2.1 Sampling sites

Observations took place at the Roberto Sarao station on the island of Lampedusa (35^∘31^′5^′′ N, 12^∘37^′51^′′ E; 20 m a.s.l.) from 11 June to 5 July 2013. Ancillary measurements for this study taken at Ersa at the northern tip of Cap Corse (42^∘58^′5^′′ N, 9^∘22^′49^′′ E; 560 m a.s.l.) are also considered. The positions of the stations are shown in Fig. 1.

Figure 1The Mediterranean basin. The two sites considered in this study, Lampedusa and Ersa, are indicated in white dots. Image is courtesy of © Google Earth 2019.

2.2 Instrumentation, measurements and data

Instruments at the Lampedusa supersite were housed in the PEGASUS (Portable Gas Field and Aerosol Sampling Unit) station, a portable observatory initiated by LISA, described in Mallet et al. (2016). Relevant to this study, a cToF-AMS (Aerodyne Inc., Billerica, USA) was used to measure the size-resolved composition of non-refractory particulate matter below 1 µm (NF-PM₁) (Drewnick et al., 2005). Data were collected with a 3 min time resolution. The cToF-AMS was operated from a certified total suspended particulate (TSP) sampling head (Rupprecht and Patashnick, Albany, NY, USA), followed by a cyclone impactor cutting off aerosol particles larger than 1 µm in aerodynamic diameter (using a flow rate of 16 lpm). A Nafion drier was used; however, the relative humidity at the inlet of the cToF-AMS was checked throughout the campaign and was always below 55 %.

A particle-into-liquid sampler (Metrohm PILS; Orsini et al., 2003) was installed on a TSP inlet and collected samples approximately every hour. Denuders to remove acid/base gases were not used. Samples were analysed for major inorganic and organic anions (F⁻, Cl⁻, ${\mathrm{NO}}_{\mathrm{3}}^{-}$, ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$, ${\mathrm{PO}}_{\mathrm{4}}^{\mathrm{3}-}$, HCOO⁻, CH₃COO⁻, (COO⁻)₂) and cations (Na⁺, ${\mathrm{NH}}_{\mathrm{4}}^{+}$, K⁺, Ca²⁺, Mg²⁺) using ion chromatography (Metrohm, model 850 Professional IC) equipped with Metrosep A Supp 7 pre-column and column for anions measurements and Metrosep C4-250 mm pre-column and column for cation measurements and a 500 µL injection loop. The device was operated with a 1 h time resolution.

A 13-stage rotating cascade impactor (NanoMOUDI model 125B; Marple et al., 1991) was used to measure the size-segregated inorganic elemental composition. The NanoMOUDI impactor, also operated from the TSP inlet, allows the separation of the particles into 13 size classes from 10 nm to 10 µm diameters with a backup stage. Each sample was collected for 3 d with a flow rate of 10 lpm to ensure enough material was collected on each impactor stage. The 47 mm diameter polytetrafluoroethylene filters (2 µm pore size) were used and coated with high-quality vacuum grease (Dekati DS-515) to avoid bouncing. They were then analysed using X-ray fluorescence (PW-2404 spectrometer by PANalytical^™) for the particulate elemental concentrations for elements from Na to Pb as described in Denjean et al. (2016).

A scanning mobility particle sizer (SMPS) measured the mobility number size distribution of aerosols every 3 min from 14.6 to 661.2 nm diameter. The instrument is composed by an X-ray electrostatic classifier (TSI Inc., model 3080) and a differential mobility analyser (DMA; TSI Inc., model 3081) and a condensation particle counter (CPC; TSI Inc., model 3775) operated at 1.5∕0.3 L min⁻¹ aerosol/sheath flows. Data were corrected to take into account the particle electrical charging probabilities, the CPC counting efficiency and diffusion losses. Each scan was recorded with a 5 min time resolution. A drier was not used on the SMPS inlet, and therefore the size distributions reported are for ambient conditions.

A GRIMM optical particle counter (OPC; GRIMM Inc., model 1.109) was used to measure the number size distribution over 31 size classes ranging from 0.26 up to 32 µm (nominal diameter range assuming the aerosol refractive index of latex spheres in the calibration protocol). The instrument was operated at a 6 s resolution and data were acquired as 3 min averages.

The equivalent black carbon mass concentration (eBC) was determined by the measurement of light attenuation at 880 nm performed by a spectral aethalometer (Magee Sci. model AE31) operated at a 2 min time resolution and equipped with a TSP particle inlet. As the evaluation of eBC is used as a qualitative tracer of pollution, the factory mass conversion factor of 16.6 m² g⁻¹ was applied to the raw measurement of attenuation without further corrections.

