ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-18-5371-2018Vertical distribution of aerosol optical properties in the Po Valley during the 2012 summer campaignsAerosol particle optical properties in the Po ValleyBucciSilvias.bucci@isac.cnr.ithttps://orcid.org/0000-0002-6251-9444CristofanelliPaolohttps://orcid.org/0000-0001-5666-9131DecesariStefanoMarinoniAngelahttps://orcid.org/0000-0002-6580-7126SandriniSilviahttps://orcid.org/0000-0002-3346-6531GrößJohannesWiedensohlerAlfredDi MarcoChiara F.https://orcid.org/0000-0002-9635-8191NemitzEikohttps://orcid.org/0000-0002-1765-6298CairoFrancescoDi LibertoLucaFierliFedericof.fierli@isac.cnr.itInstitute for Atmospheric Sciences and Climate of the National Research Council, (ISAC-CNR), Rome, ItalySc Dept. of Physics, Ferrara University, Ferrara, ItalyInstitute for Atmospheric Sciences and Climate of the National Research Council, (ISAC-CNR), Bologna, ItalyLeibniz Institute for Tropospheric Research, Leipzig, GermanyNatural Environment Research Council, Centre for Ecology & Hydrology, Penicuik, UKFederico Fierli (f.fierli@isac.cnr.it) and Silvia Bucci (s.bucci@isac.cnr.it)20April2018188537153893March20176June201719February201826February2018This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/18/5371/2018/acp-18-5371-2018.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/18/5371/2018/acp-18-5371-2018.pdf
Studying the vertical distribution of aerosol particle physical and chemical
properties in the troposphere is essential to understand the relative
importance of local emission processes vs. long-range transport for
column-integrated aerosol properties (e.g. the aerosol optical depth, AOD,
affecting regional climate) as well as for the aerosol burden and its impacts
on air quality at the ground. The main objective of this paper is to
investigate the transport of desert dust in the middle troposphere and its
intrusion into the planetary boundary layer (PBL) over the Po Valley (Italy),
a region considered one of the greatest European pollution hotspots for the
frequency that particulate matter (PM) limit values are exceeded. Events of
mineral aerosol uplift from local (soil) sources and phenomena of hygroscopic
growth at the ground are also investigated, possibly affecting the PM
concentration in the region as well. During the PEGASOS 2012 field campaign,
an integrated observing–modelling system was set up based on near-surface
measurements (particle concentration and chemistry), vertical profiling
(backscatter coefficient profiles from lidar and radiosoundings) and
Lagrangian air mass transport simulations by FLEXPART model. Measurements
were taken at the San Pietro Capofiume supersite (44∘39′ N,
11∘37′ E; 11 m a.s.l.), located in a rural area relatively
close to some major urban and industrial emissive areas in the Po Valley. Mt.
Cimone (44∘12′ N, 10∘42′ E; 2165 m a.s.l.) WMO/GAW
station observations are also included in the study to characterize
regional-scale variability. Results show that, in the Po Valley, aerosol is
detected mainly below 2000 m a.s.l. with a prevalent occurrence of
non-depolarizing particles (>50 % throughout the campaign) and a
vertical distribution modulated by the PBL daily evolution. Two intense
events of mineral dust transport from northern Africa (19–21 and
29 June to 2 July) are observed, with layers advected mainly above 2000 m,
but subsequently sinking and mixing in the PBL. As a consequence, a non-negligible
occurrence of mineral dust is observed close to the ground (∼7 % of
occurrence during a 1-month campaign). The observations unambiguously show
Saharan dust layers intruding the Po Valley mixing layer and directly
affecting the aerosol concentrations near the surface. Finally, lidar
observations also indicate strong variability in aerosol on shorter
timescales (hourly). Firstly, these highlight events of hygroscopic growth of
anthropogenic aerosol, visible as shallow layers of low depolarization near
the ground. Such events are identified during early morning hours at high
relative humidity (RH) conditions (RH >80 %). The process is
observed concurrently with high PM1 nitrate concentration (up to
15 µg cm-3) and hence mainly explicable by deliquescence of
fine anthropogenic particles, and during mineral dust intrusion episodes,
when water condensation on dust particles could instead represent the
dominant contribution. Secondly, lidar images show frequent events (mean daily occurrence of
∼ 22 % during the whole campaign) of rapid uplift of mineral
depolarizing particles in afternoon–evening hours up to 2000 m a.s.l. height.
The origin of such particles cannot be directly related to long-range
transport events, being instead likely linked to processes of soil particle
resuspension from agricultural lands.
Introduction
The Po River basin in northern Italy is one of the most
important emissive areas in Europe, characterized by high concentration of
both natural and anthropogenic aerosol and trace gases . The
geographical location of this region, surrounded by two mountain ranges,
promotes frequent occurrence of stagnant meteorological conditions
, with accumulation of local pollution from industrial, urban and
agricultural emissions, and complex processes of aerosol-chemicals
transformation. At the same time, the relative proximity to the Sahara, which represents the major mineral dust source of the planet
, makes this region subject to long-range
mineral dust transport, especially during the summer season
. Such
combination leads to unusually high concentrations of atmospheric pollutants
and particulate matter, with frequent and prolonged periods of intense
pollution. The large number of people living in the region (more than 20
million potentially exposed to high pollution levels) accentuates the need
for
accurate studies on particulate variability over the Po Valley.
Local anthropogenic sources. The co-emission of particles and NOx
from combustion emissions and the widespread sources of ammonia from
agricultural activities over the region lead to the
accumulation of primary carbonaceous particles and secondary inorganic
aerosols (ammonium nitrate) in the lower layers of the atmosphere.
, during a campaign in summer 2004, observed that, under
easterly flow, ammonium sulfate and organics dominated the sub-micron
aerosol particle fraction while, under westerly anticyclonic flow, the large
NOx and ammonium emissions at the surface resulted in a large ammonium
nitrate concentration in air masses recirculating over the Po Valley. During
summer 2012, under similar meteorological features (anticyclonic conditions
and PBL air recirculation), observed a significant
enhancement of secondary organic and inorganic aerosol particle mass. They
also pointed out differences in aerosol behaviour in rural and urban areas: rural
areas, during night, were characterized by higher relative humidity and lower
temperature compared to the urban areas, and showed higher fine nitrate
nocturnal concentration and formation of ammonium nitrate in the large
accumulation mode (0.42–1.2 µm). This large amount of highly hydrophilic
compounds significantly increases aerosol particle light scattering due to
additional water uptake: ammonium nitrate-rich particles are very hydrophilic
and, in conjunction with the high relative humidity conditions often
encountered in the Po Valley floor, contribute to the build-up of hazes in
the region, also in the summer season .
