# Characterization of carbonaceous aerosols over the Northern Adriatic Sea in the JERICO-NEXT project framework

Dario Massabò, Paolo Prati, Elisa Canepa, Mauro Bastianini, Alexander M. J. van Eijk, Tathy Missamou, Jacques Piazzola

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Dario Massabò, Paolo Prati, Elisa Canepa, Mauro Bastianini, Alexander M. J. van Eijk, et al.. Characterization of carbonaceous aerosols over the Northern Adriatic Sea in the JERICO-NEXT project framework. Atmospheric Environment, 2020, 228, 10.1016/j.atmosenv.2020.117449. insu-03668099

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# Characterization of carbonaceous aerosols over the Northern Adriatic Sea in the JERICO-NEXT project framework

Dario Massabò¹, Paolo Prati¹, Elisa Canepa²*, Mauro Bastianini³, Alexander M.J. Van Eijk⁴, Tathy Missamou⁵, Jacques Piazzola⁵

¹ University of Genova, Department of Physics and INFN, 16146 Genova, Italy;
Dario.Massabo@ge.infn.it; Paolo.Prati@ge.infn.it;

² CNR/IAS Unit of Genova, 16149 Genova, Italy; elisa.canepa@cnr.it

³ CNR/ISMAR Unit of Venezia, 30122 Venezia, Italy; mauro.bastianini@cnr.it

⁴ TNO The Hague, The Netherlands & Laboratoire de Recherche en Hydrodynamique, Energétique et Environnement Atmosphérique (LHEEA UMR 6598), Ecole Centrale de Nantes, France; lex.vaneijk@tno.nl

⁵ Mediterranean Institute of Oceanography (MIO UMR 7294) - University of Toulon (Seatech)-CS 60584 83041 Toulon cedex 9, France; missamou@univ-tln.fr; jacques.piazzola@univ-tln.fr

* Corresponding author: Elisa Canepa, CNR/IAS Unit of Genova, Via De Marini 6, 16149 Genova, Italy; elisa.canepa@cnr.it; Tel. + 39 010 6475 403; Fax + 39 010 6475 400

**Abstract:** The coastal aerosol represents a complex mixing between sea spray aerosols locally generated at the sea surface and a component generated by natural and/or anthropogenic sources on the nearby land. The aim of this paper is to study the atmospheric concentrations of carbonaceous compounds as measured in the coastal zone of the Northern Adriatic Sea between April 2017 and April 2018. The results show a quite large contribution of carbonaceous compounds, even when the sampled air mass is predominantly maritime. Although it is generally assumed that 70% and 90% of bulk organic carbon (OC) and elemental carbon (EC) concentrations, respectively, are found in the fine mode, we observed higher OC and EC concentrations in the coarse fraction (up to about 37% and 25%, respectively). This confirms the occurrence of mixing between anthropogenic matter and sea spray, mainly composed of coarse aerosol particles, during the atmospheric transport over the Mediterranean basin.

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# 1. Introduction

The earth system climate prediction is of key importance to humanity and provides guidance on how and where to act to avoid future disasters. One of the main reasons for the present uncertainties in climate and earth system modelling is the lack of reliable information on aerosol, in particular the fraction of marine origin (Richter and Veron, 2016; Canepa and Builtjes, 2017). These sea spray particles generated at the air-sea interface by wave breaking represent a major aerosol emission at the global scale (Huneeus et al., 2012) and play a crucial role in the Earth's climate through reducing direct radiative forcing (Murphy et al., 1998). They also affect cloud properties due to their ability to act as cloud condensation nuclei (Partanen et al., 2014) and ice nucleating particles (DeMott et al., 2010).

The size of sea spray particles can vary over a wide range, from submicron to about 500 µm at their formation (Veron, 2015). These particles consist of a mixture of inorganic salts - primarily sodium chloride (NaCl) and small amounts of other salts such as sulphate, calcium and potassium - and organic compounds. The particles' hygroscopicity, that is to say their ability to take up water, is determined by their size and chemical composition. The hygroscopicity of sea spray particles is commonly assumed to be nearly equal to that of pure sodium chloride. However, Zieger et al. (2017) demonstrated that the hygroscopic growth of inorganic sea spray is 8% - 15% lower than NaCl in terms of diameter change, probably due to the difference in chemical composition. Swietlicki et al. (2008) attributed the observed reduction of sea spray hygroscopic growth relative to NaCl in the remote marine atmosphere to internally mixed organic substances.

