Overview of aerosols sources
In this section, the chemical properties of the aerosols measured in Ersa
(Fig. ) are first studied, revealing a significant variability in
the contribution of the different aerosol species and outlining three main
periods, dust, PMA and BBP, under the influence of different types of air
masses and particles. The aerosol physical properties are then discussed in
Sect. 3.2.3 and 3.3.1.
The mean PM10 concentration measured by the TEOM PM10 during the
ADRIMED campaign was 11.5 ± 5.4 µg m-3. For the majority of the
sampling period the mass concentration ranged from 10 to 20 µg m-3,
except for short periods when the concentration fell to 5 µg m-3.
These decreases are usually due to wet scavenging or the diurnal variation
of the boundary layer, as the Ersa station was within the boundary layer
during daytime and sometimes slightly above the boundary layer at
night-time (aerosol concentrations at night were often lower when the Ersa
site was in the free troposphere).
In parallel, the mean PM1 concentration measured by the TEOM PM1 during ADRIMED was 6.4 ± 3.2 µg m-3.
The concentration was lower during June and rises during the beginning of July to exceed 10 µg m-3.
The major chemical constituents of PM10 measured at Ersa (Fig. ) show a significant temporal variability during the campaign. A
correlation plot (Fig. ) illustrates the relationship
between the principal chemical constituents, PM1 and PM10 mass
concentrations, as well as wind speed and direction. In the figure, the order
of the variables appear due to their similarity with one another, through
hierarchical analysis . The colour and the number
represent the correlation between two variables: when close to 100, the
correlation is high. The shape of the ellipse is a visual representation of a
scatter plot. We can observe two groups of variables on this figure. The first
one is composed of Cl-, Na+, Ca2+, K and PM10 mass
concentration and related to marine or terrestrial influence, while the second
one, composed of NH4+, SO42-, BC, organics and PM1 mass
concentration, is related to pollution influence.
Three main periods under the influence of different types of air masses and aerosols have been selected here and discussed in more details below.
The first period (16 to 20 June) corresponds to a dust outbreak and is
characterised by the concentration of non-sea-salt calcium (nss-Ca2+)
concentration, a proxy of desert dust , which increases
from 0.5 to 2 µg m-3. This dust event lasted a few days, from 16 to
20 June, with nss-Ca2+ concentrations peaking on 18 June at 2 µg m-3 at the Ersa site. In addition, the concentrations of calcium
measured by the PILS-IC are relatively low for a dust event, because the
maximum concentration of dust particles was located at an altitude ranging
between 3 and 6 km . The second reason concerns the
method used by the PILS-IC instrument, which analyses only the soluble fraction
of aerosols, while a significant part of dust Ca is insoluble. In that sense,
the concentration of nss-Ca2+ determined by PILS-IC remains a
qualitative indicator of the presence of dust particles.
Ratio of inorganic sea salt mass concentration (PILS-IC) over PM10
mass concentration (TEOM PM10).
PILS-IC measurements also indicate an increase in the concentrations of
oxalate, potassium, SO42- and NH4+ from 5 to 9 July, which
correspond to BBP influences. A brief increase of NO3-
concentrations (PILS-IC) was recorded on 5 and 6 July (Fig. ).
This event was also detected by the ACSM. Indeed, there is a difference of a factor of 2 in terms of total mass concentrations
between the first part of the campaign (6 June to 4 July) and during the BBP period. During the first period (6 June to 4 July),
the mass concentration of each aerosol species is low, characterised by a mean total concentration of
3.7 ± 1.6 µg m-3. During the second identified period (4 to 13 July), the total PM1
aerosol mass concentration increases suddenly to reach a mean of 7.2 ± 1.7 µg m-3. Similarly,
the total PM1 mass concentration measured by the TEOM increases from 5.9 ± 3.0 to 8.4 ± 3.3 µg m-3.
This increase is mainly due to a large addition of the concentration of submicronic organics compounds
(from 2.1 ± 0.9 µg m-3 to 4.1 ± 1.2 µg m-3) and an increase
of SO42- (and NH4+) from 0.9 ± 0.6 (0.5 ± 0.3) to 1.8 ± 0.6 (0.9 ± 0.3) µg m-3.
organics, sulfate and ammonium concentrations remain high until 10 July, when they decrease, but to values that are still higher than
during the month of June. In parallel, the black carbon (BC) concentration is found to be low throughout the whole period of the campaign,
although we observe an increase during July (mean of 0.41 ± 0.11 µg m-3) compared to June
(mean of 0.28 ± 0.11 µg m-3). For a few days and during the PMA period, the concentration of BC is
found to be very low (0.20 ± 0.09 µg m-3) and recovers its previous concentration by 3 July.