Long-range natural sources: Saharan dust transport. Mineral dust
intrusion episodes can significantly affect public health: epidemiological
studies in Italy have revealed increased respiratory mortality
concurrently to such events. During summer months, Saharan dust
particles are uplifted to the mid-troposphere levels (up to 5 km) by
the strong surface winds and the large-scale convection that typically
involves the northern Sahara in this season . Mineral dust is
then transported over the whole Mediterranean Basin, following a usually
anticyclonic pattern of circulation triggered by the
extended subtropical anticyclone of the Atlantic Azores. Mineral dust over
the Mediterranean is found to be usually transported in the lower free
troposphere between 2 and 5 km altitude ; nevertheless, there is indication of episodes of mixing with
PBL air , with a contribution
to the surface particulate mass concentration estimated to be of the order of
10 µgm-3. While the above-mentioned studies rely on in situ
measurements, direct evidence of mineral dust entrainment in the PBL from
continuous vertically resolved observations over the Po Valley is still
lacking.
As accumulation of pollution can affect both regional climate and public
health , a better understanding of the
processes contributing to the high concentration of PM on the region is
needed. The present paper offers more details on aerosol processes related to
both local and long-range sources, and their interaction, in the Po Valley
region. The study is based on the analyses of continuous and vertically
resolved profiles of particle light scattering and depolarization: in
particular, it exploits light detection and ranging or laser imaging detection
and ranging (lidar) aerosol characterization, assisted by ground observations
and transport models, to identify the origin of the particulate entering and
mixing in the PBL during the summer season. The lidar represents a widely used
technique for studying the vertical and temporal distribution of particulate
matter optical properties e.g..
Coupled with in situ observations and model analysis, lidar observations can
also be used to derive transport pathways and physical–chemical processes
e.g.. In this work, lidar observations
are compared with near-surface measurement techniques from sites at different
altitudes, and supported by Lagrangian model simulations, to provide new
insights into the processes that affect aerosol particle variability in the Po
Valley and improve the understanding of the possible role of different local
and remote sources on the PM level over the region. The focus is on the
observation of mineral aerosol in the PBL, on the analysis of its origins and
on its mixing and interaction with local aerosol particles and anthropogenic
pollution. Please note that, from here on, by “aerosol” we will refer to just
the aerosol particle phase, excluding the carrier gas.
Observations and methodology
Observations were performed in the framework of the SuperSito project,
coordinated by the Regional Agency of Prevention and Environment and funded
by Region Emilia-Romagna (ARPA-ER, Italy, www.supersito-er.it, last access: 07 March 2018), and of the FP7
European project PEGASOS (Pan-European Gas-AeroSOl-climate interaction Study,
pegasos.iceht.forth.gr, last access: 07 March 2018).
Measurement stations
The San Pietro Capofiume (hereafter SPC) station (44∘39′ N, 11∘37′ 0 E; 11 m a.s.l.) is located in the south-eastern part
of the Po Valley, at a flat rural background site relatively close to densely
populated cities and industrial sites (i.e. 30 km NE from the Bologna
urban area, with about 500 000 inhabitants, and 20 km S from Ferrara,
with around 150 000 inhabitants). SPC is included in the list of WMO/GAW regional
stations, being representative of the surrounding wider region. The Mt.
Cimone (44∘12′ N, 10∘42′ E; 2165 m) WMO/GAW global
station (hereafter CMN), located 100 km south-west from SPC, is
instead situated at the top of the highest peak of the northern Apennines.
For the greater part of the year, CMN observations can be considered
representative of the background conditions of the southern European free
troposphere , while, during warm months, it can be
considered at a transition level between the uplifted boundary layer and the
free troposphere . Under summer anticyclonic conditions,
polluted air masses transport from the regional boundary layer can be
detected at CMN, due to thermal transport processes and PBL growth
. CMN also represents the first
mountain ridge impacted by Saharan air masses on their way across the central
Mediterranean Basin to Europe.
The lidar system operating in SPC, described in , uses a
532 nm Nd:YAG pulsed laser source, with pulse duration of 1 ns,
400 µJ of energy and repetition rate of 1 kHz. The optical
receiver of the lidar is a Newtonian telescope. Taking into account the
distance between the telescope and the laser beam, the overlap of the laser
within the field of view begins at few tens of metres (around 50 m) from the
system and is complete at around 300 m. Experimental correction allows the
reconstruction of the lidar backscattering profile down to around 100 m,
with an acceptable uncertainty (close to 10 %) on the backscatter ratio
precision seeand its supplementary material. Profiles
are collected every 10 min with a vertical resolution of 7.5 m extending,
on average, up to 7 km. In the following discussion, we will make use of the
aerosol linear depolarization ratio (δa) and the aerosol
backscattering ratio Ra=R-1 derived from the total
backscattering ratio (R), defined as δa=βaperpβaparR(r)=βa(r)+βm(r)βm(r),
and therefore
Ra(r)=R(r)-1=βa(r)βm(r).βm(r) represents the backscatter coefficient from molecular
contribution while βaperp and βapar are the
backscattered signal components from aerosol particle light scattering, with
polarization respectively perpendicular and parallel to the polarization of
the emitted light. The inversion of the lidar signal is accomplished with the
Klett method , finding a suitable region of the
profile that is supposed to be free of aerosol to calibrate the signal, and
using piecewise constant extinction-to-backscattering ratio (lidar ratio,
LR) values. Using volume depolarization (δ(r); see
) as a proxy of aerosol linear depolarization ratio, we
select different values for LR following what reported in the literature:
highly depolarizing desert dust (δ(r)<10%) is associated with
a LR equal to 50 sr , while for low depolarizing
aerosol we assume the values typical for anthropogenic aerosol,
LR ∼ 60–70 sr . In
addition we considered different values for water cloud (LR =20 sr)
and ice clouds (LR =30 sr) . A more
detailed description of the methods used to perform the inversion of lidar
data, together with a thorough uncertainty analysis, performance in conditions
close to the SPC site and additional experimental setup details, is given
in and and its Supplement.
