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Characterization of aerosols above the Northern Adriatic Sea: Case studies of offshore and onshore wind conditions
Atmospheric Environment 132 (2016) 153e162
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Characterization of aerosols above the Northern Adriatic Sea: Case studies of offshore and onshore wind conditions J. Piazzola a, *, N. Mihalopoulos b, E. Canepa c, G. Tedeschi a, P. Prati d, P. Zarmpas b, M. Bastianini e, T. Missamou a, L. Cavaleri e a
University of Toulon, Mediterranean Institute of Oceanography, MIO-UM 110, France University of Crete, Chemistry Department, Greece CNR-ISMAR, Genova, Italy d University of Genova, Department of Physics and INFN, Italy e CNR-ISMAR, Venice, Italy b c
h i g h l i g h t s This paper is a contribution of the study of coastal aerosol properties over the Mediterranean. We study the sea-spray and the anthropogenic contributions in the aerosol concentrations measured in the Mediterranean coast. One of the objectives is to implement an accurate source term for the sea-spray aerosols in the aerosol transport model.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 November 2015 Received in revised form 9 February 2016 Accepted 26 February 2016 Available online 2 March 2016
Aerosol particles in coastal areas result from a complex mixing between sea spray aerosols locally generated at the sea surface by the wind-waves interaction processes and a continental component resulting from natural and/or anthropogenic sources. This paper presents a physical and chemical analysis of the aerosol data acquired from May to September 2014 in the Adriatic Sea. Aerosol distributions were measured on the Acqua Alta platform located 15 km off the coast of Venice using two Particle Measuring System probes and a chemical characterization was made using an Ion Chromatography analysis (IC). Our aim is to study both the sea-spray contribution and the anthropogenic influence in the coastal aerosol of this Mediterranean region. To this end, we focus on a comparison between the present data and the aerosol size distributions measured south of the French Mediterranean coast. For air masses of marine origin transported by southern winds on the French coast and by the Sirocco in the Adriatic, we note a good agreement between the concentrations of super-micrometer aerosols measured in the two locations. This indicates a similar sea surface production of sea-spray aerosols formed by bubble bursting processes in the two locations. In contrast, the results show larger concentrations of submicron particles in the North-Western Mediterranean compared to the Adriatic, which result probably from a larger anthropogenic background for marine conditions. In contrast, for a coastal influence, the chemical analysis presented in the present paper seems to indicate a larger importance of the anthropogenic impact in the Northern Adriatic compared to the North-Western Mediterranean. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Coastal aerosols Anthropogenic compounds Atmospheric transport
1. Introduction The estimation of the atmospheric aerosol impact on climate remains an important scientific challenge (Intergovernmental Panel on Climate Change (IPCC)). In particular, in contrast with the
* Corresponding author. E-mail address: [email protected] (J. Piazzola). http://dx.doi.org/10.1016/j.atmosenv.2016.02.044 1352-2310/© 2016 Elsevier Ltd. All rights reserved.
radiative forcing attributed to greenhouse gases, the uncertainties related to aerosol radiative forcing remain very large (IPCC, 2013). This is due to their heterogeneous spatial and temporal distribution, their different origins (natural and anthropogenic) and their physical and chemical behaviour in the free troposphere. In coastal areas, aerosols may be of either natural or anthropogenic origin. The sea-spray aerosols represent a major contribution to the coastal aerosol mass (Yoon et al., 2007; Piazzola et al., 2009). Sea-spray
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particles result from the primary production through breaking waves (e.g., Monahan et al., 1986), which provide aerosols of diameters 0.5 mm. In addition, a generation of smaller particles results from secondary production due to gas-particle conversion (e.g., Fitzgerald, 1991). Sea-spray is made primarily of sodium chloride (NaCl) and small amounts of other salts such as sulfate, calcium and potassium, but it can also contain significant amounts of organic carbon. The organic component was mainly attributed to bubble bursting processes, due to the predominantly insoluble and surface active character of organic carbon in the marine aerosol particles (Ceburnis et al., 2008). Although it is well-recognized that marine aerosols have a significant influence on the coastal urban air quality through their ability to have chemical and physical interactions with gases and other aerosol species (Knipping and Dabdub, 2003), their interactions with anthropogenic pollutants in coastal areas are still largely unknown. In this context, we need first to obtain a good knowledge of the sea-spray generation processes and its atmospheric transport. To this end, Tedeschi and Piazzola (2011) developed the Marine Aerosol Concentration Model (MACMod) dedicated to the atmospheric transport of marine aerosols in the Marine Atmospheric Boundary Layer (MABL). Such a transport model needs to have accurate input conditions, as the source terms characteristics of the sea-spray production. However, uncertainties on the sea-spray source function (hereinafter S3F) are still large (De Leeuw et al., 2011). Demoisson et al. (2013, hereinafter, D13) recently proposed a new formulation of the S3F characteristics of the North-Western Mediterranean using the aerosol size distributions measured at different locations south of the French coast on board the ship Atalante in May 2008. The implementation of this latter sea-spray formulation in MACMod contributed to increase its performance for the Mediterranean (D13). One of the questions we need to reply is to what extent this formulation can be used in other Mediterranean areas. In addition, a better knowledge is required on atmospheric interactions between the sea-spray and other atmospheric compounds. Physicochemical analysis of the aerosol in different Mediterranean areas is then needed. The aim of this paper is to study the sea-spray contribution and the anthropogenic influence in the coastal aerosol measured in the Mediterranean. To this end, we used the aerosol size distributions acquired in the Northern Adriatic between May and September 2014 on board the Acqua Alta platform located near the coast of Venice. The results are compared to the aerosol size distributions recorded on board the ship Atalante south of the French coast in May 2008 that were used for the implementation of the S3F proposed by D13 for the Mediterranean. In addition, to evaluate the anthropogenic influence in the aerosols measured in the Northern Adriatic, we present a chemical characterization of aerosols concentrations. The composition of aerosols in the study area is also then compared to measurements made in May 2007 on the island of Porquerolles located south of the French coast (Piazzola et al., 2012; hereinafter P12). In Section 2, we describe the field sites of the present study, while Section 3 deals with the variation of the aerosol size distributions. Section 4 presents the chemical analysis, which allows for a good appreciation of the degree of anthropogenicity of the Western Mediterranean. 2. Field sites and experiments and general characterization of the areas Our aim is to study the aerosol properties in the Northern Adriatic and compare the results to the data obtained south of the French Mediterranean coast. Alternatively, these two coastal regions are under influence of continental and marine air masses depending on the wind direction. The variation in wind direction
can also be accompanied by changes of the fetch, which represents the distance over water for which the wind has blown constantly. Continental winds may then blow over the sea, giving the transported air masses a mixed character (defined as “coastal”). For our analysis, we have selected episodes characteristics of coastal and marine influence, i.e., short and large fetches, as found in coastal Mediterranean areas. Large fetches deal with sea conditions for which the waves tend to be fully developed. For the wind speed conditions analyzed in Section 3, this corresponds to fetches larger than 150 km, following the criterion proposed by (Hsu, 1986). 2.1. The Acqua Alta field site Measurements in the Northern Adriatic took place on the Acqua Alta platform, which is an oceanographic tower (Fig. 1a) located in the Northern Adriatic Sea, 15 km off the coast of the Venice lagoon (Fig. 1b). The tower, which has been operational since the early 70s, is managed by the Institute of Marine Sciences - National Research Council (ISMAR-CNR) and commonly hosts a large variety of instruments. The main structure is a quadratic template jacket steel structure with four 0.60-m diameter main legs fixed to inserted steel pipes driven 22 m into the sea bed (Cavaleri, 2000). The aerosol probes (see below) were fixed on the second floor, at 7 m above marine surface level, on which there is a 5.5 4.0 m platform. These were connected to power generators and to an indoor room for data acquisition. 2.2. Sampling and analytical procedures During the experimental campaigns conducted in the Northern Adriatic and in the North-Western Mediterranean, the aerosol data were acquired in the 0.1e45 mm size ranges using two particle measuring systems (PMS): one active scattering spectrometer probe (ASASP) and one classical scattering spectrometer probe (CSASP). The data were stored as the average over a 4-min interval. Prior to the experiments, the probes had been calibrated with latex particles of known sizes. It should be noted that a minimum of a factor of 3 is expected between the aerosol concentration simultaneously measured using two probes of the same type at the same location (Reid et al., 2006). To ensure a rather accurate comparison between the two sites (see Section 3), we used the same probes (not only the same model but the same devices) for the two experimental campaigns in order to have the same measurement error. However, the absolute concentration registered by a PMS probe may change over time and after servicing due to decreasing laser power or contamination of optics (Reid et al., 2006). The probes were then checked in the laboratory at the end of each series of experiments. The differences noted for each probe before and after the experiment were less than 15% for aerosol diameters smaller than 10 mm. This can be assumed as accurate since our aim was to study more specifically the sea-spray aerosols issued from bubble-bursting processes which deal, for a larger part, with particles of sizes smaller than 10 mm. Prior to the plots, all the aerosol size distributions were normalized to a relative humidity of 80%. The experimental campaign in the Northern Adriatic consisted of three intensive measurement periods of 7e10 days between May and September. Among them, we have selected a number of ensembles of measurements performed during windy period of nearly constant direction (throughout the paper we use incoming direction). Each aerosol spectrum reported in Section 3 is an average of four to six aerosol size distributions. For chemical characterization reported in Section 4, aerosols were sampled with a low pressure cascade impactor (Dekati) from 14.45 of 18th of June to 24.00 of 19th of June 2014 (about 33 h) and from 14.30 of 23rd of September to 12.00 25th of September 2014
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Dekati lps response function. 2.3. Campaign overview in the Adriatic The data were acquired during three experimental campaigns of seven to ten days made between May and September 2014. The study focuses on two particular types of meteorological conditions that often occur in the study area, and which should yield different aerosol characteristics (see Section 3 and Section 4). As outlined above, the wind direction is the first parameter to consider in our analysis, since changes in wind direction may result in transport from different aerosol source regions. In addition, the wind speed is an important parameter in marine areas for its role in the production of sea-spray aerosols and dispersion of the continental ones in marine atmosphere (e.g., Piazzola and Despiau, 1997a). First, we study typical coastal conditions that are characterized by offshore winds (which travel from land to the sea) and air masses coming from the north of Italy. Under these conditions, we expect a strong continental and/or anthropogenic background with a possible mixing with marine aerosols if white-capping occurs along the fetch. The selected episodes for our analysis correspond to periods for which the wind has blown long enough in a constant direction at a given wind speed (and hence at a given fetch). This methodology ensures that the measured aerosol size distributions are rather representative of one specific meteorological period. As an example, Fig. 2a and Fig. 2b show the numerical calculations of the air mass back trajectories, using the hysplit model (Draxler and Rolph, 2003), typical of coastal conditions in the study area induced by an offshore wind. In addition, this study will also focus on the aerosol characteristics observed during southern winds, which correspond to marine air mass conditions with Sirocco winds coming from the southern Mediterranean Sea. These conditions generally correspond to large fetches for which the waves tend to be fully developed, as noted above. Most of the time however, for southern wind direction in the coast of Venice, a part of the air mass trajectory may have crossed land a few days before arriving in the study area, as shown in Fig. 3a, where we show the back trajectory analysis for the episode analyzed in Section 3. In a few cases, a southern wind can also transport air masses originally coming from the European continent, as shown in Fig. 3b which correspond to one of the episodes used for the chemical characterization reported in Section 4. 2.4. Meteorological characteristics of the French Mediterranean coast
Fig. 1. Field site: (a) the Acqua Alta platform and (b) the study area in the Adriatic Sea with the platform location (square). V denote the city of Venice.