The meteorological measurements (air pressure, temperature, relative humidity, wind direction and speed and precipitation) were collected by a Vaisala Milos 500 station with a sampling rate of 10 min. The wind sensor was installed on a 10 m meteorological tower, while the air temperature and humidity were measured at a height of 2 m.

2.3 Data analysis

3.1 Analysis of local and synoptic meteorology

The analysis of the hourly resolved 144 h air-mass back trajectories provides an indication of the origin of the air masses sampled at Lampedusa during the field campaign. Six distinct clusters are identified (Fig. 2). Cluster 1 (eastern Mediterranean) is representative of air masses that circulate around the eastern–central Mediterranean basin before arriving at the Lampedusa site (average altitude of 400 m). Cluster 2 (central Europe) is representative of air masses arriving from central Europe (average altitude of 800 m). Cluster 3 (Atlantic) is representative of more marine-like air masses that predominately originate over the Atlantic Ocean, pass over the Strait of Gibraltar between Spain and Morocco, and cross the western Mediterranean basin (average altitude of 500 m). Cluster 4, western Europe, cluster 5, mistral (low), and cluster 6, mistral (high), all have similar angular trajectories but are distinguishable by their different wind speeds and altitudes (although the Euclidian method of cluster analysis only considers differences in horizontal distances). The two mistral clusters typically originate over the northern Atlantic Ocean, travel over France at a high altitude before descending over the western Mediterranean and travelling with relatively higher wind speeds towards Lampedusa. The altitude of mistral (high) was, on average, higher than mistral (low) (1400 and 1000 m, respectively) and also coincided with higher wind speeds at Lampedusa (13 and 9 m s⁻¹, respectively). In comparison, the trajectories of the western Europe cluster spent much more time circulating at lower altitudes (700 m, on average) over the western Mediterranean basin and, to a certain extent, east of the Lampedusa site.

Figure 2(a) Hourly 144 h (6 d) back trajectories from Lampedusa from 10 June to 5 July 2013, cut off at 120 h (5 d). Colours represent the assigned cluster. (b) The same as panel (a) but cut off at 24 h.

As a complement, the pressure, temperature, relative humidity, wind speed and direction time series recorded at the station are shown in Fig. 3. Two main weather regimes are observed: the former characterized by intense (up to nearly 20 m s⁻¹) north-westerly winds, persisting for several days (10–13 and 22–30 June) and cool temperatures, whereas the latter associated with low-gradient anticyclonic conditions and light winds from the east or south-east, also favouring warmer temperatures (14–21 June and 1–3 July). Temperatures were relatively stable over the sampling period, fluctuating between approximately 18.5 and 28.2 ^∘C. The relative humidity typically ranged between 70 % and 82 % with very few and very brief episodes of drier air masses (relative humidity close to or below 50 %). The wind speed and direction distributions during the campaign can be compared to the June–July climatology from 1999 to 2012 (Fig. 4). During the sampling period of this study, the frequency of winds from the north-westerly sectors was nearly double the average when compared to normal conditions, approaching 40 % with high winds speeds exceeding 10 m s⁻¹ observed during more than 20 % of the time.

Figure 3Meteorological conditions (wind direction and speed, relative humidity, pressure and temperature) measured at the Lampedusa site during SOP-1a.

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Figure 4Wind speed and direction at Lampedusa during the period from 5 June to 5 July during the years 1999–2012 (a) and during this campaign (b).

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Data of sea level pressure and 1000 mbar meridional wind component composite anomalies obtained from the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis (Kalnay et al., 1996) indicate that this particular situation was induced by a dipolar pattern, characterized by positive pressure anomalies in the western Mediterranean and negative ones in the eastern part of the basin (see Fig. S1 in the Supplement). This produced a persisting, stronger-than-normal gradient over southern Italy. As a consequence, surface dust episodes typically driven by strong south or south-easterly winds, associated to cyclonic systems moving along northern African coasts, were basically absent during the campaign.

