on board the ships as well as fuel sulfur content modelling separately for
the main and auxiliary engines (; ).
Detailed vessel characteristics have been gathered for more than 90 000 individual
ships, reported by IHS Fairplay and other ship classification societies. The
AIS system provides automatic updates of the positions and instantaneous speeds
of ships at intervals of a few seconds. For this study, archived and downsampled
(approx. 4 min update rate) AIS messages provided by the Baltic Sea riparian
states were used for 2012 and 2014, containing several hundred million AIS
messages annually.
The shipping emission inventory consist of hourly updated 2×2 km2
gridded data for NOx, SOx, CO and particulate matter, which
is further divided into elementary carbon (EC), organic carbon (OC),
sulfate (SO4) and mineral ash. For the North Sea, ship emissions from 2011 were
adopted for 2012; total ship emissions of NOx were almost unchanged between
the 2 years. For Baltic Sea ship emissions are from 2012 and were provided for two
vertical layers (below 36 m, from 36 to 1000 m). In CMAQ,
SOx was attributed completely to SO2 and a NO:NO2
ratio of 95 : 5 was applied. Ship emissions below 36 m were attributed to the
lowest vertical model layer. Ship emissions above 36 m were attributed to the
second lowest layer, which appears to be justified based on findings with ship plume
simulations , showing that plume dispersion in the convective
boundary layer (BL) is insensitive to the initial buoyancy flux.
Future scenarios for shipping emissions
Shipping in the Baltic Sea in the future is modelled in a number of scenarios taking
into account the development of traffic and transport work, fleet development for
different ship types (number and size), changes in fuel mixture and regulations
influencing emissions and fuel consumption. Due to the long lifetime of ships it will
take about 30 years after the NECA entry date until the entire ship fleet is renewed and follows the Tier 3 emission regulation for
NOx. It was decided that the future regional CTM simulations for
2040 would be performed in order to see the full effect of the NECA.
Future baseline scenario BAU 2040
The baseline scenario for the future situation in 2040 is the so-called business-as-usual
(BAU) scenario, which is constructed as a reference scenario
(BAU 2040) for all other future scenarios. It accounts for current trends of economic growth and
development of shipping and takes into account predefined regulations. Regarding
regulations affecting emissions in air, the following are the most important ones in
BAU:
Sulfur regulation: the Baltic and North seas are sulfur emission control areas (SECAs), where the
maximum allowed sulfur (S) content in marine fuel has been gradually lowered, reaching 0.1 % S from 2015.
For sea areas outside SECAs the maximum fuel sulfur content will be 0.5 % S from 2020. These
regulations directly influence the emissions of SOx and have a strong impact on the particulate matter emissions.
NOx regulation: NOx emissions from marine engines are regulated with Tier 1 for
new ships from 2000 and Tier 2 from 2011. Tier 3 is applied in NOx emission control areas and
is applied for new ships in the Baltic and North seas from 2021.
Fuel efficiency: the regulation by IMO on Energy Efficiency Design Index (EEDI)
requires new ships to become gradually more fuel efficient. The EEDI regulation was enforced for new ships from 2015
onwards. The EEDI will influence engine emissions in a similar way to the regulations on sulfur and NOx.
The BAU scenario assumes a share of ships driven by liquefied natural
gas (LNG) of about 10 % in the ship fleet in 2040. This is modelled as a
fraction of new ships introduced each year that will use LNG, since retrofitting
of existing ships from fuel oil to LNG is assumed to be less likely due to high costs.
Since LNG is used as a means to comply with the sulfur regulations, ship types
that operate mainly within SECAs are modelled as more likely to use LNG.
The fuel efficiency for new ships in BAU is assumed to improve more than what
is required from the EEDI regulation, following recent trends and assumption
from , assuming that further technical improvements and more
efficient operation take place. The traffic volumes are expected to continue to grow
with about 1 % yr-1 on average (it varies with ship type); the current trend of
using larger vessels is expected to continue as well.
Future scenario NoNECA 2040
The other two future scenarios, NoNECA 2040 and EEDI 2040, are deviations from
the development given by the BAU scenario. In the NoNECA scenario,
the nitrogen emission control area is assumed not to be implemented,
i.e. all new ships up to 2040
are assumed to follow the Tier 2 NOx standard.
The difference with the BAU scenario is
then that new ships from 2021 follow the Tier 2 standard rather than Tier 3.
The same introduction of LNG as in BAU is assumed, since the use
of LNG is mainly motivated by the SECA regulation.
From the difference between BAU and NoNECA, the effect of implanting the NECA on
emissions can be deduced.
Future scenario EEDI 2040
In the EEDI scenario, improvements in fuel efficiency strictly follow
the requirements of the EEDI regulation. Annual efficiency increases of
0.65 % to 1.04 %, depending on ship type, are assumed in the EEDI
scenario, while the corresponding values in the BAU scenario are 1.3 % to
2.25 %. From the difference between BAU and EEDI, the effect of a
lower fuel efficiency increase than expected based on the continuation of the current trend
can be deduced.
Table provides emission scaling factors used in the three
scenarios for future shipping emissions.
Future scenario emissions: emission scaling factors used in the
three scenarios for shipping emissions for the relevant air pollutants.
PM-other includes EC, OC and mineral ash. The emission
scaling factors give the respective emissions in 2040 in relation to the
emissions in 2012.
Scenario
CO
PM-other
SO4
SOx
NOx
BAU
0.679
0.351
0.088
0.088
0.207
NoNECA
0.679
0.351
0.088
0.088
0.505
EEDI
0.923
0.490
0.121
0.207
0.285
Future land-based emissions
The three scenarios studied here (BAU, NoNECA and EEDI) for
future shipping emissions are combined with land-based emissions for 2040, which follow
the currently decided emission regulations in Europe. The future land-based emission
dataset for the year 2040 was created based on the present-day SMOKE-EU emission
dataset (Sect. ) using growth factors for each source sector and each
species. The employed emission scaling factors are based on the trend between annual
total emissions from the 2012 SMOKE-EU inventory and 2040 baseline emissions of
the current legislation (CLE) scenario from ECLIPSE v5 .
