The origin of air masses impacting Ersa for the three different periods depicted in Fig.  has been investigated using FLEXPART model in order to characterise the transport time and the emitting sources of these aerosols.

Figure a represents the time series computed from clusters of FLEXPART back trajectories. The upper one represents the transport of the air masses passing over different regions before reaching the Ersa station. These different zones take into account the regions influenced by anthropogenic pollution, biomass burning and marine influences and are represented in Fig. . They represent the most probable influence on air masses arriving in Cap Corse due to their close locations and specific emissions. For each day during the campaign (bottom axes), the upper figure indicates the different zones through which the air masses passed before reaching Ersa. The bottom figure indicates the transport time from these zones to the Ersa sample site.

In general, the Ersa station was influenced by air masses coming from the west and south during the first part of the field campaign (from 6 to 26 June), and was more influenced by air masses coming from the east and north during the last part of the campaign (26 June to 13 July). During the campaign, Ersa was always affected by air masses that passed over French or Italian coastal areas. The influence of the Mediterranean coasts of Spain is also very present during the campaign, especially in the first part of June.

In terms of transport time, FLEXPART simulations indicate that air masses spent a few hours to several days over the sea after leaving the French or Italian coasts, and 2 to 6 days from continental European sources. Coastal regions are likely the source of anthropogenic-pollution impacted air masses, because they are highly industrialised and populated and are the last major source of anthropogenic aerosols before transport over the Mediterranean Sea. At the local scale, Ersa was mostly under the influence of a westerly wind (≃ 270∘; Fig. ) from the beginning of the campaign to the beginning of July, expect for a few days during the dust outbreak (from 16 to 20 June), where it was under a south-eastern influence (≃ 150∘). Finally, from 4 to 9 July, Ersa was mostly experiencing an easterly wind (≃ 100∘).

The influence of southerly air masses is marked by the passage of air masses above northern Africa, at the beginning of the campaign (19 June, Fig. ). The transport time of air masses from northern Africa to Ersa ranges between 2 and 6 days. Such air masses contain significant concentrations of mineral dust particles, which are usually transported at higher altitudes over the Mediterranean Basin, in the free troposphere and up to 9 km in altitude . Thus, we also performed simulations starting at 4000 m a.s.l., that show that the air masses arriving at Cap Corse on 19 and 20 June were within the boundary layer (< 1000 m) over Tunisia and Algeria from 2 to 3 days before. At the Ersa site, during the dust outbreak around 19 June, the wind speed reached 15 m s-1.

Besides the coastal anthropogenic influence observed during the first week of July, Fig.  shows that the air masses came from eastern Europe from 7 to 12 July, and in particular from Ukraine, 3–4 days before reaching Ersa. The date at which the air masses passed over these regions corresponds to significant emissions of biomass burning observed near the Black Sea as shown by the MODIS satellite retrievals (http://rapidfire.sci.gsfc.nasa.gov/cgi-bin/imagery/firemaps.cgi).

FLEXPART back-trajectory simulations also show that during the PMA period, air masses were coming from the north-west of Cap Corse, including the Gulf of Lion. This is consistent with a higher PMA concentration in Ersa, as a longer fetch leads to higher mass concentration. Our simulations reveal that these air masses were also influenced by anthropogenic sources from France and Italy. The study of the transport of air masses passing over maritime zones (Fig. b), especially the Gulf of Lion and North Atlantic zones, gives us information about the transport time from the source regions to Ersa and the changes in altitude. Our simulations indicate that the mean transport time from the Gulf of Lion is less than a day for the whole period except for the last day, 26 June. Whereas from the North Atlantic zone (Bay of Biscay), the transport time is more than 2.5 days and increases up to 4.5 days for 26 June (Table ). Almost no precipitation occurred during this period between the Bay of Biscay and Corsica, so these air masses were likely not impacted by wet scavenging.