Aerosol particle number size distribution: APSS and OPSS
Aerosol concentrations at the ground are obtained from an aerodynamic
particle sizer spectrometer (APS; TSI, model 3321), operating at SPC,
that provides real-time aerodynamic measurements of particles from 0.5 to 10 µm at 1 min time resolution. The aerodynamic diameter is defined
as the physical diameter that a unit density sphere will have if it settles
through the air with a velocity equal to the one of the sampled particle. The
aerodynamic diameters of particles are established by measuring their transit
time between two points when accelerated singly through a well-defined flow
field. The aerodynamic diameters are here converted to volume-equivalent
particle diameters, following and assuming an effective
particle density of 1.8, during dust days, and 1.55 during the remaining
days, accordingly to the values retrieved by . The near-surface aerosol number concentrations at the free tropospheric level are
instead derived from an optical particle size spectrometer (OPSS; Grimm, model 1.108) operating at the CMN station. The OPSS
provides particle counts in the diameter (Dp) range of 0.3 µm<Dp<10µm. The instrument is based on the
quantification of the 90∘ scattering of light by aerosol. According
to the specifications, the reproducibility of the OPSS in particle counts is
±2 % . Such measurements allow the determination of
the fine (0.3 µm<Dp<1µm) and coarse
(1 µm<Dp<10µm) aerosol fractions with
a 1 min time resolution. For the purpose of the paper we make use of the
time series of coarse (Dp>1µm) aerosol concentration observed
at CMN, without any further correction to the “optical” diameter, to provide
a clear indication on the presence of mineral dust layer in the free
troposphere .
Chemical composition: MARGA
The Monitor for AeRosol and GAses (MARGA, Metrohm Applikon B.V. Schiedam NL)
is a wet-chemistry instrument that provides continuous measurements of the
water-soluble inorganic gases and aerosol components that might have a direct
effect on air quality . The analytical system
allows for the characterization of inorganic aerosol (Cl-,
NO3-, SO42-, NH4+, K+, Ca2+, Mg2+, Na+) and
gases (NH3, HNO3, SO2, HONO, HCl) at hourly resolution
. In the sampling box, the air passes through a wet rotating
denuder (WRD) where water-soluble gases are stripped from
the air stream and collected in water. The sampled air then continues through
a steam-jet aerosol collector SJAC;,
where the water-soluble aerosols are separated from the air stream and
collected. The gas and aerosol samples are then analysed by online ion
chromatography with high accuracy (detection limits as low as 0.01 µgm-3; ). Size-selective particle cyclones are used in
front of the two MARGA sampling boxes so that the size of the particles for
analysis can be limited to an aerodynamic diameter of less than 10 (PM10) or
1 (PM1) µm.
Transport modelling
We make use of the FLEXPART Lagrangian particle dispersion model (version 9.02) and references therein to characterize the
transport during the campaign period. FLEXPART is a widely used model system
to simulate synoptic and mesoscale transport and diffusion of aerosol and
trace gases, as well as loss processes such as dry and wet deposition or
radioactive decay , and has been validated using large-scale
tracer experiments . In our case, the model is
driven by pressure level data from NCEP Global Forecast System (GFS)
(rda.ucar.edu, last access: 01 March 2017). Meteorological input is provided every 6 h (00:00, 06:00,
12:00 and 18:00 UTC) at a resolution of 0.5∘×0.5∘.
FLEXPART output is the footprint of the retroplume, namely the time of
residence of the air parcels over each geographical grid point during the 5 days prior to the moment of the trajectories' release. Such a quantity,
expressed in seconds, gives an indication of which emissive regions are going
to contribute (and in which extent) to the mineral dust enrichment of the air
parcels, therefore influencing dust burden at the time and the position of
trajectories release. To give an estimate of the variability in the mass of
mineral dust advected over SPC, we compute, for each release, the mass
fraction of trajectories that encounter dust emissive regions with respect to the
total mass of the released cluster. Dust is considered to be injected into the
atmosphere when the trajectories are crossing the PBL height (as extracted by
FLEXPART itself from the GFS input meteorological field; see
) over the Sahara. The back-trajectory clusters were
released every 6 h, along the whole campaign period, from the SPC station
concurrently to the 1000–2000 and 3000–4000 m
atmospheric layers.
Lidar aerosol type classification
Lidar observations have been extensively used to identify mineral dust layers
and discriminate among different typologies of aerosols, based on a choice of
specific ranges of optical parameters considered representatives of distinct
aerosol types. Examples are shown in , where the
classification among eight different types of aerosol is derived from volume
depolarization ratio (δ), lidar ratio (LR) and the colour ratio (CR).
In the categorized aerosol types are sea salt, mineral
dust and mixed dust based on LR and aerosol linear depolarization ratio
δa. The estimate of LR (which requires independent
information on the extinction that should be derived from the Raman signal),
as well as an evaluation of the CR (based on the adoption of two-wavelength
channel), is not possible with single-wavelength elastic lidar like the one
deployed at SPC during the PEGASOS campaign. Nevertheless, some typologies of
aerosol show distinct values of aerosol linear depolarization ratio. At
532 nm, values of δa around or higher than 30 % are
generally associated with layers of nearly pure mineral dust, while smaller
values (around 8–10 %) are often detected concurrently to mixture
of mineral dust and non-depolarizing particles
. By contrast, smoke and other
anthropogenic aerosols exhibit low values of δa (less than
5 %) .
Here we implement a three-type aerosol discrimination scheme, based on the
different statistical distribution of optical properties of each class (see
Fig. ), to characterize the vertical and temporal aerosol
variability over the region along the campaign period (15 June 2012–5 July 2012). The reader should notice that the lower δa values that we
observe with respect to what usually found in the literature (especially for the
dust layers) are likely linked to the calibration process, and in particular
to the difficulty in individuating completely aerosol-free layers in the
vertical span of the adopted lidar system (from the ground to 7 km).
In this work, δ has been calibrated following the “0∘
calibration” or the “atmospheric calibration” procedure, i.e. making use of a
low aerosol height range in the lidar signal, where only the molecular
contribution could be considered. There, the volume depolarization ratio has
been forced to assume a reference value for the air molecule linear
depolarization ratio derived from the literature, in our case δmol=1.3 %, given the instrumental setup of the lidar and the measurement
conditions; see . We acknowledge that this calibration is
unsatisfactory for producing quantitative results, as the possible residual
presence of small amounts of highly depolarizing aerosol, in the assumed
clean range, can easily compress the range of variability of the volume
depolarization and underestimate the final depolarization products
. However this possible source of
inaccuracy does not compromise the purpose of this work. The lidar
classification, based on the statistical distribution of the overall observed
δa and Ra values, is in fact applied here to overcome such
limitations. The robustness of the results are then further supported by
comparison with Lagrangian analysis and in situ measurements.
Probability density function of aerosol particle optical properties
over 15 June–5 July 2012 as a function of Ra/(Ra+1) and δa
parameters. The colour code indicates the frequency of occurrence.