CLE assumes efficient enforcement of committed legislation but delays in
introducing or enforcing particular laws are considered when such information was
available. The scaling factors for land-based emissions, given as average of the Baltic
Sea riparian states for CO, PM-other, SO4, SO2,
NOx and NH3 are 0.75, 0.70, 0.45, 0.45, 0.40 and 0.80,
respectively.
Ship emissions from the STEAM database were merged with the land-based
emissions from the SMOKE-EU database for the Baltic Sea region and
interpolated to the corresponding CMAQ domain sizes and resolutions. Total
annual emissions of NOx in 2012 and in 2040 (BAU scenario) prepared
for the CMAQ simulations are shown on geographic maps in Fig. .
Annual total emissions of NOx
(mg(N)m-2) in the surface layer for the Baltic Sea region:
(a) in 2012 and (b) in 2040 for the BAU scenario.
Gridded emissions from the STEAM and SMOKE emission databases
interpolated to a grid resolution of 4×4 km2 and
transformed to a Lambert conformal projection for the two CMAQ
high-resolution domains. Grid lines mark a lat–long grid with 0.5×0.5∘ cells.
Future scenario model results
Air quality changes in 2040 compared to the present day
Future air quality situation
In the BAU 2040 scenario (future reference simulation), with the introduction
of the NECA in 2021, NOx emissions from ship traffic in the Baltic
Sea are reduced by 79 % in 2040 compared to 2012, because most ships of the
Baltic Sea ship fleet will then fulfil the Tier 3 regulation. In the
NoNECA scenario, the NECA is not established, but all other
developments (economic growth, fleet renewal and efficiency increase) are as in the
BAU scenario, still leading to a reduction in NOx emission from
ships by 50 %. In the EEDI scenario, fuel efficiency increase follows
the EEDI regulation, thus remaining below the efficiency increase assumed
for the BAU scenario, resulting in an overall reduction in NOx
emissions from ships by 71 % compared to 2012. The spatial maps of average
summer (JJA) concentrations of daily maximum O3, NO2,
SO2 and PM2.5 in the three future scenarios for 2040 are
compared to the present-day results in Fig. .
Future air quality situation in the Baltic Sea region in summer
(JJA) compared to the present day. CMAQ model results for present-day
(first column), for BAU 2040 (second column), for NoNECA 2040 (third
column) and for EEDI 2040 (fourth column) are shown for
(a) daily maximum O3, (b) NO2,
(c) SO2 and (d) PM2.5.
Over most parts of the Baltic Sea region, the summer mean of daily maximum
O3 in BAU 2040 decreases by 10 %–25 % compared to 2012, as
a consequence of the NECA and reduced land-based emissions of NOx
(Fig. a). The future change in ozone is similar in
EEDI 2040, implying that the effect of increased fuel efficiency is less
pronounced and that the NOx reduction through establishing the
NECA has a much greater influence on future ozone levels in the Baltic
Sea region. In the NoNECA scenario, daily maximum O3 over land
will decrease less than in the BAU scenario, but still an average ozone
reduction by 15 % in 2040 is predicted for large parts of Sweden and the Baltic
Sea compared to the present day.
In the BAU 2040 scenario, summer mean NO2 concentrations are
drastically reduced by ∼80 % over most parts of the Baltic Sea and by up
to ca. 90 % in the northern Baltic Proper compared to 2012
(Fig. b). This appears to be a result of the combined emission
reductions through the NECA and the regulation of land-based emissions
(Sect. ), leading to a shift in the overall atmospheric
photochemical regime due to the lower abundance of NOx in the future.
A strong reduction is also seen in EEDI 2040, where NO2 levels over
the Baltic Sea decrease by ∼80 % compared to 2012. NoNECA 2040 results
in a reduction in NO2 by ∼50 % over the entire Baltic Sea.
BAU 2040 adopts the agreed SOx emission reduction measures;
i.e. the SECA limit of 0.1 % S in fuel from 2015 onwards and the global
limit of 0.5 % S in fuel from 2020 onwards. The other two future scenarios also
implement the two sulfur regulations. In 2040, summer mean SO2 levels
drop by 80 %–90 % over the entire Baltic Sea compared to the present day.
Summer mean PM2.5 levels in 2040 decrease by 50 %–60 % along the main
shipping routes and by 40 %–50 % in the other parts of the Baltic Sea compared
to 2012. The EEDI scenario involves lower primary PM emission reductions
(by 51 %) than in BAU 2040 and NoNECA 2040 (by 65 %). However, as
for the other air pollutants, no large differences in the spatial concentration
distributions in summer 2040 are seen between the EEDI and the BAU
scenarios, suggesting that the lower fuel efficiency increase has only marginal
implications on the future air quality in the Baltic Sea region.
Influence of ship emissions in the BAU future scenario
Figure summarizes the predicted ship contribution in
summer 2040 according to the BAU 2040 scenario, which is analogous to
Fig. for the present-day ship contribution.
As a result of the introduction of the NECA in 2021, the future impact of
ship emissions on O3 levels in the Baltic Sea region diminishes. In 2040,
the ship contribution to summer mean daily maximum O3 concentrations
is highest over the Gotland Basin (range: 5–6 ppbv), while it is smaller for
all over parts of the Baltic Sea region, not exceeding 4.5 ppbv. Overall,
the model simulations predict that shipping emissions will still influence ozone
levels over the Baltic Sea and in the coastal areas in 2040, with relative
contributions in the range of 10 %–20 % to daily maximum O3.
Future (2040) ship contribution in the Baltic Sea region in summer
(JJA) from CMAQ model results for the BAU 2040 scenario: ship-related
concentration (left) for gaseous pollutants (in ppbv) and for
PM2.5 (in µgm-3), percentage ship contribution (right)
for (a) daily maximum O3, (b) NO2,
(c) SO2, and (d) PM2.5. Ship-related
contribution only shown for the high-resolution area. The same scales as in
Fig. were used to facilitate comparison of the concentration
and contribution maps. The sharp change in the O3 ship contribution
north of 58.8∘ N is an artefact of the averaging in the overlap area
of the two 4 km resolution grids.