The mean altitude for the air masses coming from the Gulf of Lion is close to 1000 m (972 m ± 753) for the 5 days, with minima mainly below 500 m, while the mean altitude from the North Atlantic is 1374 m (±828) and the minimum is below 800 m only for the first 3 days. reported that the concentration of sea salt aerosols associated with emissions was highest up to altitudes of 600–700 m, which typically correspond to the marine boundary layer (MBL) height. Thus an influence from the North Atlantic Ocean would occur more likely during the first 3 days of the period, when the air masses lay within the MBL. Concerning the Gulf of Lion, the altitudes of the air masses are low enough to bring sea salt aerosols in Ersa. While vertical transport is not well captured in the model, the FLEXPART model indicates that most of the PMA aerosol mass is transported in the MBL.

To summarise, our FLEXPART simulations clearly indicate that the Ersa site was impacted by a disperse set of air masses from different regions transporting different types of aerosols. These FLEXPART results are consistent with chemical measurements obtained at Ersa station, as well as the three periods discussed here.

The following sections will focus on the optical, physical and chemical properties of aerosols sampled during the PMA period. The dust and BBP events will be used as a comparison for different states of the atmosphere impacting the Ersa site.

As reported by and , the ratio of the concentration of Cl- over Na+ is an indicator of the chloride depletion that happens when PMAs react with acidic gases like HNO3 and H2SO4 according to the chemical Reactions (R1, R2, R3):

H2SO4+2NaCl→Na2SO4+2HClNaCl+H2SO4→NaHSO4+HClNaCl+HNO3→HCl+NaNO3.

These reactions result in a loss of particulate chloride in PMAs during transport. The typical mass ratio of Cl- / Na+ of the sea water is 1.8 ; however, the study of PM1 PMA in the Mediterranean Basin by shows a Cl- / Na+ ratio of 1.2. Numerous values are referenced over the Mediterranean Basin: 0.6 for long-term measurements (July 2012–April 2013) in Ersa station , 0.49 by , 1.00 by , 1.2 during summer by in Finokalia (eastern Mediterranean, Crete) and 1.2 during summer in the eastern Mediterranean coast of Turquey by . These values are found to be low compared to the seawater ratio, especially those by , which is probably related to the high reactivity of chloride with acidic gases that are present in relatively high concentrations in the Mediterranean atmosphere . A good correlation was found between Na+ mass concentration and the sum of Cl- + NO3- mass concentrations (PM10 measurements; r2=0.87) indicating that NO3- is the main component that interacts with sea salt.

During the PMA period, the Cl- / Na+ mass ratio varies between 0.13 and 1.3 (Fig. ), with a mean of 0.59 ± 0.23. This result is consistent with the long-term measurement taken between July 2012 and April 2013 at Ersa . This indicates that PMA measured in Ersa (and throughout the Mediterranean Basin) were predominantly aged.

To distinguish mostly aged and mostly fresh PMA, we used a spectral analysis of the ATOFMS measurements. The terms “fresh” and “aged” PMA, which will be used from now in this text correspond to the classification made with the ATOFMS. During the ChArMEx-ADRIMED campaign, an alternation between these two states of PMA was detected.

The size distribution of these two ATOFMS sea salt types were fitted according to a sum of log-normal modes. The fresh PMA were characterised by one mode with a vacuum aerodynamic diameter of 1.29 µm and a standard deviation σ of 1.34, while the aged PMA were characterised by three different modes, as detailed in Table .

Our results show that during the campaign aged PMA are dominant, but during the PMA period (22–26 June) when the wind near Cap Corse is higher (Sect. 3.2.2), there is an alternation of short events of fresh or aged PMA, with a dominance of fresh PMA. The comparison of the ATOFMS and PILS data show a relatively good agreement between the two instruments regarding the dominance of fresh and aged PMA (Fig. ).

To compare the two instruments, we looked at the count ratio of aged to fresh PMA, and attributed a state to the Cl- / Na+ ratio measured by the PILS-IC. For a large number of measurements only aged aerosols were detected and were labelled “only aged”. When the count ratio of aged aerosols over fresh aerosols was higher than one, the measurements were characterised as mostly aged, and less than one the PMA were considered mostly fresh. One can observe in Fig.  that the Cl- / Na+ ratio is higher when the ATOFMS distinguished fresh PMA, and lower when the ATOFMS distinguished aged PMA. We then determined the mean Cl- / Na+ ratio for mostly aged (0.38 ± 0.15) and mostly fresh PMA (0.62 ± 0.17). In our observations, the mostly fresh PMA ratio remains low compared to the initial ratio of 1.8 or even 1.2 for PM1 PMA (Schwier et al., 2016), revealing that even though PMA are characterised as fresh, they have undergone chemical reactions before reaching Ersa station.

