Introduction
The trace gas sulfur dioxide (SO2) is important for atmospheric
chemistry (Charlson and Rodhe, 1982) as a principal air pollutant (e.g.,
contributor to acid rain). Atmospheric oxidation of SO2 leads to
sulfate aerosols, which influence the Earth's radiative balance directly by
scattering incoming radiation and indirectly by affecting cloud formation
(Charlson et al., 1987). Important natural sources of SO2 include the
atmospheric oxidation of dimethyl sulfide (DMS, which is formed by marine
biota) and volcanic eruptions. Anthropogenic fossil fuel combustion also
produces SO2. SO2 is removed from the lower atmosphere by dry
deposition and oxidation in both the gas phase and the aqueous phase. The
relatively slow gas phase oxidation of SO2 leads to sulfuric acid
vapor, which usually condenses upon pre-existing aerosols but can nucleate
to form new particles under specific conditions (e.g., Clarke et al., 1998).
The much faster aqueous phase oxidation of SO2 takes place primarily in
cloud water (e.g., Hegg, 1985; Yang et al., 2011b) and leads to particulate
sulfate, which is removed from the atmosphere mainly by wet deposition.
SO2 production from DMS occurs principally via daytime oxidation by the
hydroxyl radical (OH). From observations in the equatorial Pacific, Bandy et al. (1996) and Chen et al. (2000) reported a clear increase in SO2
mixing ratio and coincidental decrease in DMS during the day. This
anticorrelation confirmed that DMS oxidation by OH is an important source
of SO2 over the remote ocean. Yang et al. (2011b) showed that DMS
remains the predominant sulfur precursor in the marine atmospheric boundary
layer of the relatively unpolluted southeast Pacific.
In the last few decades, SO2 emissions from terrestrial combustion
sources such as power plants and ground transportation have been subject to
strict regulation (e.g., UK Clean Air Acts). These forms of legislation have
significantly reduced the atmospheric sulfur burden over land in North
America and Europe (e.g., Lynch et al., 2000; Malm et al., 2002; Vestreng et al., 2007).
Unlike terrestrial SO2 emissions, ship emissions were for a
long period excluded from international environmental agreements. This
allowed ships to burn low-grade fuels with high sulfur content (i.e., heavy
fuel oils), which resulted in large SO2 emissions from ship engine
exhausts (hereafter ship emissions). In addition to SO2, ship exhausts
also contain carbon dioxide (CO2), nitrogen oxides, carbon monoxide,
heavy metals, organic toxins, and particulates such as black carbon (e.g.,
Agrawal et al., 2008). Closer to the coast and near shipping lanes, ship
emissions can be an important contributor to the atmospheric sulfur budget
(Capaldo et al., 1999; Dalsoren et al., 2009). Eyring et al. (2005a) estimated
that global, transport-related emissions of SO2 from ships in the year
2000 were approximately 3-fold greater than from road traffic and aviation
combined. Air-quality models predict that aerosols resulting from ship
emissions contribute to tens of thousands of cases of premature mortality
near coastlines (Corbett et al., 2007). Impacts may be further exacerbated
as the global population expands and shipping-based trade increases (Eyring
et al., 2005b).
In January 2015, new air-quality regulations from the International Maritime
Organization (IMO), an agency of the United Nations, came into force. These
regulations aim to reduce sulfur emissions in sulfur emission control areas
(SECAs) by decreasing the maximum allowed sulfur content in ship fuel from
1 % (regulation since 2010) to 0.1 % by mass. The English Channel and
the surrounding European coastal waters are within a SECA. The IMO further
intends to reduce the ship's fuel sulfur content (FSC) in the open ocean from the
current cap of 3.5 to 0.35 % by 2020. As an alternative to burning
lower sulfur fuel, these regulations also allow ships to use scrubber
technology to reduce SO2 emissions. Winebrake et al. (2009) estimated
that such reduced emissions would approximately halve the premature
mortality rate in coastal regions. A decrease in anthropogenic sulfur
emission is also expected to make DMS a relatively more important sulfur
source in regions such as the North Atlantic.
There have been few direct measurements of ship emissions that are relevant
for a regional scale. Kattner et al. (2015) reported large reductions of
SO2 in ship plumes from 2014 to 2015 near the mouth of the Hamburg
harbor on the river Elbe, which is about 100 km away from the North Sea.
