Particulate matter with aerodynamic diameters lower than 10
The PM
The evolution of soluble V and Ni concentrations (typical markers of heavy
fuel oil combustion) was related to meteorology and ship traffic intensity
in the Straight of Sicily, using a high-resolution regional model for
calculation of back trajectories. Elevated concentration of V and Ni at Capo
Granitola and Lampedusa are found to correspond with air masses from the
Straight of Sicily and coincidences between trajectories and positions of large
ships; the vertical structure of the planetary boundary layer also appears
to play a role, with high V values associated with strong inversions and
a stable boundary layer. The V concentration was generally lower at Lampedusa
than at Capo Granitola V, where it reached a peak value of 40 ng m
Concentrations of rare earth elements (REEs), La and Ce in particular, were used to
identify possible contributions from refineries, whose emissions are also
characterized by elevated V and Ni amounts; refinery emissions are expected
to display high La
Based on the sampled aerosols, ratios of the main aerosol species arising
from ship emission with respect to V were estimated with the aim of deriving
a lower limit for the total ship contribution to PM
Ship emissions may significantly affect atmospheric concentrations of
several important pollutants, especially in maritime and coastal areas
(e.g. Endresen et al., 2003). Main emitted compounds are carbon dioxide (CO
Heavy oil fuels used by ships contain varying transition metals. The aerosol
emitted by ship engines is formed at high temperatures (
In spite of the large amount of gas and particulate emitted by ships, maritime transport is relatively clean if calculated per kilogram of transported good. However, maritime transport has been increasing with respect to air and road transport (Micco and Pérez, 2001; Grewal and Haugstetter, 2007). In addition, emissions from other transport sectors are decreasing due to the implementation of advanced emission reduction technologies, and the relative impact of shipping emissions is increasing.
Regulations aiming at reducing emissions based on restrictions on the fuel sulfur content (sulfur emission control areas, or SECAs) have been implemented in several regions. Although the legislation is focussed on sulfur emissions, the overall health and environmental effects depend in a complex way on the physical and chemical properties of the emissions (WHO, 2013). Several studies have been carried out to determine the detailed chemical composition of shipping emissions (Agrawal et al., 2008a, b; Moldanová et al., 2009; Murphy et al., 2009; Lyyränen et al., 1999; Cooper, 2003; Sippula et al., 2014); however, the ships emissions are still poorly characterized with respect to on-road vehicles.
A large variety of anthropic sources (refineries, power plants intense ship traffic, etc.) and natural emissions make the Mediterranean region one of the most polluted in the world (e.g. Kouvarakis et al., 2000; Marmer and Langmann, 2005). The multiplicity of Mediterranean sources (some of which with the same markers of ship aerosol) makes the quantification of ship contribution to the total aerosol amount difficult (e.g. Becagli et al., 2012).
The contribution of ships and harbour emissions to local air quality, with specific focus on atmospheric aerosol, has been investigated using models (Trozzi et al., 1995; Gariazzo et al., 2007; Eyring et al., 2005; Marmer et al., 2009), experimental analyses at high temporal resolution (Ault et al., 2010; Contini et al., 2011; Jonsson et al., 2011; Diesch et al., 2013; Donateo et al., 2014), receptor models based on the identification of chemical tracers associated with ship emissions (Viana et al., 2009; Pandolfi et al., 2011; Cesari et al., 2014) and integrated approaches with receptor and chemical transport models (Bove et al., 2014). Few studies exist in open sea (Becagli et al., 2012; Schembari et al., 2014; Bove et al., 2016).
In this context, studies performed at Mediterranean sites where it is possible to distinguish ship emission from other sources of heavy fuel oil combustion, are important to investigate the current impact of the ship emissions on primary and secondary aerosols. This study contributes to the identification and characterization of the emissions from ships and the impact on the aerosol distribution in the central Mediterranean. The experiment was set up with the aim of unambiguously recognizing the ship source by a combination of methods.