Complementary to the FLEXPART results, we used wind measurements at the Semaphore station, at the Gulf of Lion buoy and at the Bay of Biscay buoy to investigate the possible relationship between the increase in PMA concentration observed in Ersa and the wind speed at these stations, as well as to better assess the origin of sea salt aerosols at Ersa.

During the ChArMEx-ADRIMED campaign, the majority of analysed air masses containing PMA came locally from the west and the concentration of marine particles increased with wind speed (Fig. a). The wind direction is constant around 270∘ for 6 days (21–26 June), and fluctuates afterwards between eastern and western origins. The maximum wind speed (20 m s-1) encountered during the campaign was observed on 24 June, coinciding with the highest sea salt mass concentration measured.

To investigate the relationship between wind speed and concentration of PMA measured in Ersa, we averaged its concentration by wind bins of 1 m s-1 for different cases. We first looked at the relationship between the concentration of PMA in Ersa and the wind speed measured at the Semaphore for the whole period of the campaign (Fig. a). The result indicates a relationship between the wind speed and PMA concentration and the best fit (r2=0.92) is presented in the form of ln [PMA] = a × WS + ln(M0), where WS corresponds to the wind speed and M0 (µg m-3) to the concentration that corresponds to a wind speed WS = 0. The error bars correspond to 2σ rms (root mean square). Above 13 m s-1, the concentration starts to rise rapidly. The relationship described here is compared with fit parameters found by , and chosen because the time resolution of the measurements were similar to those in Ersa and wind speed encountered during their measurements were in the same range as in Ersa during the campaign. Despite the high correlation between PMA concentration and wind speed shown here, our results yield mass concentrations at least an order of magnitude lower than other studies shown in Fig. . This difference is probably related to the sampling altitudes, which for our study was 533, and ∼ 10 m a.s.l. for and . This is contrary to , who did not find a significant correlation between PMA concentrations and wind speed in Cabo Verde, even though they found an increase of PMA concentration on days of higher wind speeds. had difficulties establishing a relationship between local emissions of PMA and wind speed measurements using instrumentation with a long integration time during the FETCH campaign, in accordance with previous results of . found that wind speed was a good indicator for a measuring period but not for a specific case.

To investigate the origin of PMAs as a function of their ageing, we distinguished the air masses that contain fresh or aged PMA, using the method defined in Sect. 3.2.1, for the whole campaign. We observe that the concentration of aged PMA (Fig. b) is constant and does not depend on the local wind speed, which suggest that the Ersa site is always impacted by long-range transport containing aged PMA, even if the concentration is low (0.6 ± 0.2 µg m-3). On the contrary, we observe that fresh PMA concentration measured at Ersa (Fig. b) is highly dependant on the wind speed, following a fit of the form ln [PMA] = a × WS + ln(M0) with a correct correlation (r2=0.59). This result indicates that the highest concentration of PMA measured in Ersa during the campaign corresponds to fresher aerosols and is dependent on the local meteorological conditions.

We then compared the wind speed at the two probable regions of emission, the Gulf of Lion and the Bay of Biscay, to the concentration of PMA, using FLEXPART results, for the PMA period (22–26 June). To account for the transport time of PMA, we added a delay of 12 h, which corresponds to the mean transport time from the Gulf of Lion to Ersa modelled with FLEXPART for the PMA period, and 60 h for the Bay of Biscay (Fig. b and c). This work was done for the PMA period from 22 to 26 June. In Fig. a, the correlation between the mass concentration of PMA and the wind speed at Ersa is good for fresh PMA (r2=0.71; Fig. a) as presented in the previous paragraph for the whole campaign. For the Gulf of Lion (Fig. b), the correlation is good for aged PMA (red curve, r2=0.87) while there is no correlation following this fit for fresh PMA and wind speed at the Gulf of Lion. The same analysis was done for the Bay of Biscay (Fig. c) but no correlation was found for fresh, aged PMA or all the PMA regardless of their ageing.