Using the SO2 : CO2 ratio in ship plumes, they found a high
compliance rate of ∼ 95 % after the stricter regulation in
January 2015. Based on SO2 and CO2 measurements from the Saint
Petersburg Dam, which spans the Gulf of Finland and separates the Neva Bay
from the rest of the Baltic Sea, Beecken et al. (2015) found a compliance
rate of 90–97 % in 2011 and 2012. They observed a bimodal distribution in
the ship's FSC, with the lower mode centering around
∼ 0.2 % by mass and the higher mode centering around
∼ 0.9 %. Compliance checks are mainly limited to manual
checking of fuel logs and fuel quality certificates when ships are in port.
Global and regional ship emission estimates have typically been scaled from
inventories for individual vessels in combination with information about ship
traffic (e.g., Endresen et al., 2003; Collins et al., 2008; Matthias et al.,
2010; Whall et al., 2010; Aulinger et al., 2016; Jalkanen et al., 2016).
Accurate assessment of the success of IMO regulations requires long-term
continuous observations at strategic locations. Here we present 1.5 years of
continuous atmospheric SO2 measurements from the Penlee Point
Atmospheric Observatory (PPAO) in the English Channel, one of the busiest
shipping lanes in the world. The PPAO measurements date back to seven months
before recent IMO sulfur regulations came into force, providing a reference
point for future changes in emissions. The unique location of PPAO (Fig. 1;
http://www.westernchannelobservatory.org.uk/penlee/) allows us to
partition the atmospheric SO2 budget to natural (mostly dimethyl sulfide)
and anthropogenic (mostly ship emission) sources.
Wind rose at Penlee Point Atmospheric Observatory from 2014 to 2015
overlaid on a map of the British Isles (left), and a map showing the
observatory (yellow circle) on the western side of the 4 km wide Plymouth Sound
(right). The English Channel lies between the UK and northern France.
Colors on the spokes correspond to wind speeds in units of m s-1 and
concentric circles indicate frequency of occurrence in 2.5 % intervals
(outer circle = 12.5 %). Winds predominantly came from the
west/southwest at speeds of 4–12 m s-1.
Experiment
The PPAO is located on the western
side of the mouth of Plymouth Sound (Fig. 1). See Yang et al. (2016)
for detailed site description. About 11 m above mean sea level and
∼ 30 m away from the high water mark, the site is exposed to
air that has traveled over water across a wide wind sector (from northeast
to southwest). Near-continuous measurements of SO2, CO2, ozone
(O3), methane (CH4), as well as standard meteorological parameters
have been made at the PPAO since May 2014.
SO2 mixing ratio was measured every 10 s by an enhanced trace
level pulsed fluorescence SO2 analyzer (Thermo Scientific,
model 43i). The instrument noise level is about 0.06 parts per billion (ppb)
at a 1 min averaging interval and 0.025 ppb at a 5 min averaging
interval. O3 mixing ratio was measured every 10 s by a dual-beam,
UV absorption ozone monitor (2B Technologies, Inc., model 205), which has a
noise level of about 2.2 ppb at a 1 min averaging interval and 1.0 ppb
at a 5 min averaging interval. The SO2 and O3 sensors shared
the same air inlet, which consisted of a ∼ 4 m long 0.64 cm
outer diameter PFA (perfluoroalkoxy) tubing that extended just outside the
air vent of the building (∼ 2 m above ground and ∼ 13 m above mean sea level).
A 5 µm diameter Teflon filter was installed
upstream of the instruments to reduce particulates in sampled air. The
O3 instrument sampled through an additional 5 µm particle filter.
These filters were replaced approximately every 2–4 weeks. Blank
measurements of SO2 and O3 were made simultaneously by directing
sample air through the existing particle filters, ∼ 8 m of
0.64 cm outer diameter copper tubing (which efficiently removed SO2),
and an O3 scrubber. Linearly interpolated blanks are subtracted from
the raw data. We checked the calibration of the SO2 instrument twice a
year with a SO2 gas standard diluted in nitrogen (100 ppb, BOC). The
measured SO2 mixing ratio was within a few percent of the gas standard.
An O3 calibration device was unavailable during the duration of these
measurements. However, an intercomparison with a recently calibrated 2B
O3 Monitor showed that the accuracy of the O3 measurement at PPAO
was within ±5 %.
CO2 mixing ratio was measured by a Picarro cavity ring-down analyzer
(G2311-f) every 0.1 s (see Yang et al., 2016 for details). Ambient air
was drawn from a mast on the rooftop of the observatory (nominally at
∼ 18 m above mean sea level) through a ∼ 18 m long 0.95 cm outer diameter PFA tubing at about
15–30 L min-1 by a dry vacuum pump. The Picarro analyzer subsampled from this
main flow via a ∼ 2 m long 0.64 cm outer diameter Teflon PFA
tubing and a high throughput dryer (Nafion PD-200T-24M) at a flow rate of
∼ 5 L min-1. The total delay time from the inlet tip to
the analyzer was 1.9–3.3 s. The instrument calibration was checked
with a CO2 gas standard (BOC) and the accuracy was within 0.5 %. The
instrument noise for 1 min averaged CO2 is less than 0.05 ppm.