In a previous study (Becagli et al., 2012), we used measurements of PM
Map of the study area with the sites of Lampedusa (LMP) and Capo Granitola (CGR) are displayed in the left panel. A–C indicate the three sites selected to study the stability of the boundary layer in the Straight of Sicily (see Sect. 3.2.2). The ship routes in the study area during the first 10 days of June 2013 are displayed in the right panel. Red and blue dots show the routes of merchant and fishing vessels, respectively.
PM
Lampedusa is a small island in the Central Mediterranean sea, more than 100 km far from the nearest Tunisian coast. At the station for climate Observations, which is located on a plateau 45 m a.s.l on the northeastern coast of Lampedusa, continuous observations of aerosol properties (di Sarra et al., 2011, 2015; Becagli et al., 2013; Marconi et al., 2014; Calzolai et al., 2015; Sellitto et al., 2017), aerosol radiative effects (e.g. Casasanta et al., 2011; di Sarra et al., 2011; Meloni et al., 2015) and other climatic parameters are carried out. Figure 1 shows the map of the central Mediterranean with the measurement stations.
PM
Sampling strategy and chemical measurements carried out on each filter for the two sites: Lampedusa (LMP) and Capo Granitola (CGR). The sampling time interval is at local time (LT).
The two channels operated in parallel and were loaded with different types
of filters: the first one with 47 mm diameter, 2
The sampling site at CGR is located at Torretta Granitola, a Research Center of the Italian National Research Council, in southwestern Sicily (12 km from Mazara del Vallo). The sampler was installed on the roof of one of the research centre buildings at about 20 m a.s.l., directly on the coastline, facing the Straight of Sicily.
At CGR PM
The PM
A quarter of each Teflon filter from LMP and a 1.5 cm
Blank values were negligible with respect to the concentration in the samples for Teflon filters. Blank values for quartz filters were negligible for most of the analysed species. For some species characterized by high blank values, always lower than the 25th percentile value, they were subtracted from the measured concentrations.
Another quarter of the Teflon filter from LMP and another 1.5 cm
The remaining half Teflon filter from Lampedusa, another punch of the quartz
filter from CGR, was used for the determination of metals by ICP-AES through
the solubilisation procedure reported in the EU EN14902 (2005) rule, by
concentrating sub-boiling distilled HNO
The overall aerosol sampling and analytical strategy for the two sites are reported in Table 1.
Numerical simulations with a non-hydrostatic mesoscale atmospheric model
were used to characterize the meteorological conditions in the Straight of Sicily
during the campaign and to support the interpretation of the
experimental results. The Weather Research and Forecasting (WRF) model
(Skamarock et al., 2008) outputs, provided by the Department of Physics of
the University of Genoa, Italy, were used, covering the entire Mediterranean
with a grid spacing of 10 km and hourly temporal resolution. Initial and
boundary conditions to drive WRF simulations were obtained from the Global
Forecast System operational global model (Environmental Modeling Center,
2003) outputs (0.5
In particular, the WRF 3-D hourly meteorological fields were used to calculate backward trajectories with the NOAA HYbrid Single-Particle Lagrangian Integrated Trajectory Model (HYSPLIT; Stein et al., 2015). The trajectories were used to assess the origin of the air masses impacting the monitoring sites and to support the source attribution suggested by the analysis of specific markers (see Sect. 3.2.2). The use of a high-resolution regional atmospheric model for trajectory calculations allows for a better representation of boundary layer properties and mesoscale phenomena such as land and sea breezes, which can have a relevant impact especially in complex topography coastal sites like CGR.
Also, the high temporal resolution of meteorological data (hourly instead of three- or six-hourly products typically available from global models) permits a better description of diurnal cycles and a more accurate trajectory computation without time interpolation between subsequent atmospheric fields (Solomos et al., 2015).
Specifically, 48 h-long back trajectories arriving at LMP and CGR were computed from a reference height of 10 m above the ground level, starting every six hours for the whole period of the campaign, from 10 June to 31 July 2013.
