ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-16-4725-2016Long-range transport and mixing of aerosol sources during the 2013 North American
biomass burning episode: analysis of multiple lidar observations in the
western Mediterranean basinAncelletGerardgerard.ancellet@latmos.ipsl.frhttps://orcid.org/0000-0002-1542-6085PelonJacquesTotemsJulienhttps://orcid.org/0000-0002-1038-455XChazettePatrickhttps://orcid.org/0000-0002-6230-2982BazureauArianeSicardMichaëlhttps://orcid.org/0000-0001-8287-9693Di IorioTatianaDulacFrancoisMalletMarcLATMOS/IPSL, UPMC Univ. Paris 06 Sorbonne Universités, UVSQ, CNRS, Paris, FranceLSCE, Laboratoire des sciences du Climat et de l'Environnement, CEA, Université Versailles St-Quentin, CNRS/INSU, Gif-sur-Yvette, FranceRSLab/CTE-CRAE-IEEC, Universitat Politècnica de Catalunya, Barcelona, SpainENEA, Agenzia nazionale per le nuove tecnologie, l'energia e lo sviluppo economico sostenibile, Rome, ItalyLaboratoire d'Aérologie, Université Paul Sabatier, CNRS/INSU, Toulouse, FranceGerard Ancellet (gerard.ancellet@latmos.ipsl.fr)15April2016167472547429October201518November201521January201622March2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/16/4725/2016/acp-16-4725-2016.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/16/4725/2016/acp-16-4725-2016.pdf
Long-range transport of biomass burning (BB) aerosols between North America
and the Mediterranean region took place in June 2013. A large number of
ground-based and airborne lidar measurements were deployed in the western
Mediterranean during the Chemistry-AeRosol Mediterranean EXperiment (ChArMEx)
intensive observation period. A detailed analysis of the potential North
American aerosol sources is conducted including the assessment of their
transport to Europe using forward simulations of the FLEXPART Lagrangian
particle dispersion model initialized using satellite observations by MODIS
and CALIOP. The three-dimensional structure of the aerosol distribution in
the ChArMEx domain observed by the ground-based lidars (Minorca, Barcelona
and Lampedusa), a Falcon-20 aircraft flight and three CALIOP tracks, agrees
very well with the model simulation of the three major sources considered in
this work: Canadian and Colorado fires, a dust storm from western US and the
contribution of Saharan dust streamers advected from the North Atlantic trade
wind region into the westerlies region. Four aerosol types were identified
using the optical properties of the observed aerosol layers (aerosol
depolarization ratio, lidar ratio) and the transport model analysis of the
contribution of each aerosol source: (i) pure BB layer, (ii) weakly dusty BB,
(iii) significant mixture of BB and dust transported from the trade wind
region, and (iv) the outflow of Saharan dust by the subtropical jet and not mixed
with BB aerosol. The contribution of the Canadian fires is the major aerosol
source during this episode while mixing of dust and BB is only significant at
an altitude above 5 km. The mixing corresponds to a 20–30 % dust
contribution in the total aerosol backscatter. The comparison with the MODIS
aerosol optical depth horizontal distribution during this episode over the western
Mediterranean Sea shows that the Canadian fire contributions were as large as
the direct northward dust outflow from Sahara.
Introduction
Forest fires are a significant source of tropospheric aerosol particles at
northern latitudes in spring and summer , and
many studies project higher temperatures and longer growing season
. The focus of biomass burning emission impact
on the atmospheric composition is often on the effect of these fires on the
aerosol distribution in North America and Siberia
. Long-range transport of biomass burning plumes
has been also recognized as a significant source of aerosol in the
mid-latitude free troposphere over Europe . Air mass aging related to long-range transport also
leads to aerosol optical and chemical properties different from results
obtained when looking at observations close to the fire region
. As an example, the absorbing
efficiency in the visible spectral range is known to significantly increase
in case of internally mixed BC (coating with secondary compounds) compared to
externally mixed BC . So far little attention has been
paid to the frequent mixing of dust and biomass burning (BB) aerosol
occurring during their transatlantic long-range transport while lidar data
analysis has shown that such a mixing will likely modify the extinction to
backscatter ratio often called lidar ratio (LR) and then the aerosol optical
depth (AOD) . Results of
also show that the solubility of iron is enhanced by the mixing with biomass
burning aerosols, while aerosol deposition may influence the rate of nitrogen
fixation by microorganisms, and subsequently the global carbon cycle
. Although episodic, such long-range transport of smoke
aerosols over the Mediterranean can also impact the regional energy budget by
changing the distribution of solar energy. Indeed, for an aged BB plume,
report a net shortwave radiative forcing over the sea
(daytime average) up to -64 W m-2, at the surface and up to
-22 W m-2, at the top of the atmosphere (for an AOD of 0.40 at
550 nm). The large concentration of absorbing material (BC particles) within
smoke plumes leads to significant absorption of solar radiations within the
atmospheric layer where smoke resides, which could perturb the relative
humidity and temperature vertical profiles. In the framework of the
Chemistry-Aerosol Mediterranean Experiment/Aerosol Direct Radiative Impact in
the Mediterranean (ChArMEx/ADRIMED) experimental campaign, many aerosol lidar
and aircraft measurements were made in June–July 2013 in the
Mediterranean region during a case of intense biomass burning transport from
North America to Europe . Only a
few studies report such long-range transport observations from North America
to Europe or even the eastern Mediterranean
.