Under certain conditions, such as high biological activity in the water, the organic species make up a substantial fraction of the total particle mass, especially for small particles (<1 µm). As an example, measurements in the North Atlantic coastal area showed a marked seasonal variation in the marine aerosol chemical characteristics related to biological activity of the ocean (Cavalli et al.
58 2004, O'Dowd et al. 2004). Specifically, during winter the sea spray composition was 59 predominantly sea salt while, during periods of high biological activity, the inorganic sea spray 60 signal was progressively replaced by organic matter and non-sea-salt (nss)-sulphate, mainly in the 61 submicrometric region. While measurements like these have demonstrated that sea spray can take 62 up substantial concentrations of organic material, our still limited understanding of the mechanisms 63 that control the transfer of organic matter from seawater to sea spray prevents us from predicting 64 these concentrations (Wang et al., 2015a).

65 Sea spray has a significant influence on the coastal urban air quality (e.g., Knipping and Dabdub, 66 2003) through its chemical and physical interactions with continental aerosols and gases of both 67 natural and anthropogenic origin. Along the north-western Adriatic coast of Italy, the industrial area 68 of Porto Marghera with numerous chemical and oil refining plants is only a few kilometres away 69 from the popular tourist city of Venice. Consequently, significant efforts to measure the levels of 70 atmospheric chemical pollutants and the concentration and composition of aerosols in the Venice 71 lagoon have been carried out (Donateo et al., 2012; Salameh et al., 2015). The work of Piazzola et 72 al. (2016), in the framework of the JERICO project (http://www.jerico-ri.eu/previous-project/jerico- 73 fp7/, accessed 20/08/2019), was a novelty in this context. For the first time aerosol samples were 74 collected on the Acqua Alta platform located 15 km off the coast of Venice. The study was 75 conducted from May to September 2014 and included both physical and chemical analysis (Cl, 76 NO₃, SO₄, Oₓ, Na, NH₄, K, Ca) of aerosol samples.

77 Field observation of carbonaceous aerosols in coastal areas reported in the literature are scarce 78 and only span limited time periods. The present work, in the framework of the JERICO-NEXT 79 project (http://www.jerico-ri.eu/, accessed 20/08/2019), extends the previous efforts by focussing on 80 carbonaceous aerosols, both elemental carbon (EC) and organic carbon (OC), fractions that were 81 not considered in the JERICO measurements. This paper discusses a chemical analysis of aerosol 82 concentrations sampled during four episodes between April 2017 and April 2018 with different 83 meteo-marine conditions and air mass history in the Northern Adriatic.
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<figure><img src="image_2.png" /><figcaption>**Figure 1:** The Acqua Alta platform. *colour online only*</figcaption></figure>

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<figure><img src="image_3.png" /><figcaption>**Figure 2:** The study area in the Adriatic Sea with the position of the Acqua Alta platform (black square in the right figure). *colour online only*</figcaption></figure>

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## 2. Material and Methods

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### 2.1 The Acqua Alta platform

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The Acqua Alta platform is an oceanographic tower (Figure 1) located in the northern part of the Adriatic Sea (Figure 2, left panel), a basin oriented from north-west to south-east, about 750 km long and 200 km wide connected at its southern end with the Ionian Sea, and hence with the Mediterranean Sea, through the Otranto strait, which is about 80 km wide. The depth of the northern part of the Adriatic Sea is rather limited (Figure 2, right panel), while about 150 km further south it suddenly deepens. The Adriatic Sea is mostly surrounded by mountains, the Dinaric Alps north-eastward and the Apennines south-westward along the Italian peninsula. Flat areas can be found only along the north-western and south-western coast of the basin, in correspondence of the Po Valley in Northern Italy and the Apulia Region in Southern Italy, respectively.