3.2 Aerosol composition

The dry NR-PM₁ concentrations measured at Lampedusa by the cToF-AMS ranged from 1.9 to 33.4 µg m⁻³, with a mean of 10.2 µg m⁻³ over the sampling period. Sulfate contributed the most to the measured NR-PM₁ mass (41 % ± 9 % on average), followed by significant contributions from organics (31 % ± 8 %) and ammonium (17 % ± 3 %). The eBC, nitrate and sea salt (scaled from the NaCl component of m∕z 58) contributed 6 % (±4 %), 1 % (±0.4 %) and 3 % (±2 %), respectively. Figure 5 shows the total PM₁ concentration (calculated as the sum of the individually measured species), with contribution from each of the species, as well as the calculated PM₁ mass concentration from the SMPS (assuming an average density based on the composition data). There was reasonable agreement (slope of 0.62; R²=0.67) between the PM₁ mass concentration calculated from composition measurements and the SMPS, with discrepancy observed during periods of high sulfate concentrations from the eastern Mediterranean. This could be due to a combination of a broader accumulation mode exceeding the upper size limits of the cToF-AMS inlet, differences in sampling line relative humidity or unaccounted-for variations in the collection efficiency. This figure also contains an indication of the air-mass origin over the sampling period.

Figure 5The time series of PM₁ mass concentration, coloured by the relative contribution from each species. The top bar is coloured according to the air-mass origin.

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During most of the campaign there was reasonable agreement (slope of 1; R²=0.6) between the PM₁ ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ (cToF-AMS) and the TSP ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ (PILS) concentration, with the exception of periods of high sea salt concentrations when the TSP ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ were significantly higher (slope of 0.5; R²= 0.2 for TSP Cl concentrations >10 µg m⁻³; see Fig. S2). Supporting measurements of the size-segregated composition from the cascade impactor corroborate this, indicating a higher contribution of elemental sulfur in the coarse mode during periods of higher sea salt (Fig. S3). These periods corresponded to the mistral air masses, characterized by higher wind speeds and indicated the role of coarse sea salt particles in acting as a condensation sink for sulfate species. If these events are frequent, this could have important implications for the radiative properties of these aerosols by altering the scattering properties and potentially cloud condensation nuclei concentrations and composition.

Figure 6 shows the organic mass, split according to different OA factors from the PMF of the OA peaks. From the unconstrained PMF of the UMR and peak-fitted organic mass spectra, the most meaningful solution was found from a four-factor solution of the UMR analysis (see Figs. S4 and S5 for the mass reconstruction and time series' residuals). This has resulted in one factor resembling a primary organic aerosol and three oxygenated organic aerosol (OOA) factors. Herein, we label these factors HOA (hydrocarbon-like OA), MO-OOA (more oxidized OOA), LO-OOA (less oxidized OOA) and MSA-OOA (methanesulfonic-acid-related OOA).

Figure 6The time series of the PM₁ more oxidized OOA (MO-OOA), less oxidized (LO-OOA) and sulfate (a) and methanesulfonic-acid-related OOA (MSA-OOA), hydrocarbon-like OA (HOA) and eBC (b).

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These factors were compared with ambient organic mass spectra listed in the AMS Spectral Database (Ulbrich et al., 2009). The spectra for the OOA factors (see Fig. 7) were strongly correlated with each other (R>0.97) and all three were similar to a continental OOA factor observed in a ship campaign in the Arctic (Chang et al., 2011), as well as a low-volatility OOA factor identified in Paris (Crippa et al., 2013). Despite the similarities in their mass spectra, they exhibited different diurnal trends (Fig. 7) and time series that were associated with different wind directions and air masses and were therefore not recombined into a single OOA factor.

Figure 7The mass spectra for the four PMF factors (HOA: hydrocarbon-like organic aerosol, LO-OOA: less oxidized OOA, MO-OOA: more oxidized OOA, MSA-OOA: methanesulfonic-acid-related OOA) retrieved from the PMF analysis of unit-mass resolution data.

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The MO-OOA factor was the most dominant factor during the field campaign (∼53 % of the total OA mass) and was typical of low-volatility/highly oxidized OOA observed in many other studies, in the Mediterranean, with high contributions of m∕z 44 (f₄₄=0.31) (e.g. Hildebrandt et al., 2010, 2011). This factor was the most prominent during air masses from the eastern Mediterranean, central Europe and Atlantic (contributing to 71 %, 58 % and 55 % to OA, respectively) and was strongly correlated with ammonium sulfate (R²=0.68) over the whole campaign. It also had a distinct diurnal trend, with concentrations increasing during daylight hours, indicative of photochemical processing. The LO-OOA factor was slightly less oxygenated (f₄₄=0.26) and exhibited a different time series (R²=0.04), related to different air masses, than the MO-OOA. The less oxygenated OOA factor has also been associated with semi-volatile species and is often labelled as SV-OOA (Jimenez et al., 2009). Despite a distinct diurnal profile similar to previously reported SV-OOA factors (with a peak in the early morning), we refrain from labelling our LO-OOA this way because the mass spectrum was generally much more oxygenated and contained less f₄₃ than typically reported SV-OOA (see Fig. 9).