The purpose of this paper is to analyze the transatlantic long-range
transport of BB and dust aerosol sources from North America during this
period. The context of our study is described in Sect. by
describing the main characteristics of the summer 2013 BB episode in North
America and the observation network considered for the analysis of the
aerosol distribution in the Mediterranean region. The aerosol sources are
identified using satellite observations, and the transport of dust or BB
plumes is calculated with the FLEXPART Lagrangian model (see
Sect. ). The aerosol lidar observations are discussed in
Sect. , where the contribution of the different aerosol sources
is assessed using the comparison of the spatial distribution of the layers
with the FLEXPART model simulations (forward from the source regions and
backward to calculate the potential emission sensitivity for each observed
aerosol layers). The mixing between dust and BB plumes is mainly derived from
the analysis of the aerosol layer optical properties. The Minorca and
aircraft lidar observations during ChArMEx are thoroughly described in a
companion paper submitted with this paper and in a paper
in preparation by .
ContextThe 2013 North American biomass burning period
June 2013 was on the drier side in the US High Plains region with most areas
receiving less than 70 % of normal precipitation. It was especially dry for
most of Colorado and Wyoming, which received less than 50 % of normal
precipitation, and many locations in the western areas of those states
received little to no precipitation. As a consequence many fires took place
in North America. Fire started in the state of Colorado on 10 June and lasted until
22 June 2013 . Two large fires in southern
Colorado even produced pyrocumulonimbus clouds and very large smoke plumes on
19 and 20 June 2013 in the West Fork Complex and in the East Peak.
In Canada there were also many fires (334) during the period 13 to
26 June 2013 burning 632 000 ha. The seasonal fire occurrence was below
average while the area burned was more than twice the 10-year average, due to
large fires burning in Québec. The majority of fires were spread between
Manitoba, Alberta, Yukon and Northwest Territories, and Québec, while
75 % of the area burned was in Québec and 20 % in Manitoba
. The total amount of area burned was around
500 000 ha for the period 12 to 25 June 2013, i.e., more than twice the
10-year summer average for the same period. The eastern Canadian fires at 80
and 100∘ W took place during 4–6 days between 18 and 24 June
while the fires west of 120∘ W took place during 2–3 days
starting on 17 June in Alaska and 22 June in the Mackenzie Mountains.
The 2013 Mediterranean lidar observation network
During ChArMEx an intensive observation period took place in western
Mediterranean region from 11 June to 5 July 2013 (SOP-1a) when airborne
measurements were made by two aircraft (ATR42 and F20) and ground-based
observations at four sites in Lampedusa, Corsica, Barcelona and Minorca
. During ChArMEx, aerosol backscatter vertical profiles
were made by airborne and ground-based lidar systems, which provide a very
good opportunity to characterize the vertical distribution of the North
American BB plume over the Mediterranean region. The map of the ChArMEx lidar
observation network is shown in Fig. . The Falcon 20 aircraft was
equipped with an airborne lidar LNG providing attenuated
backscatter vertical profiles at three wavelengths (1064, 532 and 355 nm). It
was based in Cagliari, Sardinia. The LNG lidar has been mainly used with a
downward-looking mode. Two tracks have been made in late June during the
passage of the BB plume over the western Mediterranean: a transect between
Cagliari and Minorca on 27 June 2013 and a loop around Sicily on 28 June.
Only the loop on 28 June is considered in this work because the 27 June data
will be discussed in a future paper by on the
airborne observations during ChArMEx. The ground-based lidars are located in
Minorca (40∘ N, 4∘ E), Barcelona (41.4∘ N,
2∘ E) and Lampedusa (35.5∘ N, 12.5∘ E). The
Minorca lidar works at 355 nm, while the Barcelona and Lampedusa lidars
measure the atmospheric backscatter signal at 532 nm. The ground-based lidar
systems are respectively described in ,
, and . The airborne lidar LNG was also run
every morning in Cagliari (39∘ N, 9∘ W) from 24 to
30 June 2013 pointing upward from the surface. All the lidars can record also
the depolarization ratio between the signal polarized parallel and
perpendicular to the plane of the outgoing beam. For the ground-based lidar,
the uncertainty on the aerosol depolarization ratio is of the order of 1–2 %
as explained in . For the airborne lidar LNG, the
depolarization ratio is measured at 355 nm and it is calibrated on molecular
scattering using a value of 1.5 ± 0.3 % for clean air, corresponding
also to 1–2 % error on the aerosol depolarization ratio.
In addition to ground-based and airborne lidar, the observations of the
spaceborne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) are
known to be very useful to track aerosol plumes . Three
CALIOP nighttime tracks shown in Fig. on 27 and 28 June 2013
are ideally located above the ChArMEx area when the BB plume is expected over
Europe.
Map of the ChArMEx lidar observations. The colored vertical lines are
the positions of the nighttime CALIOP tracks on 27 and 28 June 2013. The red
thick line shows the loop followed by the Falcon 20 aircraft on 28 June 2013
from 13:00 to 15:00 UT, while the blue crosses are for the Minorca,
Barcelona and Lampedusa ground-based lidar.
Aerosol sources and transportMethodology
Satellite remote sensing was considered for the BB aerosol source
identifications: both Moderate Resolution Imaging Spectroradiometers (MODIS)
on Terra and Aqua platforms, and CALIOP. The distribution of the fires was
taken from the NASA Fire Information for Resource Management System (FIRMS),
which provides the analysis of the MODIS hot spots in terms of fire radiative
power (FRP) given in MW. Only fire areas with FRP > 0.8 GW are included
in this analysis. The MODIS 0.5 µm AOD daily product is also
considered to estimate the horizontal extent of BB plume when large
AOD > 0.3 is found near the spots with elevated FRP. Both MODIS
instruments on Aqua and Terra are considered to derive the daily mean. The
mean error on the MODIS AOD daily product is 0.03 with a root mean square
error of 0.14 according to . When a CALIOP overpass is found
near the MODIS BB plume, the lidar vertical cross section is used to specify
the vertical extent of the MODIS BB plume.