According to these results, during the PMA period, the PMA that were measured in Ersa were a mixture of fresh PMA emitted near the Ersa station and of aged PMA emitted from the Gulf of Lion. It should be noted that measurements of PMA have also been made when the air masses were coming from the east, but the concentrations were lower (< 2 µg m-3).

From these results, the most probable zone that brings PMA to Ersa during ADRIMED regarding altitude, transport time of air masses and local wind speed would be the Gulf of Lion and the sea close to Ersa, considering that the buoy at the Bay of Biscay represents the wind speed of the area. Beyond the scope of this work, an analysis of the emission and transport of marine aerosols during this PMA period is ongoing and uses the Meso-NH model.

This PMA period represents the background atmospheric conditions that affect Ersa most of the time. In this section, after an overview of the ADRIMED field campaign, the number and volume size distribution of PMA are investigated, as they are fundamental parameters which estimate the aerosol radiative effects. A comparison with two sporadic events (dust and BBP), which influence Ersa principally in spring and summer, is also carried out.

The total number concentration (CPC + OPS) during the campaign observes a mean value of 1900 ± 920 cm-3 with several short episodes (few hours) of high concentrations (> 5000 cm-3) at the end of June. Thus, the background number concentration is higher than what is usually measured at a pristine marine site (300–600 cm-3; ) and denotes contamination by other sources, principally from continental Europe, as Ersa is not affected by immediately local sources. In parallel, the number size distributions measured by the SMPS show that the particles detected during these short episodes of high concentration have diameters below 50 nm and probably correspond to new particles during transport over the Mediterranean sea.

During this field campaign, the fine and accumulation modes (10 nm < Dp < 600 nm) were dominant in number. Furthermore, the concentration of these two modes rises at the beginning of July, particularly the accumulation mode, following the scheme already mentioned in the previous section for PM1 particles. Hence, the ratio of the number concentration from 4 to 13 July over the number concentration from 6 June to 3 July is greater than 2 for particle diameters greater than 0.24 and 0.52 µm. This ratio reaches its highest value for particles with diameters of 0.4 µm.

Before conducting comparisons on the physical properties of sea salt, the PMA period was divided into several shorter periods according to their ageing (see Sect. 3.2.1), that will be called “ageing periods”. In addition, we chose a supplementary period (1–4 July) corresponding to low PMA concentration when it does not exceed the background concentration (0.76 µg m-3). The number and volume size distribution were averaged over the ageing periods and fitted under the assumption that the distribution is a sum of log-normal modes to investigate whether the ageing of PMA could be characterised by their size distributions. Three to six modes were necessary to fit the observed dry size distributions.

For the number size distribution, a large variety of Aitken and accumulation mode can be derived when comparing the different periods. They show a large variety of diameters and concentration whether they contain low or high PMA concentration, aged or fresh. However, a coarse mode (modal diameter of 1.2 µm) appears for all the size distributions containing PMA, for both aged and fresh aerosols. This mode does not exist when the concentration of PMA is within the background. The concentration of this mode seems to be higher for fresh than aged PMA which is probably due to dry deposition during transport. As we did not find any significant difference between the size distributions of aged and fresh PMA, they are merged for the sequence of the analysis as PMA size distribution over the PMA period.