Position and main characteristics of the ships travelling in the central
Mediterranean were derived from the MarineTraffic database
(
Three classes of ships defined by the AIS classification were considered:
all the ships, the merchant ships (i.e. cargo and tanker) and the fishing
vessels. Merchant and fishing vessels are the most frequent ships in the
Straight of Sicily; merchant ships are expected to produce the highest impact
due to their higher emissions (
Time series of the main aerosol components at LMP
Scatter plot of OC vs. EC at LMP
Mean and standard deviation of PM
The sea salt aerosol (SSA) component of PM
Figure 2 shows the time series of the main PM
Average concentrations of PM
The largest PM
SSA accounted for about 26 and 24 % of PM
Nitrate concentrations, although relatively high at both sites, are in agreement with the long term measurements performed at Lampedusa (e.g. Calzolai et al., 2015) and with data from other remote sites in the western (Mallorca; e.g. Simo et al., 1991) and eastern Mediterranean (Finokalia; e.g. Mihalopoulos et al., 1997).
Organic aerosol (OA) was the most abundant component at CGR, where its mean
concentration was
Elemental carbon and organic carbon show higher values at CGR than LMP. At
CGR, moderate and elevated values of OC and EC appear correlated
(
Thus, we used a conversion factor of 1.8 (typical for urban background sites, Turpin and Lim, 2001) at CGR, and of 2.1 (typical for remote sites characterized by the high impact of secondary sources, Turpin and Lim, 2001) at LMP to estimate the total organic aerosol amount from the OC measured values. Once we estimated OA with this method, the sum of the various species accounted for more than 85 % of the measured mass at both sites. The unreconstructed mass could be due to an underestimation of OA from OC, or to the presence of bound water not removed by the desiccation procedure at 50 % relative humidity (Tsyro, 2005; Canepari et al., 2013).
Several studies focussed on the identification of shipping emission specific tracers (Viana et al., 2008; Becagli et al., 2012; Isakson et al., 2001; Hellebust et al., 2010). Vanadium and Nickel are generally considered the best markers for this source because, after sulfur, they are the main impurities in heavy fuel oil (Agrawal et al., 2008a, b). The soluble fraction of these metals is even more representative for ship sources (Becagli et al., 2012).
Following Becagli et al., (2012), we used measurements of the V and Ni
soluble fractions (V
Table 3 reports slope, correlation coefficient and number of samples of the
linear correlation between V
Correlation parameters between V and Ni at LMP and CGR calculated for all the samples, and for samples with V concentration higher than 6 ng m
Time series of LCR and LVR at
V
A combination of methods is thus used in this study to unequivocally identify the ship source. The analysis is based on: additional chemical tracers, such as the rare earth elements, whose behaviour is specific for the refinery and the ship sources; high-resolution back-trajectories, based on data from the high-resolution regional model; information on the vertical mixing in the atmospheric boundary layer; and coincidences between the high-resolution back-trajectories and the position of different types of ships in the Straight of Sicily.
As discussed above, anthropogenic V and Ni originate from heavy oil combustion and may only be considered markers of the ship source when other sources can be excluded. Few studies propose the use of lanthanoid elements (La to Lu) to distinguish refinery from ship emissions (Moreno et al., 2008a, b; Du and Turner, 2015; Kulkarni et al., 2006).
In particular, the ratio between the La and Ce concentrations (La
Crustal aerosols are characterized by LCR ranging from 0.4 to 0.6 and LVR
usually in the range of 0.2–0.3 (Moreno et al., 2008a, b). LCR depends
weakly on differences in dust source area and collected aerosol size
fraction, contrarily to LVR, which reaches 0.9 for large (
Elevated values of LCR (from 1 to 13) are associated with emissions from refineries (Moreno et al., 2008a; Du and Turner, 2015). This is because zeolitic fluidised-bed catalytic cracking (FCC) units enriched in La are used to crack long-chain olefins in crude oil to shorter-chain products (Bozlaker et al., 2013; Du and Turner, 2015; Kulkani et al., 2006; Moreno et al., 2008a, b).