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The two most prominent types of meteorological and wave conditions of the Adriatic basin, especially in its northern part, are characterized by two different wind regimes that are named bora and sirocco (Pomaro et al., 2017). Bora is blowing from the north-east and is a cold, gusty and often intense wind. Due to its short fetch - which represents the distance over water for which the wind has blown constantly - of about 100 km, it prevalently induces young and steep waves that frequently break, with period ranging between 5 and 7 s. Sirocco blows from south-east along the main axis of the basin, thus with a long fetch of about 750 km. Less intense than bora, it generates high and long - but not steep - waves, with periods ranging between 7 and 10 s. Therefore, the Northern Adriatic Sea is under the influence of continental and marine air masses depending on the wind direction. Continent-originating winds may blow over the sea, giving the transported air masses a mixed character (defined as “coastal”).

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Acqua Alta is placed in this complex area, 15 km off the coast in the Venice lagoon, at 45°18.83' N, 12°30.53' E (black square in the right panel of Figure 2). It has been operational since the early 70s and it is currently managed by the Institute of Marine Sciences of the National Research Council of Italy (ISMAR-CNR). It is a steel jacket type platform (Cavaleri, 2000). The first floor is
located at 6.5 m above Mean Sea Level (MSL). The second floor, at 9 m above MSL, contains two power generators, batteries, and a small workshop; besides a peripheral catwalk, there is a 5.5 x 4.0 m² platform where several instruments can be deployed. The 5.0 x 7.0 m² hut at the third floor, at 12 m above MSL, hosts the living quarters: up to five persons can stay on board for several days. On the 70 m² top terrace, 15.5 m above MSL, two 2 m³ water tanks, an 8 m-high meteorological mast, and two wind generators are positioned; the remaining space is used for scientific instrumentation. Multiple power sources can be controlled onboard or remotely.

The Acqua Alta oceanographic tower allows for long-term deployment of automated instrumentation - meteorological, oceanographic, physical and chemical instruments - connected in real time with the ashore control station, as well as for manned operations during dedicated campaigns (Pomaro et al., 2018).

## 2.2 *Sampling and analytical procedures*

A 13-stage low pressure cascade impactor (DLPI, Dekati), with quartz fiber substrates, was installed on the top level of the Acqua Alta platform to sample directly in the atmosphere. The DLPI is a multi-jet device which operates at a nominal flow rate of 18.34 l/min with cut-off diameters (D50%) of 0.03, 0.06, 0.11, 0.17, 0.27, 0.41, 0.66, 1.0, 1.7, 2.5, 4.1, 6.9 and 10.6 µm (DLPI impactor data-sheet, Dekati). The impactor segregates particulate matter (PM) depending on its aerodynamic diameter: impaction stages consist of circular quartz filters (diameter = 25 mm) and PM is collected, on each stage, to form a round non-uniform deposit (maximum diameter: 8 mm). Four size-segregated sets of aerosol samples were collected during the present experiment; hence a total of 52 quartz filters was deployed. The organic carbon (OC) and elemental carbon (EC) analysis was made through an off-line Thermal Optical Transmittance (TOT) analysis of the exposed quartz filters by a SUNSET OC/EC analyser (Birch and Cary, 1996) operated with the EUSAAR_2 protocol (Cavalli et al., 2010). Quality control has been guaranteed by a calibration with a standard sucrose solution before each measurement run, has requested by the Sunset EC/OC analyser manual. After the sampling, a rectangular punch including the whole aerosol deposit has
been cut from each quartz filter and sent to the TOT analysis. OC and EC blank levels were subtracted by measuring, for every series, the corresponding field blank. No data reduction was performed on the cascade impactor data and the size distributions presented in the paper are thus based on raw data (i.e. the OC/EC mass detected in each impaction stage) without any correction for the impactor response function. In the following, the main modes are identified and comparatively discussed: the lack of a refined correction for the impactor response function does not prevent to draw conclusions on the main modes of the PM size distribution and on its correlation with meteorological patterns.