The number and volume size distribution have been averaged for each periods: dust, PMA and BBP (Fig. ). We chose to average the most intense part of each period to extract the representative properties of each aerosol type. Thus, although the dust event starts on 16 and ends on 20 June, we analysed the size distribution obtained from 17:00 to 19 June at 00:00 UTC. Likewise, the study of the size distribution for PMA and BBP aerosols are from 23 June at 00:00 to 25 June at 00:00 UTC and from 9 July at 00:00 to 11 July 0:00 UTC respectively. The results are summarised in Table . The highest number concentration for PMA period was for particles of modal diameter of 40 nm, followed by a mode at 130 nm and a third mode at 1.2 µm. We find a good agreement of modal diameters with the size distribution measured by in the parameterisation of the emission of PMA from the Atlantic Ocean. Furthermore, our results agree with measurements taken in the Mediterranean Sea by . We observed a high number concentration of fine particles during the PMA period, which is consistent with measurements reported in Fig.  a. The modal diameter of these fine particles is situated at 40 nm. This mode was also measured by at d=37.5 nm ± 1.4 during PMA flux measurements from Mediterranean waters. The second mode has a modal diameter of 130 nm, which is somewhat higher than the 90 nm mode found by , which is related to the presence of aged particles during our study.

We find important distinctions between the three different periods reported in Fig. a. As expected, during the dust event, the number size distribution is higher for the largest particles (3 to 10 µm size range). During the BBP period, the dominant mode of the number size distribution is located around 200 nm and the number concentration of particles greater than 500 nm is found to be low (65 ± 14 cm-3). This result is consistent with the typical number concentration of biomass burning aerosols that peaks in the size range of 100–200 nm . These hydrophilic aerosols are subject to increases in size when they age during transport , which is consistent with our observations of a mode centred at 200 nm as they were transported for 3–4 days before reaching Ersa.

Looking at the volume size distribution is a way to distinguish the particles that have the greatest impact on mass concentration, i.e. the coarser particles. On average, during the ADRIMED period the mean total volume concentration (CPC + OPS) is 40 ± 16 µm3 cm-3, and the volume concentration of smallest particles (d< 500 nm) is 22 ± 11 µm3 cm-3, while that of the coarser particles (d> 500 nm) is 16 ± 9 µm3 cm-3.

The volume size distribution shows different patterns for the dust, PMA and BBP periods. We distinguish a coarser mode between 20 and 27 June, including the PMA period, with a modal diameter of 1.6 µm. Marine aerosols with diameters greater than 1 µm are largely inorganic sea salt . A coarse mode is also observed around 19 June (dust period) with diameters between 5 and 7 µm, which probably corresponds to mineral dust particles in accordance with the volume size distributions measured on board the ATR-42 aircraft . Figure b shows two dominant modes during the dust period: one at a dry diameter of 0.18 µm and the second one around 2.4 µm. Finally, the BBP event is found to be dominated by a mode at 320 nm, and the volume concentration of the coarse mode is here very low.

We have also compared the results of the in situ surface volume size distributions with AERONET/PHOTONS retrievals (Fig. ). AERONET data are compared with the in situ measurements, as they are derived from an algorithm, averaged over a few days and have a limited number of measurements (seven available for the PMA period). Concerning dust and PMA periods, the coarse modes measured by OPS and SMPS are consistent with the atmospheric column volume size distribution and contribute to the largest fraction of aerosol mass, even though a fine mode is also detected during dust periods. During the BBP period, both observations (in situ and AERONET) clearly indicate volume size distributions that are largely dominated by the fine mode. The difference of size distribution between the three periods is higher for AERONET data than for the in situ data. For the dust period, the reason is that the main part of the dust plume was situated at an altitude of 3 to 6 km. For the PMA period, the hygroscopic growth of marine aerosols can explain a shift in the diameter modes. There also might be a loss of supermicron mode particles before they reach the OPS, which has an impact on the PMA and dust periods.

These three periods are characterised by different volume size distributions (in situ measurements), as summarised in Table . The dust and PMA periods are characterised by coarser particles, with a modal diameter of 2.4 and 1.6 µm respectively, while the BBP period is characterised by particles in the accumulation mode with modal diameter of 320 nm.

In addition to chemical and size distribution aerosol properties, we also determined optical properties, providing AOD at the measuring site, as well as the SSA and AE obtained for the whole atmospheric column from AERONET/PHOTONS observations and their spectral dependences in the solar spectral region. These results are summarised in Table . As in the previous part, a comparison with dust and BBP period was also realised.