SO2, O3, and meteorological parameters data were logged and
time stamped by the same computer. Given the short inlet tubing length and the
specified flow rates, the calculated delay time for these gases from the
inlet tip to the sensors was less than 4 s. CO2 data were
logged on a separate computer (Picarro internal PC). Both computers were
synchronized to network UTC clocks once a week. The time difference between
the SO2/O3/meteorology measurements and the CO2 measurements
was less than 1 min. We note that the gas inlets for both the SO2
and the CO2 instruments (∼ 13 and 18 m a.m.s.l.) were within the surface layer of the marine atmosphere; they should
be sampling the same air mass (and the same ship plume) under typical
meteorological conditions. Dispersion modeling from von Glasow et
al. (2003)
predicts that just 10 s after emission, a ship plume will have already
expanded vertically to a height of 20 m from the surface.
Averaged SO2 mixing ratio and relative humidity vs. wind
direction for year 2014 and 2015. Error bars on SO2 indicate 2
standard errors. Elevated humidity marks the marine-influenced wind sector to
be between about 60 and 260∘. Higher and more variable SO2
mixing ratios were observed from the southeast, particularly in
2014. Icelandic volcano plumes (e.g., Fig. A1) were excluded from averaging.
Example of local ship plumes on a day of southeasterly winds. Sharp
peaks in SO2 and CO2 generally coincided with sudden depletions in
O3 (1 min average).
Results
Figure 1 shows the frequency distribution of winds from 2014 to 2015 (wind
distributions were nearly identical for these 2 years). Winds came
predominantly from west/southwest at moderate to high speeds, which were
typically associated with low-pressure systems in the North Atlantic and
generally carried low mixing ratios of SO2 and CO2 (Yang et al., 2016).
The wind sector between 110 and 250∘ is unobstructed by
land. The Rame Head peninsula sits ∼ 1.5 km west of PPAO,
beyond which lie ∼ 40 km of coastal seas, the county of
Cornwall (width of ∼ 30 km), and the North Atlantic Ocean. The
north/northeastern sector is more influenced by emissions from terrestrial
anthropogenic sources, while winds from 50 to 110∘ face the eastern
side of Plymouth Sound, which is busy with ship traffic. The
southeasterly sector is likely affected by emissions from local ships as
well as distant pollution from the English Channel and continental Europe.
According to the Devonport Naval Base Ship Movement Report, the total number
of ships in Plymouth Sound varies from about 4000 per month in winter to
6000 per month in summer. The volume of ship traffic in the English Channel
is about 15 000 per month (Maritime and Coastguard Agency, 2007).
Atmospheric SO2 and humidity varied significantly at PPAO depending on
wind direction (Fig. 2). Increased relative humidity clearly indicates the
marine-influenced wind sector from northeast to west/southwest. SO2
mixing ratios were higher and more variable when the air mass had traveled
from the southeast than when it had come from the southwest. This elevated
SO2 signal was more pronounced in 2014 (averaged between May and
December) than in 2015 (averaged between January and November). The lowest
SO2 mixing ratios were observed in the western wind sector in both
years. In the appendix, we show episodes of large SO2 plumes from the
Icelandic volcano Bárdarbunga as observed at PPAO. These volcanic events do
not affect our analysis of SO2 in the marine atmosphere since winds
were from the northwest.
Ship plumes and SO2 frequency distributions
Figure 3 shows a ship plume-influenced time series during a period of
southeasterly winds. Sharp spikes in SO2 coincided with spikes in
CO2 (e.g., at about 12:00 and 15:40 UTC), which lasted for just a few
minutes and likely corresponded to local (within a few kilometers) ship emissions.
O3 was significantly depleted in these plumes because of its reaction
with nitrogen oxides (NOx) emitted from ships. A lower, broader hump in
SO2 can also be observed between about 18:30 and 20:00 UTC. This was
likely due to more distant ship emissions that have been diluted and mixed
in the atmosphere. A concurrent increase in CO2 was not obvious during
these 1.5 h. Given the high background mixing ratio of CO2
(∼ 400 ppm), ship plumes result in much smaller (additional)
signal : background ratios for CO2 than for SO2. As a result,
CO2 emitted from point sources tends to quickly become
indistinguishable from the background with increasing distance (i.e., greater
air dilution/dispersion). In Sect. 4.2, we use the ratio between the
SO2 and CO2 peaks to estimate the ship's apparent FSC.