Future (2040) change in the ship-related contribution in summer
(JJA) in percent compared to 2012, given as the relative difference between the
ship contribution from the BAU 2040 simulation and the ship contribution
from the present-day simulation: (a) daily maximum O3,
(b) NO2, (c) SO2 and
(d) PM2.5. Not coloured (empty) areas indicate grid cells with
a ship contribution in BAU 2040 of less than 1.0 ppbv,
0.1 ppbv, 0.01 ppbv and 0.005 µgm-3, for daily
max O3, NO2, SO2 and PM2.5, respectively.
Ship-related contribution only shown for the high-resolution area. Note the
different scale for daily max O3 (from -100 % to
100 %).
The absolute ship contribution to summer mean NO2 concentrations in 2040
drops substantially compared to 2012. The ship-related NO2 concentration
decreases from ca. 3 ppbv in the present-day situation to 0.5–1.5 ppbv
in the BAU scenario, along the main shipping routes. Even with the NECA
established, emissions from ship traffic remain the dominant contributor to
atmospheric NO2 over the Baltic Sea in 2040.
The absolute ship contribution to SO2 concentrations in summer 2040 is
less than 0.1 ppbv. However, the ship influence on ambient SO2
concentrations has not completely vanished in 2040. Along the main shipping routes
throughout the Baltic Sea, the relative contribution remains high.
The absolute ship contribution to PM2.5 in summer 2040 is predicted to be
≤0.2 µgm-3 over most parts of the Baltic Sea region, with higher
values over the Belt and Kattegat (0.4 µgm-3). The ship
influence is substantially weakened compared to the present-day situation: the relative contribution
peaks along the shipping routes (15 %–25 %) and is below 10 % over land.
Future change in the ship contribution
Figure shows the future change in the ship contribution in
summer 2040 compared to 2012 when following the BAU 2040 scenario. Future
changes in the ship contribution to daily maximum O3 are divided into
two regions with opposing signs, one with a relative increase over the central
shipping routes, and one with a relative decrease outside the ship tracks and over
the coastal regions. Over the ship lanes, ozone recovers due to reduced titration
of ozone in the ship plumes following the lower emissions of NO from ships.
At a greater distance from the ship lanes, photochemical production of ozone declines
compared to the present day, giving rise to lower O3 concentrations.
The ship contribution to NO2 decreases by 80 %–85 % over the Baltic
Sea, but is slightly more than linear with the reduced NOx emissions from
shipping. The decrease is smaller (∼77 %) in some port cities like Gdansk and
St Petersburg and in areas with a high density of ship traffic. The reduced
NOx emission from ships causes an increase in the ratio of [NO2]
to [NO] (short: NO2-to-NO ratio) in the ship plumes.
Although the NO2-to-NO ratio at the ship stack is the same (equal
to 5 : 95), it becomes higher, as NO2 from the background air entrains into
the plume, than in the present-day situation. According to the photostationary state
relation, the increased ratio causes a higher steady-state O3 concentration
in the ship plume. With the local increase in O3, the reaction of
NO with the hydroperoxyl (HO2) radical giving NO2 starts
to compete with the titration reaction (reaction of NO with O3).
In the reaction of NO with HO2 an additional ozone molecule
is produced, as the resulting NO2 molecule photolyses, amplifying the
ozone production in the plume. Hence the smaller decrease in the NO2
ship contribution is due a change in the photochemistry regime in the ship plumes
accompanied with a higher conversion of NO to NO2.
For the ship contribution to SO2, a uniform decline of around 90 %
is seen for the entire Baltic Sea in accordance with a linear decrease following
the reduction in SOx emissions from shipping of 91.2 % between 2012
and 2040 in BAU 2040. Note that ship emissions of SOx were attributed
completely to SO2. As for the NO2 ship contribution, the decrease
is slightly higher than expected due to the reduction in ship emissions. Due to
the drastic decrease in nitrogen oxides, the atmospheric oxidation capacity
increases in the future scenario simulation, leading to more efficient oxidation
of pollutants and higher availability of photo-oxidants (OH and
HO2 radicals). Hence, the removal rates of SO2 and NO2
by reaction with photo-oxidants and the rate of SO2 oxidation in clouds
are slightly increased in 2040 compared to 2012.
The ship-contributed summer mean PM2.5 between 2012 and 2040 (BAU 2040)
is reduced by 75 %–90 %, with largest reductions over the southern part of the
Baltic Sea and in the coastal regions. This is more than can be explained by the
reduction in primary PM emissions (by 65 %) from shipping. Thus a
substantial fraction of the changed ship contribution is caused by changes in the
secondary aerosol production. The future ship contribution to PM2.5 is
affected by reduced SOx emissions from ships, as a result of the regulations
for lower sulfur fuel content and by reduced NOx emissions due to
the NECA.
Together, the regulations lead to a decline in the atmospheric formation of
sulfate and nitrate particles related to shipping. In the southern part of the
Baltic Sea region, especially over Denmark and northern Germany, the ship-related
formation of secondary aerosol is also affected by the lower NH3
emissions from agriculture. Decreasing atmospheric ammonia concentrations reduce
the formation of ammonium nitrate particles, since their formation is limited by
the availability of NH3.
Effect of establishing the NECA (in 2021) on the future air quality
in summer (JJA) 2040 in the Baltic Sea region as relative difference (in
percent) between the scenario simulations BAU 2040 and NoNECA 2040:
(a) daily maximum O3, (b) NO2,
(c) SO2 and (d) PM2.5. Not coloured (white)
areas indicate grid cells with ship contribution in BAU 2040 of less than
1.0 ppbv, 0.1 ppbv, 0.01 ppbv and 0.005 µgm-3, for daily max O3, NO2, SO2 and
PM2.5, respectively.