Figure 8A summary of studies that have investigated NR-PM₁ composition (including PMF of OA) around the Mediterranean basin. Only studies that have investigated PMF-based OA source apportionment are reported. Pie charts display the average concentration during each study, where green corresponds to organics, red to sulfates, orange to ammonium, blue to nitrate, pink to either chlorides or sea salt and black to elemental or black carbon. The OA fraction acronyms correspond to the following: HOA: hydrocarbon-like organic aerosol, SV-OOA: semi-volatile oxygenated organic aerosol, LV-OOA: low-volatility oxygenated organic aerosol, BBOA: biomass burning organic aerosol, COA: cooking organic aerosol, OOA: oxygenated organic aerosol, F4: factor 4 (unidentified PMF factor), IndOA: industry-related organic aerosol, OB-OA: olive-branch organic aerosol. See Tables S1 and S2 for further details about the sampling locations, instruments used and pie chart values. ¹ This study collected on PM_2.5 filters and nebulized into an HR-ToF-AMS. ² Excludes fire periods.

Figure 9f₄₃ (the ratio of m∕z 43 to the total OA) against f₄₄ (ratio of m∕z 44 to the total OA). The triangle is considered to encapsulate typical atmospheric values of OA according to Ng et al. (2010). The values for the various PMF factors from this study and other studies conducted in the remote Mediterranean are also displayed.

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The MSA-OOA factor contributed approximately 12 % of the total OA during the campaign and is likely related to the biogenic emission and processing of dimethyl sulfide (DMS) from phytoplankton in the Mediterranean. This factor was also highly oxygenated (f₄₄=0.20) but contains key peaks related to the fragmentation of MSA from the electron impact of the cToF-AMS. The most prominent of these peaks were m∕z 96 (${\mathrm{CH}}_{\mathrm{4}}{\mathrm{SO}}_{\mathrm{3}}^{+}$), 79 (${\mathrm{CH}}_{\mathrm{3}}{\mathrm{SO}}_{\mathrm{2}}^{+}$), 78 (${\mathrm{CH}}_{\mathrm{2}}{\mathrm{SO}}_{\mathrm{2}}^{+}$), 65 (${\mathrm{HSO}}_{\mathrm{2}}^{+}$) and 45 (CHS⁺). A similar factor was observed at Bird Island in the south Atlantic Ocean (Schmale et al., 2013), albeit without the contribution of significant m∕z 44, suggesting a more aged or mixed aerosol during this campaign. A distinct diurnal pattern for the MSA-OOA was not observed.

While the MO-OOA, LO-OOA and MSA-OOA factors represent secondary organic aerosol and were the most dominant contribution of OA during the campaign (92 % on average), a primary organic aerosol factor was observed and identified here as HOA. The mass spectrum of the HOA factor was characteristic of spectra observed in other studies, with prominent peaks at m∕z 95, 91, 83, 81, 71, 69, 57, 55, 43 and 41. Although HOA is typically associated with emissions from incomplete combustion (Zhang et al., 2005), it was not well correlated with other expected tracers such as eBC, CO and NO_x. This HOA was typically associated with south-westerly winds of low speed (<5 m s⁻¹; see Fig. 10) and peaked at approximately 06:00 LT each morning. The poor correlation between the HOA factor and eBC could have been due to a variety of local sources with different HOA and eBC emission factors, a mixing of the PMF factor with some small peaks not associated with combustion processes or from regional HOA that has undergone some transport without significant oxidation. The signal fraction of each m∕z of the mass spectrum of the HOA factor however had strong correlations (0.69<R<0.89) with numerous HOA factors from other studies (Hersey et al., 2011; Ulbrich et al., 2009; Ng et al., 2011; Lanz et al., 2007; Zhang et al., 2005).

Figure 10Bivariate polar plots of mean concentrations of PM₁ species and f₄₄ at Lampedusa. The angle represents the arrival wind direction, the radius represents the wind speed, and the colours represent the mean concentrations for the respective wind directions and winds.