Figure reports the probability density function, along the
whole measurements campaign, of the aerosol occurrence, expressed as a
function of Ra/ (Ra+1) (ranging from 0 in aerosol-free conditions, i.e.
Ra=0, to 1 in the presence of an opaque aerosol layer, when Ra tends to
infinity) and δa. The analysis includes 10 min resolution
observations from the ground up to 5000 m height, for a total of about
1.5×107 sampling points. The different aerosol classes can be
discerned in three distinct patterns:
0.1<Ra/(Ra+1)<0.8 and low values of δa (< 3 %); such
low values of aerosol linear depolarization ratio are indicative of spherical
particles. These particles may be composed of anthropogenic pollution and,
for higher values of Ra, by droplets, and are defined as non-depolarizing.
0.3<Ra/(Ra+1)<0.7 and high values of δa (> 10 %); in this class we
find the highest values of aerosol linear depolarization ratio (mainly
ranging from 10 to 20 %) and this can be indicative of the
presence of mineral dust particles. This class is defined as depolarizing.
0.2<Ra/(Ra+1)<0.6 and intermediate δa values (3 % <δa<10 %) which,
based solely on Ra and δa, cannot be considered as indicative of
the dominance of a defined aerosol type unless coupled to a more thorough
correlation with additional observations. We will refer to this type as
intermediate depolarizing.
Lidar observations along the whole campaign. (a) Vertical
profiles of aerosol particle types resulting from the classification
methodology described in Sect. 3: non-depolarizing aerosol (yellow),
depolarizing aerosol (orange) and intermediate depolarization aerosol
(brown). (b) Vertical profiles of Ra/ (Ra+1). (c) Vertical
profiles of δa. The coloured bars above panel (a) give reference for
the presence of dust or mixed dust above 2000 m (orange bar) and below
2000 m (brown bar) to compare with the in situ measurements. See also
Figs. and .
The boxes in Fig. delimit the Ra/ (Ra+1) and
δa ranges for each of the three classes used to derive the
aerosol mask for the whole campaign. The results are reported in Fig. together with the profiles of Ra/ (Ra+1) and
aerosol linear depolarization ratio δa. Overall,
non-depolarizing particles (type 1) are dominant throughout the campaign with
a total occurrence of 49 % of the measurements in the 100–5000 m range.
They are observed prevalently below 2000 m and are associated with enhanced
values of Ra (Ra/ (Ra+1) ranging between 0.3 and 0.6). Due to the
presence of such particles, the vertical gradient of Ra marks, when not
masked by the presence of mineral dust layers or clouds, the PBL evolution.
Two events of depolarizing aerosol (19–21 June and 29 June–2 July),
recognized as type 2 and likely related to mineral dust presence, are
observed between 2000 and 5000 m. Such events are also clearly visible as
enhancement of Ra in the free troposphere with values of Ra/ (Ra+1)
ranging from 0.6 and 0.8. The intermediate class (detected with an occurrence
of around 19 % during the whole campaign) is found in close proximity to
the depolarizing aerosol and within the PBL. The observed vertical and time
distribution of these intermediate type particles indicates the possibility
of mixing of the dust depolarizing layers with local non-depolarizing
particulate. Nevertheless, intermediate δa values are also
observed systematically after 12:00 UTC (Universal Time Coordinated), between
0 and 1500 m height, for the majority of dust-free days. The nature of such
intermediate non-dust depolarizing aerosol is further discussed in
Sect. .
(a) Wind provenance direction. (b) Wind speed. (c)
Ground temperature at 12:00 UTC. Dust/mixed dust presence, as seen by lidar,
is indicated as in Fig. .
Meteorology and synoptic aerosol regimes
The evolution of the meteorological conditions at SPC is reported in Fig. . Vertical profiles from the ground up to 4 km height
of wind speed and direction come from radiosondes (Vaisala RS92) launched
daily at 00:00, 06:00 and 12:00 UTC. Figure c also reports the evolution of ground temperature at 12:00 UTC. PBL evolution
description is referred to the PBL height time series presented in
and also reported in the Supplement (see
Fig. S1). The observation of wind profiles over SPC highlights a sequence of
distinct meteorological regimes:
Stagnation period, 15 June–19 June: this first phase is characterized by a situation of stagnant
conditions (wind speed less than 4 m s-1 below 2000 m) with a progressive warming of the air
masses (from 29 to 34 ∘C at the ground). The PBL top is limited to below 1500 m
until 19 June, when it reaches 2000 m.
South-westerly winds, 20 June–21 June and 30 June–5 July: during the 20 June–21 June
phase, higher wind speeds (between 11
and 16 m s-1) are observed above 2000 m. Correspondingly, wind
direction profiles indicate a prevalent south and south-west provenance. The
arrival of warm Saharan air masses (temperature at the ground around
32–33 ∘C) leads to a more intense PBL
development (up to 2000 m) with respect to the previous stagnation phase. During the last days of the campaign (30 June–5 July), strong winds (between 12 and
15 m s-1 with a peak of 20 m s-1 on 3 July) are observed
above 1500 m. While during 30 June winds are coming mainly from the south, the
following days are characterized by a change of direction to south-westerly (1 July) and
then westerly flow (2 and 3 July). During this phase, temperature at the
ground reaches its highest values (35–37 ∘C)
but the dust layer presence made it difficult to unambiguously retrieve the
PBL top. During the immediately following days (3–4 July), the PBL top was
detected above 2000 m.
North-easterly winds, 22–29 June: the radiosounding profiles indicate a prevalence of south-easterly or
easterly winds above 2000 m a.g.l. Northerly/north-easterly winds are instead often visible at lower altitudes,
in particular between 23 and 24 June below 1000 m, on 26 June below 500 m (also associated with wind intensities
up to 10 m s-1) and on 27 June between 500 and 1800 m. During this period, ground temperature first
decreases to 28 ∘C and then increases again after 26 June, reaching 35 ∘C on 29 June. The PBL maximum height
varies between 1500 and 2000 m a.g.l. Such conditions are favourable for the export of the Po Valley
pollution toward the Tyrrhenian Sea and will be extensively discussed in a future paper.
The evolution of the observed size distribution and optical classification
during the distinct meteorological regimes is presented in the following
sections.