First, the AOD retrievals provide information about the loading of aerosols within the atmospheric column. During the ChArMEx-ADRIMED campaign, AOD (at 500 nm) was found to be moderate, with an average of 0.15 ± 0.08 (Fig. a). Such values are consistent with the site location and aerosol concentration (see Sect. 3.3.1), Ersa not being impacted by local pollution or high anthropogenic sources. In that sense, the AOD background is low, typical of a rural site. However, from the beginning of July to the end of the campaign, the AOD increases to values up to 0.6 with a higher wavelength dependency.

AOD is lowest during the PMA event (22–26 June), with a mean value of 0.11 ± 0.08 at 500 nm, close to those reported over the Mediterranean Basin . found that for clean oceanic conditions, AOD was below 0.1 (at 550 nm) and found an average value of 0.11 for the same marine conditions. In the Mediterranean sea in particular, reported a value of 0.15 in Crete in the background corresponding to marine aerosols and found a mean AOD over the eastern Mediterranean for marine aerosols (June–August 2010) of 0.06 ± 0.01. Furthermore, it should be noted that AOD is not very sensitive to the wavelengths during these 5 days due to the presence of coarse particles.

AOD is higher during the dust event, reaching 0.3 (at 500 nm; Fig. ), corresponding to a relatively low value for a dust outbreak occurring over the Mediterranean Basin . AOD can reach values above one (, over the western Mediterranean) and even up to two ( over Lampedusa). As observed during the PMA period, AOD is not sensitive to wavelengths during the dust event, denoting the presence of coarse particles. AOD showed a very different pattern during the last part of the campaign, reaching higher values and showing a strong dependence to the wavelengths. AOD thus exceeds 0.4 in the middle of July and is higher for shorter wavelengths. It denotes a significant contribution of small particles to the solar extinction, in accordance with the SMPS and TEOM PM1 observations previously presented and during the BBP period. The AOD values are much higher for these two periods than for the PMA period.

For the PMA episode, AE varied between 0.4 and 2, with a mean value of 1.3, which is below the mean value of the ADRIMED campaign (1.8; Fig. b). AE also decreased to 1.15 for 24 June, when PMA concentration is highest. Such a value is characteristic of clean ocean regions as reported by , who found values between 0.3 and 0.7. In addition, and reported AE between 0.7 and 1 for background marine atmosphere over the central and eastern Mediterranean. The AE measured at the Ersa station during the PMA event is not as low as these referenced values and could indicate a possible mixing between sea salt and other aerosols, as the western Mediterranean is under the permanent influence of continental sources. This point is also consistent with the observed number size distribution, which showed that the number concentration of fine particles was high during the PMA event, indicating pollution particles from the European continent.

A high variability was also found during dust periods. Indeed, AE fluctuated between 1 and 2. These are not typical values observed for desert dust particles, which generally tend toward values less than one, denoting a majority of coarse particles . In that sense, the higher values observed at Ersa could be due to the possible mixing of particles in the atmosphere during these days, by the weak intensity of the dust outbreak observed during ADRIMED or by the possible deposition of the coarser dust particles during transport. Finally, and during the BBP period, AE was found to be mostly above two. Its pattern follows a clear diurnal variation, with a maximum around 12:00 UTC and a minimum in the beginning and in the end of the day. AE observed during this period is stable for almost a week, from 4 to 10 July. The largest difference noted for AE between dust, PMA and BBP periods is in their internal variability. For the first two periods, a mixing and high variability is found while for the last period, AE is constant for more than 5 days, showing that the atmosphere is mostly under the influence of the same aerosol type.

Overall, SSA observed during the campaign remained relatively high, with values above 0.90 for most of the period, associated with a spectral dependence less than 0.05 (from 440 to 870 nm). In that sense, the presence of absorbing particles is shown to be sporadic and lasted no more than a few hours. During the PMA period, SSA was found close to unity (mean of 0.98 ± 0.02; not shown here), indicating significant scattering optical properties, consistent with marine aerosols optical properties in the solar range . During the dust period 16–20 June, SSA decreased to values between 0.90 and 0.95 (at 440 nm), indicating moderate absorbing properties, which are characteristics of desert dust over the Mediterranean Basin . Finally, during the BBP period, we observed a higher wavelength dependency, with SSA values oscillating between 0.90 and 1.0 (at 440 nm).