(about 46 h). The impactor was sampling directly in the atmosphere and was a 20 lpm 13-stages low pressure cascade impactor which cut-off aerodynamic diameters were 0.03, 0.06, 0.108, 0.17, 0.26, 0.4, 0.65, 1, 1.6, 2.5, 4.4, 6.8 and 9.97 mm. The collection plates are custom-made out of aluminium foil for subsequent Ion Chromatography (IC) analysis. No data reduction has been performed on the cascade impactor data and the size distributions presented in the paper are just based on raw data without any correction for the
One of the aims of this paper is to provide a comparison between the aerosol properties measured in the Northern Adriatic and those observed in another Mediterranean area south of the French coast (P12 and D13 for instance). In Section 3, the present data are compared to aerosol size distributions measured during an experimental campaign which took place in May 2008 on board the ship Atalante during the experimental campaign MIRAMER (D13). Measurements of aerosol concentrations have been conducted from May 15 to 28, south of the Toulon bay in the French Mediterranean, between 5.4 and 6.25 east longitude and between 42.2 and 43.2 north latitude. During the campaign, the Atalante cruised at different distances, between 10 and 100 km south from the coast of Toulon. The study area then roughly deals with a limited geographic space located south of the Toulon bay, as indicated in Fig. 4a where is reported the trajectory of the ship during the campaign. The ship regularly stopped to allow acquisition of aerosol concentrations. In Section 4, the chemical analysis made on board the Aqua Alta platform is compared to the data reported by P12 on the island of Porquerolles in May 2007. As noted above, our
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Fig. 2. Examples of calculated air mass backward trajectories for coastal wind regime characteristics of the Northern Adriatic: wind (a) from Eastern Europe and (b) from Northern Italy.
Fig. 3. Examples of calculated air mass backward trajectories for southern wind episodes (Sirocco): (a) along the Italian Adriatic coast and (b) initially above the European continent.
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Fig. 4. Examples of calculated air mass backward trajectories south of the French coast for: (a) a coastal wind episode initially above the industrialized Rhone valley, as denoted R on the map and (b) pure maritime air masses. The letter P shows the location of the island of Porquerolles while the reported form indicates the trajectory of the Atalante during the MIRAMER campaign.
analysis focuses on two typical meteorological conditions which occur in such coastal zones, i.e., both the coastal and the maritime episodes. South of the coasts of Toulon, an urbanized coastal influence can be induced by winds blowing from northwest to east, while air masses of marine origin are transported by west to southeast local winds. Two distinct cases are studied in the present paper: first of all, a local west-southwest direction which brings air masses that have passed over the very industrialized and urbanized Rhone valley located northeast of Marseille, before turning eastward over the Mediterranean Sea (see Fig. 4a). In this case, the sampled aerosols are likely to be of anthropogenic origin (see Section 3). A second type of air mass can occur in a few cases when a local southwest wind episode transport pure marine air masses with winds coming from the Sea, as shown in Fig. 4b. These latter conditions are studied in more detail when discussing the chemical analysis reported in Section 4.
3. The aerosol size distributions 3.1. Northern Adriatic general situation This section deals with the aerosol size distributions measured on the Acqua Alta platform between May and September 2014. First of all, Fig. 5 shows averaged aerosol size distributions for each different wind direction characteristics of the two meteorological conditions of the study area investigated in this paper, as described in Section 2. One aerosol size distribution deals with Sirocco conditions, which corresponds to a large fetch in the study area, while the other one was acquired for northwest incoming direction, i.e.,
Fig. 5. Aerosol size distributions measured at the Acqua Alta platform for two different wind directions, i.e., the Sirocco (black line) and a northwest direction (blue line) for a wind speed of 7 ms1. The aerosol size distributions were normalized to a relative humidity of 80%. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
for a fetch of about 13 km. The two aerosol size spectra reported in Fig. 5 were recorded for a wind speed of about 7 ms1. We can note in Fig. 5 a net distinction between the concentrations of both the sub-micrometer and super-micrometer particles. The concentrations of sub-micron particles decrease as the fetch increases, whereas the concentrations of the larger ones increase with increasing fetch. The super-micron portion of the aerosol size spectrum consist mainly of sea-spray aerosols produced at the airsea interface through breaking wave processes, while the sub-
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micron aerosols are issued from continental and/or anthropogenic sources (see Piazzola and Despiau, 1997a). The concentrations of sub-micrometer particles then decrease at larger fetch, in accordance with the transition from a continental to a marine aerosol as the air mass is advected over the sea (e.g., Van Eijk and De Leeuw, 1992).
3.2. Comparison with the French Mediterranean coast One of the objectives of the present paper is to compare data acquired in the Northern Adriatic with aerosol size distributions measured in the North-Western Mediterranean using the same instruments. One of the questions that arise from Fig. 5 is to what extent the contribution of the marine and continental/anthropogenic sources in the aerosol size spectra measured in the Adriatic Sea is different from other Mediterranean areas. In particular, can we trust sea-spray source term validated in other geographical locations for implementation in the aerosol transport model? To this end, we have plotted in Fig. 6 the averaged aerosol size distribution acquired south of the French coast on board of the ship Atalante in May 2008 for a mean wind speed of 8 ms1 (D13) and the one recorded on Acqua Alta platform in May 2014 at the same wind speed. The averaged aerosol size distribution measured south of the French coast results from data recorded under conditions as depicted in Fig. 4a, while the Adriatic data deal with the Sirocco episode for which the air mass backtrajectory is reported in Fig. 3a. Fig. 6 shows slightly larger aerosol concentrations in the supermicron size range for the Adriatic, whereas they are considerably less for the smaller sizes in the Adriatic compared to those measured south of the French coast. The surplus in sea-spray concentrations measured in the Northern Adriatic compared to those previously reported south of the French coast can be first due to “height above the sea” of the aerosol acquisition, which was closer to the sea surface in the Northern Adriatic. The averaged aerosol size distribution reported in Fig. 6 for the North-Western Mediterranean results from data recorded at 12-m height above the sea surface, as acquired on board the ship Atalante, while the data for Adriatic were recorded at 7-m height (second floor of the Acqua Alta platform). In the marine atmospheric boundary layer, an exponential decay of aerosol concentrations with altitude is generally assumed that can be modelled using Toba (1965) as a kernel. Ninety percent of the vertical gradient of the aerosol vertical profiles occurs in the first twenty meters. For the surface layer, the
Fig. 6. Comparison between the averaged aerosol size distributions acquired on board of the ship Atalante (D13) south of the French coast (the blue line) and at the Acqua Alta (the red line) for a mean wind speed of 8 ms1. The aerosol size distributions were normalized to a relative humidity of 80%. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
model of Toba (1965) has been revisited by De Leeuw (1989) and Piazzola and Despiau (1997b). Thus, the vertical profiles of aerosol concentrations have been approached as a logarithmic law, as:
z s Nz ¼ expð sðz=10ÞÞ ¼ 10 N10
(1)
where N10 and Nz are the concentrations at 10-m height and at a height z above the sea surface, respectively. The coefficient s is the slope of the profile. On the basis of the experimental study of the vertical distribution of sea-spray concentrations in the marine surface layer in the Indian Ocean, Piazzola et al. (2015) showed that the coefficient s tends to a constant value of about 0.75 for particles of radius > 0.5 mm and for wind speeds between 7 and 10 m s 1. Piazzola et al. (2015) attest the accuracy of this approach using the vertical profiles of 0.532 mm aerosol particle extinction coefficient obtained from lidar data provided by the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) and also the data provided by the Aerosol Robotic Network (AERONET). In view of the shape of the vertical profile of aerosol concentrations measured by Piazzola et al. (2015) in the surface layer for moderate wind speeds of marine origin, the extrapolation of this factor until the height of 7 m seems accurate. By applying this correction factor to the aerosol concentrations measured on board the ship Atalante to get aerosol size distributions reported at 7-m height, we can note that the differences between the two averaged spectra are much reduced (Fig. 7). To ensure the accuracy of the comparison, however, the possible influence of secondary parameters on the sea-spray production and fluxes has to be studied. Among these parameters, we have to evaluate more specifically the effect of the sea surface temperature on the whitecap coverage and/or the bubble distributions (e.g., Bowyer et al., 1990) from which aerosols are produced and the airsea temperature difference for its influence on their vertical motion (e.g., Fairall and Davidson, 1986). In addition, we could suspect seasonal variations in biological activity of the sea water to affect the sea-spray production (e.g., Thorpe et al., 1992), but not in the case of Figs. 6 and 7 that deals with aerosol data measured in May for the two areas (2008 for the French Mediterranean data and 2014 for the Adriatic campaign). Concerning the sea-surface temperature, recent publications show that the sea temperature must substantially vary to have a measurable effect on the production of sea-spray aerosols (e.g., Salter et al., 2014). The Northern Adriatic data reported in Fig. 6 were recorded on May 18 for a sea surface temperature of about 19 C near the Aqua Alta platform, as derived from the onboard meteo-oceanographic measuring station, while
Fig. 7. Comparison between the averaged aerosol size distributions acquired on board the ship Atalante (D13) south of the French coast (dashed line), after correction using the mean slope of the vertical exponential decay, and at the Acqua Alta (red line) for a wind speed of 8 ms1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Blot (2009) reports a sea temperature of 18 C on May 21 in the North-Western Mediterranean. We can think that the influence of the sea-surface temperature could be neglected. As for the air-sea temperature difference, which represents another important parameter for the aerosol concentrations in the lower atmospheric layer, we need to consider the air temperature. Blot (2009) reports 19.5 C in the afternoon of the 21 May south of the French Mediterranean coast while the air temperature was reported equal to 17.5 C at the Aqua Alta platform. We can then assume near-neutral conditions for the atmospheric stability. This shows that the results reported for the North-Western Mediterranean by Piazzola et al. (e.g., 2003) on the variation of aerosol size distributions can probably be generalized to other Mediterranean areas for the super-micron particles characteristics of the sea surface production, more particularly those issued from bubble bursting processes. The larger sub-micron aerosol concentrations measured south of the French coast can be explained in light of the air mass backtrajectory reported in Fig. 4a. In this case, the aerosols concentrations are influenced by the strong anthropogenic activities taking place in the Rhone valley. The influence of this industrial area induces a persistent anthropogenic character of the south travelling air masses over the Mediterranean. This is rather different from the aerosol properties investigated in Section 4. 4. Chemical analysis To understand the differences noted in the submicronic size range between the aerosol size spectra recorded in the Northern Adriatic and in the North-Western Mediterranean, a chemical characterization of aerosols has been made. One of our objectives is to estimate the degree of anthropogenic influence in the samples. The chemical analysis focuses on the two particular meteorological episodes as described in Section 2 which deals with experimental measurements made in June and September 2014 on the Acqua Alta platform. The first episode, covering the period from 18/06/2014 to 19/06/2014, deals with typical coastal conditions with air masses coming from the north of Italy (Fig. 2a). Under these conditions, we expect a strong continental background mixed with marine aerosols. The second studied episode covers the period from the 24 to the 25 September 2015. This episode corresponds to marine air mass conditions with winds coming from the southern Mediterranean Sea, as shown in Fig. 3b. This gave us the opportunity to investigate the physicochemical properties of aerosols for long fetch with a maritime character for which the question remains about a possible subsidence of a continental influence. Indeed, as shown in Fig. 3b, the air masses travelled over the European continent few days before. Total concentrations recorded for major species in the study area in June and September 2014 are reported in Table 1. In addition, the aim of this section is also to compare the aerosol mass distributions of the different species recorded for these two characteristic episodes to the ones recorded in the NorthWestern Mediterranean by P12 on the island of Porquerolles south of the French coast for air masses with approximately the same character. To this end, Table 2 shows the concentrations of different species in the Porquerolles Island reported in P12. The coastal conditions in Table 2 deal with a light offshore wind (see P12),
Table 1 Concentrations of major species (in mg m3) as measured in the Northern Adriatic in June and September 2014 for coastal and marine (Sirocco) conditions. An overall 10% uncertainty should be considered for each concentration value.