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3.3 Comparison with other observations around the Mediterranean

There are many factors that could influence the composition, the concentration and oxidation level of different aerosol species over the Mediterranean. These include different aerosol sources which can follow different seasonal or yearly trends (e.g. biogenic emissions) as well as the existing aerosol load and the meteorological conditions that drive transport, dilution and ageing processes. The majority of previous studies of detailed PM₁ aerosol composition have been taken at coastal sites around the Mediterranean (Mohr et al., 2012; Minguillón et al., 2015, 2016; El Haddad et al., 2013; Bozzetti et al., 2017) which could be expected to observe higher concentrations than at Lampedusa due to proximity to sources (e.g. traffic, fossil fuel use, heating, biomass burning, industrial activities). Aside from Lampedusa and the observations presented in this study, measurements at Finokalia and Cap Corse could be considered the most remote sites where these measurements have been taken. Similar to the comparison of NR-PM₁ measurements from Europe, North America and east Asia made by Zhang et al. (2007), Fig. 8 summarizes the recent observations of NR-PM₁ composition from measurements in and around the Mediterranean basin (see Tables S1 and S2 for more details that are displayed in Fig. 8).

The PM₁ mass loading observed at Lampedusa is comparable to most of these other studies performed at both remote marine sites and coastal sites in the Mediterranean. With the exception of sites in the eastern Mediterranean, OA was the dominant NR-PM₁ constituent and summertime OA was generally considered mostly secondary, comprised of SV-OOA and LV-OOA with small contributions of HOA. For remote sites, the results are consistent with a predominance of OA in PM₁ fraction in summer. PMF analysis of Q-AMS measurements at the Finokalia remote site in the eastern Mediterranean in the summer of 2008 showed two OOA factors and a distinct lack of HOA (Hildebrandt et al., 2010). A more recent study during the late 2012 summer at Finokalia observed periods influenced by biomass burning but otherwise also observed mostly oxygenated organic aerosol (Bougiatioti et al., 2014). Measurements undertaken in the western Mediterranean at Cap Corse from 11 June to 6 August 2013 encompassed the sampling period of this study. For the period from 15 July to 5 August, PMF analysis showed 55 %, 27 % and 13 % contributions of organic matter, sulfate and ammonium to non-refractory PM₁ (Michoud et al., 2017). Secondary oxygenated volatile organic compounds (VOCs) dominated the VOC spectrum during the campaign and were very well correlated with submicron organic aerosol. PMF analysis on the OA revealed a three-factor solution where SV-OOA and LV-OOA were dominant, contributing by 44 % and 53 %, respectively, with a 4 % HOA contribution. From the same measurements but reported over the extended period from 11 June to 5 August, there was a higher LV-OOA contribution (62 %) (Arndt et al., 2017) which is in agreement with our observations of MO-OOA at Lampedusa. The OA was mostly portioned into MO-OOA and LO-OOA (81 %), indicative of well-aged or oxidized secondary organic aerosol from long-range transport of pollutants.

Figure 9 displays the behaviour of the f₄₄ and f₄₃ fragments obtained during the field campaign. f₄₄, a proxy for OA oxidation (Jimenez et al., 2009), is calculated as the ratio of the mass at m∕z 44 (mostly ${\mathrm{CO}}_{\mathrm{2}}^{+}$) to the total OA, while f₄₃, equal to the ratio of m∕z 43 (mostly C₂H₃O⁺) to the total OA, typically represents less-aged OA. f₄₄ was ∼0.26 for the majority of the sampling period (Q1: 0.25; Q3: 0.27), while f₄₃ was 0.036 (Q1: 0.028; Q3: 0.041). The campaign values are compared to values from the spectra for the four PMF factors and those observed in other field campaigns in the remote Mediterranean. The dotted lines (the so-called Ng triangle) encapsulate the f₄₄ and f₄₃ values of atmospheric OA from a vast number of studies (Ng et al., 2010), with the most aged OA in the top left corner and the most fresh in the bottom right. The high f₄₄ values and the dominance of the highly oxygenated MO-OOA and LO-OOA factors show that the organic aerosol was extremely aged compared to other measurements.

3.4 Links to meteorology

The contribution of the major submicron chemical species and OA sources is further explained in the following by linking the measured and apportioned concentrations to the local meteorology (i.e. wind speed and wind direction) and to the air-mass back trajectories to account for the long-range transport of aerosol as well as more distant sources. The bivariate polar plots of these PM₁ species and f₄₄ as a function of wind speed and direction are shown in Fig. 10.