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## 3 Measurements overview and results

The chemical part of the JERICO-NEXT measurements consisted of the collection of four size-segregated sets of aerosol samples. Each set consisted of 13 quartz filters, which resulted in a total of 52 filters that were collected, and it was exposed during one of four scheduled measurement periods between April 2017 and April 2018. These periods were characterized by different meteorological and wave conditions, as specified below (and in Table 1):

- set A: from 11:00 UTC of April 19th to 11:30 UTC of April 20th, 2017 (sampling time = 24 h and 30 min); prevailing north-east wind direction with wind speeds up to approximately 14 m s⁻¹ (intense Bora conditions); the average significant wave height was 0.66 m with a maximum value of 1.15 m;

- set B: from 12:05 UTC of March 1st to 12:20 UTC of March 2nd, 2018 (sampling time = 24 h and 15 min); wind direction was from north-east to north-west with wind speeds up to approximately 11 m s⁻¹; the average significant wave height was 0.77 m with a maximum value of 0.96 m; during the data collection period, there was a light snowfall;

- set C: from 10:40 UTC of June 14th to 10:50 UTC of June 15th, 2017 (sampling time = 24 h and 10 min); wind direction and speed were variable with prevailing northern winds in the second part of
the period, the average wind speed for the period was about 3 m s⁻¹; the average significant wave height was 0.25 m with a maximum value of 0.48 m;

- set D: from 10:00 UTC to 20:00 UTC of April 15th, 2018 (sampling time of 12 h - it should be noted that this sampling time was about half of the others); wind direction was mainly from north-east; the average wind speed was about 5 m s⁻¹; the average significant wave height was 0.26 m with a maximum value of 0.34 m.

<table><caption>**Table 1:** Summary of the meteo-marine conditions during the sampling periods ($U_{max}$ = maximum wind speed; $U_{av}$ = average wind speed; $H_{Sav}$ = average significant wave height; $H_{Smax}$ = maximum significant wave height)</caption><thead><tr><th>SET</th><th>Wind conditions</th><th>Wave conditions</th><th>Air temperature</th><th>Atmospheric pressure at sea level</th></tr></thead><tbody><tr><td>A</td><td>prevailing north-east wind direction,<br/>$U_{max}$ ~ 14 m s⁻¹<br/>(intense Bora conditions)</td><td>$H_{Sav}$ = 0.66 m<br/>$H_{Smax}$ = 1.15 m</td><td>9.2 °C – 13.4 °C</td><td>1014.8 hPa –<br/>1023.1 hPa</td></tr><tr><td>B</td><td>north-east to north-west wind direction,<br/>$U_{max}$ ~ 11 m s⁻¹</td><td>$H_{Sav}$ = 0.77 m<br/>$H_{Smax}$ = 0.96 m</td><td>- 0.7 °C – 3.3 °C</td><td>995.3 hPa –<br/>1012.4 hPa</td></tr><tr><td>C</td><td>variable wind direction, prevailing northern winds in second period part,<br/>$U_{av}$ ~ 3 m s⁻¹</td><td>$H_{Sav}$ = 0.25 m<br/>$H_{Smax}$ = 0.48 m</td><td>18.8 °C – 24.5 °C</td><td>1011.0 hPa –<br/>1014.9 hPa</td></tr><tr><td>D</td><td>prevailing north-east wind direction,<br/>$U_{av}$ ~ 5 m s⁻¹</td><td>$H_{Sav}$ = 0.26 m<br/>$H_{Smax}$ = 0.34 m</td><td>16.2 °C – 18.4 °C</td><td>1009.4 hPa –<br/>1012.2 hPa</td></tr></tbody></table>
<figure><img src="image_4.png" /></figure>

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182 **Figure 3:** Air mass backward trajectory calculated using the NOAA HYSPLIT model for the 2017 sets of data: set A (left), and set C (right). *colour online only*

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<figure><img src="image_5.png" /></figure>

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186 **Figure 4:** Air mass backward trajectory calculated using the NOAA HYSPLIT model for the 2018 sets of data: set B (left), and set D (right). *colour online only*