Coastal Marine
Cl
NO3
SO4
Ox
Na
NH4
K
Ca
HPO4
Mg
0.44 0.47
3.70 1.1
2.76 4.24
0.33 0.094
0.37 0.55
1.28 0.82
0.16 0.1
0.97 0.63
0.05 0.06
0.18 0.15
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Table 2 Concentrations of major species (in mg m3) as measured in the North-Western Mediterranean by P12 in May 2007 for coastal and marine conditions. An overall 10% uncertainty should be considered for each concentration value.
Coastal Marine
Cl
NO3
SO4
Ox
Na
NH4
K
Ca
0.84 1.84
1.52 3.48
3.10 2.67
0.17 0.37
0.79 1.79
0.48 0.21
0.13 0.16
0.30 1.78
while the maritime episode deals with the air mass trajectory reported in Fig. 4b, for which aerosols are not influenced by the industrialized zone located in the Rhone valley. This is in contrast with the episode analyzed in Section 3 (Fig. 5). In case of coastal conditions, characterized by offshore blowing winds and short fetches, we can see that the nitrate concentrations are found substantially larger in the Adriatic compared to the ones measured in the West-Northern Mediterranean. This could indicate a larger importance of the anthropogenic impact in the Northern Adriatic for coastal influence. This is confirmed by the NH4 concentrations, which for the two episodes were found larger in the Adriatic (Table 1) compared to the ones recorded in the Porquerolles Island (Table 2). It should be noted that a comparison of the Porquerolles results reported in Table 2 with other measurements conducted in that area by Sellegri et al. (2001) showed similar values for anthropogenic tracers, as NH4þ and NO3- (P12). For marine air masses, the nitrate concentrations were found larger on the island of Porquerolles, although the episode investigated corresponds to pure marine air masses which has travelled from the south Mediterranean (see Fig. 4b). For a detailed view of the variation in the different size ranges, we have reported in Fig. 8 a comparison of the mass distributions of nitrates recorded for the marine conditions in the two geographical locations. Fig. 8a reports
a
b
Fig. 8. Mass distribution of nitrate aerosols for (a) coastal influence and (b) for marine influence. The dashed blue curve shows the Adriatic data while the red one is the North-Western Mediterranean (Piazzola et al., 2012). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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the mass distributions with a dominating coastal influence, while Fig. 8b was recorded during Sirocco conditions. The peak of nitrate in the submicron range located at 0.3e0.5 mm observed on the Adriatic samples can result from nitric acid/ammonia reaction leading to the formation of ammonium nitrate (Seinfeld and Pandis, 1998). In contrast, Fig. 8b shows that nitrate is found at its maximum level in super-micron sizes in the two locations when measured in oceanic condition. It should be noted that the maximum of the nitrate concentrations was found at the same particle size in the both locations, i.e., around 4 mm. However, while the concentration peak was found equal to 0.266 mgm3 in the Adriatic, the maximum in nitrate concentration was equal to 1 mgm3 on the island of Porquerolles. The occurrence of a concentration peak in the super micron size range during marine episodes, which are expected to deal with the cleanest conditions, seems characteristics of the role of the fixation of nitrogen species by aged sea-salts through the reaction of NaCl with HNO3 on the particles surface, as shown in recent observations (P12; Sellegri et al., 2001). This indicates that the reaction of sea salt and nitric acid with evaporation of hydrochloric acid (product of the reaction) is in these cases a major sink for particulate Cl. As noted in P12, this peak characterizes the ability of the anthropogenic influence to persist in the aerosol properties after long range transport over the ocean. We can expect an enhancement of the production of nitrogen species through the mentioned reaction above in case of larger oceanic path for air mass transport. This could then explain the larger nitrate concentrations found on the island of Porquerolles since they were measured where the air masses had spent more than 4 days above the sea, as shown in Fig. 4b that shows the air mass back trajectory calculated for the maritime episode reported in Table 2. Indeed, the longer the transport time above sea bodies more important is the reaction of sea-salt with gaseous nitric acid leading to nitrate formation (Sellegri et al., 2001). On the contrary, as shown in Fig. 3b, the Adriatic case deals with air masses which have travelled over land for two days before reaching our site. This is in accordance with the larger concentrations in sodium found in Porquerolles compared to the Adriatic in case of Sirocco episode (see Tables 1 and 2). The sulfate concentrations are in the same order of magnitude for coastal air masses in the Adriatic and in the French Mediterranean but slightly larger for Sirocco than for marine air masses on the island of Porquerolles. However, sulfate concentrations measured at the Acqua Alta are then rather low compared to those measured in the Eastern Mediterranean by Eleftheriadis et al. (2006) and in the coastal site of Finokalia by Bardouki et al. (2003), i.e., 10.12 ± 1.10 mg m3 and 6.88 ± 0.96 mg m3, respectively. The above comparison could indicate use of lower sulfur content in fuel for upper Adriatic Sea (in particular thanks to a voluntary agreement to preserve the area of the Venetian lagoon) compared to the Eastern and Western Mediterranean where sulfate concentration values ranging from 2 to 8 mg m3 have been repeatedly measured along touristic ship cruises (Bove et al., 2016). In Fig. 9, the mass distributions of S04 are reported for coastal influence, they are very similar. On the contrary, for the marine ones (Fig. 10) a peak in the super-micron size range is observed for the North-Western Mediterranean. This peak on the super-micron size range was also observed in the mass distribution measured in July 2013 the Mediterranean for similar air mass origin (unpublished results) as shown in Fig. 10. A similar reaction, as we observed between sea-spray and nitrates (Fig. 8b) could take place on the sulfates, as proposed by Reid et al. (2006). Actually, previous observations of the correlation between coarsesulphur and sea-salt tracers as Na and Cl have been reported several times in the literature (e.g., Schembari et al., 2014). This reaction could be impeached in the Adriatic due to the portion over land (Fig. 2a). To evaluate the anthropogenic part of the super-micron
Fig. 9. Mass distribution of sulfate aerosols for coastal influence. The dashed blue curve shows the Adriatic data, while the red one is for the North-Western Mediterranean (Piazzola et al., 2012). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. Mass distribution of sulfate aerosols for marine influence. The dashed blue curve shows the Adriatic data, while the red one is for the North-Western Mediterranean (Piazzola et al., 2012). The solid black line corresponds to data recorded south of the French coast for maritime air masses during the summer 2013 (Piazzola, person. communication). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
sulfates, non sea-salt sulfate (nss SO2 4 ) was calculated from the þ mass ratio of ðSO2 Þ/Na using the following expression: 4
h i h i nssSO2 ¼ SO2 4 4
tot
io n h 0:252* Naþ
seawater
(2)
Fig. 11. Mass distribution of nss-sulfates measured in the Mediterranean for both a marine (blue line) and a coastal influence (black line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 11 shows the mass distribution of nss-sulfates measured in the North-Western Mediterranean for both the marine and coastal influence. Fig. 11 shows that the concentrations in the supermicron size range are larger for marine influence compared to the coastal one. The non sea-salt origin varies between 40 and 70% from the supermicron SO4. In other words, 40e70% of sulfur could be of anthropogenic origin. This in accordance with the results published by Schembari et al. (2014), who reported mass concentrations over the Mediterranean using a cruise on board the ship “Costa Pacifica”. The results reported in Fig. 11 could attest the occurrence of an atmospheric reaction between sea-spray and sulfates for marine air masses. However, it should be noted that the number of samplings of the present chemical analysis is rather limited to extract a conclusive picture. 