In addition to the atmospheric column information, over the entire period of the campaign, nephelometer measurements reveal that the scattering due to particles was relatively low (mean of 37 Mm-1 ± 20, at 550 nm) and not sensitive to wavelength during June, particularly during dust and PMA periods (Fig. c). This is in contrast to July, when higher scattering coefficients (mean of 48 Mm-1 ± 24) associated with higher AE (AE July mean of 2.1 ± 0.2, AE June mean of 1.6 ± 0.5) are observed.

During dust and PMA period, the scattering coefficient remains low (mean of 28 ± 11 and 28 ± 11 Mm-1 respectively). The PMA period is characterised by a relatively weak wavelength dependency (Fig. c). While the mixing of dust with fine particles, previously shown by the AERONET volume size distribution, is shown here by a relatively high wavelength dependency (mean of 20 ± 9 Mm-1). On the contrary, during the BBP period, the wavelength dependency is highest (mean of 49 ± 15 Mm-1 between 450 and 700 nm), and the scattering coefficient reaches its highest values (up to 137 Mm-1). This clearly indicates that aerosols are smaller in size during this period, which is consistent with AERONET/PHOTONS data and PM1 concentrations obtained at Ersa station.

The optical characteristics (AOD, SSA and AE) of the air masses during the PMA event are found to be consistent with the literature , even though a mixing with continental fine particles was also detected.

In parallel to optical properties observations, the local 1-D (clear-sky) direct radiative effect (DRE) in the shortwave (SW) spectral region has been estimated using AERONET/PHOTONS retrievals for each identified period. DRE is calculated here at two different atmospheric levels: at the surface (bottom of the atmosphere, BOA) and at the top of the atmosphere (TOA). Figure b indicates the SW DRE at BOA for different AOD and different solar angles observed during the experiment. The estimated values show significant variability with instantaneous DRE between -5 and -40 W m-2, depending on the aerosol regimes. Figure a and b) indicates that PMA period is characterised by moderate TOA DRE (mean of -8 ± 3 W m-2) and BOA DRE (mean of -11 ± 4 W m-2). Such estimates at Ersa station are found to be consistent with direct SW effects of sea salt documented by , who used the COSMO-ART model over the Mediterranean Basin and reported a SW DRE from -5 to -10 W m-2 at the surface and an AOD between 0.1 and 0.2 (at 550 nm).

The highest values of BOA DRE correspond to the highest AOD observed during dust event. For this specific event, values with peak maxima of -43 W m-2 are in the same range of magnitude of values reported for mineral dust aerosols over the Mediterranean Basin by . Intermediate BOA DRE are calculated under polluted and biomass burning influence (from 5–12 July), and range from -13 to -38 W m-2. Such values are classically derived over the western Mediterranean for polluted particles .

In addition, the calculated SW DRE at TOA is reported in Fig. a, showing negative effects in all conditions due to the moderate absorbing ability of aerosols associated with a low surface albedo at Ersa (Nicolas et al., 2017) and leading to cooling at TOA. It should be noted that the DRE of aerosols in the longwave (LW) spectral range, which can counterbalance a part of the SW cooling at TOA, is not estimated here. Contrary to the LW DRE of mineral dust exerted near dust sources, this effect is generally lower than SW DRE during the transport of mineral dust over the Mediterranean Basin . In the same way as at the surface, Fig. b indicates that higher TOA DRE occur during the mineral dust event, with values as large as -20 to -25 W m-2, but due to the spread of the values during the episode, the mean value of TOA DRE is in the same range as for BBP period. Finally, we report logically intermediate TOA DRE (mean of -15 ± 4 W m-2) between 5 and 12 July, when Ersa station is affected by pollution and smoke aerosols.

To conclude, PMA SW DRE at TOA and BOA is 2 or 3 times lower than what we encounter during events like dust outbreaks and biomass burning, which occur principally in spring and summer. However, the influence of marine aerosols is permanent, depending particularly on wind speed.