Left: MODIS fire radiative power from 17 to 25 June 2013. Areas
with red dots are considered as significant fires. The blue boxes correspond
to area chosen for the release of particles in the FLEXPART forward
simulation. Right: daily AOD 0.5 µm measured by MODIS on
22 June 2013. The CALIOP tracks used to estimate the height of layer
influenced by the fires are shown in red on 22 June near Hudson Bay fires and
21 June 2013 near Colorado fires.
For the dust aerosol sources, two main information sources were considered:
(i) North American dust storms identified in the NAAPS (Navy Aerosol Analysis
and Prediction System) Global Aerosol Model simulations and
(ii) 0.5 µm AOD anomalies from the MODIS daily products. AOD
streamers transported from the tropical Atlantic belt of elevated
0.5 µm AOD to the mid-latitudes are related to the transport of
Saharan dust across the Atlantic. CALIOP overpasses near the AOD anomalies
again provide the vertical extent of dust aerosol layers.
In this work we use the new CALIOP level-1 (L1) version 4.0 attenuated
backscatter coefficients β1064 and β532 because they
correspond to a better calibration of the lidar data. They are averaged using
a 10 km horizontal resolution and a 60 m vertical resolution
. Before making horizontal or vertical averaging, the
initial 333 m horizontal resolution (1 km above the altitude 8.2 km) is
filtered to remove the cloud layer contribution . This
cloud mask makes use of the version 3 level-2 (L2) cloud layer data products
. Our scheme for distinguishing cloud and aerosol is
described in . Although the LR is available
from the CALIOP version 3 L2 aerosol layer data products, it is often based
on an aerosol classification algorithm . In our work the
LR is recalculated by using the aerosol layer transmittance and
the integrated attenuated backscatter in the aerosol layer following the
method described in . To reduce the error when using
high-horizontal-resolution CALIOP profiles, β532 is averaged over 80 km
to compute the plume transmittance whenever it is possible. The attenuated
backscatter is then corrected for the molecular and aerosol attenuation using
a forward Fernald inversion before calculating the
backscatter ratio R(z)=(βa+βR)/βR at 532 and
1064 nm using the CALIOP atmospheric density model to calculate the
βR Rayleigh backscatter vertical profiles. The aerosol
depolarization ratio δ532 is also calculated using the
perpendicular to the parallel plus perpendicular polarized aerosol
backscatter coefficient. The calibration of the relative ratio between the
two 532 nm channels is based on regular use of a pseudo-depolarizer located
ahead of the beam splitter which separates the signal polarized parallel and
perpendicular to the plane of the outgoing beam . We have
also derived the color ratio defined as the ratio of the aerosol backscatter
coefficients at 1064 and 532 nm (Ca(z)=βa1064/βa532=(R1064(z)-1)/[16(R532(z)-1)]). The aerosol
color ratio can be also written as Ca(z)=2-k, where k is an exponent
depending on the aerosol microphysical properties . The
exponent k varies from 0 to 2 when increasing the fine-mode aerosol
contribution. These two ratios are provided only for R(z)>1.3 because the
uncertainty on the depolarization and color ratios are large for weak aerosol
layers. Whenever it is possible, the use of nighttime overpasses is
preferred to improve the signal-to-noise ratio (SNR).
CALIOP vertical cross section of backscatter ratio (top), aerosol
depolarization ratio (middle), and aerosol color ratio (bottom) for the two
tracks shown in Fig. on 21 (left) and 22 (right) June 2013.
Depolarization and color ratios are only reported for backscatter ratio
> 2. Aerosol layers on 21 and 22 June 2013 are near the Colorado and
Canadian fires, respectively.
The transport of the aerosol sources is analyzed using the FLEXPART model
version 8.23 driven by 6-hourly ECMWF analysis (T213L91)
interleaved with operational forecasts every 3 h. The model is run using a
forward simulation with a tracer released within a volume estimated from the
satellite observations. The release time period ranges from 1 to 3 days
according to the MODIS AOD observations. The total mass of the tracer emitted
is estimated using the aerosol concentration given in the NAAPS Global
Aerosol Model simulations and FLEXPART calculates the gridded tracer
concentration in ng m-3. Considering the uncertainty in the estimate of
the emitted tracer mass, the tracer distribution in the ChArMEx domain is
analyzed using a relative mass fraction between the emitted mass and the
calculated mass within the model grid cell. A factor is applied to calculate
this ratio in order to take into account on the one hand the difference between
the emission volume (≈ 5 × 105 km3) and the grid
cell volume of the tracer concentration field
(≈ 2 × 103 km3) and, on the other hand, the time
difference between the emission period (1–3 days) and the integration time
(6 h) used for the calculation of the tracer gridded concentration. The
relative mass fraction is 100 % when the air mass is advected above the
0.5∘× 0.5∘ grid cell chosen for the gridded
concentration calculation, without dilution (< 100 %) or concentration
(> 100 %) of the tracer.
North American biomass burning aerosol
The MODIS FRP distributions are plotted in Fig. from 17 to
25 June 2013 showing the six main fire regions over Canada and Colorado. The
map of the 0.5 µm daily AOD MODIS also show aerosol plumes on
22 June near Hudson Bay, Colorado, and over the Atlantic Ocean where the AOD
is > 0.4. The white area on the daily mean MODIS map often corresponds to
the cloud distribution, which was high over Québec explaining the lack of
large AOD daily mean values near this strong BB source. Nearby CALIOP tracks
on 21 June over Colorado and 22 June over Canada show the vertical extent of
the aerosol layers related to the fires (Fig. ). The aerosol
layers reach 8 km over Colorado, while they remain below 4 km over Canada.