5. Conclusion This paper presents a comparison between the aerosol concentrations measured in the Northern Adriatic and in the French Mediterranean for coastal and marine episodes, resulting from offshore winds and onshore winds, respectively. The first case deals with short fetches while the second one results from winds blowing on large fetches over open water. In this latter case, which deals with air masses of marine origin transported by the Sirocco in the Adriatic and by southern winds in the French coast, we note a good agreement between the concentrations of super-micrometer aerosols measured in the two locations. In view of the moderate wind speed periods analyzed in the present paper, this then indicates a similar sea surface production of sea-spray aerosols issued from bubble bursting processes in these two locations. By discussing the possible influence of different parameters on the sea-spray production and fluxes, as the sea temperature, the air-sea temperature difference or the seasonal effects, we conclude that the results reported for the NorthWestern Mediterranean on the variation of aerosol size distributions (e.g., D13) can be generalized to the Adriatic for the supermicron particles characteristics. For marine air masses, however, we note larger aerosol concentrations for sub-micron particles south of the French coast compared to the Northern Adriatic region. Since these particles are strongly dependent on the air mass characteristics, we noted that the industrial activities of the Rhone valley in the south of France induce larger anthropogenic aerosols for marine air masses. In contrast, for a coastal influence, the chemical analysis presented in Section 4 seems to indicate a larger importance of the anthropogenic impact in the Northern Adriatic compared to the West-Northern Mediterranean. In the both locations, however, the results confirm the occurrence of reactions between sea-spray and the organic matter present in the atmosphere. Acknowledgements The authors wish to express their gratitude to the staff of CNRISMAR in Venice for their contribution to the experimental campaign and for their technical support. Many thanks also to the staff of TNO The Hague and more particularly, Leo Cohen for the calibration of the aerosol probes. This study received funding from the European Union Seventh Framework Programme (FP7/ 2007e2013) under grant agreement n 262584FP7, JERICO. References Bardouki, H., Liakakou, H., Economou, C., Sciare, J., Smolik, J., Zdimal, V., Eleftheriadis, K., Lazaridis, M., Dye, C., Mihalopoulos, N., 2003. Chemical composition of size resolved atmospheric aerosols in the eastern
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Mediterranean during summer and winter. Atmos. Environ. 37 (n 2), 195e208. http://dx.doi.org/10.1016/S1352-2310(02)00859-2. rosols Blot, R., 2009. Etude des variations spatio-temporelles des distributions d’ae ^ tie re me diterrane enne (PhD Thesis of the University of Toulon). en zone co Bove, M.C., Brotto, P., Calzolai, G., Cassola, F., Cavalli, F., Fermo, P., Hjorth, J., , D., Nava, S., Piazzalunga, A., Schembari, C., Prati, P., 2016. PM10 source Massabo apportionment applying PMF and chemical tracer analysis to ship-borne measurements in the Western Mediterranean. Atmos. Environ. 125, 140e151. http://dx.doi.org/10.1016/j.atmosenv.2015.11.009. ISSN: 1352 -2310. Bowyer, P.A., Woolf, D.K., Monahan, E.C., 1990. Temperature dependence of the charge and aerosol production associated with breaking wave in a whitecap simulation tank. J. Geophys. Res. 95 (No. C4), 5313e5319. Cavaleri, L., 2000. The oceanographic tower Acqua Alta d activity and prediction of sea states at Venice. Coast. Eng. 39, 29e70. http://dx.doi.org/10.1016/S03783839(99)00053-8. Ceburnis, D., O'Dowd, C.D., Jennings, G.S., Facchini, M.C., Emblico, L., Decesari, S., Fuzzi, S., Sakalys, J., 2008. Marine aerosol chemistry gradients: elucidating primary and secondary processes and fluxes. Geophys. Res. Lett. 35, L07804. http://dx.doi.org/10.1029/2008GL033462. De Leeuw, G., Andreas, E.L., Anguelova, M.D., Fairall, C.W., Lewis, E.R., O'Dowd, C., Schulz, M., Schwartz, S.E., 2011. Production flux of sea spray aerosol. Rev. Geophys. 49, RG2001. http://dx.doi.org/10.1029/2010RG000349. De Leeuw, G., 1989. Modeling of extinction and backscatter profiles in the marine mixed layer. Appl. Opt. 28 (7), 1356e1359. http://dx.doi.org/10.1364/ AO.28.001356. Demoisson, A., Tedeschi, G., Piazzola, J., 2013. A model for the atmospheric transport of sea-salt particles in coastal areas. Atmos. Res. 132e133, 144e153. http:// dx.doi.org/10.1016/j.atmosres.2013.04.002. Draxler, R.R., Rolph, G.D., 2003. HYSPLIT (Hybrid Single-particle Lagrangian Integrated Trajectory) Model (NOAA Air Resour. Lab., Silver Spring, MD). http:// www.arl.noaa.gov/ready/hysplit4.html. Eleftheriadis, K., Colbeck, I., Housiadaa, C., Lazaridis, M., Mihalopoulos, N., Mitsakou, C., Smolık, J., Zdımal, V., 2006. Size distribution, composition and origin of the submicron aerosol in the marine boundary layer during the eastern Mediterranean ‘‘SUB-AERO’’ experiment. Atmos. Environ. 40, 6245e6260. Fairall, C.W., Davidson, K.L., 1986. Dynamics and modeling of aerosols in the marine atmospheric boundary. In: Monahan, E.C., MacNiocaill, G. (Eds.), Oceanic Whitecaps. Reidel, Hingham, Mass, pp. 195e208. Fitzgerald, J.W., 1991. Marine aerosols : a review. Atmos. Environ. 25 (A), 523e545. http://dx.doi.org/10.1016/0960-1686(91)90050-H. Hsu, S.A., 1986. A mechanism for the increase of wind stress (drag) coefficient with wind speed over water surfaces : a parametric model. J. Phys. Oceanogr. 16, 144e150. Intergovernmental Panel on Climate Change, 2013. 30 September 2013 working group I contribution to the IPCC fifth assessment report-climate change 2013. In: The Physical Science Basis- Final Draft Underlying Scientific- Technical Assessment, Accepted but Not Approved in Detail by the 12th Session of Working Group I and the 36th Session of the IPCC on 26 September 2013 in Stockholm, Sweden. Knipping, E.M., Dabdub, D., 2003. Impact of chlorine emissions from sea-salt aerosol on coastal urban ozone. Environ. Sci. Technol. 37 (2), 275e284. http:// dx.doi.org/10.1021/es025793z. Monahan, E.C., Davidson, K.L., Spiel, D.E., 1986. A model of marine aerosol generation via whitecaps and wave disruption. In: Monahan, E.C., Mac Niocaill, G. (Eds.), Oceanic Whitecap and Their Role in Air-sea Exchange Processes. Reidel, Dordrecht, pp. 167e174. http://dx.doi.org/10.1007/978-94-009-4668-2_16. Piazzola, J., Tedeschi, G., Demoisson, A., 2015. A model for the transport of sea-spray aerosols in the coastal zone. Bound-Lay.Meteorol. 155 (Issue 2), 329e350. http://dx.doi.org/10.1007/s10546-014-9994-3. Piazzola, J., Sellegri, K., Bourcier, L., Mallet, M., Tedeschi, G., Missamou, T., 2012. Physicochemical characteristics of aerosols measured in the spring time in the Mediterranean coastal zone. Atmos. Environ. 54, 545e556. http://dx.doi.org/ 10.1016/j.atmosenv.2012.02.057. Piazzola, J., Forget, P., Lafon, C., Despiau, S., 2009. Spatial variation of sea-spray fluxes over a mediterranean coastal zone using a sea-state model. BoundLay.Meteorol. 132 (1), 167e183. http://dx.doi.org/10.1007/s10546-009-9386-2. Piazzola, J., Bouchara, F., Van Eijk, A.M.J., De Leeuw, G., 2003. Development of the Mediterranean extinction code MEDEX. Opt. Eng. 42 (4), 912e924. http:// dx.doi.org/10.1117/1.1556765. Piazzola, J., Despiau, S., 1997a. Contribution of marine aerosols in the particle size distributions observed in Mediterranean coastal zone. Atmos. Environ. 31 (18), 2991e3009. http://dx.doi.org/10.1016/S1352-2310(97)00088-5. Piazzola, J., Despiau, S., 1997b. Vertical distribution of aerosol particles near the airesea interface in coastal zone. J. Aerosol Sci. 28, 1579e1599. http:// dx.doi.org/10.1016/S0021-8502(97)00020-7. Reid, J.S., Brooks, B., Crahan, K.K., Hegg, D.A., Eck, T.F., O'Neill, N., De Leeuw, G., et al., 2006. Reconciliation of coarse mode sea-salt aerosol particle size measurements and parameterizations at a subtropical ocean receptor site. J. Geophys. Res. 111 (D02202) http://dx.doi.org/10.1029/2005JD006200. Salter, M.E., Nilsson, E.D., Butcher, A., Bilde, M., 2014. On the sea-water temperature dependence of the sea spray aerosol generated by a continuous plunging jet. J. Geo Phys. Res. Atmos. 119, 9052e9072. http://dx.doi.org/10.1002/ 2013JD021376. , D., Navac, S., Schembari, C., Bove, M.C., Cuccia, E., Cavalli, F., Hjorth, J., Massabo
162
J. Piazzola et al. / Atmospheric Environment 132 (2016) 153e162
Udisti, R., Prati, P., 2014. Source apportionment of PM10 in the Western Mediterranean based on observations from a cruise ship. Atmos. Environ. 98, 510e518. http://dx.doi.org/10.1016/j.atmosenv.2014.09.015. Seinfeld, J.H., Pandis, S.N., 1998. Atmospheric Chemistry and Physics: from Air Pollution to Climate Change. Wiley-Interscience, New York. ISBN: 978-0-47172018-8. Sellegri, K., Gourdeau, J., Putaud, J.-P., Despiau, S., 2001. Chemical composition of marine aerosol in a Mediterranean coastal zone during the FETCH experiment. J. Geophys. Res. 106 (No. D11), 12023e12038. http://dx.doi.org/10.1029/ 2000JD900629 (AGU). Tedeschi, G., Piazzola, J., 2011. Development of a 2D marine aerosol transport model: application to the influence of thermal stability in the marine atmospheric boundary layer. Atmos. Res. 101 (no. 1e2), 469e479. http://dx.doi.org/
10.1016/j.atmosres.2011.04.013. Toba, Y., 1965. On the giant sea-salt particles in the atmosphere: II theory of the vertical distribution in the 10-m layer over the ocean. Tellus 17, 365e382. http://dx.doi.org/10.1111/j.2153-3490.1965.tb01429.x. Thorpe, S.A., Bowyer, P., Woolf, D.K., 1992. Some factors affecting the size distributions of oceanic bubbles. J. Phys. Oceanogr. 22, 382e389. Van Eijk, A.M.J., De Leeuw, G., 1992. Modeling aerosol particle size distributions over the North Sea. J. Geophys. Res. 97 (C9), 14417e14429. http://dx.doi.org/10.1029/ 92JC01214. Yoon, Y.J., Ceburnis, D., Cavalli, F., Jourdan, O., Putaud, J.P., Facchini, M.C., Decesari, S., Fuzzi, S., Sellegri, K., Jennings, S.G., O'Dowd, C.D., 2007. Seasonal characteristics of the physicochemical properties of North Atlantic marine atmospheric aerosols. J. Geophys. Res. 112, D04206. http://dx.doi.org/10.1029/2005JD007044.