Mixing of aerosol from different sources may produce a large variability of LCR, with larger values corresponding to a stronger impact from refineries.
The time series of LCR and LVR at LMP and CGR are displayed in Fig. 4. The range of values expected for crustal aerosol is highlighted in the figure. Please, note that the uncertainty on LCR is very large when La and Ce concentrations are close to the detection limit. These cases may produce very large values of LCR which are not significant; and were removed from the time series.
LCR at LMP and CGR was generally around the value expected for crustal
aerosol (dashed grey area in Fig. 4); 10 samples from LMP and 2 samples
from CGR show values of LCR higher than 1. LCR is
Moreno et al., (2008b) have shown that it is possible to identify aerosol from refineries based on the V-La-Ce-three-component plot. This type of plot is shown in Fig. 5 for the data from LMP and CGR. La and Ce were scaled in order to have the typical UCC composition in the centre of the plot.
Three-component Ce-La-V plot for LMP and CGR. Literature data for different aerosol types are also shown.
The compositions of UCC (Henderson and Henderson, 2009), African desert dust
(Castillo et al., 2008; Moreno et al., 2006), FCC (Kulkarni et al.,
2006), La-contaminated (refinery) Asian dust collected at Mauna Loa, Hawai'i (Olmez
and Gordon, 1985), and PM
The data from CGR and LMP are grouped in a region with elevated values of V, and La and Ce generally lower than in refinery and dust cases.
Data from Puertollano shown in Fig. 5 are relative to days characterized by winds originating from sectors where refineries are located; however, these samples are affected by a mix of particles from several sources, including refineries. Aerosol samples from Spain affected by refineries, in most cases display larger LCR and LVR ratios than those found at LMP and CGR. The composition of all samples collected in this period at LMP is consistent with a large impact from ship emissions. Some cases at CGR may suggest the simultaneous occurrence of crustal and ship aerosols, or dominant crustal components (orange open dots in Fig. 5). Therefore, these cases display a relatively low V concentration and are mainly associated with the mistral event. A limited crustal contamination may possibly occur at CGR in these cases, due to resuspension due to the strong wind.
Cases with LCR
All the trajectories arriving at LMP and CGR, calculated with the HYSPLIT model driven by WRF meteorological fields (see Sect. 2.3), are shown in an aggregated way in Fig. 6, where the trajectory frequency at each point of the computing grid is shown for the whole period (upper panels) and for the 10–30 June interval (lower panels). The trajectory frequency pattern is elongated in the NW–SE direction at LMP, while it is distributed over a wider range of directions at CGR, despite a general prevalence of northerly sectors. The predominance of air masses coming from the northwest is particularly evident in June (Fig. 6c and d), when areas with trajectory frequencies exceeding 10 % are found farther to the north, up to the Gulf of Lion.
During the first part of the campaign (June 2013) the synoptic situation was characterized by a “dipolar” sea level pressure anomaly pattern, with positive anomalies in the western Mediterranean and negative ones in the eastern part of the basin (Denjean et al., 2016). This situation induced stronger and more frequent than usual northwesterly winds (i.e. mistral episodes, see Sect. 3.1) over Sardinia and Straight of Sicily.
To further investigate the mechanisms determining the presence of ship emissions markers at the two sites, we investigated the relationships among the amount of V, the back-trajectory pattern, the effective number of ships influencing the air mass, the stability of the boundary layer in the ship source region (i.e. the Straight of Sicily) and the REE to V ratios discussed in Sect. 3.1.2.
All back-trajectories arriving at LMP and CGR were considered and all trajectory-ship coincidences occurring within the last 36 hours before sampling were taken into account.