The aerosol depolarization ratio is less than 7 ± 3 % for the layers
over Canada, while it is near 9 ± 3 % in the mid-troposphere over
Colorado. The uncertainties on the CALIOP aerosol depolarization ratio averaged
over the two layers are calculated using the error on the 532 nm backscatter
signals. Notice also the high depolarization ratio (> 15 %) over
Colorado below 3 km showing that the BB plume overlays dust layers in the
lower troposphere. The six areas shown in blue in Fig. are
considered for a forward run of FLEXPART in order to study the long-range
transport of the Canadian and Colorado biomass burning tracer. The depth of
the volume is set according to the CALIOP vertical distributions shown in
Fig. . The parameters of the different BB sources considered in
the FLEXPART simulations are given in Table .
The map of the biomass burning tracer plume over the ChArMEx domain on 27 and
28 June is shown in Fig. using the relative fraction between
the emitted mass and the simulated mass in the grid cell of the tracer field
as explained before. Two different maps are given for the Canadian and
Colorado fire contributions respectively. The Canadian plume crossed the
whole western Mediterranean basin, being over Minorca already on 27 June at
06:00 UT and passing over Sicily on 28 June during the day. The Colorado
fires do not play a major role in the aerosol layers observed on 27 June, but
according to the transport model they could be observed on 28 June mainly
over Spain and also in a 200 km wide strip parallel to a line from Gibraltar
to Messina. The vertical cross sections (Fig. ) show that
the front edge of the Canadian fires is above 4 km on 27 June while the
tails brought aerosol at lower altitudes in the 1–4 km range on
28 June. The Colorado fires can be only detected above 5 km. The relative
mass fraction is larger than 30 % in the Canadian fire plume showing that a
significant part is indeed advected above the Mediterranean while the
remaining part is transported to central Europe as observed by the EARLINET
lidar network in Germany . The relative fraction of the
Colorado fires remains in the range 20–30 % because the major part of the
plume remains over Spain and the Atlantic Ocean.
Map of the relative fraction of the FLEXPART biomass burning tracer
plume in % for the Canadian (top) and Colorado (bottom) fires on
27 June 2013 at 06:00 UT (left) and 28 June at 18:00 UT (right). The altitude
range corresponds to the vertical levels included in the calculation of the
tracer relative fraction.
Vertical cross section of the relative fraction of the FLEXPART
biomass burning tracer in % for the Canadian fires on 27 June 2013 at 06:00 UT
(top left), 28 June at 18:00 UT (top right), and the Colorado fires on 28 June at 18:00 UT
(bottom).
North American dust layers
Modeling and satellite observations suggest that the western USA is a
significant contributor to the global mineral dust aerosol budget
, and mineral dust emissions from this source region may
have increased during the last 20 years . Several dust
blows hit Utah, Colorado and Wyoming in June 2013 due to the very dry
conditions and strong winds, which were also the cause of the Colorado forest
fires . The NAAPS aerosol transport model simulations
indicate elevated surface dust concentrations
(> 300 µg m-3) from 19 to 22 June 2013 in a region almost
similar to the large MODIS AOD area related to the Colorado fires. It also
explains the aerosol layers with large depolarization seen by CALIOP on
21 June 2013 at 41∘ N at 3 km below the Colorado fires
(Fig. ). In addition to the local sources coming from western USA,
the MODIS maps on 20 and 21 June also show that dust streamers are
transported at latitudes north of 30∘ N from the large-scale Saharan
dust plume, crossing the Atlantic because of the trade winds. Three streamers
are shown in Fig. over the Atlantic Ocean, where the
0.5 µm AOD is enhanced with values > 0.3. Nearby CALIOP tracks
on 20 and 21 June show that the AOD enhancement is indeed related to the
contribution of aerosol layers with large depolarization > 20 %
(Fig. ). The uncertainty on the average depolarization ratio for
the dust layers is of the order of 5 %. The three areas shown in blue in
Fig. are considered in our study in order to analyze the role
of dust layers over the Atlantic in the aerosol distribution over the
Mediterranean Sea. According to the CALIOP vertical cross sections, the
northern layer at 42∘ N was already uplifted in the altitude range
3–5 km while the dust plumes near 30∘ N remain below 3 km. Four
areas are then selected for a FLEXPART forward run of dust tracers (see
Table ). The emission volume is set according to the MODIS AOD
anomaly horizontal extents and the CALIOP vertical
distribution of the dust layers. The emission period
is chosen between 20 and 22 June for the dust layers over the Atlantic when
the AOD anomalies are observed with MODIS, while the time frame for the High
Plains region dust source is set according to the NAAPS model simulations.
Daily AOD 0.5 µm measured by MODIS on 20 June 2013. The
CALIOP tracks used to estimate the heights of the dust layers over the
Atlantic Ocean are shown in red on 20 June at 42∘ N and 21 June at
30∘ N. The blue boxes correspond to areas chosen for the release of
particles in the FLEXPART forward simulation.
Same as Fig. for the two tracks shown in
Fig. on 20 (left) and 21 (right) June 2013. Dust layers are
seen above the Atlantic Ocean in the altitude range 1–4 km, near 32 and
42∘ N, on 20 and 21 June 2013, respectively.
The amount of tracer related to the High Plains dust sources was found to be
negligible over the ChArMEx area during the period 27 and 28 June (mass
fraction < 10 %), and it will not be considered any further. It may have
been however mixed with the lower boundary of the Colorado fire plume seen at
higher altitudes as shown in the previous section. The maps of the Atlantic
dust tracer plume over the ChArMEx campaign domain are shown on 27 and 28 June
in Fig. . The values of the mass fraction are significant
(> 30 %), showing that the contribution of long-range transport of dust
cannot be neglected even during the event of biomass burning aerosol
transport to Europe. A first plume of dust was advected across the western
Mediterranean basin already before 27 June, and a second crossed the basin on
28 June. The tail of the first one is at relatively low altitude (< 4 km)
on 27 June while the second one is above 5 km on 28 June.