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Characterization of aerosols above the Northern Adriatic Sea: Case studies of offshore and onshore wind conditions J. Piazzola a, *, N. Mihalopoulos b, E. Canepa c, G. Tedeschi a, P. Prati d, P. Zarmpas b, M. Bastianini e, T. Missamou a, L. Cavaleri e a
University of Toulon, Mediterranean Institute of Oceanography, MIO-UM 110, France University of Crete, Chemistry Department, Greece CNR-ISMAR, Genova, Italy d University of Genova, Department of Physics and INFN, Italy e CNR-ISMAR, Venice, Italy b c
h i g h l i g h t s This paper is a contribution of the study of coastal aerosol properties over the Mediterranean. We study the sea-spray and the anthropogenic contributions in the aerosol concentrations measured in the Mediterranean coast. One of the objectives is to implement an accurate source term for the sea-spray aerosols in the aerosol transport model.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 November 2015 Received in revised form 9 February 2016 Accepted 26 February 2016 Available online 2 March 2016
Aerosol particles in coastal areas result from a complex mixing between sea spray aerosols locally generated at the sea surface by the wind-waves interaction processes and a continental component resulting from natural and/or anthropogenic sources. This paper presents a physical and chemical analysis of the aerosol data acquired from May to September 2014 in the Adriatic Sea. Aerosol distributions were measured on the Acqua Alta platform located 15 km off the coast of Venice using two Particle Measuring System probes and a chemical characterization was made using an Ion Chromatography analysis (IC). Our aim is to study both the sea-spray contribution and the anthropogenic influence in the coastal aerosol of this Mediterranean region. To this end, we focus on a comparison between the present data and the aerosol size distributions measured south of the French Mediterranean coast. For air masses of marine origin transported by southern winds on the French coast and by the Sirocco in the Adriatic, we note a good agreement between the concentrations of super-micrometer aerosols measured in the two locations. This indicates a similar sea surface production of sea-spray aerosols formed by bubble bursting processes in the two locations. In contrast, the results show larger concentrations of submicron particles in the North-Western Mediterranean compared to the Adriatic, which result probably from a larger anthropogenic background for marine conditions. In contrast, for a coastal influence, the chemical analysis presented in the present paper seems to indicate a larger importance of the anthropogenic impact in the Northern Adriatic compared to the North-Western Mediterranean. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Coastal aerosols Anthropogenic compounds Atmospheric transport
1. Introduction The estimation of the atmospheric aerosol impact on climate remains an important scientific challenge (Intergovernmental Panel on Climate Change (IPCC)). In particular, in contrast with the
* Corresponding author. E-mail address: [email protected] (J. Piazzola). http://dx.doi.org/10.1016/j.atmosenv.2016.02.044 1352-2310/© 2016 Elsevier Ltd. All rights reserved.
radiative forcing attributed to greenhouse gases, the uncertainties related to aerosol radiative forcing remain very large (IPCC, 2013). This is due to their heterogeneous spatial and temporal distribution, their different origins (natural and anthropogenic) and their physical and chemical behaviour in the free troposphere. In coastal areas, aerosols may be of either natural or anthropogenic origin. The sea-spray aerosols represent a major contribution to the coastal aerosol mass (Yoon et al., 2007; Piazzola et al., 2009). Sea-spray
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particles result from the primary production through breaking waves (e.g., Monahan et al., 1986), which provide aerosols of diameters 0.5 mm. In addition, a generation of smaller particles results from secondary production due to gas-particle conversion (e.g., Fitzgerald, 1991). Sea-spray is made primarily of sodium chloride (NaCl) and small amounts of other salts such as sulfate, calcium and potassium, but it can also contain significant amounts of organic carbon. The organic component was mainly attributed to bubble bursting processes, due to the predominantly insoluble and surface active character of organic carbon in the marine aerosol particles (Ceburnis et al., 2008). Although it is well-recognized that marine aerosols have a significant influence on the coastal urban air quality through their ability to have chemical and physical interactions with gases and other aerosol species (Knipping and Dabdub, 2003), their interactions with anthropogenic pollutants in coastal areas are still largely unknown. In this context, we need first to obtain a good knowledge of the sea-spray generation processes and its atmospheric transport. To this end, Tedeschi and Piazzola (2011) developed the Marine Aerosol Concentration Model (MACMod) dedicated to the atmospheric transport of marine aerosols in the Marine Atmospheric Boundary Layer (MABL). Such a transport model needs to have accurate input conditions, as the source terms characteristics of the sea-spray production. However, uncertainties on the sea-spray source function (hereinafter S3F) are still large (De Leeuw et al., 2011). Demoisson et al. (2013, hereinafter, D13) recently proposed a new formulation of the S3F characteristics of the North-Western Mediterranean using the aerosol size distributions measured at different locations south of the French coast on board the ship Atalante in May 2008. The implementation of this latter sea-spray formulation in MACMod contributed to increase its performance for the Mediterranean (D13). One of the questions we need to reply is to what extent this formulation can be used in other Mediterranean areas. In addition, a better knowledge is required on atmospheric interactions between the sea-spray and other atmospheric compounds. Physicochemical analysis of the aerosol in different Mediterranean areas is then needed. The aim of this paper is to study the sea-spray contribution and the anthropogenic influence in the coastal aerosol measured in the Mediterranean. To this end, we used the aerosol size distributions acquired in the Northern Adriatic between May and September 2014 on board the Acqua Alta platform located near the coast of Venice. The results are compared to the aerosol size distributions recorded on board the ship Atalante south of the French coast in May 2008 that were used for the implementation of the S3F proposed by D13 for the Mediterranean. In addition, to evaluate the anthropogenic influence in the aerosols measured in the Northern Adriatic, we present a chemical characterization of aerosols concentrations. The composition of aerosols in the study area is also then compared to measurements made in May 2007 on the island of Porquerolles located south of the French coast (Piazzola et al., 2012; hereinafter P12). In Section 2, we describe the field sites of the present study, while Section 3 deals with the variation of the aerosol size distributions. Section 4 presents the chemical analysis, which allows for a good appreciation of the degree of anthropogenicity of the Western Mediterranean. 2. Field sites and experiments and general characterization of the areas Our aim is to study the aerosol properties in the Northern Adriatic and compare the results to the data obtained south of the French Mediterranean coast. Alternatively, these two coastal regions are under influence of continental and marine air masses depending on the wind direction. The variation in wind direction
can also be accompanied by changes of the fetch, which represents the distance over water for which the wind has blown constantly. Continental winds may then blow over the sea, giving the transported air masses a mixed character (defined as “coastal”). For our analysis, we have selected episodes characteristics of coastal and marine influence, i.e., short and large fetches, as found in coastal Mediterranean areas. Large fetches deal with sea conditions for which the waves tend to be fully developed. For the wind speed conditions analyzed in Section 3, this corresponds to fetches larger than 150 km, following the criterion proposed by (Hsu, 1986). 2.1. The Acqua Alta field site Measurements in the Northern Adriatic took place on the Acqua Alta platform, which is an oceanographic tower (Fig. 1a) located in the Northern Adriatic Sea, 15 km off the coast of the Venice lagoon (Fig. 1b). The tower, which has been operational since the early 70s, is managed by the Institute of Marine Sciences - National Research Council (ISMAR-CNR) and commonly hosts a large variety of instruments. The main structure is a quadratic template jacket steel structure with four 0.60-m diameter main legs fixed to inserted steel pipes driven 22 m into the sea bed (Cavaleri, 2000). The aerosol probes (see below) were fixed on the second floor, at 7 m above marine surface level, on which there is a 5.5 4.0 m platform. These were connected to power generators and to an indoor room for data acquisition. 2.2. Sampling and analytical procedures During the experimental campaigns conducted in the Northern Adriatic and in the North-Western Mediterranean, the aerosol data were acquired in the 0.1e45 mm size ranges using two particle measuring systems (PMS): one active scattering spectrometer probe (ASASP) and one classical scattering spectrometer probe (CSASP). The data were stored as the average over a 4-min interval. Prior to the experiments, the probes had been calibrated with latex particles of known sizes. It should be noted that a minimum of a factor of 3 is expected between the aerosol concentration simultaneously measured using two probes of the same type at the same location (Reid et al., 2006). To ensure a rather accurate comparison between the two sites (see Section 3), we used the same probes (not only the same model but the same devices) for the two experimental campaigns in order to have the same measurement error. However, the absolute concentration registered by a PMS probe may change over time and after servicing due to decreasing laser power or contamination of optics (Reid et al., 2006). The probes were then checked in the laboratory at the end of each series of experiments. The differences noted for each probe before and after the experiment were less than 15% for aerosol diameters smaller than 10 mm. This can be assumed as accurate since our aim was to study more specifically the sea-spray aerosols issued from bubble-bursting processes which deal, for a larger part, with particles of sizes smaller than 10 mm. Prior to the plots, all the aerosol size distributions were normalized to a relative humidity of 80%. The experimental campaign in the Northern Adriatic consisted of three intensive measurement periods of 7e10 days between May and September. Among them, we have selected a number of ensembles of measurements performed during windy period of nearly constant direction (throughout the paper we use incoming direction). Each aerosol spectrum reported in Section 3 is an average of four to six aerosol size distributions. For chemical characterization reported in Section 4, aerosols were sampled with a low pressure cascade impactor (Dekati) from 14.45 of 18th of June to 24.00 of 19th of June 2014 (about 33 h) and from 14.30 of 23rd of September to 12.00 25th of September 2014
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Dekati lps response function. 2.3. Campaign overview in the Adriatic The data were acquired during three experimental campaigns of seven to ten days made between May and September 2014. The study focuses on two particular types of meteorological conditions that often occur in the study area, and which should yield different aerosol characteristics (see Section 3 and Section 4). As outlined above, the wind direction is the first parameter to consider in our analysis, since changes in wind direction may result in transport from different aerosol source regions. In addition, the wind speed is an important parameter in marine areas for its role in the production of sea-spray aerosols and dispersion of the continental ones in marine atmosphere (e.g., Piazzola and Despiau, 1997a). First, we study typical coastal conditions that are characterized by offshore winds (which travel from land to the sea) and air masses coming from the north of Italy. Under these conditions, we expect a strong continental and/or anthropogenic background with a possible mixing with marine aerosols if white-capping occurs along the fetch. The selected episodes for our analysis correspond to periods for which the wind has blown long enough in a constant direction at a given wind speed (and hence at a given fetch). This methodology ensures that the measured aerosol size distributions are rather representative of one specific meteorological period. As an example, Fig. 2a and Fig. 2b show the numerical calculations of the air mass back trajectories, using the hysplit model (Draxler and Rolph, 2003), typical of coastal conditions in the study area induced by an offshore wind. In addition, this study will also focus on the aerosol characteristics observed during southern winds, which correspond to marine air mass conditions with Sirocco winds coming from the southern Mediterranean Sea. These conditions generally correspond to large fetches for which the waves tend to be fully developed, as noted above. Most of the time however, for southern wind direction in the coast of Venice, a part of the air mass trajectory may have crossed land a few days before arriving in the study area, as shown in Fig. 3a, where we show the back trajectory analysis for the episode analyzed in Section 3. In a few cases, a southern wind can also transport air masses originally coming from the European continent, as shown in Fig. 3b which correspond to one of the episodes used for the chemical characterization reported in Section 4. 2.4. Meteorological characteristics of the French Mediterranean coast
Fig. 1. Field site: (a) the Acqua Alta platform and (b) the study area in the Adriatic Sea with the platform location (square). V denote the city of Venice.