For the other two future scenarios, NoNECA 2040 and EEDI 2040, changes in
the ship-contributed pollutant concentrations compared to the present day are smaller
than in BAU 2040. In the scenario without the implementation of NECA,
NoNECA 2040, the ship contribution to NO2 in 2040 decreases by
50 %–60 % over the Baltic Sea (Fig. S13). The ship contribution to ozone
increases widely by more than 10 % compared to the present day, indicating enhanced
ozone production due to shipping activities in 2040, mainly over sea and the coastal
areas of Sweden, Denmark and Poland. The EEDI scenario, with lower fuel
efficiency, results in a significantly smaller reduction in ship-contributed
PM2.5 than the BAU scenario. Still, the ship-contributed summer
mean PM2.5 between 2012 and 2040 is reduced by 65 %–80 % over the
impacted areas (Fig. S14).
Future air quality: effect of the NECA
The difference between the two future scenarios BAU 2040 and NoNECA 2040 is
the higher emission reduction in NOx from shipping in the BAU
scenario through the establishment of the NECA. Figure
illustrates the effect of introducing the NECA in 2021 into major air quality
components compared to a future situation without NECA, determined based on
the difference between modelled concentrations in the BAU 2040 and
NoNECA 2040 scenarios. Land-based emissions are the same in both scenarios;
therefore changes are solely due to different ship emissions in the two future
scenarios.
The result of the NECA in 2040 is a reduction in NOx emissions
from shipping by 59 % on average, corresponding to the difference between a
Tier 3-dominated ship fleet with the NECA and Tier 2-dominated ship
fleet without the NECA. The reduction in NOx emissions from
shipping primarily translates into a ∼60 % decrease in NO2
summer mean concentrations within a wide corridor of the ship routes. In addition,
the population in coastal areas in northern Germany, Denmark and western Sweden will
be less exposed to NO2 in 2040 due to the introduction of the NECA.
Due to the lower atmospheric NOx levels, less ozone is formed, and daily
maximum O3 concentration over the Baltic Sea in summer 2040 is on average
6 % lower than without the NECA. In the areas close to the main shipping
routes, ozone is almost unchanged despite the sharp reduction in NOx
emissions, probably due to compensating effects between changed titration losses
and changed photochemical ozone production. As expected, levels of atmospheric
SO2 are largely unaffected by the NECA (<±2 %).
Effect of lower fuel efficiency on the future air quality in summer
(JJA) 2040 in the Baltic Sea region as relative difference (in percent)
between the scenario simulations EEDI 2040 and BAU 2040:
(a) daily maximum O3, (b) NO2,
(c) SO2 and (d) PM2.5. Not coloured (white)
areas indicate grid cells with ship contribution in BAU 2040 of less than
1.0 ppbv, 0.1 ppbv, 0.01 ppbv, 0.005 µgm-3, for daily max O3, NO2, SO2 and
PM2.5, respectively.
A secondary effect of the NECA is a reduction in the formation of particulate
nitrate. Due to the non-linearity of the atmospheric particle mass formation
(i.e. photochemistry and gas-to-particle conversion depend on precursor concentrations
and existing particulate matter in a non-linear fashion) the impact of reducing
gaseous precursors does not result in a linear reduction in future PM2.5
levels. Figure d shows the change in summer mean PM2.5
concentration pattern due to the NECA. Note that primary emissions of
PM2.5 are the same in BAU and NoNECA; thus changes are
solely attributed to modified particulate nitrate concentrations. The largest
decrease in PM2.5, by up to 8 %, occurs over the Danish islands, where the
abundance of ammonium nitrate is highest.
Future air quality: effect of lower fuel efficiency
The BAU scenario assumes an improvement in the marine fuel efficiency beyond
that required by the EEDI regulation for new ships. With the difference
between the EEDI 2040 and BAU 2040 scenarios (land-based emissions are the
same in both scenarios), the effect of a slower rate of fuel efficiency improvement
compared to the projections in the BAU scenario on the air quality in 2040
is determined. The lower fuel efficiency affects the ship engine emissions and leads
to NOx, SO2 and PM2.5 emissions from ships
that are on average 37.9 %, 36.8 % and 39.6 % higher in 2040,
respectively, compared to the BAU scenario. As a consequence of the lower
fuel efficiency, modelled summer mean concentrations of NO2 and
SO2 along the main shipping routes in 2040 are higher by 40 % and
25 % than in BAU, respectively (Fig. ).
Nitrogen deposition in summer (JJA) 2040: (a) accumulated
total deposition of nitrogen (in mg(N)m-2) in scenario BAU 2040, (b) percentage change in the ship contribution to nitrogen
deposition in scenario BAU 2040 compared to the present day,
(c) effect of the NECA on nitrogen deposition, and
(d) effect of the lower efficiency of EEDI on nitrogen
deposition. Not coloured (empty) areas indicate grid cells with ship
contribution in BAU 2040 of less than 6.0 mg(N)m-2 for total
nitrogen deposition. The ship-related contribution is only shown for the
high-resolution area.
The lower fuel efficiency has little influence on daily maximum ozone
concentrations over the Baltic Sea. Further, the influence of the changed fuel
efficiency on atmospheric secondary particle formation is rather limited (not shown).
For PM2.5, the higher primary particle emissions compared to BAU
do not fully propagate into surface air concentrations (increase by less than
10 %). A large fraction of the ship-related PM2.5 is from secondary
formation, which does not increase proportionally with the increased primary
PM emissions, for example due to the limited availability of NH3.
Future nitrogen deposition
Summer-accumulated total nitrogen deposition to seawater in 2040 according to
BAU 2040 is below 100 mg(N)m-2 in most parts of the Baltic Sea,
with highest deposition remaining in the Belt Sea (Fig. a).
The average summer deposition rate for the Baltic Sea is 48 mg(N)m-2.
The ship contribution to total nitrogen deposition in summer is massively reduced
(by more than 60 %) in the coastal areas of the Baltic Sea region compared to
2012 (Fig. b). Over sea, largest reductions of the ship
contribution take place in an area extending from Kattegat to the Arkona basin.
Future (2040) annual and seasonal nitrogen deposition amounts
(ktN) to the seawater of the Baltic Sea and ship-related nitrogen
deposition according to scenario BAU 2040, taken from the CD04
grid. Values in brackets denote the change (in ktN) compared to
2012. Amounts refer to a Baltic Sea surface area of 431 390 km2,
including the western part of Skagerrak.