It was assumed that the ship plume influenced the sampled air mass if:
the trajectory passed within 15 km of the position of a ship; the corresponding air mass altitude was less than 500 m.
The total number of ships fulfilling these criteria was associated with each
trajectory. The analysis was based on the available 1 h time resolution
meteorological fields (a ship influencing a trajectory was counted once every hour).
To further explore the impact of different types of ships, the analysis was carried out considering the following three ship categories: all the ships, the merchant (i.e. cargo and tanker) and the fishing vessels.
Trajectory frequency computed at each grid cell with starting points
at LMP
Time series of Vanadium concentration (black line with dots) and number
of ships affecting the air masses sampled at CGR (upper panel) and LMP (lower
panel). Green, red and blue lines indicate, respectively, the total number of
ships and the number of merchant (i.e. cargo and tanker) and fishing vessels.
The time evolution of the temperature inversion index (d
The atmospheric stability is also expected to play a large role in
modulating the ship impact (for an example of its influence on V amounts, see Becagli et
al., 2012). A temperature inversion (TI) index was calculated based on the
3-D atmospheric fields of the WRF model at three sites in the Straight of Sicily.
The temperature inversions were used as a proxy to identify periods
characterized by a stable boundary layer. The three sites, A (37.2
Figure 7 summarizes the results of this analysis. It shows the time
series of the number of the ships influencing the trajectories arriving at LMP and
CGR, respectively, and the corresponding measured values of V. Samples which
show a limited influence from ship emissions, determined on the basis of the
La-Ce-V composition (see Sect. 3.1.2), are highlighted with arrows (orange
arrows for samples with La-Ce-V ratios typical for crust; pink and gray for
sample possibly influenced by refineries, i.e. with LCR
In general, there is a rather good correspondence between the cases classified as influenced by ships emissions and the number of ships encountered along the associated air mass trajectory at CGR. The correspondence is somewhat less evident at LMP. As discussed above, the V concentration ascribed to ships (data points without arrows in Fig. 7) is generally higher at CGR than at LMP. Part of this difference may be ascribed to the shorter distance between CGR and the main shipping route crossing the Straight of Sicily with respect to LMP, the consequent larger number of encountered ships and an aerosol dilution effect during transport from the sources to LMP.
Maxima of V attributed to ships occurred between 19 and 20 June at CGR
(about 42 ng m
A possible explanation for this behaviour is provided by the temporal evolution of TI in the Straight of Sicily. The temperature inversion started to develop on 14 June and gradually increased in intensity until 22 June; the TI persistence and progressive increase in intensity provided suitable conditions for the ship plumes being trapped in the boundary layer, with a consequent build-up of the ship aerosol and V concentration. This process appears particularly efficient at CGR between 21 and 25 June.
A similar combined dependency on number of ships and TI appears also at LMP around 7 July. It is interesting to note that V from ships seems to depend more directly on the number of merchant ships (see, for example, the lack of V peaks on 17 June, 12 and 29 July at LMP, when the number of fishing vessels was high and the number of merchant ships was low) than on the total or the number of fishing ships.
Thus, the trajectory analysis, carried out in combination with the available information on the ship tracks, confirms that ship emissions are the main factors responsible for most of the moderate and elevated values of V measured at LMP and CGR during the campaign and in particular for those cases with LCR compatible with the ship source. This analysis also clearly suggests that the boundary layer structure plays a very important role in determining the impact produced by the emissions. This simplified approach confirms the importance of carefully characterizing the emission scenario and the meteorological conditions in studies on the ships' emissions impact on air quality.
SO
A previous study based on five years of data from Lampedusa (Becagli et al.,
2012) has shown that the non-sea salt sulfate behaviour is not directly
correlated with V and Ni because several other SO
The same study suggested a lower limit of about 200 for the
nssSO
Figure 8a and 8b shows nssSO
The nssSO
The calculated lower limit of the sulfate to V ratio at LMP is 207, in
agreement with the values of 200 estimated by Becagli et al., (2012). The
nssSO
NO
Here we try to use the same approach used for sulfate for the determination
of a lower limit for the NO
Scatter plots of nssSO
Figure 8c and d show the NO
Elemental and Organic Carbon are also present in the ship plume (Shah et al., 2004). In particular, OC constitutes about 15–25 % and EC is generally lower than 1 % of the PM sampled at the plume of main ship engine powered by heavy fuel oil (Agrawal et al., 2008b).