(about 46 h). The impactor was sampling directly in the atmosphere and was a 20 lpm 13-stages low pressure cascade impactor which cut-off aerodynamic diameters were 0.03, 0.06, 0.108, 0.17, 0.26, 0.4, 0.65, 1, 1.6, 2.5, 4.4, 6.8 and 9.97 mm. The collection plates are custom-made out of aluminium foil for subsequent Ion Chromatography (IC) analysis. No data reduction has been performed on the cascade impactor data and the size distributions presented in the paper are just based on raw data without any correction for the
One of the aims of this paper is to provide a comparison between the aerosol properties measured in the Northern Adriatic and those observed in another Mediterranean area south of the French coast (P12 and D13 for instance). In Section 3, the present data are compared to aerosol size distributions measured during an experimental campaign which took place in May 2008 on board the ship Atalante during the experimental campaign MIRAMER (D13). Measurements of aerosol concentrations have been conducted from May 15 to 28, south of the Toulon bay in the French Mediterranean, between 5.4 and 6.25 east longitude and between 42.2 and 43.2 north latitude. During the campaign, the Atalante cruised at different distances, between 10 and 100 km south from the coast of Toulon. The study area then roughly deals with a limited geographic space located south of the Toulon bay, as indicated in Fig. 4a where is reported the trajectory of the ship during the campaign. The ship regularly stopped to allow acquisition of aerosol concentrations. In Section 4, the chemical analysis made on board the Aqua Alta platform is compared to the data reported by P12 on the island of Porquerolles in May 2007. As noted above, our
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Fig. 2. Examples of calculated air mass backward trajectories for coastal wind regime characteristics of the Northern Adriatic: wind (a) from Eastern Europe and (b) from Northern Italy.
Fig. 3. Examples of calculated air mass backward trajectories for southern wind episodes (Sirocco): (a) along the Italian Adriatic coast and (b) initially above the European continent.
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Fig. 4. Examples of calculated air mass backward trajectories south of the French coast for: (a) a coastal wind episode initially above the industrialized Rhone valley, as denoted R on the map and (b) pure maritime air masses. The letter P shows the location of the island of Porquerolles while the reported form indicates the trajectory of the Atalante during the MIRAMER campaign.
analysis focuses on two typical meteorological conditions which occur in such coastal zones, i.e., both the coastal and the maritime episodes. South of the coasts of Toulon, an urbanized coastal influence can be induced by winds blowing from northwest to east, while air masses of marine origin are transported by west to southeast local winds. Two distinct cases are studied in the present paper: first of all, a local west-southwest direction which brings air masses that have passed over the very industrialized and urbanized Rhone valley located northeast of Marseille, before turning eastward over the Mediterranean Sea (see Fig. 4a). In this case, the sampled aerosols are likely to be of anthropogenic origin (see Section 3). A second type of air mass can occur in a few cases when a local southwest wind episode transport pure marine air masses with winds coming from the Sea, as shown in Fig. 4b. These latter conditions are studied in more detail when discussing the chemical analysis reported in Section 4.
3. The aerosol size distributions 3.1. Northern Adriatic general situation This section deals with the aerosol size distributions measured on the Acqua Alta platform between May and September 2014. First of all, Fig. 5 shows averaged aerosol size distributions for each different wind direction characteristics of the two meteorological conditions of the study area investigated in this paper, as described in Section 2. One aerosol size distribution deals with Sirocco conditions, which corresponds to a large fetch in the study area, while the other one was acquired for northwest incoming direction, i.e.,
Fig. 5. Aerosol size distributions measured at the Acqua Alta platform for two different wind directions, i.e., the Sirocco (black line) and a northwest direction (blue line) for a wind speed of 7 ms1. The aerosol size distributions were normalized to a relative humidity of 80%. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
for a fetch of about 13 km. The two aerosol size spectra reported in Fig. 5 were recorded for a wind speed of about 7 ms1. We can note in Fig. 5 a net distinction between the concentrations of both the sub-micrometer and super-micrometer particles. The concentrations of sub-micron particles decrease as the fetch increases, whereas the concentrations of the larger ones increase with increasing fetch. The super-micron portion of the aerosol size spectrum consist mainly of sea-spray aerosols produced at the airsea interface through breaking wave processes, while the sub-
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micron aerosols are issued from continental and/or anthropogenic sources (see Piazzola and Despiau, 1997a). The concentrations of sub-micrometer particles then decrease at larger fetch, in accordance with the transition from a continental to a marine aerosol as the air mass is advected over the sea (e.g., Van Eijk and De Leeuw, 1992).
3.2. Comparison with the French Mediterranean coast One of the objectives of the present paper is to compare data acquired in the Northern Adriatic with aerosol size distributions measured in the North-Western Mediterranean using the same instruments. One of the questions that arise from Fig. 5 is to what extent the contribution of the marine and continental/anthropogenic sources in the aerosol size spectra measured in the Adriatic Sea is different from other Mediterranean areas. In particular, can we trust sea-spray source term validated in other geographical locations for implementation in the aerosol transport model? To this end, we have plotted in Fig. 6 the averaged aerosol size distribution acquired south of the French coast on board of the ship Atalante in May 2008 for a mean wind speed of 8 ms1 (D13) and the one recorded on Acqua Alta platform in May 2014 at the same wind speed. The averaged aerosol size distribution measured south of the French coast results from data recorded under conditions as depicted in Fig. 4a, while the Adriatic data deal with the Sirocco episode for which the air mass backtrajectory is reported in Fig. 3a. Fig. 6 shows slightly larger aerosol concentrations in the supermicron size range for the Adriatic, whereas they are considerably less for the smaller sizes in the Adriatic compared to those measured south of the French coast. The surplus in sea-spray concentrations measured in the Northern Adriatic compared to those previously reported south of the French coast can be first due to “height above the sea” of the aerosol acquisition, which was closer to the sea surface in the Northern Adriatic. The averaged aerosol size distribution reported in Fig. 6 for the North-Western Mediterranean results from data recorded at 12-m height above the sea surface, as acquired on board the ship Atalante, while the data for Adriatic were recorded at 7-m height (second floor of the Acqua Alta platform). In the marine atmospheric boundary layer, an exponential decay of aerosol concentrations with altitude is generally assumed that can be modelled using Toba (1965) as a kernel. Ninety percent of the vertical gradient of the aerosol vertical profiles occurs in the first twenty meters. For the surface layer, the
Fig. 6. Comparison between the averaged aerosol size distributions acquired on board of the ship Atalante (D13) south of the French coast (the blue line) and at the Acqua Alta (the red line) for a mean wind speed of 8 ms1. The aerosol size distributions were normalized to a relative humidity of 80%. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
model of Toba (1965) has been revisited by De Leeuw (1989) and Piazzola and Despiau (1997b). Thus, the vertical profiles of aerosol concentrations have been approached as a logarithmic law, as:
z s Nz ¼ expð sðz=10ÞÞ ¼ 10 N10
(1)
where N10 and Nz are the concentrations at 10-m height and at a height z above the sea surface, respectively. The coefficient s is the slope of the profile. On the basis of the experimental study of the vertical distribution of sea-spray concentrations in the marine surface layer in the Indian Ocean, Piazzola et al. (2015) showed that the coefficient s tends to a constant value of about 0.75 for particles of radius > 0.5 mm and for wind speeds between 7 and 10 m s 1. Piazzola et al. (2015) attest the accuracy of this approach using the vertical profiles of 0.532 mm aerosol particle extinction coefficient obtained from lidar data provided by the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) and also the data provided by the Aerosol Robotic Network (AERONET). In view of the shape of the vertical profile of aerosol concentrations measured by Piazzola et al. (2015) in the surface layer for moderate wind speeds of marine origin, the extrapolation of this factor until the height of 7 m seems accurate. By applying this correction factor to the aerosol concentrations measured on board the ship Atalante to get aerosol size distributions reported at 7-m height, we can note that the differences between the two averaged spectra are much reduced (Fig. 7). To ensure the accuracy of the comparison, however, the possible influence of secondary parameters on the sea-spray production and fluxes has to be studied. Among these parameters, we have to evaluate more specifically the effect of the sea surface temperature on the whitecap coverage and/or the bubble distributions (e.g., Bowyer et al., 1990) from which aerosols are produced and the airsea temperature difference for its influence on their vertical motion (e.g., Fairall and Davidson, 1986). In addition, we could suspect seasonal variations in biological activity of the sea water to affect the sea-spray production (e.g., Thorpe et al., 1992), but not in the case of Figs. 6 and 7 that deals with aerosol data measured in May for the two areas (2008 for the French Mediterranean data and 2014 for the Adriatic campaign). Concerning the sea-surface temperature, recent publications show that the sea temperature must substantially vary to have a measurable effect on the production of sea-spray aerosols (e.g., Salter et al., 2014). The Northern Adriatic data reported in Fig. 6 were recorded on May 18 for a sea surface temperature of about 19 C near the Aqua Alta platform, as derived from the onboard meteo-oceanographic measuring station, while
Fig. 7. Comparison between the averaged aerosol size distributions acquired on board the ship Atalante (D13) south of the French coast (dashed line), after correction using the mean slope of the vertical exponential decay, and at the Acqua Alta (red line) for a wind speed of 8 ms1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Blot (2009) reports a sea temperature of 18 C on May 21 in the North-Western Mediterranean. We can think that the influence of the sea-surface temperature could be neglected. As for the air-sea temperature difference, which represents another important parameter for the aerosol concentrations in the lower atmospheric layer, we need to consider the air temperature. Blot (2009) reports 19.5 C in the afternoon of the 21 May south of the French Mediterranean coast while the air temperature was reported equal to 17.5 C at the Aqua Alta platform. We can then assume near-neutral conditions for the atmospheric stability. This shows that the results reported for the North-Western Mediterranean by Piazzola et al. (e.g., 2003) on the variation of aerosol size distributions can probably be generalized to other Mediterranean areas for the super-micron particles characteristics of the sea surface production, more particularly those issued from bubble bursting processes. The larger sub-micron aerosol concentrations measured south of the French coast can be explained in light of the air mass backtrajectory reported in Fig. 4a. In this case, the aerosols concentrations are influenced by the strong anthropogenic activities taking place in the Rhone valley. The influence of this industrial area induces a persistent anthropogenic character of the south travelling air masses over the Mediterranean. This is rather different from the aerosol properties investigated in Section 4. 4. Chemical analysis To understand the differences noted in the submicronic size range between the aerosol size spectra recorded in the Northern Adriatic and in the North-Western Mediterranean, a chemical characterization of aerosols has been made. One of our objectives is to estimate the degree of anthropogenic influence in the samples. The chemical analysis focuses on the two particular meteorological episodes as described in Section 2 which deals with experimental measurements made in June and September 2014 on the Acqua Alta platform. The first episode, covering the period from 18/06/2014 to 19/06/2014, deals with typical coastal conditions with air masses coming from the north of Italy (Fig. 2a). Under these conditions, we expect a strong continental background mixed with marine aerosols. The second studied episode covers the period from the 24 to the 25 September 2015. This episode corresponds to marine air mass conditions with winds coming from the southern Mediterranean Sea, as shown in Fig. 3b. This gave us the opportunity to investigate the physicochemical properties of aerosols for long fetch with a maritime character for which the question remains about a possible subsidence of a continental influence. Indeed, as shown in Fig. 3b, the air masses travelled over the European continent few days before. Total concentrations recorded for major species in the study area in June and September 2014 are reported in Table 1. In addition, the aim of this section is also to compare the aerosol mass distributions of the different species recorded for these two characteristic episodes to the ones recorded in the NorthWestern Mediterranean by P12 on the island of Porquerolles south of the French coast for air masses with approximately the same character. To this end, Table 2 shows the concentrations of different species in the Porquerolles Island reported in P12. The coastal conditions in Table 2 deal with a light offshore wind (see P12),
Table 1 Concentrations of major species (in mg m3) as measured in the Northern Adriatic in June and September 2014 for coastal and marine (Sirocco) conditions. An overall 10% uncertainty should be considered for each concentration value.