Histogram distributions of SO2 mixing ratios in 2014
(a) and 2015 (b) from the southeastern and southwestern wind
sectors (5 min average). Distributions are normalized to the total number of
observations from the respective wind sectors.
Average diel cycles of SO2 mixing ratio, separated into the
southeastern and southwestern wind sectors for 2014 (a) and 2015 (b). SO2 in
the southwestern sector showed diel variability that is largely consistent with
DMS oxidation. SO2 in the southeastern sector was significantly lower and
less variable in 2015 than in 2014.
Figure 4 shows the histograms of SO2 mixing ratios (from 5 min
averages) in 2014 and 2015. We have separated the data into two wind
sectors, southeast (80–170∘) and southwest (210–250∘).
The southeastern sector encompasses the English Channel while avoiding most of
the UK landmass. The southwestern sector, with largely an unobstructed oceanic
fetch of thousands of kilometers, bypasses the northwestern coast of France
as well as the busiest part of the shipping lanes (see ship's Automatic
Identification System maps from Jalkanen et al., 2016). On average, winds
came from our SW sector 7.5 days a month and came from the SE sector 2.1
days a month. The SO2 distributions shifted towards lower mixing ratios
in 2015 compared to 2014, especially for the southeastern wind sector. For
example, SO2 mixing ratios from the southeast exceeded 0.5 ppb
∼ 1 % of the time in 2015 (compared to ∼ 11 % in 2014).
Diel variability in SO2
We compute the mean diel cycles of SO2 mixing ratio in the southeastern
and the southwestern sectors for both 2014 (May to December) and 2015 (January
to November), which are shown in Fig. 5. The long averaging periods help
to reduce measurement noise and also allow variability caused by horizontal
transport to largely cancel. SO2 from the southwest shows a very tight
diel cycle and low variability (relative standard errors less than 10 %).
SO2 mixing ratio was the lowest near sunrise, increased throughout the
day, and decreased after sunset. This diel cycle suggests that SO2 from
the southwest came primarily from the photooxidation of biologically derived
DMS. Differences in the mean SO2 diel cycles in 2014 and 2015 for the
southwestern sector are largely due to the different months used in the
averaging. Considering only the months of May to November, mean SO2
mixing ratios in this wind sector for the 2 years differ by only
∼ 0.01 ppb (see Sect. 4.1).
SO2 from the southeast was about 3 times higher and also more
variable than from the southwest in 2014. Peaks in SO2 were observed in
the morning, mid-afternoon, and early evening. These timings are consistent
with the schedule of channel-crossing ferries, which enter Plymouth
Sound at least once a day from approximately due south. In 2015, SO2
from the southeast was about 2 times higher than from the southwest and
variability in SO2 mixing ratio was reduced. In both years, SO2
from the southeast shows an underlying diel trend (i.e., increasing during
the day and decreasing at night) that suggests contributions from DMS
oxidation. This implies that SO2 from the southeast is made up of at
least two major components: ship emissions and DMS oxidation.
SO2 from DMS oxidation
The SO2 diel cycle from the southwestern wind sector (Fig. 5) is
consistent with daytime SO2 production from DMS oxidation by the OH
radical (e.g., Bandy et al., 1996; Yang et al., 2009). DMS was measured using a
high-resolution proton-transfer-reaction mass spectrometer (Ionicon,
Austria) from the rooftop of the PML building in Plymouth (∼ 6 km north/northeast of PPAO) in 2012 (Yang et al., 2013) and in 2015.
Recently calibrated transmission efficiencies from the manufacturer and
kinetic reaction rates from Zhao and Zhang (2004) were used to derive the
DMS mixing ratio (uncertainty ≤ 40 %). DMS levels in marine air from
the southwest and southeast were comparable during the 2015 measurement
period (21 April to 15 May). The mean DMS diel cycle from the marine sector
(Fig. 6) clearly shows an anticorrelation with shortwave irradiance,
implying daytime oxidation by OH (mostly to SO2). The diel amplitude in
DMS mixing ratio was ∼ 0.09 ppb. A day/night difference in
atmospheric DMS was also observed from the PML rooftop in June 2012 from
marine air (Yang et al., 2013).