Nitrogen deposition
Year
JFD
MAM
JJA
SON
All emissions
Oxidized
35.7
10.9
5.6
6.9
12.3
(-58.8)
(-12.2)
(-10.5)
(-16.2)
(-19.8)
Reduced
52.9
8.1
15.3
13.9
15.6
(-11.6)
(-1.0)
(-3.1)
(-3.6)
(-3.9)
Total
88.6
19.0
20.9
20.8
27.9
(-70.3)
(-13.2)
(-13.6)
(-19.8)
(-23.7)
Ship emissions
Total
4.9
0.8
0.9
1.8
1.4
(-17.6)
(-3.1)
(-3.4)
(-6.7)
(-4.4)
The introduction of the NECA causes a maximum reduction in the summer-accumulated
nitrogen over seawater by 18 % compared to not introducing the NECA in
2021 (Fig. c). This means that the Tier 2 fleet in
NoNECA 2040 already accomplishes a large reduction in nitrogen deposition
compared to today. The effect of the lower fuel efficiency in 2040 (according to
EEDI 2040) is an increase in nitrogen deposition compared to BAU, mainly
over the northern Baltic Proper and over coastal areas. The relative increase is up
to 12 % (Fig. d).
Table shows the BAU 2040 annual and seasonal nitrogen
deposition sums to the entire Baltic Sea seawater surface, for total, oxidized
and reduced nitrogen. The ship-related annual nitrogen deposition is reduced by
17.6 ktN, while the overall nitrogen deposition is reduced by
70.3 ktN compared to 2012. Thus the reduction in NOx emissions
over the continent, in accordance with a current legislation scenario for land-based
emissions in the Baltic Sea region, has a larger impact on the future nitrogen input
to the Baltic Sea than the shipping fleet.
Summary and discussion
Changes in the air quality in the future scenarios
In the BAU scenario, with the introduction of the NECA in 2021,
NOx emissions from ship traffic in the Baltic Sea are reduced by about
80 % in 2040 because most ships of the Baltic Sea ship fleet will then fulfil
the Tier 3 regulation. With the NoNECA scenario, the entire ship fleet
follows Tier 2 regulations for NOx in 2040 and, in conjunction with
the fuel efficiency increase, leads to an overall NOx emission reduction
from the ship fleet by about 50 %.
Table presents the relative changes in annual mean concentrations
of air pollutants in the Baltic Sea region between 2012 and 2040 (as average of
the CD04 grid domains). Annual mean NO2 decreases by 61 %–72 %
between 2012 and 2040 in the Baltic Sea region, depending on the shipping scenario,
with the smallest decrease in the NoNECA scenario.
The BAU scenario adopts the agreed SOx emission abatement
regulations: the pre-established SECA limit of 0.1 % S in fuel
from 2015 onwards followed by the global limit of 0.5 % S in ship fuels
from 2020 onwards.
On average, annual mean SO2 decreases by ∼60 % between 2012
and 2040, independent of the shipping scenario.
Consequently, particulate sulfate decreases
by 50 %–60 % over the Baltic Sea between 2012 and 2040 (not shown) in all three
scenarios. The burden of PM2.5 over the Baltic Sea region decreases by
35 %–37 % between 2012 and 2040 (Table ). The reduction in PM2.5 is larger over sea, where it drops by 50 %–60 % along the main
shipping routes, and is smaller over the coastal areas. The large drop over sea is due
to the reduction in particulate matter emissions from ships and the lower production
of sulfate and nitrate related to reduced emissions of primary precursor gases
(NOx and SOx) from ship traffic. In most coastal areas the
decreased PM2.5 is mainly a consequence of the abatement measures on land.
Summary of overall changes in future scenarios. Changes (in percent)
in spatial average for all future scenarios compared to the present day
(simulations with all emissions): annual means of NO2,
SO2, PM2.5 and the daily maximum O3 within the
4 km resolution area (CD04 grid domains) and annual sum of
nitrogen deposition to seawater.
Scenario
NO2
SO2
PM2.5
O3
N
daily max
deposition
BAU 2040
-72
-61
-37
-4
-44
NoNECA 2040
-61
-61
-35
-3
-40
EEDI 2040
-69
-60
-37
-3
-43
On an annual average, the daily maximum O3 decreases only slightly over
the Baltic Sea region, but the summer average decreases by 10 %–25 %, depending
on the shipping scenario, in large parts of Sweden and the Baltic Sea compared
to the present day.
Overall, a lower fuel efficiency increase than in BAU has only marginal
implications on the future air quality in the Baltic Sea region.
Changes in the ship contribution in the future scenarios
The absolute ship contribution to ambient levels of NO2 and SO2
between 2012 and 2040 changes slightly more than expected due to the reduction in
ship emissions. The lower abundance of NOx in the future atmospheric
background increases the oxidation capacity of the atmosphere and leads to more
efficient oxidation of pollutants via gas-phase reactions and in-cloud processing.
Table presents the relative changes in the annual mean absolute
ship contributions in the Baltic Sea region between 2012 and 2040.
A consequence of establishing the NECA is the reduction in the ship
contribution to daily maximum ozone by 18 % on average compared to the present
situation. If the NECA is not implemented, an increase in the ship-related
daily maximum ozone by 31 % results compared to the present day. The introduction
of NECA is hence critical for abating ship emissions of NOx to
levels that are low enough to sustainably dampen ozone production in the Baltic Sea
region. A second important effect of the NECA over the Baltic Sea region is a
reduction in secondary formation of particulate nitrate. The introduction of the
NECA reduces the ship-related PM2.5 by 72 % in 2040 compared
to the present day, while it is reduced by only 48 % without implementation of the
NECA.
The effect of the lower fuel efficiency on the absolute ship contribution of
air pollutants is limited. Still, the annual mean ship contributions in 2040 to
the four pollutants are significantly higher than in the BAU scenario.