Summer stagnant conditions: 15–19 June
Meteorological evolution is compared with the aerosol optical variability
from lidar (see Fig. ) and with ground aerosol number
concentration and volume size distribution (estimated as the volume of a
sphere with diameter corresponding to the volume-equivalent particle
diameter) at SPC and CMN (see Fig. ). Figure a shows the time trend of
small particle concentration (297 nm <Dp<420 nm) from the APSS at SPC. The stagnation period
(15–19 June), typical for the Po Valley on hot summer days
, is characterized by a marked daily cycle in the aerosol
concentration and by a progressive day-by-day accumulation of particles in
the PBL. This is noticeable in the increase in the particle number size
distribution of small particles from APSS (from 5 to nearly 20 cm-3)
during the early morning hours (00:00–06:00 UTC), when the lower troposphere is
stably stratified. The lidar observations (Fig. ) show a
persistent layer of non-depolarizing particles up to 2 km height,
attributable to anthropogenic aerosol and modulated vertically by the PBL
daily cycle. APSS data in Fig. b show a bimodal
aerosol distribution with a clear increase in volume mode due to submicron
particles (0.5 µm<Dp< 1 µm) growing from
0.4 to more than 1 µm3 cm-3. During this period of the
campaign, the OPSS at CMN (Fig. c) indicates coarse-particle number concentrations below 0.4 cm-3.
(a) APSS particle counts at 297–420 nm of diameter.
(b) Time series of the volume size distribution of aerosol particles
as observed by the APSS. The y axis indicates the volume-equivalent particle
diameters in micrometres while colours report the corresponding volume
concentration. (c) Time series of coarse (Dp> 1 µm) particle
number concentration observed at CMN. Dust/mixed dust presence, as seen by
lidar, is indicated as in Fig. .
Vertical profiles of aerosol particle types (upper panels), as in
Fig. , and APSS aerosol time series of the volume size
distribution of aerosol particles (lower panels), as in Fig. ,
for the first (a) and the second (b) dust event.
Saharan dust events: 19–21 June and 29 June–3 July
During the first event (19 to 21 June) strong south-westerly winds (with
speeds greater than 10 m s-1 above 2000 m) are associated with a
stable anticyclonic circulation centred above the southern Mediterranean and
Tunisia, leading to an efficient south/south-westerly circulation. Mineral
dust can be clearly observed as an enhancement in lidar Ra profiles (Fig. b) above 2000 m until 21 June, while, below that
height, it is not possible to discern any deviation from the background
aerosol signal. The enhancement in Ra is accompanied with increased aerosol
linear depolarization ratio (δa∼10–15%)
during the whole event, with values up to 20 % above 3000 m during
20 June, resulting in a coherent layer of type 2 particles visible in the
aerosol mask (Fig. a). In correspondence of the
presence of dust aerosol at 2000 m level, the OPSS at CMN (Fig. c) detects an increase in coarse-particle concentration
up to 1.8 cm-3. The peak seen on 21 June at around 09:00 UTC (greater
than 5 particles cm-3) should be attributed to an enhancement in
aerosol load, which can be caused by an intensification of mineral dust
burden or by mixing with pollution from the regional PBL
, as suggested by the observed corresponding
increase in black carbon concentration at CMN (see also
). Intermediate depolarization aerosol type 3 is
observed below the depolarizing layer, throughout the mineral dust event, and
persisting until 22–23 June. As mentioned in the previous section, this can
be a signature of mineral dust mixing with local non-depolarizing
particulate. A direct comparison of the aerosol layer structure with the in
situ measurement is presented in Fig. , focusing on the days
of dust transport. From the comparison it is indeed possible to observe a
simultaneous enhancement of coarse (2 µm<Dp<5 µm) particle detection by the APSS on 20–22 June (Fig. a) concurrently to the identification of type 3
class near the ground, suggesting mineral dust presence in the layer
composition. It is worth noting that, while ground measurements do not
indicate a clear coarse-particle enhancement after 22 June, the lidar still
observes a lofted layer of intermediate depolarizing aerosol until 23 June.
During the second event (29 June to 3 July) high wind speeds above 2000 m
(up to 20 m s-1) are associated with a high-pressure area centred
above central Italy, leading again to favourable south-westerly circulation.
Lidar data show a second layer of enhanced Ra (Ra/ (Ra+1) around
0.6) lasting from 28 June at 23:00 UTC to 3 July at 00:00 UTC.
Depolarization reaches values higher than in the previous event (with mean
values of 15 % and maximum exceeding 20 %); this is again visible as
a thick and persistent layer of type 2 aerosol that, in this case, extends
down to the ground on 1 July (see Fig. b). As in
the previous case, it is possible to observe the presence of intermediate
depolarization particles (type 3) close to the depolarizing layer (type 2).
The dust layer appears to be characterized by a more intense contribution of
coarse particles with respect to the previous event, visible both at lofted level
and ground. APSS observations in fact show an increase in coarse-particle
volume simultaneously to detection of type 2 and 3 particles close to the
ground, with values higher than in the previous event (> 1 µm3 cm-3) and increased contribution from the intermediate
particle size (1–2 µm). Similarly, concurrently to the
presence of the depolarizing layer at 2000 m, the OPSS (Fig. c) shows coarse-particle concentrations nearly doubled
compared to the previous event (concentration between 2 and
3 particles cm-3).
The upper panels of Fig. show the footprints of the 5 days
retroplumes released on 20 June at 18:00 UTC (panel a) and on 29 June at
12:00 UTC (panel b). The transport for the first event has a more direct
pathway, with an average transport time of 2 days from the northern Sahara. The
second event appears instead to originate from the western Sahara and has a
longer pathway revolving around the anticyclonic circulation (around 4 days).
Aerosol optical depth from multi-model forecasts (SDS-WAS Sand and Dust Storm
WMO warning advisory and assessment system, visible at
http://sds-was.aemet.es/forecast-products/dust-forecasts/compared-dust-forecasts,
last access: 01 March 2017) indicates a spatial
distribution in agreement with the FLEXPART footprints for the two events.
The simulated mineral dust mass fraction over the SPC site (Fig. c) allows providing an estimate of the evolution (with a
time step of 6 h) of the mineral dust contribution on the SPC site. The
simulation confirms the presence of the two desert dust transport periods and
the progressive descent of the dust layers advection from 3000–4000 to
1000–2000 m heights, simultaneously to what is shown by lidar observations.
The maximum mineral aerosol fraction from FLEXPART analysis occurs on
20 June, both at the upper layer (9 %) and at the bottom layer (9 %
also). According to FLEXPART, the import of mineral dust at the lower layer
persists until the morning of 23 June, when dust presence is not
unambiguously inferable from observations, but the aerosol mask still
indicates the presence of intermediate depolarizing particles below 2000 m.