Saharan dust
Although the synoptic wind conditions (northwesterly flow) from 25 to
29 June 2013 were not favorable for the export of Saharan dust to the basin
as explained in , it is important to set the northern limit
of the area influenced by the northward transport of Saharan dust. The
characteristics of the dust emissions were estimated using the Multi-angle
Imaging SpectroRadiometer (MISR) AOD maps for the period 22 to 28 June (not
shown) because the multiangle observations are better suited to distinguish
surface and dust contribution to the solar reflection. The depth of the
Saharan dust layer has been estimated looking at several CALIOP overpasses
above northern Africa during the same period. A FLEXPART forward run with a
Saharan dust tracer was made for a wide area over northern Africa in the box
(24–34.5∘ N, 0–10∘ E; 0–6 km) from 23 to 28 June 2013.
The vertical layering of the Saharan dust tracer over the ChArMEx domain is
shown on 28 June in Fig. . As expected for a nearby source,
the relative mass fraction is very large (> 100 %). Although the dust
outflow from Sahara is transported above Lampedusa, it remains south of
36.6∘ N between Lampedusa and Cagliari. No Saharan dust is expected
above Minorca. The altitude of the dust plume is between 2.5 and 4.5 km
because the uplifting in the westerly flow is very limited.
Aerosol observations in the Mediterranean basin
In this section, the ChArMEx aircraft or ground-based lidar observations and
the CALIOP vertical cross sections on 27 and 28 June 2013 are compared with
the expected contributions of the different aerosol sources transported
across the Atlantic.
Characteristics of the biomass burning tracer emission used for the
forward FLEXPART simulation. The emitted mass is only a rough estimate
explaining the use of relative mass fraction in the simulation analysis.
Altitude is given above ground level.
Aerosol sourceRelease timeHorizontal domainVerticalEmittedrange, kmmass, kgQuébec BB18–24 June 201380/70∘ W, 51/54∘ N0–33 × 107Manitoba BB20–24 June 2013102/95∘ W, 57/61∘ N0–32.5 × 107NWT BB22–24 June 2013128/121∘ W, 61/65∘ N0–52 × 107N. Alaska BB17–19 June 2013160/154∘ W, 60/64∘ N0–51.9 × 107S. Alaska BB19–22 June 2013154/148∘ W, 65/69∘ N0–53.6 × 107Colorado BB19–22 June 2013105/96∘ W, 37/41∘ N0–65 × 107
Same as Table 1 for the dust tracer emission.
Aerosol sourceRelease timeHorizontal domainVerticalEmittedrange, kmmass, kgDust High Plains19–22 June 2013105/99∘ W, 37/40∘ N0–35 × 107Dust over Atlantic20–21 June 201360/50∘ W, 37/43∘ N1–55 × 107Dust over Atlantic20–21 June 201369/59∘ W, 25/33∘ N1–45 × 107Dust over Atlantic20–21 June 201348/38∘ W, 25/33∘ N1–45 × 107
Map (top) and vertical cross section (bottom) of the relative
fraction of the FLEXPART Atlantic dust tracer in % on 27 June 2013 at 06:00 UT
(left) and 28 June at 18:00 UT (right). The altitude ranges in the top figures
correspond to the vertical levels included in the calculation of the tracer
relative fraction.
Spatial distribution of the aerosol layers
Three nighttime CALIOP overpasses are suitable for a comparison with the
different BB plumes: 27 June at 10∘ W and at 10∘ E, and
28 June at 0∘ W. The backscatter ratio R(z) and the aerosol
depolarization ratio δ532 are shown in Fig. in the
latitude range where cloud-free sky made possible the observations of aerosol
layers. The R(z) values are larger than 3 in these layers. On 27 June, the
layers are in the altitude range 5 to 7.5 km at 10∘ E while it is
between 2 and 5 km at 10∘ W. In both cases low δ532
values (< 10 %) are found, showing that the plumes are not mixed with
a significant amount of dust (except at 10∘ W where δ532 may
reach 10 % in some layers). The uncertainty for δ532 is of the
order of 3 %. These results are in good agreement with the characteristics
of the Canadian fire plumes discussed in Sect. . Indeed it
was found that the front edge of the plume was at 10∘ E on 27 June
with an altitude range 4–7 km, while the tail is at 10∘ W in
the altitude range 2–5 km (see vertical cross section on 28 June in
Fig. ). Although the Colorado fires may be present in the
27 June CALIOP cross section at 10∘ W according to the FLEXPART
simulations, the altitude range of the observed aerosol layers is not
consistent with the influence of the Colorado BB plume, which is expected at
an altitude above 5 km. On 28 June at 0∘ W, the CALIOP observations
show also aerosol layers in the 3–5 km altitude range, with slightly higher
depolarization ratio (≈ 10 ± 3 %), but still in the range
expected for biomass burning aerosol . The altitude range
is again in good agreement with a major role of the tail of the Canadian fire
plume.
Same as Fig. for the FLEXPART Saharan tracer on
28 June 2013 at 18:00 UT.
CALIOP vertical cross section of backscatter ratio (left),
aerosol depolarization ratio (right) for the three tracks shown in
Fig. on 27 June 2013 at 03:00 UT at 10∘ W (top),
on 28 June 2013 at 02:00 UT at 0∘ W (middle), and on 27 June 2013
at 01:00 UT at 10∘ E (bottom).