The highest BC concentration (0.75 µg m-3) was reached on 5 July. The ACSM observations clearly
indicate that concentrations of all the chemical components during the BBP episode are twice the concentration they
had during the first period of the ADRIMED campaign. Similar episodes of biomass burning events of European origin
were studied by at a Mediterranean site, with an increase in the concentration of PM10
nitrate (6 times higher than the annual average), sulfate (about 3 times higher than the annual average), ammonium (more than
4 times higher than the annual average), OM (about 2 times higher than the annual average) and potassium (2 times higher than the annual average).
Our observations reveal that the mean concentration of inorganic PMA
(averaged for the months of June and July 2013) was found to be low with a
value of 0.76 ± 1.04 µg m-3 (Fig. ). However, during
the PMA period, the concentration of PMA increases up to 6.5 µg m-3,
with a mean concentration of 3.2 ± 1.8 µg m-3. For this specific
period, the mass of PMA represents, on average, 22 % of the total mass
measured by the TEOM PM10 instrument, while the average contribution was
about 7 % for the whole period of observations. At the Ersa station, the
highest concentration of PMA was reached on 24 June, when PMA concentration
represented 40 % of hourly PM10 mass concentration for 25 % of the data
(Fig. ). Even though the mean values of PMA mass
concentration measured at Ersa were low compared to values referenced at
other Mediterranean sites, this contribution still remains significant.
Indeed, Pey et al. (2009) reported a ratio of 10 % of sea spray (sum of Na
and Cl- mass concentrations from Quartz fibre filter) to PM10
(annual mean = 2.9 µg m-3) in Mallorca (117 m a.s.l.), while
found a contribution of 40 % of PMA in the coarse mode
of inorganic ions during summer at Finokalia (150 m a.s.l., Crete).
analysed the chemical composition of PM10 aerosols in
the Mediterranean Basin and found a mean annual contribution of sea spray to
PM10 that did not exceed 24 %. So the contribution of inorganic sea salt
to the PM10 mass concentration in the Mediterranean Basin is on average
lower than 20 % but can reach 40 % during particular events such as the one
observed at Ersa in June 2013. Moreover, sea spray likely comprises a
substantial fraction of organic PMA and hence may represent a larger fraction
of PM10 than the one estimated solely from the inorganic fraction
. Furthermore, while the contribution of PMA to
PM10 mass concentration is high during the PMA period, the mass contribution
of nss ions to the total ionic content is relatively low during the PMA
period (53 ± 11 %). In comparison, the mass contribution of nss-ions to
the total ionic content is 84 ± 5 % for the ADRIMED field campaign, and
is 82 ± 14 and 92 ± 3 % for the dust and BBP periods
respectively. Furthermore, the Ca2+ concentration measured during the
PMA period (up to 2 µg m-3) indicates the presence of dust
particles, probably related to strong winds lifting soil/dust in the vicinity
of the Ersa station . However, unlike the dust period, they
do not represent the dominant aerosol influence during the PMA period.
Maps representing the different zones used for the study of the origin of air masses
with FLEXPART. (a) Anthropogenic and desert zones (red for Spanish coasts, dark blue for French coasts,
green for Italy, blue for Greece, orange and yellow for northern Africa. (b) Marine zones
(G corresponds to Gulf of Lion, B to Bay of Biscay).
However, unlike the dust period (16–20 June), the major aerosol influence
during the PMA period is PMA, with a mass concentration reaching 6.5 µg m-3, which is 3 times higher than nss-Ca2+ mass concentration.
Indeed, during the dust period, PMA mass concentration is low (0.4 ± 0.3 µg m-3) and so the main aerosol mass contribution are dust
particles.
Origins and time of residence of the different air-masses observed at Ersa
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.
Time series of air mass sources derived from the FLEXPART back-trajectory simulations at
500 m from 7 June 2013 to 13 July 2013. The top panel (a) represents the passage of an air mass through the
different zones before they reached Ersa. Panel (b) represents the transport time of the air masses from each
zone in (a) to Ersa.
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.
FLEXPART maps representing for (a, b, c) the probability density of the back trajectories for the three periods: dust, PMA and BBP. The bottom panels (d, e, f) represent
the mean transport time from the Ersa station for the three periods: dust, PMA and BBP. Each
back trajectory starts at 500 m from the Ersa measurement site.
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.
Transport time (mean ± standard deviation) from the Gulf of Lion and North Atlantic Ocean to the Ersa station
obtained from FLEXPART simulations analyses (see Sect. 3.1.1).
22 June
23 June
24 June
25 June
26 June
Gulf of Lion: mean transport time
(days)
0.72±0.69
0.49±0.29
0.21±0.09
0.55±0.30
1.32±0.50
North Atlantic Ocean: mean transport time
(days)
2.88±0.87
2.50±0.72
2.94±1.5
3.23±1.26
4.45±1.22
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.