The second desert dust event predicted by FLEXPART again shows the same
timing with respect to observation and also confirms the presence of a thick
layer of dust that involves, at the same time, the 1000–2000 and
3000–4000 m layers. The estimated mass fraction contribution, however, especially in the lower layer (between 2 and 4 %), appears
inferior with respect to the previous events. It should be emphasized that a
quantitative assessment of the mineral aerosol transport from this method
would be difficult, due to uncertainties related to the model estimate of PBL
height over the desert, to the variability in the emissions of mineral dust
and to the uncertainties on the trajectory dynamics .
Nonetheless, despite such limitations, the model offers a robust
characterization of the dynamics and timing of the events, supporting the
interpretation of the data analysis.
FLEXPART back-trajectories over GFS meteorological input: panels (a)
and (b) show the footprint (in nanoseconds of residence over each bin) of the
trajectories released over SPC at 3000 m. Black triangle indicates the
point of release, and black squares mark the position of the centre of mass every
24 h. The pattern of trajectories released on 20 June at 18:00 UTC are
shown on the left (a), while pattern released on 29 June at 12:00 UTC
are on the right (b). The simulated mass fraction contribution of dust
over the SPC site is reported in (c) with a time step of 6 h. The
black line is relative to the particles released in the 1000–2000 m
atmospheric layer and the blue line to the release at 3000–4000 m.
Mean daily variability
Figure reports the frequency of observations for each of the
three classes, integrated for the period 15 June–5 July. Depolarizing
aerosols (Fig. a) are associated with the two events
of desert dust and hence are mostly observed between 1500 and 5000 m height
with a frequency of occurrence ranging between 15 and 30 %.
Non-negligible occurrences (∼ 10 %) are also observed close to the
ground and can be attributed to the mineral dust descent during the second
event. Non-depolarizing aerosol (Fig. c) is dominant
throughout the campaign (occurrence up to 80 % below 2000 m); this class
of aerosol appears to be mainly confined below the PBL top (derived by the
analysis shown in Fig. S1 of the Supplement and traced with a
black dashed line). During the campaign, the diurnal PBL starts to develop on
average at 06:00 UTC and reaches its maximum vertical extension, up to 2 km
height, between 17:00 and 18:00 UTC. A high occurrence of non-depolarizing
particles marks the PBL average daily evolution both during the diurnal
formation and at night-time (21:00 to 05:00 UTC), forming the residual
layer. A clear minimum of non-depolarizing particles occurrence is observed
in the afternoon between 16:00 and 19:00 UTC, when lidar indicates instead a
maximum (50 % of observation) of intermediate depolarization type
occurrence (Fig. b). Such enhancement of type 3
particle detection during late afternoon appears frequently along the
campaign (clearly visible on 13 days over 21; see Fig. ). It
should be noted that on 20–22 June and 30 June–2 July the presence of
mineral dust can mask any δa enhancement in the PBL. On
average the vertical extent of such a layer of intermediate depolarization is
limited within the PBL below 1500 m (Fig. b).
Daily average evolution of particle volume size distribution, relative to
dust-free days, is reported in Fig. . Fine-particle
(Dp< 1 µm) volume shows a semidaily cycle, corresponding to
the daily cycle of non-depolarizing aerosol near the ground, with
concentration increasing during the stable nocturnal layer phase (late
night–early morning) and strongly decreasing during the stage of
well-developed PBL. The larger particle mode shows two maxima: a first one
(volumes < 0.4 µm3 cm-3), concurrently to the
uplift of the PBL layer around 09:00 UTC, and a second one forming at
15:00 UTC, with a maximum (volumes > 0.5 µm3 cm-3)
at 20:00 UTC, showing a similar timing than the depolarization enhancement
described above. Further analysis of the afternoon PBL aerosol composition is
reported in the following section.
Mean daily frequency of the vertical distribution of each aerosol
class (computed as the ratio between the number of aerosol class detections
and the number of days of measurements): depolarizing (a), intermediate
depolarizing (b) and non-depolarizing (c). The mean PBL height,
derived from lidar analysis, is reported as a black dashed line over the
non-depolarizing particle distribution.
Non-desert-dust depolarizing aerosol
We report in Fig. the
δa profiles of a representative case study (3 July) of the
late afternoon occurrence of intermediate depolarizing aerosol. The aerosol
linear depolarization ratio indicates that the plume starts to develop from
15:00 to 20:00 UTC and reaches the maximum height of 1500 m in the late
evening, with a vertical structure suggesting a possible uplift of particles
from the ground. An increase in aerosol linear depolarization ratio in a
regime of convective PBL has already been observed by . Their
results show a positive correlation of enhanced δa with an
increase in vertical wind velocity, possibly indicating a source emission of
particles transported upward by convection. The actual nature of the aerosol
plume cannot be assessed solely by lidar depolarization. An increase in
depolarization can be due to the presence of irregularly shaped particles
that can belong to a wide range of aerosol types, from soil and desert dust
to marine aerosol and ash particles
. The hourly time resolution measurements of PM1 and PM10
aerosol chemical compositions, provided by the MARGA analyser, show no
evident correlation between the depolarization increase and the presence of
sea salt (not reported here). Similarly, no correlation was found with
absorbing aerosol (black carbon), investigated by means of a multi-angle
absorption photometer MAAP; (also not shown). By
contrast, MARGA observations highlight a maximum in PM10 calcium
concentrations, simultaneously to the afternoon increase in ground
depolarization (starting between 15:00 and 20:00 UTC; see Fig. b) and in the detection of larger particles from APSS (maxima
between 18:00 and 20:00 UTC, Fig. ). The daily mean
evolution of the calcium ion (Ca2+) fraction, calculated over the
total PM10 ions, shows a marked increase after 10:00 UTC with a maximum in
the late afternoon (17:00–20:00 UTC, up to 0.35; see
Fig. a). This daily behaviour is in agreement with the
enhancement in aerosol volume contribution from large particles with respect to
the fine ones, shown in Fig. b. These results
reinforce the hypothesis of the crustal origin of the intermediate
depolarizing particles observed by the lidar. It is possible therefore to
explain, at least on a qualitative basis, the recurrent detection of the
afternoon aerosol plumes as emissions and resuspension of soil particles from
dried land sources. The frequent occurrence of such events during the
observational campaign indicates that the Po Valley can effectively act as a
source of mineral particles, likely originating from agricultural soils, that
under convective atmospheric conditions can be uplifted at the PBL top in
late afternoon hours. This is further confirmed by the daily evolution of
non-desert-dust coarse-particle concentration at CMN (Fig. c), which indicates an enhancement in the coarse-particle fraction in
late afternoon/evening. Hence, recurrent vertical transport from the Po
Valley, triggered by thermal air mass, can uplift mineral aerosol over the
mountains ridge, and potentially impact on particulate transport up to the
regional scale .