Several ground-based lidar observations have also identified aerosol plumes
possibly related to the transatlantic transport. The characteristics of the
aerosol layers are summarized in Table . The Minorca lidar data
are discussed in a companion paper by . An aerosol layer
between 3 and 5 km seen in Minorca is quite similar to the CALIOP observations
on 28 June. A second layer between 5 and 7 km is also seen in Minorca with a
noticeable depolarization (δ355> 12 ± 1 %). The upper
layer is not seen by CALIOP because it is expected at latitudes higher than
40∘ N and is masked by overlaying clouds. In Minorca the vertical
profiles of the water vapor mixing ratio were also measured during the night
showing elevated mixing ratio > 1 g kg-1 above
5 km and values near 0.5 g kg-1 in the aerosol layer observed around
4 km. The time series of the Minorca lidar is also useful to estimate the
horizontal range of BB plume. The plume is observed for 24 h from 27 June
at 00:00 UT to 28 June at 12:00 UT, and the wind speed at 4 km is between
30 and 40 km h-1. Therefore, the plume zonal extent is of the order of
1200 km. It is very similar to the size of the Canadian tracer plume obtained
in the FLEXPART simulations (15∘ longitude difference between the
front edge and the tail).
As expected the Barcelona lidar detects similar features: a strong layer
between 5 and 7 km with δ532≈10 % and an optically thin
layer between 3 and 5 km with δ532<10 %. The spectral variation of
the aerosol depolarization ratio between Barcelona and Minorca cannot be
accurately estimated but is less than 1.5. It is consistent with a small
influence of urban aerosol . When looking at the Lampedusa
lidar data at 35∘ N, a layer is seen between 2 and 4 km on 28 June,
which is influenced by the Saharan dust outflow discussed in
Sect. since δ532≥30 %, i.e., a value similar
to other dust layers observed over Minorca during ChArMEx
.
The Lampedusa lidar measures aerosol layers in the 2–4 km altitude range on
both days, but with very different optical characteristics. A dust layer with
δ532>30 % on 28 June at 12:00 UT while a mixture of dust and BB
aerosol is seen on 27 June from 08:00 to 16:00 UT. The aerosol layer seen by
CALIOP on 27 June at 01:00 UT near 36∘ N has optical characteristics
close to the layer observed in Lampedusa on 27 June (Fig. ),
i.e., a depolarization between 10 and 15 % and LR between 50 and 55 sr.
The LNG airborne lidar data obtained during ChArMEx will be thoroughly
discussed in a forthcoming paper by . Here we will only consider the vertical structure of the
aerosol layers observed on 28 June 2013 along the loop shown in
Fig. . The three corresponding vertical cross sections of attenuated
R(z) at 532 nm are shown in Fig. . Three interesting regions can
be identified:
the 38.2∘ N layer at 2–4 km on the Cagliari–Lampedusa section
and at 11–14∘ E on the return section between Messina and Cagliari;
the upper altitude layer in the 4–6 km altitude range covering a
southwest (36∘ N, 12∘ E) to northeast (39∘ N,
15∘ E) band, the width of which is of the order of 100 km;
a low-altitude layer between 2 and 4 km south of 36∘ N, which
corresponds to the layer seen by the Lampedusa lidar.
The spatial distribution of the aerosol layers seen by the LNG lidar
corresponds quite well to the position of North American BB plumes and the
expected latitudinal extent of the Saharan dust calculated with the FLEXPART
simulation in Sect. . Indeed layer A is related to the
tail of the Canadian BB plume. Layer B is also in the latitude range of
the Canadian BB plume, possibly mixed with the Colorado BB plume present
between Gibraltar and Messina (see Fig. ). Layers A and B seen
by the LNG airborne lidar on 28 June are also consistent with the
superposition of two different aerosol layers seen above Minorca 24 h
before. In layer B, δ355≈10±1 %, i.e., higher than the
low values found in layer A (δ355≤5±2 %).
Characteristics of the aerosol layers observed in the free
troposphere by the ground-based lidars listed in Fig. on 27 and
28 June 2013.
Optical properties of the four types of aerosol encountered during the
passage of the BB plume: depolarization ratios
(δ532, δ355),
lidar ratios (LR532, LR355), and color ratios (CRa1064/532, CRa532/355).
Airborne lidar vertical cross sections of attenuated backscatter
ratio at 532 nm on 28 June 2013 along the loop shown in Fig. :
(top) Lampedusa–Cagliari around 13:00 UT, (middle) Lampedusa–Messina around
14:00 UT, and (bottom) Cagliari–Messina around 14:40 UT.
Left: FLEXPART potential emission sensitivity (PES) in seconds for
three aerosol layers identified by CALIOP and Minorca lidar: 27 June 2013
at 02:00 UT, 10∘ W, 43∘ N (top); 27 June at 12:00 UT, in
Minorca (middle); and 27 June at 01:00 UT, 10∘ E, 39∘ N (bottom).
The PES vertical cross section are along the red line following the North
American east coast (right).
Same as Fig. for the three aerosol layers identified by
the Falcon 20 lidar on 28 June 2013: layer A (top), layer B (middle), and
layer C (bottom).