Summary of ship contribution changes in future scenarios. Changes
(in percent) in spatial average of the ship contributions for all future
scenarios compared to the present day (simulations with all emissions): annual
means of NO2, SO2, PM2.5 and the daily maximum
O3 within the 4 km resolution area (CD04 grid
domains) and annual sum of nitrogen deposition to seawater.
Scenario
NO2
SO2
PM2.5
O3
N
daily max
deposition
BAU 2040
-82
-91
-72
-18
-78
NoNECA 2040
-55
-90
-48
31
-46
EEDI 2040
-75
-88
-61
-1
-69
Contribution of ship emissions to nitrogen deposition
A previous study estimated the contribution of airborne
nitrogen from international ship traffic to the oxidized nitrogen deposition
in the Baltic Sea basin to be about 8 % to 11 % (period: 1997–2006)
on an annual average. The contribution from ships with a range from 12 % to
14 % has been reported for the period 2008 to 2011 . In the
present study, the relative ship contribution to the deposition of oxidized
nitrogen is 24 % (Table ), about twice as high as the previous estimates.
However, the total annual nitrogen deposition for 2012 in
the present study is 29 % lower compared to the EMEP MSC-W model
used by HELCOM . Taking the literature value of 14 %
and the oxidized nitrogen deposition flux in 2012 reported by HELCOM
(128.9 ktNyr-1; ), an absolute ship
contribution of 18 ktNyr-1 is derived, which is only slightly lower
than our estimate of 22.5 ktNyr-1.
The relative ship contribution to the total nitrogen deposition is 14 % on
annual average and 21 % in summer in the present-day situation
(Table ). The ship contribution drops to 5.6 % in 2040 (9 %
in summer) when following the BAU scenario (Table ).
Between 2040 and 2012 the ship-related deposition of oxidized nitrogen decreased
by 78 %. In BAU 2040 the ship contribution to the annual deposition of
oxidized nitrogen over the Baltic Sea is only 14 %.
Nitrogen deposition to the seawater of the Baltic Sea decreases on average by
40 %–44 % between 2012 and 2040 (Table ).
Depending on the future shipping
scenario, the decline in the ship-related nitrogen deposition varies between
46 % and 78 % (Table ). In the EEDI scenario, when
the NECA is established but fuel efficiency increase is lower than in
BAU, nitrogen deposition in most ship-influenced areas decreases less than
in the BAU scenario. The weakest reduction is found for the NoNECA
scenario, in which nitrogen deposition decreases by only 30 % over coastal areas
of Denmark, Germany and western Finland. The western part of the Baltic Sea would be most
affected if the NECA is not implemented (Fig. c).
Prognosis of the total nitrogen deposition to the Baltic Sea
A linear relationship was found between the emissions of NOx from the
Baltic Sea ship fleet and the annual ship-related nitrogen deposition to Baltic Sea
seawater (spatial average) based on the results of the present-day simulation and the
future scenario simulations (Fig. ). Because the changes in
the nitrogen deposition attributed to shipping (Fig. b) between
2012 and 2040 are mainly confined to the Baltic Sea and the surrounding coastal areas,
it was expected that the changes in the ship-related deposition flux are proportional
to the atmospheric input of oxidized nitrogen via ship emissions. An important link
between the ship emissions and the deposition of nitrogen is the formation of
HNO3, which constitutes the most important removal pathway for nitrogen
in the atmosphere .
The relationship presented above is useful for a quick evaluation of the ship-related
nitrogen deposition in future shipping scenarios. Cumulative scenarios based on
Shared Socioeconomic Pathways (SSPs) with respect to future ship emission in
the Baltic Sea region were designed in the SHEBA project. In
scenario SSP3 (regional rivalry), which represents a world with much less
international trade and low mitigation capacity , future
shipping deviates largely from the predefined regulations but growth of shipping
is slower than in BAU by 0.5 % yr-1. The fuel efficiency development
is lower by 1 % yr-1 than in EEDI. Use of LNG is similar as
in BAU. The Tier 2 regulation is not enforced in SSP3; i.e. the entire
ship fleet applies the Tier 1 standard for NOx emissions. Ship NOx
emissions in SSP3 are 143 ktNyr-1, somewhat lower than in the
current situation. Based on the linear model the ship-related nitrogen deposition is
estimated to be 21.5 ktNyr-1.
Relationship between emissions of NOx (in
ktNyr-1) from the Baltic Sea ship fleet and the annual
ship-related nitrogen deposition (in ktNyr-1) to the Baltic
seawater (on spatial average) based on the model results of the present-day
simulation and the model results of the future scenario simulations. Red-filled circle indicates the ship contribution in scenario SSP3
predicted from the linear fit to the relationship.
Thus, in this quick assessment, SSP3 brings a slight improvement in 2040
compared to the current situation. The comparison of the simulated future scenarios
to SSP3 also underlines the potential of the Tier 2 standard regulation
for newly built ships (as in NoNECA 2040) to reduce the future impact from
shipping, compensating, together with the faster fuel efficiency development, the
projected higher ship traffic growth.
Discussion of uncertainties and limitations
The ship contribution to air pollutants and nitrogen deposition in the
present study was computed using a zero-out method; i.e. the ship emissions
were removed in one simulation. An alternative brute-force method would be
the perturbation of the emissions, for example reduction by 20 %, which
might be more careful with respect to the non-linearity of the involved
photochemistry. However, our goal was to derive the impact of shipping in
different scenarios, while perturbing emissions is mainly used to investigate
short-term responses to expected (small) changes in a sectoral emissions. A
previous study by applied the so-called tagging method to
assess the ship contribution from each riparian state of the Baltic Sea.
Tagging requires adding auxiliary variables to the CTM itself to track
pollution. While tagging for inert primary pollutants is straightforward;
methods for addressing secondary pollutants require an analysis of the
limiting reagents to avoid tagging all possible follow-up products in the
gas-phase, aerosol phase and cloud water. Differences between tagging and
brute-force methods are usually observed in secondary processes involving
precursors from different sources. Some comparison studies (;
) indicate that tagging is advantageous for source allocation
rather than for predicting responses to emissions changes.