Mean daily evolution of aerosol particle volume size distribution on
dust-free days from the APSS at SPC.
Vertical profiles of lidar aerosol linear depolarization ratio on 3 July 2012.
Effect of aerosol hygroscopic growth on scattering and depolarization
Lidar data (Fig. ) frequently
show, during early morning hours, a shallow layer of non-depolarizing aerosol
below 300 m, more easily visible during days characterized by desert dust
and mixed dust events (see for instance 00:00–06:00 UTC of 19 June and
between 00:00 and 08:00 UTC of 30 June). The decrease in depolarization is less
evident in dust-free atmosphere but is nevertheless observed in several other
days of the campaign (18, 21, 22 and 29 June, 1, 4 and 5 July), always below 300 m, before 08:00 UTC and usually associated with
high values of Ra(Ra/(Ra+1)>0.6). The average profiles of
δa, relative humidity (RH) and potential temperature
(θ) are reported in Fig. 11 along the whole vertical range of lidar
observations. The dust days are selected concurrently to enhanced
presence of coarse aerosol at the ground as seen from the APSS, i.e. when for
Dp> 2 µm the volume size distribution reaches values higher than
0.5 µm3 cm-3 (20–22 June and 30 June–2 July; see
also Fig. ).
Panel (a) shows the mean daily evolution of the ratio of PM10
concentration of the calcium ion (Ca2+) over the total PM10 ion concentration
(Ca2++ PM10/ total PM10) in SPC. Panel (b) reports daily mean of the ratio of
large particles (1 µm<Dp< 5.5 µm) over the fine ones
(0.5 µm<Dp<1µm) at SPC, while panel (c) shows the
daily mean of coarse (Dp> 5 µm) particles at CMN. Each mean is
computed on dust-free days.
During dust-free days (Fig. a), values of
δa stay on average below 1 % in the upper layer (above 2000 m) and below 2 % in the lower layer (below 2000 m).
Starting from 800 m, RH increases with decreasing altitude, and this is
associated with a progressive decrease in the δa values. During
dust days (Fig. b) a more defined stratification of
the atmosphere is observed. This is also visible in the profiles of θ
that, compared to the dust-free days, shows a clear passage from a stable
layer below 500 m to a more unstable one concurrently to the
depolarizing aerosol. The mineral dust plume is visible between 1500 and 4000 m height, associated with a layer of higher depolarization
(δa>10 %) and drier air with respect to the lowermost layer
(with RH increasing with height from 40 to 60 %). Below 1500 m, RH varies between 55 and 60 % and δa
shows lower values compatible with dust presence mixed with local pollution
(around 6 %).
Figure reports the mean vertical profiles of δa, RH and θ
for dust-free (a) and dust days (b) at 05:00 UTC. Please
note
the different horizontal axis for δa.
Conversely, the effect of possible phenomena of hygroscopic growth begins to
be visible in the lowermost troposphere (below 500 m). Such a layer,
marked by the increased stability shown by the θ profile, is characterized
by a sharp decrease in δa (from 7 down to 3 %)
associated with the increase in RH (from 50 to 75 %). The
study indicates in both cases a decrease in the depolarization in the lower
layers for RH > 60 %. During dust-free days the affected aerosol layer
(below 400 m) shows a depolarization decrease of about 1 %
(from an average value of 2 to values around 1 % and less).
During dust days the process influences the aerosol layers at different
levels: for RH values between 60 and 65 %, depolarization
values decrease is around 2 % (from more than 7 to around
5.5 %). Closest to the ground, the effect is more evident with
increasing RH (up to ∼ 70 %), corresponding with a decrease in
depolarization up to 3.5 %. This low depolarization near ground
suggests the presence of increasingly spherical particles, which can be
originated by two different processes:
The presence of fine particles of anthropogenic origin that may deliquesce:
the stagnant meteorological conditions that characterize the Po Valley during
anticyclonic phases are favourable for the formation of secondary inorganic
aerosols (especially ammonium nitrate) and of secondary organic aerosol
. A recent study showed that, during
the same 2012 campaign at SPC, the aerosol liquid water was mainly driven by
locally formed nitrate; hence the growth of spherical non-depolarizing
aerosol could occur due to deliquescence of fine particles of anthropogenic
origin, of which nitrates were the dominant compound.
Mechanisms explaining the increase in scattering of mineral dust particles, along with a
reduction of their depolarization ratio, can also be hypothesized
. It should
be emphasized that, during the analysed case study, high relative humidity
values (80 % or more) are observed in the lowermost non-depolarizing
layer (see Fig. ), suggesting that condensation of water
around mineral dust particles coated with (or simply enriched in) hydrophilic
components may play a role in the modification of the optical properties of
desert dust in this atmospheric layer. Indeed, even if mineral dust is
primarily a hydrophobic aerosol, it can become hydrophilic due to chemical
reactions occurring on the particle surfaces during long-range transport
or locally from the condensation of inorganic
and organic soluble materials from ground sources.
During the summer 2012 campaign, under the observed conditions, both
processes may have played a relevant role: Fig. shows that,
during the stagnation phase (from 14 until 19 June), aerosol nitrate
(NO3-) concentration in both fine- and large-particle MARGA channels
(PM1 and PM10) increases with a marked daily cycle peaking at night. The
submicron fraction of nitrate dominates the concentration of PM10 nitrate
during this phase. The APSS submicron particle volume concentration follows a
daily variability and a buildup similar to that shown by the APSS nitrates
concentration, reaching maxima during 19 June (APSS volume concentration up
to 25 µm m-3 and NO3- PM1 and PM10 concentration up to
15 and 18 µg m-3, respectively).
Such an increase, evident during early morning hours in small aerosol from APSS and
in PM1 nitrate from MARGA, supports the hypothesis of anthropogenic fine-particle deliquescence, therefore explaining the low values of
δa observed by the lidar in the surface layer during
dust-free days. Similarly, in the presence of dust, the low depolarization values
can be related to external mixing of dust depolarizing particles with such
locally formed spherical particles. Nevertheless, after the end of the
stagnation phase (19 June) the aerosol nitrate concentration decreased and,
during the observed desert dust episodes, the difference between the nitrate
PM10 and PM1 fractions became more evident, with PM10 prevailing over PM1.