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Figures 3 and 4 show air mass backward trajectories calculated using the NOAA HYSPLIT model (Stein et al., 2015; Rolph et al., 2017). The four analysed episodes range from long distance (set A) to local transport (set D), and comprise two mixed episodes (set B and C). All trajectories are partly over land, but the times spent over land differ as does the geographical origin of the air masses. During the sampling of set D, the back-trajectories reaching the Aqua Alta site passed at low levels over the sea while during sampling A they passed at high levels. During the sampling of set B and C, the back-trajectories showed a mixed behaviour: the last section of all of them was low for set B, while some of them were at high levels and some other at low levels for the entire path for set C. During the sampling periods of sets A and B, local sea spray production was likely because of the high winds and waves, while during the sampling periods of set C and D sea spray production should be more limited.

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In order to check that our measurements were not perturbed by the presence of relevant mineral dust concentrations, we verified that both the AOD (Aerosol Optical Depth) and the Angstrom coefficient values measured using the sun photometer located in the eastern Italian coastal zone excluded the presence of Saharan dust intrusion. In particular, the photometer located in Lecce (https://aeronet.gsfc.nasa.gov/cgi-bin/data_display_aod_v3?site=Lecce_University&nachal=2&level=2&place_code=10) was running at the same dates and no dust was detected there.

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Total concentrations recorded for carbonaceous species for the four episodes are reported in Table 2. The reported main random uncertainties are calculated from the propagation of two kind of errors. The first one is related to the sampled air volume, and can be estimated to be ±3%. The second one is related to the instrumental measurement error and is provided directly by the EC-OC analyser software, which considers hundreds of replicate runs (relative standard deviation is typically 5%) as well as instrument blanks obtained from analysing previously run filters without re-opening the oven.
In accordance with findings reported earlier in the literature, in all episodes the mass concentrations of OC were substantially larger than those of EC. For both species, the larger concentrations were recorded during episodes A et D (with a maximum of about 11 µg m⁻³ of OC for episode D), which corresponded to a long-range northern origin of the air mass, and to a local meridional flow, respectively. The episodes B and C corresponded to a flow that was basically zonal, and yielded smaller concentrations of OC and EC.

<table><caption>**Table 2:** EC and OC concentrations measured during the four sampling periods. First two rows: total concentration in all 13 stages of the DLPI-Dekati impactor; row three: OC/EC ratio of the total concentration in all 13 stages of the DLPI-Dekati impactor; row four and five: sub-micrometric fraction (8 stages, from 0.032 µm to 1.03 µm included); last two rows: percentages of EC and OC in the sub-micrometric mode.</caption><thead><tr><th>SET</th><th>A</th><th>B</th><th>C</th><th>D</th></tr></thead><tbody><tr><td>EC (µg m<sup>-3</sup>)</td><td>0.42 ± 0.07</td><td>0.12 ± 0.04</td><td>0.15 ± 0.04</td><td>0.44 ± 0.09</td></tr><tr><td>OC (µg m<sup>-3</sup>)</td><td>6.9 ± 0.9</td><td>4.0 ± 0.6</td><td>3.9 ± 0.5</td><td>11.2 ± 1.4</td></tr><tr><td>OC/EC</td><td>16.4 ± 3.5</td><td>33.3 ± 12.2</td><td>26.0 ± 7.7</td><td>25.4 ± 6.1</td></tr><tr><td>EC<sub>Subµ</sub> (µg m<sup>-3</sup>)</td><td>0.35 ± 0.05</td><td>0.09 ± 0.03</td><td>0.13 ± 0.04</td><td>0.36 ± 0.06</td></tr><tr><td>OC<sub>Subµ</sub> (µg m<sup>-3</sup>)</td><td>4.8 ± 0.6</td><td>2.6 ± 0.4</td><td>2.8 ± 0.3</td><td>7.1 ± 1.0</td></tr><tr><td>EC<sub>Subµ</sub> (%)</td><td>83 ± 3</td><td>75 ± 12</td><td>86 ± 12</td><td>81 ± 5</td></tr><tr><td>OC<sub>Subµ</sub> (%)</td><td>70 ± 2</td><td>65 ± 2</td><td>71 ± 1</td><td>63 ± 1</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Figure 5:** EC mass collected in each stage of the DLPI impactor. DLPI stages are identified with the corresponding cut-off diameter. Set A: long distance transport, and local sea spray production; set B: long distance and local transport, and local sea spray production; set C: long distance and local transport; set D: local transport. *colour online only*</figcaption></figure>