European regions that are affected by a high density of ship traffic, such as the UK,
France, western Germany, North Sea, the southern part of the Baltic Sea and along
the ship tracks in the Mediterranean, are currently in a NMVOC-limited regime
with respect to ozone formation (Beekmann and Vautard, 2010). In northern Europe,
except for the region of the English Channel and parts of the North Sea, a transition
from NMVOC-limited to NOx-limited regime is projected
until 2020 and the next decades .
In a NMVOC-limited regime the production of ozone is sensitive to emissions of
NMVOC, while increasing NOx leads to a reduction in ozone by
titration. In the NOx-limited regime, ozone is sensitive to emissions of
NOx, while it is hardly affected by additional NMVOC emissions.
In the simulations for the future scenarios in 2040, a transition
towards a NOx-limited regime most certainly happens in the currently
NMVOC-limited areas of the Baltic Sea, in particular along the ship tracks in
the southern part. This is clearly seen in the BAU 2040 scenario, where a relative
increase in the ship-related daily maximum ozone occurred (due to less titration) over
the central shipping routes, whereas the ship-related ozone decreased in the
NOx-limited areas outside the ship tracks and over the coastal regions.
However, predicted changes in the daily maximum ozone concentrations due to shipping
are uncertain because of the lack of data on NMVOC emissions from shipping in
the STEAM inventory that were used in the CTM calculations.
We have reduced land-based emissions in the future scenarios in order to obtain a
more realistic estimation of the consequences of regulations on shipping emissions
on the future air quality in the Baltic Sea region. Based on the model results for
the future ship contribution, it is obvious that reduced land-side emissions of
primary gaseous precursors amplified the decline in secondary aerosols related to
shipping, in particular over the coastal areas. However, the reduction in land-side
emissions has a very small effect on the determined ship contributions to
NO2 and SO2 over the Baltic Sea (Fig. S15).
The reason for the underestimation of WNO3 and WNH4 in the
CMAQ simulations compared to observations of the regional background
monitoring stations of the EMEP network, could not be fully resolved.
The formation of particulate nitrate involves complex chemistry of several compounds
in the gas-phase and multi-component solution systems on aerosols. The simulation of
nitrate is highly uncertain because it requires accurate computation of the
concentrations of the precursors, e.g. HNO3, NH3, dust and
sea salt. The joint underestimation of WNO3 and WNH4 was found
in the statistical analysis of model–observation pairs and also in the comparison of
modelled and observed seasonal averages. The most convincing explanation at the
current stage is that the oxidative conversion of NOx to HNO3
occurs at a too-slow rate in the model, combined with too little particulate ammonium
from the regional background that is advected into the Baltic Sea region.
An alternative explanation might be that the wet removal of NO3-
and NH4+ in CMAQ is not efficient enough. In addition, the
evaluation of simulated precipitation amounts and frequency showed that the southern
part of the Baltic Sea receives too little rainfall in summer. For the other seasons
and in the northern part the precipitation bias is positive. Too-low precipitation
in the southern part, where modelled concentrations of NO3-
and NH4+ are much higher compared to the northern part, could be
responsible for an average underestimation of the total nitrogen wet deposition to
the Baltic Sea.
Coarse-mode particles are removed much faster than fine-mode particles; therefore
the deposition of particulate nitrate crucially depends on the uptake to larger
particles. Heterogeneous chemical production of nitrate on coarse-mode particles
has been found to control the atmospheric nitrate production to a very large extent
. The hydrolysis of N2O5 to produce HNO3
is considered in CMAQ by uptake coefficients depending on temperature, RH and
particle composition, using the parameterization by , but only for
fine-mode aerosols. The Davis parameterization tends to predict too-high
N2O5 uptake coefficients near the surface, especially over marine and
coastal areas, where relative humidity is high .
CMAQ allows for dynamic mass transfer to coarse particles and therefore takes into account the
reactive uptake of HNO3 by sea salt particles. Meanwhile, resuspension of
mineral dust was not activated in the simulations, and the missing heterogeneous
reaction on dust particles surfaces may have contributed to the underestimation
of WNO3.
Conclusions
The impact of ship emissions on the present-day (2012) and future (2040) air
quality and nitrogen deposition was evaluated with a regional atmospheric CTM.
The meteorological fields, the emissions from ship traffic and the emissions from
land-based sources are considered at a grid resolution of 4×4 km2
for the innermost model domain covering most of the Baltic Sea region. Ship emissions
from the STEAM model based on ship movements from AIS records and
detailed ship characteristics in combination with solving atmospheric chemistry and
transport at high resolution, enable a better treatment of the plumes from ship
traffic compared to previous CTM studies in the Baltic Sea region.
The effect of future legislation related to shipping and of future changes in the
ship fuel efficiency of the ship fleet on air quality and deposition in 2040 in the
Baltic Sea region was determined based on computational results from regional
CTM simulations. Future air quality and nitrogen deposition is analysed
in order to investigate (1) the effect of establishing the NECA in 2021
compared to a future situation without NECA and (2) the effect of a lower
fuel efficiency increase than expected based on continuation of the current trend.
A BAU scenario has been designed in which the NECA is implemented and
the fuel efficiency for new ships improves more than required by
IMO′s Energy Efficiency Design Index regulation.
Establishing the NECA in 2021 has several benefits for the Baltic Sea
environment. One important effect of the NECA is a reduction in secondary
formation of particulate nitrate. The introduction of the NECA reduces the
ship-related PM2.5 by 72 % in 2040 compared to the present day, while it
is reduced by only 48 % without implementation of the NECA. A major
consequence of establishing the NECA is a reduction in the ship contribution
to daily maximum ozone in 2040 compared to the present situation. If the NECA
is not implemented, an increase in the ship-related daily maximum ozone results
compared to the present day. The introduction of NECA is thus critical for abating
ship emissions of NOx to levels that are low enough to sustainably dampen
ozone production in the Baltic Sea region. Overall, the introduction of the
NECA is expected to be beneficial for avoiding future health impacts of ozone
and PM2.5 in coastal areas of the southern part of the Baltic Sea region.