Coastal Marine
Cl
NO3
SO4
Ox
Na
NH4
K
Ca
HPO4
Mg
0.44 0.47
3.70 1.1
2.76 4.24
0.33 0.094
0.37 0.55
1.28 0.82
0.16 0.1
0.97 0.63
0.05 0.06
0.18 0.15
159
Table 2 Concentrations of major species (in mg m3) as measured in the North-Western Mediterranean by P12 in May 2007 for coastal and marine conditions. An overall 10% uncertainty should be considered for each concentration value.
Coastal Marine
Cl
NO3
SO4
Ox
Na
NH4
K
Ca
0.84 1.84
1.52 3.48
3.10 2.67
0.17 0.37
0.79 1.79
0.48 0.21
0.13 0.16
0.30 1.78
while the maritime episode deals with the air mass trajectory reported in Fig. 4b, for which aerosols are not influenced by the industrialized zone located in the Rhone valley. This is in contrast with the episode analyzed in Section 3 (Fig. 5). In case of coastal conditions, characterized by offshore blowing winds and short fetches, we can see that the nitrate concentrations are found substantially larger in the Adriatic compared to the ones measured in the West-Northern Mediterranean. This could indicate a larger importance of the anthropogenic impact in the Northern Adriatic for coastal influence. This is confirmed by the NH4 concentrations, which for the two episodes were found larger in the Adriatic (Table 1) compared to the ones recorded in the Porquerolles Island (Table 2). It should be noted that a comparison of the Porquerolles results reported in Table 2 with other measurements conducted in that area by Sellegri et al. (2001) showed similar values for anthropogenic tracers, as NH4þ and NO3- (P12). For marine air masses, the nitrate concentrations were found larger on the island of Porquerolles, although the episode investigated corresponds to pure marine air masses which has travelled from the south Mediterranean (see Fig. 4b). For a detailed view of the variation in the different size ranges, we have reported in Fig. 8 a comparison of the mass distributions of nitrates recorded for the marine conditions in the two geographical locations. Fig. 8a reports
a
b
Fig. 8. Mass distribution of nitrate aerosols for (a) coastal influence and (b) for marine influence. The dashed blue curve shows the Adriatic data while the red one is the North-Western Mediterranean (Piazzola et al., 2012). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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the mass distributions with a dominating coastal influence, while Fig. 8b was recorded during Sirocco conditions. The peak of nitrate in the submicron range located at 0.3e0.5 mm observed on the Adriatic samples can result from nitric acid/ammonia reaction leading to the formation of ammonium nitrate (Seinfeld and Pandis, 1998). In contrast, Fig. 8b shows that nitrate is found at its maximum level in super-micron sizes in the two locations when measured in oceanic condition. It should be noted that the maximum of the nitrate concentrations was found at the same particle size in the both locations, i.e., around 4 mm. However, while the concentration peak was found equal to 0.266 mgm3 in the Adriatic, the maximum in nitrate concentration was equal to 1 mgm3 on the island of Porquerolles. The occurrence of a concentration peak in the super micron size range during marine episodes, which are expected to deal with the cleanest conditions, seems characteristics of the role of the fixation of nitrogen species by aged sea-salts through the reaction of NaCl with HNO3 on the particles surface, as shown in recent observations (P12; Sellegri et al., 2001). This indicates that the reaction of sea salt and nitric acid with evaporation of hydrochloric acid (product of the reaction) is in these cases a major sink for particulate Cl. As noted in P12, this peak characterizes the ability of the anthropogenic influence to persist in the aerosol properties after long range transport over the ocean. We can expect an enhancement of the production of nitrogen species through the mentioned reaction above in case of larger oceanic path for air mass transport. This could then explain the larger nitrate concentrations found on the island of Porquerolles since they were measured where the air masses had spent more than 4 days above the sea, as shown in Fig. 4b that shows the air mass back trajectory calculated for the maritime episode reported in Table 2. Indeed, the longer the transport time above sea bodies more important is the reaction of sea-salt with gaseous nitric acid leading to nitrate formation (Sellegri et al., 2001). On the contrary, as shown in Fig. 3b, the Adriatic case deals with air masses which have travelled over land for two days before reaching our site. This is in accordance with the larger concentrations in sodium found in Porquerolles compared to the Adriatic in case of Sirocco episode (see Tables 1 and 2). The sulfate concentrations are in the same order of magnitude for coastal air masses in the Adriatic and in the French Mediterranean but slightly larger for Sirocco than for marine air masses on the island of Porquerolles. However, sulfate concentrations measured at the Acqua Alta are then rather low compared to those measured in the Eastern Mediterranean by Eleftheriadis et al. (2006) and in the coastal site of Finokalia by Bardouki et al. (2003), i.e., 10.12 ± 1.10 mg m3 and 6.88 ± 0.96 mg m3, respectively. The above comparison could indicate use of lower sulfur content in fuel for upper Adriatic Sea (in particular thanks to a voluntary agreement to preserve the area of the Venetian lagoon) compared to the Eastern and Western Mediterranean where sulfate concentration values ranging from 2 to 8 mg m3 have been repeatedly measured along touristic ship cruises (Bove et al., 2016). In Fig. 9, the mass distributions of S04 are reported for coastal influence, they are very similar. On the contrary, for the marine ones (Fig. 10) a peak in the super-micron size range is observed for the North-Western Mediterranean. This peak on the super-micron size range was also observed in the mass distribution measured in July 2013 the Mediterranean for similar air mass origin (unpublished results) as shown in Fig. 10. A similar reaction, as we observed between sea-spray and nitrates (Fig. 8b) could take place on the sulfates, as proposed by Reid et al. (2006). Actually, previous observations of the correlation between coarsesulphur and sea-salt tracers as Na and Cl have been reported several times in the literature (e.g., Schembari et al., 2014). This reaction could be impeached in the Adriatic due to the portion over land (Fig. 2a). To evaluate the anthropogenic part of the super-micron
Fig. 9. Mass distribution of sulfate aerosols for coastal influence. The dashed blue curve shows the Adriatic data, while the red one is for the North-Western Mediterranean (Piazzola et al., 2012). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. Mass distribution of sulfate aerosols for marine influence. The dashed blue curve shows the Adriatic data, while the red one is for the North-Western Mediterranean (Piazzola et al., 2012). The solid black line corresponds to data recorded south of the French coast for maritime air masses during the summer 2013 (Piazzola, person. communication). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
sulfates, non sea-salt sulfate (nss SO2 4 ) was calculated from the þ mass ratio of ðSO2 Þ/Na using the following expression: 4
h i h i nssSO2 ¼ SO2 4 4
tot
io n h 0:252* Naþ
seawater
(2)
Fig. 11. Mass distribution of nss-sulfates measured in the Mediterranean for both a marine (blue line) and a coastal influence (black line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 11 shows the mass distribution of nss-sulfates measured in the North-Western Mediterranean for both the marine and coastal influence. Fig. 11 shows that the concentrations in the supermicron size range are larger for marine influence compared to the coastal one. The non sea-salt origin varies between 40 and 70% from the supermicron SO4. In other words, 40e70% of sulfur could be of anthropogenic origin. This in accordance with the results published by Schembari et al. (2014), who reported mass concentrations over the Mediterranean using a cruise on board the ship “Costa Pacifica”. The results reported in Fig. 11 could attest the occurrence of an atmospheric reaction between sea-spray and sulfates for marine air masses. However, it should be noted that the number of samplings of the present chemical analysis is rather limited to extract a conclusive picture. 