The conversion efficiency from DMS to SO2 due to OH oxidation is about
70–90 % (Davis et al., 1999; Chen et al., 2000; Shon et al., 2001). At a
conversion efficiency of 80 %, 0.09 ppb of oxidized DMS would lead to
about 0.07 ppb of SO2 produced during the day. In comparison, over the
1.5 years of observations at PPAO the mean amplitude of the SO2 diel
cycle from the southwest was ∼ 0.06 ppb. This comparison is
qualitative because DMS was only measured during the spring/summer periods
and at a different location. Nevertheless, it appears that within the
measurement uncertainties DMS oxidation can account for the vast majority of
the observed SO2 diel cycle from the southwestern wind sector.
Removal of SO2 from the marine atmosphere
SO2 is mainly removed from the marine boundary layer via aqueous
oxidation (e.g., cloud processing), deposition to the ocean, and possibly
dilution by the free tropospheric air. We can approximate the total loss of
SO2 from the nighttime change in the averaged SO2 mixing ratio
(daytime SO2 oxidation is very slow). This calculation assumes no
temporal trend in any nocturnal ship emissions (on a diel timescale) and a
constant marine boundary layer height. In a polluted marine environment, a
small amount of SO2 could be formed at night via DMS oxidation by the
nitrate radical (NO3; Yvon et al., 1996), a process we neglect here. A
linear fit to the nighttime decrease of SO2 from the southwest in 2014
(Fig. 5a) yields a total loss rate of about 0.2 ppb per day. The average
SO2 mixing ratio was about 0.1 ppb in the evening hours, which implies
a SO2 residence time of approximately 0.5 d. In 2015, the total loss
rate was about 0.1 ppb per day, which also implies a SO2 residence time
of ∼ 0.5 d. This residence time is in close agreement with
previous estimates in the marine atmosphere (e.g., Cuong et al., 1975; Yang et al., 2011b).
Average diel variability in DMS mixing ratio and shortwave
irradiance, measured from the PML rooftop between April and May 2015 (wind
from the marine sector). Error bars indicate 2 standard errors.
Mean diel cycles in the wind speed-dependent SO2 deposition
flux, separated into the southeastern and southwestern wind sectors for year 2014
(a) and 2015 (b). Error bars indicate 2 standard errors. SO2
deposition flux was significantly greater in the southeastern sector than in
the southwestern sector in 2014.
Monthly means and 25th/75th percentiles of SO2
mixing ratio from southeastern and southwestern wind sectors. SO2 from the
southwest shows a clear seasonal cycle, with higher values in summer/early
autumn and lower values in winter/early spring. A similar underlying
seasonal variability is also apparent in SO2 from the southeastern sector.
We compute the dry deposition flux of SO2 to the surface ocean as
-Vd ⋅ [SO2], where [SO2] is the atmospheric SO2
concentration and Vd is the deposition velocity of SO2. Upon
contact with seawater, SO2 rapidly dissociates to form HSO3-
and then sulfite (Eigen et al., 1964). The effective solubility of SO2
in seawater (pH ∼ 8) due to this chemical enhancement is very
large (dimensionless water : air solubility of about 5e8), which means
that air–sea SO2 exchange should be gas phase controlled. Oxidation to
sulfate permanently removes sulfite from the surface ocean with a timescale
of minutes to hours (Schwartz, 1992). The combination of a low aqueous S(IV)
concentration (e.g., Campanella et al., 1995; Hayes et al., 2006) and a high
effective solubility results in a near-zero interfacial SO2
concentration that is in equilibrium with seawater (e.g., Liss and Slater,
1974). We compute the deposition velocity of SO2 using the COARE gas
transfer model (Fairall et al., 2011), which utilizes the air-side diffusivity
of SO2 from Johnson (2010) and the measured wind speed at PPAO. Diel
cycles in SO2 deposition flux (Fig. 7) resemble the mirror image of
SO2 mixing ratio. For the southwestern wind sector, the average SO2
deposition flux was about -1 to -2 µmole m-2 d-1.
For the southeastern sector, deposition flux averaged about -3 µmole m-2 d-1
in 2014 and -1 µmole m-2 d-1 in 2015.
Interestingly, while SO2 mixing ratio from the southeast was still
higher than from the southwest in 2015, deposition fluxes between the two
wind sectors were comparable. This is because wind speeds were typically
lower from the southeast. Overall, dry deposition removes SO2 from a
well-mixed, 1 km deep marine atmospheric boundary layer with a timescale of
∼ 2 days at PPAO. It accounted for approximately a quarter of
the total SO2 losses, similar to previous findings (e.g., Yang et al., 2011b).