Aerosol source attribution
Although the comparison with the position of the FLEXPART tracer plumes can
already help to attribute a specific source to the observed layers in the
ChArMEx area, it can be further checked by calculating the potential emission
sensitivity (PES) values by running the FLEXPART model in the backward mode
for 10–11 days to identify the area where surface emissions may influence
the observed aerosol structure seen by CALIOP, the Minorca ground-based lidar
and the LNG airborne instrument. The PES is given in seconds in order to be
multiplied by model surface fluxes to produce concentrations at the receptor
location. The PES is calculated using 6 h averages on a three-dimensional
1∘× 1∘× 1 km grid. The results are shown
for the CALIOP and Minorca observations on 27 June 2013
(Fig. ). Similar calculations were also made for layers A, B and
C seen by the airborne lidar on 28 June (Fig. ). The
simulations for the layers seen by CALIOP on 28 June at 02:00 UT are not shown
because they are very similar to the results obtain for the Minorca lidar on
27 June at 12:00 UT or for layer B seen by LNG.
The aerosol layers observed by CALIOP along the two cross sections at
10∘ W and 10∘ E are indeed mainly related to aerosol
sources over Canada and Alaska, but the retro-plume altitude and latitude at
60∘ W are quite different when reaching the Atlantic Ocean. The
probability of dust and biomass mixing is higher for the CALIOP layers at
10∘ W, which is located in the 40–50∘ N latitude band at
lower altitude (5–7 km) than for the CALIOP layers at 10∘ E. This
may explain the slight depolarization difference for the two CALIOP tracks
since there are more layers with δ532≈ 10 % at
10∘ W than at 10∘ E. The mixing of dust layers over the
Atlantic and Canadian BB aerosol is even more explicit for the Minorca layer
at 6 km since two branches of elevated PES are seen over the two aerosol
source regions identified for this layer. It explains the relatively higher
aerosol depolarization ratio (up to 12 %) at 6 km than at 4 km in Minorca
during this episode. Such a transport pathway is also consistent with the
water vapor mixing ratio maximum > 1 g kg-1 seen by the Minorca
water vapor lidar near 6 km since uplifting of air masses from the lower
troposphere above the Atlantic Ocean is likely to increase the humidity in
the mid-troposphere.
When considering the PES related to the airborne lidar layers, layer A
PES is similar to the 10∘ E plume showing a strong influence of the
Canadian aerosol BB source, while layer B PES distribution resembles the
results obtained with the Minorca layer seen 1 day earlier. For the dusty
layer C seen both by the aircraft and at the Lampedusa station, the PES
distribution shows that there is no transatlantic transport for the period 17
to 28 June while the aerosol sources are mainly located above northern Africa
and western Europe at low altitude (< 3 km). Although air masses are
still advected from western Europe, Saharan dust emission remains the major
aerosol source since western Europe air masses were heavily influenced by
Saharan dust layers during the period 16 to 20 June . This is
consistent with the large depolarization seen above Lampedusa
(δ532≈30 %).
The ATR42 aircraft also flew between Cagliari and Lampedusa on 28 June around
12:00 UT to sample the aerosol layers with in situ measurements
. The analysis of the CO and BC in situ measurements made
on board the ATR42 shows that layers A and B correspond to a CO excess
above the background of the order of 100 ppbv while ΔCO is less than
20 ppbv for layer C (not shown). The BC variability shows also the same
pattern. This is in very good agreement with the conclusions derived from the
lidar data analysis coupled with the Lagrangian transport model simulations.
Aerosol optical properties
In this section, we will summarize the results about the aerosol layer
optical properties and the aerosol source attribution. The analysis conducted
in the previous sections leads to the identification of four different aerosol
layers during the passage of the BB plume over the ChArMEx area:
pure BB layer at 10∘ E above 4 km on 27 June 2013 at 01:00 UT
(CALIOP);
weakly dusty BB layer below 5 km observed between 10∘ W
and 10∘ E on 27 and 28 June at 02:00 UT by CALIOP, the Minorca lidar
and layer A seen by LNG flying around Sicily on 28 June;
significant mixture of BB and dust transported across the Atlantic above
5 km at Minorca on 27 June at 12:00 UT and layer B seen by LNG on 28 June
at 14:00 UT;
the outflow of Saharan dust above the sea at latitudes south of
36∘ N on 28 June at 13:00 UT (Lampedusa lidar and layer C seen by
LNG).
For layer I δ532 is < 5 %, while the LR at 532 nm is
60 ± 20 sr when using the aerosol layer transmission from the averaged
L1 CALIOP attenuated backscatter, and it is 65 sr in the level-2 (L2) CALIOP
operational aerosol data products. The color ratio is between 0.2 and 0.4.
Both LR and Ca are in the range expected for a pure BB layer
in agreement with our source identification.
For layer II δ532 and δ355 are respectively in the range
5–10 and < 8 % with the lowest values on 27 June at 45∘ N
along the 10∘ W CALIOP overpass. The LRs calculated from the
ground-based and airborne lidars are more accurate, and they are
59 ± 5 sr at 355 nm and 60 ± 5 sr at 532 nm, respectively.
The CALIOP 27 June (10∘ W) and 28 June LRs at 532 nm are estimated to
be 60 ± 20 sr and 50 ± 20 sr, respectively, using the L1 data
analysis and are of the order of 60 sr for both layers using the L2
operational products. It also gives confidence in the LR retrieval to see the
largest LR is obtained where δ532 is minimum. The CALIOP Ca is
in the range 0.4–0.5. These aerosol optical parameters are still in the
range expected for a BB layer. However differences with the optical
parameters found for layer I (higher depolarization and Ca, slightly lower
LR) are consistent with a BB mixed with a small amount of dust or an increase
in relative humidity. According to the small values (< 0.5 g kg-1)
of water vapor recorded by the Minorca lidar observations
, the mixing with a small amount of dust is more likely.