Aerosol size distributions of the EC component for each episode are shown in Figure 5. The values plotted in Figure 5 are expressed in terms of mass concentration (µg m⁻³) collected in each DLPI stage, characterized by its cut-off size (D50%). Figure 5 shows that the major contribution of the EC component is always found in the submicrometric mode, but the mass size distributions differ from each other. Sets A and D show higher EC concentration than sets B and C, and the maximum EC values, between 0.08 and 0.10 µg m⁻³, are recorded in the stages 0.409 and 0.662 µm. Set B yields the lowest EC values with a maximum of about 0.03 µg m⁻³ in the stage 0.063 µm. Set C produces low values as well, with maximum values of about 0.03 µg m⁻³ through the stages 0.266 and 0.662 µm.
<figure><img src="image_7.png" /><figcaption>**Figure 6:** OC mass collected in each stage of the DLPI impactor. DLPI stages are identified with the corresponding cut-off diameter. Set A: long distance transport, and local sea spray production; set B: long distance and local transport, and local sea spray production; set C: long distance and local transport; set D: local transport. **colour online only**</figcaption></figure>

Figure 6 reports the aerosol size distribution of OC species collected in each DLPI stage. Compared to EC, OC concentrations between the various sets differ less, but the same order is found: set D followed by set A yield higher concentrations than sets B and C. For set D the highest OC values, about 1.2 µg m⁻³, are between 0.266 and 0.662 µm. Set A shows three different modes, of decreasing amplitude, at the stages 0.063, 0.409 and 6.87 µm, in the concentration range of about 0.7-0.9 µg m⁻³. Set B shows an almost flat distribution (except for the smallest stages, where the maximum value of nearly 0.6 µg m⁻³ is reached), while set C exhibits a fluctuating behaviour with the highest values of about 0.5 µg m⁻³ in the submicrometer region.
<figure><img src="image_8.png" /><figcaption>**Figure 7:** EC:OC concentration ratio measured in each stage of the DLPI impactor. DLPI stages are identified with the corresponding cut-off diameter. Set A: long distance transport, and local sea spray production; set B: long distance and local transport, and local sea spray production; set C: long distance and local transport; set D: local transport. **colour online only**</figcaption></figure>

A comparison between Figures 5 and 6 reveals that EC concentrations reside mainly in the submicrometer mode, whereas a significant fraction of OC is also found in the supermicrometer mode, especially for set D and to a lesser extent for set A. To highlight this, Figure 7 shows the size-segregated EC:OC concentration ratio. For all sets the higher values are found in the submicrometer region. Specifically, set A shows the highest values - with the maximum at 0.662 µm - followed by sets C and D. Set C presents its maximum at 0.266 µm with a steep decrease for very small particles. A unique situation was found for set B with the EC:OC concentration ratio quite stable around 0.025, except for particles smaller than 0.174 µm where the ratio peaked at 0.08.
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## 4 Discussion

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The aim of this paper is to advance the knowledge of the atmospheric concentration of carbonaceous compounds in coastal areas. More specifically, the present study focuses on the coastal zone of the Northern Adriatic, which mainly receives air masses with a mixed character of marine and continental origin with a large anthropogenic influence. EC is not expected in nascent sea spray aerosol - it has a primary origin from incomplete combustion of organic material from anthropogenic activities like traffic, residential heating, industry, and energy production - but may be found in marine aerosol because introduced by mixture with anthropogenic aerosols (Sandrini et al., 2014; Bertram et al., 2018). On the contrary, nascent sea spray aerosol includes OC, which is emitted from the organic matter reservoir in the Earth's oceans (Ceburnis et al., 2016). Furthermore, like EC, OC is directly emitted from anthropogenic sources, but can be also formed in the atmosphere (secondary organic aerosol) through condensation to the aerosol phase of low vapour pressure compounds from precursor organic gases (Sandrini et al., 2014).