5. Conclusion This paper presents a comparison between the aerosol concentrations measured in the Northern Adriatic and in the French Mediterranean for coastal and marine episodes, resulting from offshore winds and onshore winds, respectively. The first case deals with short fetches while the second one results from winds blowing on large fetches over open water. In this latter case, which deals with air masses of marine origin transported by the Sirocco in the Adriatic and by southern winds in the French coast, we note a good agreement between the concentrations of super-micrometer aerosols measured in the two locations. In view of the moderate wind speed periods analyzed in the present paper, this then indicates a similar sea surface production of sea-spray aerosols issued from bubble bursting processes in these two locations. By discussing the possible influence of different parameters on the sea-spray production and fluxes, as the sea temperature, the air-sea temperature difference or the seasonal effects, we conclude that the results reported for the NorthWestern Mediterranean on the variation of aerosol size distributions (e.g., D13) can be generalized to the Adriatic for the supermicron particles characteristics. For marine air masses, however, we note larger aerosol concentrations for sub-micron particles south of the French coast compared to the Northern Adriatic region. Since these particles are strongly dependent on the air mass characteristics, we noted that the industrial activities of the Rhone valley in the south of France induce larger anthropogenic aerosols for marine air masses. In contrast, for a coastal influence, the chemical analysis presented in Section 4 seems to indicate a larger importance of the anthropogenic impact in the Northern Adriatic compared to the West-Northern Mediterranean. In the both locations, however, the results confirm the occurrence of reactions between sea-spray and the organic matter present in the atmosphere. Acknowledgements The authors wish to express their gratitude to the staff of CNRISMAR in Venice for their contribution to the experimental campaign and for their technical support. Many thanks also to the staff of TNO The Hague and more particularly, Leo Cohen for the calibration of the aerosol probes. This study received funding from the European Union Seventh Framework Programme (FP7/ 2007e2013) under grant agreement n 262584FP7, JERICO. References Bardouki, H., Liakakou, H., Economou, C., Sciare, J., Smolik, J., Zdimal, V., Eleftheriadis, K., Lazaridis, M., Dye, C., Mihalopoulos, N., 2003. Chemical composition of size resolved atmospheric aerosols in the eastern
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Mediterranean during summer and winter. Atmos. Environ. 37 (n 2), 195e208. http://dx.doi.org/10.1016/S1352-2310(02)00859-2. rosols Blot, R., 2009. Etude des variations spatio-temporelles des distributions d’ae ^ tie re me diterrane enne (PhD Thesis of the University of Toulon). en zone co Bove, M.C., Brotto, P., Calzolai, G., Cassola, F., Cavalli, F., Fermo, P., Hjorth, J., , D., Nava, S., Piazzalunga, A., Schembari, C., Prati, P., 2016. PM10 source Massabo apportionment applying PMF and chemical tracer analysis to ship-borne measurements in the Western Mediterranean. Atmos. Environ. 125, 140e151. http://dx.doi.org/10.1016/j.atmosenv.2015.11.009. ISSN: 1352 -2310. Bowyer, P.A., Woolf, D.K., Monahan, E.C., 1990. Temperature dependence of the charge and aerosol production associated with breaking wave in a whitecap simulation tank. J. Geophys. Res. 95 (No. C4), 5313e5319. Cavaleri, L., 2000. The oceanographic tower Acqua Alta d activity and prediction of sea states at Venice. Coast. Eng. 39, 29e70. http://dx.doi.org/10.1016/S03783839(99)00053-8. Ceburnis, D., O'Dowd, C.D., Jennings, G.S., Facchini, M.C., Emblico, L., Decesari, S., Fuzzi, S., Sakalys, J., 2008. Marine aerosol chemistry gradients: elucidating primary and secondary processes and fluxes. Geophys. Res. Lett. 35, L07804. http://dx.doi.org/10.1029/2008GL033462. De Leeuw, G., Andreas, E.L., Anguelova, M.D., Fairall, C.W., Lewis, E.R., O'Dowd, C., Schulz, M., Schwartz, S.E., 2011. Production flux of sea spray aerosol. Rev. Geophys. 49, RG2001. http://dx.doi.org/10.1029/2010RG000349. De Leeuw, G., 1989. Modeling of extinction and backscatter profiles in the marine mixed layer. Appl. Opt. 28 (7), 1356e1359. http://dx.doi.org/10.1364/ AO.28.001356. Demoisson, A., Tedeschi, G., Piazzola, J., 2013. A model for the atmospheric transport of sea-salt particles in coastal areas. Atmos. Res. 132e133, 144e153. http:// dx.doi.org/10.1016/j.atmosres.2013.04.002. Draxler, R.R., Rolph, G.D., 2003. HYSPLIT (Hybrid Single-particle Lagrangian Integrated Trajectory) Model (NOAA Air Resour. Lab., Silver Spring, MD). http:// www.arl.noaa.gov/ready/hysplit4.html. Eleftheriadis, K., Colbeck, I., Housiadaa, C., Lazaridis, M., Mihalopoulos, N., Mitsakou, C., Smolık, J., Zdımal, V., 2006. Size distribution, composition and origin of the submicron aerosol in the marine boundary layer during the eastern Mediterranean ‘‘SUB-AERO’’ experiment. Atmos. Environ. 40, 6245e6260. Fairall, C.W., Davidson, K.L., 1986. Dynamics and modeling of aerosols in the marine atmospheric boundary. In: Monahan, E.C., MacNiocaill, G. (Eds.), Oceanic Whitecaps. Reidel, Hingham, Mass, pp. 195e208. Fitzgerald, J.W., 1991. Marine aerosols : a review. Atmos. Environ. 25 (A), 523e545. http://dx.doi.org/10.1016/0960-1686(91)90050-H. Hsu, S.A., 1986. A mechanism for the increase of wind stress (drag) coefficient with wind speed over water surfaces : a parametric model. J. Phys. Oceanogr. 16, 144e150. Intergovernmental Panel on Climate Change, 2013. 30 September 2013 working group I contribution to the IPCC fifth assessment report-climate change 2013. In: The Physical Science Basis- Final Draft Underlying Scientific- Technical Assessment, Accepted but Not Approved in Detail by the 12th Session of Working Group I and the 36th Session of the IPCC on 26 September 2013 in Stockholm, Sweden. Knipping, E.M., Dabdub, D., 2003. Impact of chlorine emissions from sea-salt aerosol on coastal urban ozone. Environ. Sci. Technol. 37 (2), 275e284. http:// dx.doi.org/10.1021/es025793z. Monahan, E.C., Davidson, K.L., Spiel, D.E., 1986. A model of marine aerosol generation via whitecaps and wave disruption. In: Monahan, E.C., Mac Niocaill, G. (Eds.), Oceanic Whitecap and Their Role in Air-sea Exchange Processes. Reidel, Dordrecht, pp. 167e174. http://dx.doi.org/10.1007/978-94-009-4668-2_16. Piazzola, J., Tedeschi, G., Demoisson, A., 2015. A model for the transport of sea-spray aerosols in the coastal zone. Bound-Lay.Meteorol. 155 (Issue 2), 329e350. http://dx.doi.org/10.1007/s10546-014-9994-3. Piazzola, J., Sellegri, K., Bourcier, L., Mallet, M., Tedeschi, G., Missamou, T., 2012. Physicochemical characteristics of aerosols measured in the spring time in the Mediterranean coastal zone. Atmos. Environ. 54, 545e556. http://dx.doi.org/ 10.1016/j.atmosenv.2012.02.057. Piazzola, J., Forget, P., Lafon, C., Despiau, S., 2009. Spatial variation of sea-spray fluxes over a mediterranean coastal zone using a sea-state model. BoundLay.Meteorol. 132 (1), 167e183. http://dx.doi.org/10.1007/s10546-009-9386-2. Piazzola, J., Bouchara, F., Van Eijk, A.M.J., De Leeuw, G., 2003. Development of the Mediterranean extinction code MEDEX. Opt. Eng. 42 (4), 912e924. http:// dx.doi.org/10.1117/1.1556765. Piazzola, J., Despiau, S., 1997a. Contribution of marine aerosols in the particle size distributions observed in Mediterranean coastal zone. Atmos. Environ. 31 (18), 2991e3009. http://dx.doi.org/10.1016/S1352-2310(97)00088-5. Piazzola, J., Despiau, S., 1997b. Vertical distribution of aerosol particles near the airesea interface in coastal zone. J. Aerosol Sci. 28, 1579e1599. http:// dx.doi.org/10.1016/S0021-8502(97)00020-7. Reid, J.S., Brooks, B., Crahan, K.K., Hegg, D.A., Eck, T.F., O'Neill, N., De Leeuw, G., et al., 2006. Reconciliation of coarse mode sea-salt aerosol particle size measurements and parameterizations at a subtropical ocean receptor site. J. Geophys. Res. 111 (D02202) http://dx.doi.org/10.1029/2005JD006200. Salter, M.E., Nilsson, E.D., Butcher, A., Bilde, M., 2014. On the sea-water temperature dependence of the sea spray aerosol generated by a continuous plunging jet. J. Geo Phys. Res. Atmos. 119, 9052e9072. http://dx.doi.org/10.1002/ 2013JD021376. , D., Navac, S., Schembari, C., Bove, M.C., Cuccia, E., Cavalli, F., Hjorth, J., Massabo
162
J. Piazzola et al. / Atmospheric Environment 132 (2016) 153e162
Udisti, R., Prati, P., 2014. Source apportionment of PM10 in the Western Mediterranean based on observations from a cruise ship. Atmos. Environ. 98, 510e518. http://dx.doi.org/10.1016/j.atmosenv.2014.09.015. Seinfeld, J.H., Pandis, S.N., 1998. Atmospheric Chemistry and Physics: from Air Pollution to Climate Change. Wiley-Interscience, New York. ISBN: 978-0-47172018-8. Sellegri, K., Gourdeau, J., Putaud, J.-P., Despiau, S., 2001. Chemical composition of marine aerosol in a Mediterranean coastal zone during the FETCH experiment. J. Geophys. Res. 106 (No. D11), 12023e12038. http://dx.doi.org/10.1029/ 2000JD900629 (AGU). Tedeschi, G., Piazzola, J., 2011. Development of a 2D marine aerosol transport model: application to the influence of thermal stability in the marine atmospheric boundary layer. Atmos. Res. 101 (no. 1e2), 469e479. http://dx.doi.org/
10.1016/j.atmosres.2011.04.013. Toba, Y., 1965. On the giant sea-salt particles in the atmosphere: II theory of the vertical distribution in the 10-m layer over the ocean. Tellus 17, 365e382. http://dx.doi.org/10.1111/j.2153-3490.1965.tb01429.x. Thorpe, S.A., Bowyer, P., Woolf, D.K., 1992. Some factors affecting the size distributions of oceanic bubbles. J. Phys. Oceanogr. 22, 382e389. Van Eijk, A.M.J., De Leeuw, G., 1992. Modeling aerosol particle size distributions over the North Sea. J. Geophys. Res. 97 (C9), 14417e14429. http://dx.doi.org/10.1029/ 92JC01214. Yoon, Y.J., Ceburnis, D., Cavalli, F., Jourdan, O., Putaud, J.P., Facchini, M.C., Decesari, S., Fuzzi, S., Sellegri, K., Jennings, S.G., O'Dowd, C.D., 2007. Seasonal characteristics of the physicochemical properties of North Atlantic marine atmospheric aerosols. J. Geophys. Res. 112, D04206. http://dx.doi.org/10.1029/2005JD007044.