Chemical characterisation
This study focuses on aerosols transported by the westerlies, i.e. the
westerly airstream that flows from North America across the North Atlantic at
subtropical latitude and midlatitudes. Previous studies have shown that the Saharan
Air Layer (SAL), i.e. the dusty airstream that expands from northern Africa to
the Americas, is the most important carrier of aerosols in the tropical and
subtropical North Atlantic . Therefore, we first did a brief
comparison of the characteristics of the aerosol composition in these two airstreams, with the aim of illustrating the huge differences between them. The
frequency of the westerlies and of the SAL at Izaña is shown
in Fig. c: the westerlies occur with high frequency throughout the
year, with a maximum in April–May and a minimum in July–August when Izaña
is mostly within the SAL.
Table shows the median chemical composition and mass closure of
the PM10 and PM2.5 aerosols in samples collected at Izaña under
the SAL and the westerlies (Fig. a–b). The transport of
particulate pollutants in the SAL had already been studied by
. The SAL impacts on Izaña in July and August, and is
linked to the northern shift of the Harmattan trade winds
(Fig. a). The summer SAL occurs 1–5 km a.s.l. off northern
Africa. It is associated with air from the Mediterranean flowing south-westward to
the Sahara resulting in the emissions and export of dust to the Atlantic
above the marine boundary layer. The westerlies occur throughout the year and
are associated with airstreams from North America that, in some cases, may
have circulated around the Azores High (Fig. a).
Concentrations of bulk PM10 and PM2.5 are ∼ 20 and 10 times
higher in the SAL than in the westerlies respectively (Table ).
Mass closure of PMx accounts for a rather low fraction of the
gravimetrically determined PMx concentrations under westerly conditions
(50–70 % of PMx, Table ), compared to the SAL (70–90 %
of PMx, Table ). This is attributed to the relatively high
inaccuracy of the manual gravimetric method under low PMx concentrations
described above (see details in Sect. S1). Thus, the sum of
the main chemical components (∑ in Table ) is probably a
better proxy of the actual bulk PMx concentrations in the westerlies
than the gravimetric PM concentrations.
In the SAL, the PM10 aerosol population (median of ∑ – sum of the
main chemical components – ∼ 41 µg m-3;
Table ) is basically constituted by dust (78 %:
36 µg m-3) mixed with organic matter (4.5 %:
2.1 µg m-3), sulfate (3.8 %: 1.8 µg m-3),
nitrate (1.8 %: 0.8 µg m-3) and ammonium (0.4 %:
0.2 µg m-3). In contrast, PM10 aerosol in the westerlies
(median of ∑ – sum of the main chemical components –
∼ 1.8 µg m-3) is predominantly constituted by dust
(44.5 %: 1.1 µg m-3), organic matter (12.4 %:
0.32 µg m-3), nss-SO4= (9.8 %:
0.25 µg m-3) and ammonium (2.2 %:
0.06 µg m-3). Nitrate in the SAL mostly occurs in the coarse
range as non-ammonium salt coating dust particles (see details in
). In the westerlies nitrate concentrations tend to be
extremely low. In the few observed nitrate events, it tended to occur in the
sub-2.5 µm range, attributed to an ammonium salt. In the SAL, about
three-quarters of non-sea-salt sulfate (nss-SO4=) is present as ammonium
sulfate (a-SO4=) linked to anthropogenic sulfur emissions, with the
remaining three-quarters being non-ammonium sulfate (na-SO4=), most probably
linked to soil emissions of gypsum/anhydrite soil minerals in beds of Saharan
dry lakes ().
Aerosols in the SAL and in the westerlies also exhibit differences in terms
of size distribution. PM mass mostly occurs in the sub-2.5 µm in the
westerlies and in the coarse 2.5–10 µm range in the SAL
(Table ).
(a) Wind vector at 850 mb and (b) transport route
frequency (TRF) for January (Jan), April (Apr), August (Aug) and November
(Nov) of the period 2008–2013. The Gulf inflow is highlighted as well as the
location of Izaña (black circle).
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(a) Latitudinal ranges at which the westerlies occurs over
the eastern coast of North America. Grey circles: maximum and minimum
latitude of the outflow. Red circles: centre of the
outflow. (b) Meteorological scenarios associated with export of
pollutants (according to ) and
circulations (blue: January–May,
red: July–August) illustrated over mean SO2 values observed by
satellite by – copyright of the authors. (c) Monthly average values of the omega vertical wind
component at the 850 hPa level (negative values indicate upward movements)
calculated for the domains illustrated in
plot (d). (d) Domains 1 (32–40∘ N,
90–100∘ W), 2 (35–40∘ N, 75–90∘ W), 3
(40–43∘ N, 70–80∘ W) and 4 (46–60∘ N,
70–80∘ W).