Considering that the sampling site on Lampedusa is on the north-east tip of the island, it is evident that the ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ and ${\mathrm{NH}}_{\mathrm{4}}^{+}$ were likely a result of north-easterly marine air masses, in agreement with previous results (e.g. Bove et al., 2016). Sea salt concentrations were highest during high north-westerly wind speeds. Higher concentrations of ${\mathrm{NO}}_{\mathrm{3}}^{-}$, HOA and some of the periods with elevated eBC concentrations were observed during low-speed south-westerly winds, likely a result of the human settlements and activity on the island of Lampedusa (population of ∼6000 located to the south-west of the sampling site). Besides, the polar plot for eBC showed a patchier pattern, indicative of more local or point sources, and the elevated signals were likely due to air masses passing over ship plumes. Although the mass spectra for the LO-OOA and MO-OOA factors were very similar, their bivariate polar plots indicate different sources or photochemical processes. The MO-OOA was more prominent during north-easterly winds, indicating the most aged organics were influenced by air masses from the eastern Mediterranean, either from long-range transport or from circulation of closer pollution sources, while the LO-OOA was more dominant during north-westerly wind directions and air masses from over the western Mediterranean. Figure 11 shows the average contribution of each species during different air-mass periods (see Table S2 for the mean concentrations and standard deviations).

The highest concentrations of PM₁ were observed during eastern Mediterranean and central Europe air-mass periods, when significant lifetime over the lower-altitude marine environment and/or higher SO₂ emissions allowed the conversion and condensation of sulfate. These aged aerosols are corroborated by the high number concentrations within the accumulation mode during these periods relative to other periods, measured by the SMPS as well as the size-resolved sulfate composition, as discussed in the next section. In contrast to the eastern Mediterranean and central European air masses, sulfate concentrations were relatively low during the two mistral air masses. This behaviour has been found also in PM₁₀, with elevated values of sea salt aerosol and low non-sea-salt sulfate during mistral events (Becagli et al., 2017). The organic mass concentration was relatively uniform across the periods of different air-mass origins, with the exception of the high mistral winds which yielded OA concentrations approximately half the rest of the campaign and a higher contribution of MSA-OOA in comparison with other periods. The higher contribution of MO-OOA compared with LO-OOA from eastern air masses, and vice versa during western air masses, could be indicative of different OA sources prior to oxidation or due to different photochemical ageing between the two directions.

Figure 11The 144 h air-mass back trajectories (top) assigned to each cluster; (middle) the PM₁ composition for each air-mass cluster and (bottom) the contribution of OA factors for each air-mass cluster. The diameters for the PM₁ composition pie graphs are proportional to the total PM₁ concentration for each air-mass cluster period, and the radius for the OA factor pie graphs is proportional to the total PM₁ organic concentration for each air-mass cluster period.

Figure 12The number size distribution of PM₁ aerosol, coloured by averages for different air-mass origins: (a) mistral (high) and mistral (low); (b) Atlantic and western Europe; (c) eastern Mediterranean and central Europe; and the volume size distribution of PM₁ aerosol, coloured by averages for different air-mass origins: (d) mistral (high) and mistral (low); (e) Atlantic and western Europe; (f) eastern Mediterranean and central Europe.

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3.5 Aerosol size distributions

There are distinctions between the measured PM₁ size distributions during periods of different air-mass origins (Fig. 12). It should be noted that these size distributions are under ambient conditions without an inlet drier which could shift the size distribution to larger sizes if water is present. The ambient relative humidity for each air-mass back-trajectory cluster was eastern Mediterranean (53 %), central Europe (61 %), Atlantic (74 %), western Mediterranean (70 %), mistral (low) (67 %) and mistral (high) (74 %). Although the higher temperature inside the PEGASUS mobile laboratory could lower the relative humidity at the sampling point of the SMPS with respect to the ambient relative humidity, this was not measured or logged during the campaign.

Consistent with the higher concentrations of sulfate and ammonium species, the eastern Mediterranean and central Europe had the most pronounced accumulation modes with respect to those from other clusters due to the presence of accumulation-mode sulfate (see Fig. S6 for the size-resolved chemical composition). In contrast, the mistral (high) air masses had very few particles in the accumulation mode and were mostly dominated by nucleation-mode (>14 nm observations only) and Aitken-mode particles, in terms of number. There was only one period of mistral (high) air masses, spanning 38 h between 09:00 UTC on 24 June and 23:00 UTC on 25 June.