Discussion
Long-term changes in SO2 and ship emissions
We evaluate the long-term trends in SO2 from different wind sectors in
order to understand seasonal variability and assess any changes due to ship
emissions. SO2 mixing ratios from the southeast and the southwest are
averaged into monthly intervals (Fig. 8). Mean SO2 from the southwest
in the summer months were comparable between 2014 and 2015. In contrast,
SO2 from the southeast was significantly lower in summer 2015 than in
summer 2014. For both wind sectors, lower SO2 mixing ratios were
observed in winter/early spring compared to summer/early autumn. The
seasonal cycle in SO2 from the southwest is consistent with the
variability in surface seawater DMS concentration previously measured at the
nearby L4 mooring station (∼ 6 nM in summer and
∼ 1 nM in winter; Archer et al., 2009). Using those seawater
DMS concentrations, local wind speeds and temperatures, and the DMS air–sea
transfer velocity from Yang et al. (2011a), we predict DMS fluxes on the
order of ∼ 10 and 3 µmole m-2 d-1 for the
summer and winter, respectively. These fluxes would be sufficient to account
for the observed SO2 burden from the southwest (∼ 0.10 ppb in summer and ∼ 0.02 ppb in winter) assuming a SO2
residence time of 0.5 day in a 1 km deep marine boundary layer
(∼ 8 and ∼ 2 µmole m-2 d-1 of
sulfur required). In addition to lower seawater DMS concentrations in
winter, less incoming irradiance and shorter daylight hours in those months
will also reduce the photochemical production of SO2 from atmospheric
DMS. Overall, we see that the seasonal cycle of SO2 in air from the
southwest can largely be explained by natural variability.
The difference between monthly averaged SO2 mixing ratios
from the southeast and southwest (ΔSO2), which we consider to
approximately represent ship emissions. ΔSO2 including and
excluding local ship plumes is shown. Error bars are propagated from 2
times the standard errors from each wind sector. Solid vertical line
indicates the 1 January 2015 mandate for reduction in ship SO2
emissions.
The difference in SO2 mixing ratio between the southeastern and
southwestern
sectors (ΔSO2) is shown in Fig. 9. There is a fair amount of
scatter in ΔSO2, which is partly because SO2 measurements
from the two wind sectors were not concurrent (i.e., winds could not be
blowing from the southeast and southwest at any single moment).
Nevertheless, mean (±standard error) ΔSO2 decreased from
∼ 0.15 (±0.03) ppb in 2014 to ∼ 0.05
(±0.01) ppb in 2015, with a sharp drop off coincident with the
1 January 2015 mandate for ship sulfur emission reduction.
We attribute ΔSO2 to ship sulfur emissions based on the
following assumptions: (I) SO2 from the southwest is from DMS oxidation
only; (II) SO2 from the southeast is affected by both DMS oxidation and
ship emissions; (III) DMS oxidation contributes equally to the SO2
burden in both the southeastern and southwestern wind sectors. Data from the
southwestern sector will almost certainly include some ship contributions but
the tightly constrained diel cycle in SO2 suggests this influence is
fairly small. Automatic Identification System maps
(http://www.marinetraffic.com/) indicate lower ship density and greater
distances between the shipping lanes and PPAO (i.e., lower SO2
emissions, increased plume dilution, and greater SO2 removal) in the
southwestern sector than in the southeastern sector. This information corroborates
the idea that ship emissions from the southwest only have a minor effect on
our observations.
Entrainment from the free troposphere could bring anthropogenic SO2
into the marine boundary layer (e.g., Simpson et al., 2014), which is not
accounted for here. We further assume negligible influence of terrestrial
SO2 emissions (e.g., from continental Europe) in the southeastern sector
because of atmospheric dilution and rapid removal of SO2 from the lower
atmosphere (residence time ∼ 0.5 d, see Sect. 3.4). The
English Channel near Plymouth has a width of approximately 200 km. At a
speed of 5 m s-1, southeasterly winds blow over the channel in
approximately half a day, comparable to the removal time.
Local ship plumes and fuel sulfur content
The SO2 signals from the southeast include local ship emissions (e.g.,
from ships entering/exiting Plymouth Sound) as well as more distant
emissions from the English Channel. Local emissions usually appear as sharp
spikes, while more distant emissions tend to have plumes that are broader
and less intense due to atmospheric dilution. We use concurrent peaks in
SO2 and CO2 to estimate the ships' apparent FSC. The FSC calculation assumes that all of the carbon and sulfur in fuel
is released into the atmosphere during combustion. We use the word
“apparent” here because our calculation reflects the downstream emissions
rather than the actual fuel composition. Ships that “scrub” sulfur from
stack emissions will have apparent FSC values that are lower than the actual
FSC. To minimize the uncertainty in our estimate, we focus
on well-resolved plumes from nearby ships only.