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As shown in Table 2, the total OC and EC concentration levels vary up to 11.2 µg m⁻³ and 0.44 µg m⁻³, respectively, both recorded during episode D. Coastal environments are complex systems, where natural and anthropogenic environmental variables determine specific conditions that are unlikely to be duplicated in different locations. Nevertheless, when making a comparison with the limited literature available, this maximum OC concentration is larger than the majority of the OC values measured in coastal non-urban areas for similar diameter interval and average time. The OC concentration during episode D exceeds, by a factor of about four, the maximum OC concentrations measured by Bardouki et al. (2003) in the Eastern Mediterranean for summer and winter seasons, and by Ding et al. (2019) above the Yellow Sea and the Bohai Sea. It is about 30% larger than the maximum OC value measured by Simoneit et al. (2004) during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia). It exceeds (by a factor of more than two nearly in all cases) those measured in the north-western Mediterranean by Piazzola et al. (2012), and in the south-western Mediterranean (Sicily channel, Capo Granitola station) by Dinoi et al. (2017). It also
exceeds the majority of the OC values measured over the East China Sea, for all seasons, by Wang et al. (2015b). Nevertheless, in this last case, the OC concentrations reached about 30 µg m⁻³ (3 times our maximum) for one sample during fall, about 13 µg m⁻³ for one sample during winter, and about 10 µg m⁻³ for two samples during spring. The authors attributed these episodes to peculiar environmental conditions, that were elucidated as related to transport of air masses from regions where biomass burning, indoor heating, and dust storms were taking place, respectively. In our case, the very high concentrations during episode D, compared to the majority of the ones reported in the literature, are attributed to the low wind speed, which does not favour atmospheric dispersion, and mainly to the local low level flow pattern (Figure 4) that crosses a strongly urbanized and industrialized area.

As demonstrated by Figures 5 and 6, episode A also yielded fairly high OC and EC concentrations. From a meteorological point of view, this episode was characterized by a very high wind speed and short fetch (about 15 km). These features make it comparable to episode 3 in Piazzola et al. (2012), which presented measurements recorded on the island of Porquerolles, located in the Mediterranean just south of the French city of Toulon. Specifically, episode 3 was characterized by constant north-northwestern flux from UK travelling down south western France and with a fetch of about 25 km. During episode 3, Piazzola et al. (2012) recorded the smallest concentration of anthropogenic species, which was explained by enhanced pollutants dispersion in high winds speed episodes (Piazzola and Despiau, 1997). However, Piazzola et al. (2012) still recorded appreciable OC concentration (about 5 µg m⁻³) and therefore conjectured that a large fraction of it was of oceanic origin and picked up during the short transit over sea. Thus, we hypothesize that at least a fraction of the significant OC concentrations recorded during episode A (about 7 µg m⁻³) was attributable to oceanic origin.

Episodes B and C were both characterized by less distinctive meteorological conditions than episodes A and D. During episode B wind speed was high, but less than during episode A. During episode C wind speed was low, like during episode D, but the majority of the air back-trajectories
were at high levels. Thus, most air masses arriving at the platform had spent considerable time at higher elevations and only a few of these were capable of low-level transport of local emissions and pollutant accumulation was less likely. As a result, carbonaceous aerosols concentrations for episodes B and C were lower than for episodes A and D, but a clear relation between the meteorological scenario and these lower concentrations was not readily found.

During episode B there was a light snowfall. Unfortunately, precipitation measurements were not available at the platform. Nevertheless, from cumulated measurements recorded at the Cavallino Treporti station (courtesy of the ARPAV agency), about 17 km far, a precipitation rate of the order of 0.1 mm h⁻¹ is conceivable. Snowfall is thought to lead to even more efficient scavenging than rain for water equivalent precipitation rates of less than 1.5 mm h⁻¹ (Bernauer, 2015), but this effect was not discernible in our measurements.