Estimates of the average and maximum of the lower limit of nssSO
Figure 8 shows EC
The pattern of the ratio EC
Finally, as the limit ratios at CGR are likely affected by other sources
than shipping, we assume that the limit ratios obtained at Lampedusa for
V
With all the limitations described above, by using the lower limits for the
ratios nssSO
The minimum ratio of each species with respect to V and the minimum estimated
contribution of ship emissions, for the average amount and for the maxima,
of the total concentration of these species and of PM
The estimated minimum concentration of non-sea-salt sulfate from ship
emissions was 1.35
At CGR the minimum ship contribution to sulfate, averaged over the same time
period, is higher than at LMP (2.1
Marmer and Langmann (2005) estimate that ship emissions contribute by 50 %
to the total amount of nssSO
The estimated minimum contribution by ships to the total nssSO
Ships appear to contribute, by small fractions, to the total budget of
NO
Organic aerosol from ships also contributes significantly to the total OA amount and to the total PM; in particular, at LMP virtually all the OA present in cases with maximum ship impact may be attributed to the ship source.
By summing these three contributions, it is possible to estimate the total
aerosol mass due to ship emissions and its contribution to the total mass
of PM
These percent contributions are higher than the annual average for the Mediterranean region estimated by Viana et al., (2014). It has to be considered that these authors used data from harbour or coastal sites, which are highly affected by other sources in addition to ships, and where gas-to-particle conversion is still at its initial phase. Moreover, the percentages reported in this study are relative to the summer season, when the ship contribution in the Mediterranean region is highest (Becagli et al., 2012).
The estimated lower limit for the ship contribution in cases with maximum
ship impact was between 42 and 50 % of the total PM
In this study we have investigated the impact of the ship emissions to PM
The PM
The identification of aerosol originating from ships was based on an integrated analysis combining chemical analyses, calculations of backward trajectories using a high resolution regional model and on tracking of ship traffic in the Mediterranean through the Automatic Identification System.
The main results of this study may be summarized as follows:
Moderate and elevated values of V and Ni in the aerosol were unambiguously
associated with the ship source; this attribution was based on:
the V to Ni ratio, which corresponds to what is expected for heavy fuel oil
combustion; low amounts of La and Ce with respect to V and La coincidences between air mass trajectories and travelling ships. In addition to travelling ships, also the planetary boundary layer vertical
structure played an important role in determining the dispersion of aerosols
from the ship source; temperature inversions appeared associated with
elevated amounts of ship emissions tracers, suggesting that they favoured
the build-up of aerosol concentration in the lowest atmospheric layers. As expected, merchant ships (cargo and tankers) appeared to produce a larger
impact on the measured aerosol than fishing vessels. Lower limits for the ratios nssSO By using these ratios, the lower limits to the contribution of the ship
source to nssSO Lampedusa is a small island in the southern sector of the central
Mediterranean, relatively far from the main Mediterranean shipping route;
thus, results at Lampedusa may be taken as representative of the impact of
ships on the aerosol properties in a wide open sea area in the central
Mediterranean during summer.
All the data presented in this paper are available upon request. Please contact the corresponding author (silvia.becagli@unifi.it).
The authors declare that they have no conflict of interest.
Measurements at Lampedusa were partly supported by the Italian Ministry for University and Research through the NextData and Ritmare projects.
We thank the Institute for Coastal Marine Environment of the National Research
Council (IAMC-CNR) for hosting the instruments at Capo Granitola. Thanks are
due to MarineTraffic (