The intensified ventilation established after 19 June may in fact have
limited the accumulation of anthropogenic particles, at the same time
carrying drier African air masses and making nitric acid condensation on
coarse particles prevails over condensation on accumulation-mode aerosol.
Consequently coarse-mode nitrate would promote water condensation on the
large particles, leading to low aerosol linear depolarization ratio values
even in the presence of dust.
Conclusions
The presented
paper provided a characterization of the effects of meteorological evolution
and transport patterns on the aerosol variability, based on the observations
collected during two major field campaigns (PEGASOS and SuperSito) in the
eastern part of the Po Valley. The aim was to contribute to the understanding
of the processes that lead to the high concentration and variability of
aerosol characterizing the Po Valley during typical summer conditions.
APSS fine-particle (Dp<1µm) volume contribution
(a) compared to nitrate ion concentration (NO3-) both in the
PM1 (black) and PM10 (blue) channel (b). Zero values correspond to
missing observations.
The analysis of meteorological conditions, coupled with observations from
lidar and in situ aerosol number/size distribution spectrometers, led to the
identification of distinct meteorological regimes with a temporal and spatial
distribution of different aerosol types.
We identified a first phase (15–18 June), characterized by a stagnation
period (weak winds below 2000 m), representative of hot and polluted
conditions in the whole Po Valley area, with progressive accumulation of
locally emitted aerosol in the lower troposphere and consequent increase in
the fine-mode aerosol concentration near the ground. Particle concentration
at the ground therefore showed a clear daily cycle with maxima during the
early morning, when PBL uplift and vertical mixing were absent.
Observations and Lagrangian analysis thus allowed a detailed description of
two events of Saharan dust transport (in line with the average occurrence of
2–3 summer desert dust episodes over the region detected by satellite;
). Mineral dust layers were advected over the measurement
site from the Sahara, travelling along anticyclonic patterns at high
level (around 3000–4000 m) and carrying depolarizing aerosol. The study
offered evidence of dust transport to the ground, showing clear dust layer
intrusion in the PBL and rapid mixing with local pollution. We showed how
this mixed layer, generally characterized by lower depolarization values, can
reach the ground within a few hours and we showed, by direct comparison with ground
in situ instruments, the corresponding enhancement of particle volume size
distribution in the 2–5 µm range (leading to values higher than
1 µm3 cm-3). In both events the plumes indeed descended to
low height (with a total occurrence of depolarizing aerosol identification
inside the PBL of ∼ 7 % along the whole campaign). As, on a
climatological basis, Saharan dust advection occurs with noticeable frequency
over the northern Mediterranean (i.e. , indicated a frequency of
17 % for the 2001–2011 period), dust intrusion can represent a
significant factor in increasing PM concentration at the ground. Such results
give direct evidence to the suggestion of , who
hypothesized, based on in situ measurements in northern Italy and
back-trajectory analysis, that mineral dust events detected in the free
troposphere can lead, with non-negligible frequency, to PM10 exceedance at
the ground in the time span of some hours.
The study revealed moreover the presence of events of late afternoon
particles resuspension from the soil, not related to Saharan dust transport,
impacting on the PM concentration near the ground. The existence of a
contribution to PM10 levels from resuspension aerosol sources in European
regions has already been hypothesized by , based on chemical
transport model study. Here, several events of intermediate depolarizing
aerosol (mean daily frequency of detection ∼ 22 %), up to 2000 m height, were observed during the late afternoon (17:00–20:00) in
dust-free days. The concurrent increase in calcium particle spectroscopic
measurements (with a contribution up to 0.35 of the total PM10 fraction)
indicated the crustal nature of such an aerosol, and can therefore be reasonably
attributed to processes of vertical uplift of soil particles, likely related
to regional activities (i.e. farming or combustion processes). The vertical
extension of such plume, as observed in lidar profiles and in the daily
variability in the CMN measurements, also suggests that local pollution can
be transported above mountain peaks and hence potentially exported outside
the orographic boundaries of the region.
The combination of depolarization profiles with meteorological and aerosol
measurements also allowed highlighting the effects of the condition of high
RH (typical for this region) on the particle processes. The analysis revealed
how, in conditions of high relative humidity values (RH > 60 %) in
a shallow layer near the ground (< 500 m), the aerosol linear
depolarization ratio decreases with respect to the above atmospheric layer.
Such an effect is particularly visible when mineral dust particles are present near
the ground. During this period, the temporal evolution and the high values of
nitrates ion concentration in the PM1 and PM10 channels suggest that the
origin of such low depolarization particles can be related to processes of
secondary organic aerosol formation and hygroscopic growth on mineral dust
particles with a nitrate-enriched surface.
In conclusion, the in-depth analysis of the aerosol light backscattering
profiles provided new insights into particle behaviour from the ground up to
the
free troposphere. Results pointed out particle processes, observed
relatively frequently over the period of the campaign, that impact aerosol
variability, air quality and potentially regional climate, and that therefore deserve
more extended analysis from longer-period vertically resolved
observations (i.e. EARLINET network). The detailed information retrieved
(vertical stratification, hygroscopic growth near ground, aerosol evolution
inside the PBL) can also support larger-scale studies. As an example, we cite
here a recent study, based on MAIAC satellite information ,
that attempted to assess a method for surface PM retrieval from space
observations relying on rough approximations of PBL evolution and RH effect
on aerosols. Accurate studies on aspects such as the ones we presented here may,
therefore, represent an important contribution to the improvement of more
complex and focused atmospheric observation techniques.
Data observed at the Mt. Cimone WMO/GAW station during
PEGASOS campaign can be accessed by the MOVIDA system
http://www.isac.cnr.it/cimone/data-access (last access: 26 March 2018) and through the WDCA database at
http://ebas.nilu.no/ (last access: 31 December 2017). All other data reported in this paper come from
national and EU projects. Access can be granted by the paper's authors and
data owners.
The Supplement related to this article is available online at https://doi.org/10.5194/acp-18-5371-2018-supplement.
The authors declare that they have no conflict of
interest.
Acknowledgements
This work was partly funded by the project PEGASOS (FP7-ENV-2010-265148),
the project SuperSito by the Emilia-Romagna region (DRG no. 428/10), the EU
project StratoClim (grant agreement no. 603557), the EU FP7 grant
ÉCLAIRE (grant 282910) and the project of National Interest NextData. We
would like to acknowledge the Energy Research Centre of the Netherlands
(ECN) for providing us with a MARGA instrument to use during these campaigns.
This study also received funding from the FP7 project BACCHUS (grant
agreement 603445).
Edited by: Aijun Ding
Reviewed by: three anonymous referees
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