Primary marine aerosols
PMA ageing
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.
Comparison of the two instruments ATOFMS and PILS-IC for inorganic component of sea salt aerosols.
The time series represents the mass ratio of chloride to sodium ions calculated from the PILS-IC measurements.
The marker colour represents the degree of ageing determined by the ATOFMS.
Characteristics of the three log-normal modes of aged sea salt aerosols measured by ATOFMS at Ersa station
Mode
Aerodynamical
σ
diameter µm
1
0.46
1.28
2
1.13
1.35
3
1.95
1.23
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.
PMA concentration measured at Ersa as a function of wind speed for the ADRIMED period. The PMA concentrations
have been averaged by wind speed bins of 1 m s-1. The error bars represent ± 2σ/N (N is the
number of independent measurements). (a) The black curve correspond to measurements, while the red, blue and green
curves correspond to fits parameters by .
Panel (b) represents fresh (blue curve) and aged (red curve) PMA over the whole campaign.
PMA sources
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.
Concentration of sea salt aerosols measured by the PILS IC as a function of wind speed measured at
Ersa (a), at the Gulf of Lion buoy (b) and at the Bay of Biscay buoy (c), for the PMA period. Offsets of
12 h and 60 h have been applied between the wind speed measurements in the Gulf of Lion and the Bay of Biscay
respectively and the PMA concentrations observed at Ersa to account for transport time of the air masses.
The PMA concentrations have been averaged by wind speed bins of 1 m s-1. The error bars represent ±2σ/N.
Blue curves represents all the PMA measurements, while green and red curves
represent fresh and aged PMA.
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.
PMA physical properties in comparison with dust and BBP periods
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.
Characteristics of the fit by a log-normal distribution (N, d, σ) for the three periods: dust, PMA and BBP
Number concentration
N1
d1
σ1
N2
d2
σ2
N3
d3
σ3
N4
d4
σ4
Dust
156
0.06
1.88
389
0.13
1.51
0.11
1.16
1.3
0.02
3
1.47
PMA
1162
0.04
1.46
164
0.13
1.56
0.45
1.2
1.5
0.02
5.4
1.25
BBP
0.13
0.027
0.9
582
0.08
1.79
170
0.22
1.35
0.07
1.5
1.7
Volume concentration
V1
d1
σ1
V2
d2
σ2
V3
d3
σ3
V4
d4
σ4
Dust
0.5
0.18
1.46
0.64
0.26
1.43
0.43
2.36
1.62
PMA
0.09
0.07
1.47
0.44
0.24
1.54
1.02
1.64
1.71
0.18
6.66
1.34
BBP
0.77
0.2
1.54
1.34
0.32
1.33
0.31
2.24
1.55
0.2
6.13
1.36
Number (a) and volume (b) size distribution averaged by periods of dust, PMA, and BBP using the
SMPS and OPS instruments. The dry diameters range 10 nm to 10 µm. The first and last days of each
period were removed to capture the main feature and the maximum amplitude of the event.
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.
AERONET volume size distributions averaged for the three periods.
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.
Summary of the optical properties (mean and standard deviation) estimated for the three
different aerosols regimes: AOD, AE, SSA, scattering coefficient (in Mm-1), and instantaneous TOA and BOA radiative effect (in W m-2)
AOD
AE
SSA
Scattering coefficient
TOA
BOA
500 nm
440–870 nm
440 nm
550 nm
Dust
0.16±0.08
1.4±0.3
0.97±0.03
28±11
-14±6
-21±11
PMA
0.11±0.08
1.3±0.4
0.98±0.02
28±11
-8±3
-11±4
BBP
0.23±0.07
2.1±0.2
0.98±0.03
54±18
-15±4
-23±6
ADRIMED
0.17±0.1
1.7±0.5
0.98±0.03
37±20
-12±5
-17±8
PMA optical properties and local shortwave direct radiative effect in comparison with dust and BBP periods
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.
Time series of (a) aerosol optical depth (AOD) at three
wavelengths (440, 500 and 870 nm) measured by the radiometer from the
AERONET network situated at the Semaphore during the ADRIMED campaign,
(b) Ångström exponent calculated from AERONET data, during
the ADRIMED campaign, using the extinction measurements at 440 and 870 nm,
(c) Scattering coefficient at three wavelengths: 450 nm (blue),
550 nm (green) and 700 nm (red) measured by the nephelometer situated at
Ersa.
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.
Aerosol radiative effect at (a) the top of the atmosphere (TOA) and (b) the bottom of the atmosphere (BOA)
represented as a function of the aerosol optical thickness (AOT) for each of the major periods, retrieved from AERONET.
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.