Simple dispersion modeling predicts that local ship plumes have a typical
duration of a few minutes. For example, von Glasow et al. (2003) estimated
the horizontal dispersion of a ship plume as
H=H0⋅(t/t0)0.75.
H0 and H are the horizontal extents of a plume at the initial time
t0 (1 s) and the time of interest t. The initial plume extent
(H0) is assumed to be 10 m. We estimate the horizontal dispersion of
plumes emitted from an upwind distance of 2 km (e.g., halfway across
Plymouth Sound) and 100 km (e.g., halfway across the English Channel). At a
speed of 8 m s-1, wind travels 2 and 100 km in ∼ 4 and
200 min, respectively. Applying these timescales to Eq. (1) yields
horizontal plume extents of 600 and 11 000 m. For a ship 2 km away that is
traversing perpendicular to the mean wind at a speed of 4 m s-1, its
emission should be observable at PPAO for 2.5 min. A ship 100 km away
would have a plume that is theoretically observable for nearly an hour.
These timescales are roughly consistent with our time series observations
(e.g., Fig. 3). Ships that do not travel perpendicular to the wind will
have plumes that are observable for longer periods. Faster wind speeds or
ship speeds shorten the duration of plume detection, and vice versa.
The following steps were used to separate local ship emissions from the
background or more distant emissions based on 1 min average gas mixing
ratios. All data processing was done with Igor Pro (WaveMetrics).
Apply 2 h running-median smoothing to the SO2 time series.
Identify “no plume” times as when the 1 min average SO2 was within
0.1 ppb (i.e., < twice the instrument noise) of the smoothed
SO2.
Subtract the linear interpolation of the “no plume” SO2 time series
from the 1 min average SO2 time series to derive the SO2
deviations from the background (SO2′).
Apply the “FindPeak” function to SO2′ to identify the time, height,
leading edge, and trailing edge of a peak within non-overlapping 10 min
windows. A minimum peak height of 0.2 ppb (i.e., > 3 times the
instrument noise) was required for positive peak identification.
Integrate SO2′ between the leading edge and the trailing edge to yield
the SO2 plume peak area.
Apparent fuel sulfur content (FSC) estimated from peak areas as
well as peak heights of coincidental SO2 and CO2 plumes (1 min
averaged data). Solid horizontal lines indicate IMO fuel sulfur
emission content limits in European waters (SECA) regions: 1 % SFC in
2014 and
0.1 % SFC in 2015.
The 10 min window in step 4 was chosen to minimize the occurrence of
multiple plumes within a single window but also allow for identifications of
plumes persisting for several minutes. A total of 816 distinct SO2
plumes were identified from May 2014 to November 2015 from the marine wind
sector. The mean SO2 plume height (1 min average) was 1.35 ppb in
2014 and 0.48 ppb in 2015, with a typical plume duration of ∼ 3 min.
CO2 plumes from local ship emissions were identified in an analogous
fashion based on an analysis of the 1 min average CO2 time series
(i.e., independent of the SO2 analysis). The “no plume” CO2
threshold (step 2 above) was set to be 0.2 ppm, and the minimum peak height
required for positive peak identification (step 4) was set to be 1.0 ppm.
Based on these schemes, a total number of 1242 separate CO2 plumes were
identified from May 2014 to November 2015, with a mean plume height of 2.6 ppm.
Apparent FSC is computed from coincidental SO2
and CO2 plume heights as well as plume areas (N=245) following
Kattner et al. (2015). To account for clock drift between the computers that
recorded the SO2 and CO2 data, we allowed the times of the
SO2 and CO2 peaks to differ by up to 1 min. The results of
these calculations for the marine wind sector are shown in Fig. 10. Gaps
in observations were largely due to the CO2 analyzer malfunctioning or
winds out of sector. The peak area and peak height methods yielded similar
results. FSC from peak area appears to be slightly more variable, possibly
due to a greater sensitivity toward the definition of plume baseline. Based
on peak height, in 2014 the mean (±standard error) FSC was 0.17
(±0.03) %, with ∼ 99 % of the plumes below the IMO
threshold of 1 % FSC. About 70 % of the plumes were already below
0.1 % FSC in 2014. In 2015, mean FSC decreased to 0.047 (±0.003) %, with ∼ 99 % of the plumes below the new IMO threshold
of 0.1 %. FSC estimated from the peak area method shows slightly lower
levels of compliance (∼ 95 % for both years). The reduction
in mean FSC from 2014 to 2015 is proportionally comparable to the decrease
in ΔSO2 computed in Sect. 4.1.