For layer III mainly seen by the Minorca ground-based and airborne lidar,
δ355 values are in the range 8–12 %, while LRs are
45 ± 5 sr and 42 ± 5 sr at 532 and 355 nm, respectively. The
LR of layer B seen by LNG is calculated by including also the
contribution of the underlying layer between 3 and 4 km to get a better
molecular reference. Following the methodology proposed by ,
the aerosol depolarization value for layer III is consistent with a
contribution of 20–30 % of dust and 80–70 % of BB aerosol in the total
aerosol backscatter in layer III, if we assume that pure dust and pure BB
aerosol types have δ355 of 25 and 5 %, respectively, and LRs at 355 nm of 45 ± 10 and 60 ± 10 sr, respectively. It is also
interesting to calculate Ca between 355 and 532 nm using the Barcelona
and Minorca observations assuming that R(z) is stationary during the
advection of the aerosol layers between Barcelona and Minorca. The Ca
value is 0.74 for layer III while it is only 0.35 for layer II. It is
consistent with a larger contribution of the accumulation mode when BB is
mixed with dust, but also with a larger water vapor mixing ratio
(1 g kg-1) for layer III than for layer II.
For layer IV larger depolarization up to 30 % is seen by the Lampedusa
lidar at 532 nm. The LRs calculated by the LNG lidar and the ground-based
lidar at 532 nm are respectively 48 ± 5 and 30 ± 10 sr.
The layer optical parameters are consistent with a dust plume with a large
depolarization, while a large variability is observed for LR. The large
depolarization ratio and the low LR value at 355 nm are quite similar to
previous observations by in fresh dust exported over the
Mediterranean Sea. The strong variation in the LR values between layer C
of the LNG lidar at 13.5∘ E and the Lampedusa observations at
12.5∘ E suggests an increase of the mixing between the northward
African dust outflow and the BB plume as the aircraft moved across the
boundary between layer IV and II between Lampedusa and the southern cape of
Sicily.
The aerosol properties and spatial distribution of the four aerosol types are
summarized in Table and Fig.
respectively. The spatial distribution of the MODIS AOD at 0.5 µm
is also shown in Fig. , where the largest AOD values are seen
before the plume dispersion above northern Spain. For the type II aerosol,
i.e., an aged BB plume seen below 5 km and mixed with a small amount of dust
mainly from continental origin, two areas are distinguished for the Colorado
and Canadian fires using the results of the FLEXPART forward simulations. The
Canadian fires significantly contribute to the AOD observed by MODIS over
the Mediterranean Sea. The additional contribution of the upper aerosol
layers of type III where the BB is mixed with dust also explains the
significant AOD increase over the western Mediterranean region. The BB
contribution to AOD is as large as the northern African dust contribution
(type IV) that dominates the southern part of the domain with AOD values in
the same range of 0.3–0.4 over northeastern Algeria and Tunisia.
MODIS AOD horizontal distribution on 27 and 28 June 2013 over the
Mediterranean region. The area corresponding to the aerosol types identified
during our analysis of the BB plume passage is delimited by the red lines.
The black crosses are for the Minorca and Lampedusa stations.
Conclusions
A very interesting event of long-range transport of biomass burning (BB)
aerosols between North America and the western Mediterranean region that took
place in late June 2013 was documented during the ChArMEX/ADRIMED
campaign. Although the occurrence of such events has been discussed in
previous publications, the contribution of this work takes advantage of
a large number of ground-based and airborne lidar measurements used in
conjunction with spaceborne lidar observations by CALIOP during this period.
A detailed analysis of the biomass burning North American sources was
conducted including the assessment of their transport to Europe using forward
simulations with the FLEXPART model initialized using satellite observations.
The specific question of mixing between dust and BB particles was addressed
by considering the possible dust sources transported along the same transport
pathway. The role of mixing was quantified by considering the optical
properties of the different aerosol layers observed during two days of the
ChArMEx campaign (27 and 28 June 2013) when the biomass burning aerosol load
was at its maximum over the western Mediterranean. The three-dimensional
structure of the aerosol distribution revealed by the lidar network and the
airborne lidar flight provides a detailed assessment of the different aerosol
source contributions when it is coupled with the results of the Lagrangian
FLEXPART transport model. Four aerosol types were identified using the
depolarization ratio and the three-dimensional structure of the aerosol
plume: (i) pure BB layer, (ii) weakly dusty BB, (iii) a significant mixture
of BB and dust transported from the North Atlantic trade wind region, and (IV) the
direct northward outflow by the subtropical jet of Saharan dust not mixed
with BB aerosol. Mixing of dust and BB can correspond to a 20–30 % dust
contribution in the total aerosol backscatter. The comparison with the MODIS
AOD distribution during this episode over the western Mediterranean Sea shows
that the Canadian fire contributions were surprisingly as large as the direct
northward dust outflow from the Sahara. An additional contribution from a
mid-tropospheric aerosol layer due a mixture of dust and BB aerosol was found
in the region of higher AOD seen by MODIS. The next step will now concern the
use of all presented and analyzed data for evaluating 3-D regional models to
simulate this specific event, in terms of optical properties, possible mixing
and vertical extent of mineral dust and forest fire aerosol layers.
Acknowledgements
This work was funded by the French MISTRALS program funded by CNRS/INSU,
ADEME, CEA, Météo-France and CNES for aerosol and cloud satellite
mission validation. The NILU team led by A. Stohl is gratefully
acknowledged for distributing the FLEXPART model. The SAFIRE team, INSU DT
and D. Bruneau from LATMOS are gratefully acknowledged for the aircraft
flight operation and the LNG lidar operation. The lidar measurements in
Barcelona were supported by the 7th Framework Programme project Aerosols,
Clouds, and Trace Gases Research Infrastructure Network (ACTRIS) (grant
agreement no. 262254) and by the Spanish Ministry of Science and Innovation
and FEDER funds under the projects TEC2012-34575, UNPC10-4E-442 and
CGL2011-13580-E/CLI. Edited by: O. Dubovik
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