It is worth to note that the OC contribution, especially during episodes A and D, was significantly higher than observed during the phytoplanctonic blooming period in the Northern Atlantic coastal area (Cavalli et al., 2004) for clean marine conditions. This supports the hypothesis that in our measurements area a significant fraction of OC is likely composed of anthropogenic species. While Figure 6 shows that the major contribution of the OC compounds is found in the submicrometric mode, an appreciable supermicrometer component is present, especially for set D and to a lesser extent for set A. In the first case, this is probably due to the meteorological conditions that favoured the accumulation of pollutants, as discussed above. For episode A, characterised by very high wind speed that in principle favoured both pollutant dispersion and large sea spray production, mixing between sea spray and anthropogenic carbonaceous species can be hypothesized. In support of this hypothesis, Figure 6 suggests an OC peak around 5-6 µm, right at the centre diameter of most sea spray modes. This suggests that sea spray can act as vehicle for anthropogenic carbon. It appears to be in agreement with observation by Yan et al. (2019) over Southeast China Sea, and by Wang et al. (2015b) over the East China Sea.
Although it is generally assumed that 70% and 90% of bulk OC and EC concentrations, respectively, are found in the fine mode (Sciare et al., 2008), we can see in Table 2 that these portions are lower: the average submicrometer OC percentage is 67% and the average EC percentage is 82%. Accordingly, the OC and EC percentages in supermicrometer aerosols are higher, with an average of 33% and 18%, and peak values of 37% (set D) and 25% (set B), respectively. This again confirms the occurrence of mixing between sea spray, consisting of quite large aerosol particles, and anthropogenic matter during the atmospheric transport.

The question of the representativeness of the present study, which discusses four episodes with different meteo-marine conditions and air masses history in a specific Northern Adriatic area, can be raised. In general, the difficulties in making measurements of carbonaceous compounds in coastal zone, including high experimental cost and technical challenges, prevent exhaustive long-term and detailed data collection - in particular directly above the sea, like in the present study. Therefore, the four sets collected during JERICO-NEXT, albeit limited in number, for a specific location, and with uncertainties, provide a contribution to the sparse literature data already available in order to gain a more complete picture of the topic.

367

## 5 Conclusions

This paper presents a chemical analysis for EC and OC components in atmospheric aerosols acquired during four episodes with different meteo-marine conditions and air mass histories, in the Northern Adriatic between April 2017 and April 2018.

Our results show that OC concentrations are substantially higher than usually reported in the literature for similar maritime areas in two episodes characterized by: i) high wind speed and sea spray production, and ii) low wind speed and stagnant flow within a strongly urbanized and industrialized area. This confirms the major influence of wind conditions (e.g., Piazzola et al., 2012) and air masses trajectories (e.g., Sciare et al., 2008) on OC in coastal areas.
In the present study, the OC and EC percentages in supermicrometer aerosols were higher than generally assumed in the literature. In our opinion, this can be ascribed to the occurrence of mixing, during atmospheric dispersion, between the coarse sea spray component and the anthropogenic carbonaceous emissions. This conclusion, even if based on a limited number of samples collected in a specific location, indicates that the mixing of anthropogenic species and larger aerosols resulting from marine emission can be largely underestimated and calls for more extended and systematic studies.

## Acknowledgments

The authors wish to express their gratitude to the staff of CNR-ISMAR in Venice for their contribution to the JERICO-NEXT experimental campaign and for their technical support. The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY website (http://www ready.noaa.gov) used in this publication. The DLPI-Dekati impactor was provided by the Mediterranean Institute of Oceanography (MIO UMR 7294) - University of Toulon (Toulon, France). The filters' exposition was funded by the European Commission's H2020 Framework Programme under grant agreement No. 654410, JERICO-NEXT. The filters' analyses were funded by the Physics Laboratory for the Environment (LUNA) of the Department of Physics, University of Genova (Genoa, Italy).

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