The most pronounced new particle formation (NPF) events and subsequent growth were observed during the two mistral air masses, particularly between 25 and 27 June. Very high number concentrations in the nucleation mode were also observed in very brief periods during the Atlantic and, to an extent, the western Europe air masses. There was no trend (R²=0.03) over the whole campaign between the ratio of the particle number concentration between 14–25 nm and 14–600 nm and the fraction of ${\mathrm{CH}}_{\mathrm{3}}{\mathrm{SO}}_{\mathrm{2}}^{+}$ (a fragment of MSA, measured by the cToF-AMS) to total PM₁ organics (see Fig. S7). There was a weak positive trend during periods of the mistral (high) air mass (R²=0.39). No trends were observed between the ratio of sub-25 nm and sub-600 nm particle number concentrations and the occurrence of other cToF-AMS fragments such as amines that could be linked with biogenic gas-to-particle conversion. Furthermore, there was a weak negative trend between the ratio of sub-25 nm and sub-600 nm particle number concentrations and the calculated f₄₄ (R²=0.12). Without instrumentation to measure the concentration of clusters and smaller aerosols (<14 nm) in conjunction with organic vapours over a longer time period, it is difficult to isolate and conclude the origin of these nucleation particles in a general sense, and we will limit our analysis to the most pronounced event during the campaign. Figure 13 shows the size distribution over this period as well as SO₂, eBC and ${\mathrm{CH}}_{\mathrm{3}}{\mathrm{SO}}_{\mathrm{2}}^{+}$, as well as the back trajectory ending at 04:00 UTC on 25 June at Lampedusa. This NPF event occurred during the night and therefore in the absence of photochemistry. There was no discernible increase in eBC or SO₂ during these events and the 3 d cascade impactor sample from 25 to 28 June was characterized by the lowest concentrations of vanadium and nickel (released from heavy oil combustion events due to ship emissions) of the whole campaign. The air-mass back trajectory during this event was characteristic of the mistral (high) cluster. These are high-altitude air masses that descended over the Atlantic Ocean before having undergone a hydraulic jump over the southern France region and then a rapid descent over the western Mediterranean basin at high speed before arriving at the Lampedusa site. It is interesting to note that these air masses were anomalous for the typical June/July period at Lampedusa. Although the detected mass of ${\mathrm{CH}}_{\mathrm{3}}{\mathrm{SO}}_{\mathrm{2}}^{+}$ is likely due to the condensation of MSA on accumulation-mode particles, and considering that the cToF-AMS collection efficiency below ∼100 nm is poor, the increasing concentration of ${\mathrm{CH}}_{\mathrm{3}}{\mathrm{SO}}_{\mathrm{2}}^{+}$ did coincide with the nucleation events during this period, suggesting a possible nucleation and condensation of marine biogenic vapours. Different studies indicate that NPF events may be triggered by atmospheric mixing processes (Kulmala et al., 2004; Hellmuth, 2006; Lauros et al., 2007, 2011) due to different phenomena like the enhancement of turbulence in elevated layers (Wehner et al., 2010), the break-up of the nocturnal inversion (Stratmann et al., 2003) or the turbulence associated with the nocturnal low-level jet (Siebert et al., 2007). Nighttime NPF events have also been observed in the eastern Mediterranean (Kalivitis et al., 2012).

Figure 13(a) The number size distribution during a new particle formation event on 25 June 2013 and the corresponding concentrations of SO₂, eBC and MSA fragment, ${\mathrm{CH}}_{\mathrm{3}}{\mathrm{SO}}_{\mathrm{2}}^{+}$ and (b) the HYSPLIT air-mass back trajectory during the event.

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Furthermore, the intrusion of descending mid-tropospheric air masses in the boundary layer (Pace et al., 2015) has been linked to the occurrence of NPF events and would be consistent with the absence of high concentrations of BC, SO₂, vanadium and nickel which could be expected from ship emissions (Healy et al., 2009; Isakson et al., 2001) in the boundary layer over the Mediterranean. Pace et al. (2006) have shown that clean marine aerosol conditions are rare at Lampedusa and generally associated with north-westerly progressively descending trajectories, in agreement with the findings of this study. The relative absence of pre-existing particles acting as a condensation sink favours NPF events as observed during the field campaign.

3.6 Accumulation of sulfates across the Mediterranean

In order to investigate the ageing of aerosols across the north–south trajectory of European continental air masses, we compare the average NR-PM₁ composition measurements at Lampedusa to the concurrent measurements conducted at the Ersa site during summer 2013 (Michoud et al., 2017; Arndt et al., 2017). This is shown in Table 1.

Table 1Campaign average PM₁ concentration for the major aerosol species measured at the Ersa and Lampedusa sites during the SOP-1a period and for periods of coincident air-mass back trajectories between Ersa and Lampedusa.