Our FSC estimates illustrate a fairly similar decreasing trend to that
observed near the Hamburg harbor (Kattner et al., 2015). The mean FSC value
at PPAO is approximately half of what was estimated by Kattner et al. (2015)
for the year 2014 and is comparable to their estimate for 2015. However, our
observations are different in several aspects. The vast majority of the
ships entering/leaving Plymouth Sound are naval, commercial ferries, and
private vessels according to the Devonport Naval Base Ship Movement Report.
The number of large container ships entering Plymouth Sound is
proportionally much lower than near Hamburg. Because the distances between
ships and PPAO were not fixed (as opposed to spatially restricted sampling
locations in Kattner et al., 2015, and Beecken et al., 2015), plumes observed
at our site were usually more dilute and variable in duration. This
variability made plume identification more challenging. We expect the bulk
of the uncertainty in FSC to be due to instrument noise in SO2, which
should be within 20 % for plumes in 2015 and better in 2014. Total
uncertainty in FSC may be higher though due to inexact plume baseline
quantification, uncertainty in the threshold required for SO2 spike
detection, and the small but variable time lag between the SO2 and
CO2 measurements. A higher SO2 spike threshold could bias our FSC
estimates towards plumes with greater sulfur content, while a lower
threshold would be too close to the noise level of the measurement.
Long-term records of another tracer (e.g., nitrogen oxides or black carbon)
would allow for a more independent identification of ship plumes for the
calculation of FSC. Finally, we reiterate that our FSC estimates are for
well-resolved peaks from local ship plumes, which do not necessarily reflect
ship emissions from the main shipping lanes of the English Channel.
Top-down estimates of SO2 emissions from the
English Channel
We use ΔSO2 with local ship emissions excluded (Fig. 9) to
estimate distant anthropogenic SO2 emissions (e.g., from the English
Channel). Without local spikes (i.e., the “no plume” SO2 time series in
Sect. 4.2), the computed mean SO2 mixing ratio from the southwest is
only slightly lower, as might be expected due to the relatively low ship
density in that direction; in contrast, mean SO2 mixing ratio from the
southeast is lowered by ∼ 0.04 ppb in 2014 and ∼ 0.01 ppb in 2015. Approximately a quarter of the SO2 attributed to ship
emissions in Sect. 4.1 (ΔSO2) was due to local ship plumes.
ΔSO2 excluding local ship emissions was ∼ 0.11
(±0.03) ppb in 2014 and ∼ 0.04 (±0.01) ppb in
2015. The largest differences in ΔSO2 with and without nearby
plumes occurred in the summer for both years, when the ship traffic was the
heaviest.
We make an order of magnitude estimate for the ship emissions in the English
Channel required to sustain the observed SO2 mixing ratios. This
calculation assumes a SO2 residence time of 0.5 d in a well-mixed, 1 km
deep marine atmospheric boundary layer. For this scenario it would take
∼ 9 µmole m-2 d-1 of SO2 from ships to
account for a ΔSO2 of 0.11 ppb and ∼ 3 µmole m-2 d-1 of SO2 emission from ships to account for a
ΔSO2 of 0.04 ppb. Simplistic extrapolation of these fluxes to
the area of the English Channel (about 75 000 km2) yields a total ship
SO2 emission of ∼ 16 Gg per year in 2014 and
∼ 5 Gg per year in 2015. Compared to previous inventory-based
SO2 emission estimates for the English Channel, our 2014 value is about
40 % of that reported by Jalkanen et al. (2016) for 2011 and about a
quarter of the Ship Emissions Inventory found in the UK Department for
Environment Food and Rural Affairs (DEFRA) report (Whall et al., 2010).
Our SO2 emission estimate is low in part because we have purposely
excluded local ship plumes. Also, ΔSO2 could be a slight
underestimate of ship emissions due to the assumption of zero ship influence
in the southwestern wind sector. More accurate constraints of sulfur emissions
from the English Channel from point measurements such as at PPAO probably
require modeling of air trajectory/dispersion, detailed information about
ship traffic, and knowledge of the SO2 source area (i.e., the
concentration footprint; Wilson and Swaters, 1991; Schmid, 1994). A more
complete description of the sulfur budget in this environment would also
require sulfate concentration measurements in aerosols as well as in
precipitation droplets.