The effect of the lower fuel efficiency on the absolute ship contribution of air
pollutants is relatively small. The implementation of the NECA in 2021 can
be regarded as a safeguard for the case that the fuel efficiency increase falls below
the projected trend.
The decline in the ship-related nitrogen deposition to the Baltic Sea between 2012
and 2040 varies between 46 % and 78 % in different future scenarios.
When the NECA is established but the fuel efficiency increase is lower than
expected, nitrogen deposition in most ship-influenced areas decreases less than in
the BAU scenario. The weakest reduction is found for the scenario without
implementing the NECA, in which nitrogen deposition decreases by only
30 % over coastal areas of Denmark, Germany and western Finland. The western part
of the Baltic Sea would be most affected if the NECA is not implemented.
A prognostic relationship for a quick evaluation of the ship-related nitrogen
deposition in future shipping scenarios was derived in this work. The relationship
should be further modified to consider the interannual variability of atmospheric
deposition due to changing meteorological conditions in order to allow for more robust
projections of the ship-related nitrogen input to the Baltic Sea. However, it may be
used for estimating possible exceedances of critical loads for eutrophying substances
that are based on annual nitrogen inputs.
A limitation of the model results for regional surface concentrations of the daily
maximum ozone concentrations over the Baltic Sea region is the lack of data on
NMVOC emissions from shipping in the STEAM inventory that were used in
the CTM calculations. Additional NMVOC emissions from shipping would
serve as precursors of ozone and enhance photochemical ozone production in a
NMVOC-limited regime. In the presented model simulations, NOx
emissions from continental sources were reduced by 60 % between 2012 and 2040,
following current legislation, i.e. predefined emission abatement regulations.
The lower abundance of NOx in the future could lead to a shift in the
overall atmospheric chemical regime. To predict more accurately how such change in
the chemical regime will affect the future influence of ship emissions, a better
handle on NMVOC emissions from ships and their future development would
be important.
As a consequence of SOx emission abatement regulations for
shipping, annual mean SO2 decreases on average by ∼60 %
between 2012 and 2040, independently of the future scenario. With the reduction in SO2 emissions, less NH3 is required to neutralize the
strong acid H2SO4. The excess NH3 is available for the
formation of NO3- and NH4+ in the particulate
phase. According to , the trend of future particulate
NO3- concentrations depends on whether NOx or
NH3 are the limiting gas-phase compounds for nitrate formation.
Measurements in southern Sweden have shown that the concentrations of
NH3 and HNO3 are too low to form pure solid or aqueous
ammonium nitrate particles . Thus, in a future background
atmosphere over the Baltic Sea region, ambient levels of both gases might be
too low for ammonium nitrate formation, and the fate of these gases would be
the removal by dry and wet deposition. Meanwhile, the formulation of
heterogeneous processes related to the production of nitrate is highly
uncertain in the models, limiting the conclusions about the future transition
in the nitrate formation regime.
Use of the presented model data for health impact assessment in the densely populated
coastal areas of the Baltic Sea region is connected to uncertainties arising from
limitations of the chosen grid resolution. Despite the fine spatial resolution of
the innermost model grid, the concentration gradients between urban areas and
their surroundings (urban increment) and within harbour cities are not adequately
resolved by the simulations due to the large spatial and temporal variability of
emissions in urban areas. Ideally, a grid length of 1 km should be chosen to resolve
the urban increments in the coastal areas. However, a finer
resolution raises the need for more accurate emission data in the urban areas,
which is challenging because the compilation of urban emission inventories requires
specific information for each emitting sector .
A related study by assessed the extent of environmental
damage related to shipping on the terrestrial ecosystems surrounding the Baltic Sea.
Ecological impacts of air pollutants on land are evaluated in terms of critical load
(CL) exceedance for eutrophication. Using the latest reported CL
values for eutrophication together with the modelled deposition data of nitrogen
for 2012 and the future scenarios for 2040 of the present study,
find a significant improvement from 2012 to 2040. For the
BAU scenario, the area where the CL (eutrophication) are exceeded due
to ship-related nitrogen deposition decreased from about 20 % in 2012 to 5 %
in 2040. If the NECA is not implemented, the exceeded area due to shipping
is about 14 % in 2040, indicative of the relevance of the NECA for
coastal ecosystems surrounding the Baltic Sea. We note that the use of gridded
model data of dry deposition in the estimation of CL exceedances has
limitations. In the model simulation, dry deposition to land surfaces is weighted
for the different land use classes present in each grid cell. This might lead to
an underestimation of the eutrophication risk for forests in a grid cell which
includes other land uses, as the canopy resistance of forests is much higher than
that of grassland or other low vegetation. The CMAQ deposition data
are less affected by this problem due to the high resolution of the gridded data.
The shipping sector is an important contributor to atmospheric nitrogen
deposition in the Baltic Sea. The present study estimates a deposition flux
of oxidized nitrogen in the order of 22.5 ktNyr-1 due to
shipping emissions for the year 2012, which is slightly higher than previous estimates
(; ). Occurrences of high nutrient input to
coastal waters have been suggested to cause short-term algal blooms
. On the other hand, a study in the Kattegat showed that direct
nitrogen inputs through atmospheric deposition could not be linked to any
summer algal bloom observation, probably because the atmospheric input is
considerably diluted through mixing in the surface water layer
. The incidence of harmful algal blooms in shallow coastal waters, which damage the health
of humans and animals, has also been linked
to atmospheric nitrogen inputs . However, the relationships
between high nutrient inputs and the development of harmful algal blooms are
still not well understood .
Much stricter regulations for NOx emissions from newly built ships will
be enforced in 2021. It can be expected that significant emission reductions will
be the consequence of these regulations; however, this requires that the exhaust
gas cleaning technologies that will be implemented on board most the newly built
ships work properly. From the experiences with Euro 4 and Euro 5 diesel cars
that frequently emit much more NOx than allowed, the policy should pave the
way for extended compliance control measures. Several techniques exist on how emissions
from ships can be measured, including in situ observations at coastlines, ground-based remote-sensing techniques, sniffers on board aircraft or drones and sensors
on board the ships. The best technology needs to be tested now in order to be
prepared for the implementation of the NECA.