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On average, the PM₁ non-refractory organic mass concentrations at both sites were similar, with ∼3 µg m⁻³. ${\mathrm{NO}}_{\mathrm{3}}^{-}$ concentrations were relatively small at both sites but higher at Ersa (0.28 µg m⁻³) than at Lampedusa (0.09 µg m⁻³). Sulfate concentrations were a factor of 3.2 times higher at Lampedusa (4.5 µg m⁻³) than at Ersa (1.4 µg m⁻³), while the ammonium concentrations were a factor of 2.7.

To investigate the possible accumulation of ammonium sulfate during the transport of air masses from Europe, the hourly air-mass back trajectories from Lampedusa were filtered so that only those that passed within ±1^∘ latitude and longitude and within ±200 m altitude of the station height of Ersa (550 m) were selected. These thresholds were chosen arbitrarily since there is no clear distinction in horizontal or vertical distance from the site that would necessarily constitute a representative air mass. This resulted in a total of 192 hourly observations at the Ersa site over 32 unique air-mass back-trajectory runs (see Fig. 14).

Figure 14Hourly 120 h (5 d) back trajectories from Lampedusa that passed within ±1^∘ in latitude and longitude and ±200 m in altitude of the Ersa station. The colours represent the assigned cluster (performed on 144 h trajectories).

These trajectories were grouped mainly into the central Europe cluster (n=12). The median trajectory duration between the Ersa and Lampedusa sites was 53 h, with a minimum of 33 h and a maximum of 144 h (corresponding to the total duration of the HYSPLIT model runs in this case). Those air masses grouped in the eastern Mediterranean cluster had the longest duration time between the sites of 127 h (while coincident with the Ersa site, these air masses still spent a significant amount of time over the eastern Mediterranean), followed by central Europe (83 h), western Europe (45 h) and then the mistral (low) (38 h). In general, between the two sites, there was a 40 % enhancement in the organic mass concentration but an increase in sulfate and ammonium by a factor of 6 and 4, respectively (Table 1).

The accumulation of (NH₄)₂SO₄ between Ersa and Lampedusa appeared to be dependent on the travel time of the air mass; however, different relationships were observed during different air-mass clusters. The total sulfate concentration at Lampedusa minus the total sulfate concentration at Ersa for the same air mass and accounting for the travel time as a function of the travel time is shown in Fig. 15.

Figure 15The difference in the PM₁ ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ mass concentration at Lampedusa and Ersa as a function of the travel time of the air masses from Ersa to Lampedusa. Colours represent the air-mass origin cluster.

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There was a good positive correlation between the difference in sulfate concentrations between the two sites and the travel time for the central Europe and eastern Mediterranean air masses, while weak positive correlations were observed for the western Europe and mistral (low) clusters. It should be pointed out that the travel time was more than 110 h for the eastern Mediterranean air masses, while it was only between 33 and 58 h for the other three air-mass clusters. It is expected that the accumulation of sulfate would increase as the total travel time increases due to the opportunity for SO₂ conversion or from the addition of sulfate from separate air masses which are not accounted for in the HYSPLIT model. Although this relationship is somewhat demonstrated here, there are other factors that would influence the ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ accumulation. The sulfate concentrations presented here are measured by an Aerosol Chemical Speciation Monitor (ACSM) and cToF-AMS at the Ersa and Lampedusa sites, respectively. Both of these instruments have a 100 % inlet efficiency between ∼100 and 800 nm. The conversion of SO₂ to ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ via nucleation and condensation is dependent on the pre-existing aerosol size distribution and condensation sink. Therefore, the use of PM₁ composition can be misleading if the sulfate is condensing on coarse particles. This is demonstrated in Fig. S2, which shows the size-resolved mass distribution of sulfur and sodium collected every 3 d on multi-stage cascade impactor filters; the relative contribution of sulfur in the PM₁ is higher than that of PM₁₀ in the absence of sodium (a tracer for sea salt). Furthermore, the concentrations of ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ measured by the PILS in the PM₁₀ fraction and cToF-AMS in the PM₁ are approximately equal with low sea salt concentrations (Na+<2 µg m⁻³) but are nearly a factor of 2 higher with the PILS for higher sea salt concentrations (Fig. S3). Furthermore, the emission of SO₂, typically from ships in the Mediterranean, is not necessarily constant over time and is likely not uniformly spread over the basin and within the vertical column (e.g. Becagli et al., 2017). This could possibly explain the discrepancy between the accumulation rate of sulfate between the eastern Mediterranean and central European air-mass origins. Moreover, the sample size for this analysis is relatively small and potentially not representative of the general accumulation of ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ but nonetheless they highlight the magnitude of accumulation under different air-mass trajectories.