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North American large-scale meteorology and airstreams
The meteorological scenarios that prompt pollutant export events from North
America are described in previous studies (,
; , ;
, ; , ;
, ). Here a complementary view is
provided. We analysed how large-scale circulations over North America evolve
over the year, and more specifically how they may influence the export of
aerosols to the Atlantic. The monthly values of key meteorological fields
(geopotential heights, winds and omega at several standard levels, 925, 850
and 700 hPa and precipitation rates, e.g. Fig. a) were determined
with the National Center for Environmental Prediction/National Center for
Atmospheric Research (NCEP/NCAR) reanalysis data . To
facilitate an interpretation of how the variability in meteorology may influence
the export of aerosols we also plotted (i) the latitudinal range of the
westerlies over the eastern coast of North America (observed in the monthly
NCEP/NCAR wind fields; Fig. a), (ii) the spatial distribution of
SO2 according to (Fig. b) and of major
aerosol components according to (,
) (Fig. ) and (iii) the monthly mean values of
the omega vertical component of wind in selected domains (Fig. c
and d). Finally, in order to link the export of North American
pollutants with transatlantic transport, the transport route frequency (TRF)
field was determined for each month based on back-trajectories
(Fig. b). Figure shows examples for illustrative
months (January, April, August and November); additional material is
presented in the Supplement (Fig. S3).
During January and February, the North Atlantic anticyclone shifts southward,
expanding over the Caribbean and resulting in an intense geopotential/pressure
gradient (Fig. S2a) and westerly winds over most of North America
(Fig. a1). The main stream of the westerlies (which we refer
to as “westerly jet”) flows from western Canada (∼ 55∘ N) to
the eastern US entering the North Atlantic at relatively low latitudes
(36–38∘ N in the 850 hPa standard level; Fig. a1). We
refer to the westerly jet over the eastern coast of North America as the “North
American outflow”. Its latitudinal position over the year is plotted in
Fig. a. The TRF analysis shows that during this period air from
the central and southern US reaches Izaña (Fig. b1).
From March to June, the North Atlantic anticyclone progressively intensifies
(Fig. S2b) and the western side of its clockwise atmospheric circulation
expands from the inner Gulf of Mexico northward to the central US and then to
the eastern US, resulting in an airstream that we have called “the Gulf inflow”,
which is observed in the wind fields at the 925, 850 and 700 hPa levels
(Fig. a2). The Gulf inflow is observed from March, when the trade
winds building up results in an northward inflow across the coast of Texas,
which subsequently turns north-eastward over the central US (Arkansas to
Tennessee and Indiana) and then eastward resulting in a westerly outflow to the
Atlantic by the eastern coast of North America following the clockwise
circulation of the North Atlantic high (Fig. b2). The analysis of
the (vertical) omega component at the 925, 850 and 700 hPa levels shows
that upward movement of air occurs in the regions affected by the Gulf inflow
of warm and humid air from Texas to Indiana, in such a way that air masses
from the continental boundary layer of the central to north-eastern US may be
exported in the westerlies to the North Atlantic free troposphere
(Fig. S3b). Observe in Fig. c how omega decreases to negative
values (net upward movements) from March to May in the central US (domain 1 in
Fig. d) under the influence of the Gulf inflow. The decrease in
omega in this season is also observed to the east (domain 2 in
Fig. d). This is consistent with the fact that the storm season
occurs in this period (March to June) between the central US (northern
Texas and Kansas) and the west of the Appalachians
(Tennessee, Kentucky and Indiana), as reported by NOAA
(http://www.ncdc.noaa.gov/climate-information/extreme-events/us-tornado-climatology;
Sect. S4). Along the path of the Gulf inflow there are a
number of sources of aerosols and their precursors, including coal-fired
power plants , the emissions of which may be lifted to the
mid-troposphere during convective processes (,
; , ), and then
exported to the North Atlantic free troposphere by the westerly circulation,
which in this period tends to occur 35–45∘ N (Fig. a).
The export of pollutants from the eastern US to the Atlantic is enhanced by
eastward-moving cyclones, which have tracks that typically occur south of 40∘ N
in this season ; Fig. b shows an illustration of
this scenario () and the mean SO2 spatial distribution observed
by satellite . A dotted blue line shows the typical
eastward track of the cyclones in March–April, whereas the blue arrows
indicate a simplified scheme of the associated circulation. The satellite
detection of SO2 over the ocean off the coast of Virginia to New
Jersey (35–40∘ N in Fig. b) shows evidence of the importance of
the export of this aerosol sulfate precursor to the Atlantic. The
back-trajectory-based TRF analysis shows that air masses from the central US
(e.g. domain 1 in Fig. d) and the eastern US (e.g. domain 2 in
Fig. d) are regularly transported to Izaña (Fig. b2).
Spring (March–April) is the season of maximum frequency of the westerlies at
this observatory (23–27 days month-1; Fig. c1) and has
implications for the export of major aerosol components, which have high concentrations
in the north-eastern US (Fig. ).
In July and August, the North Atlantic anticyclone shifts northward
(Fig. S2c) resulting in intense trade winds over the Caribbean. The Gulf
inflow continues blowing northward across the great plains up to Canada where
it connects with the main westerlies jet, whereas southern winds prevail
along the eastern coast of the US (Fig. a3). In September, the
trade winds, the resulting Gulf inflow and southern winds over the eastern
coast weaken. In this season, the westerlies and the resulting North American
outflow shift northward (45–55∘ N;
Figs. a3–b3, a); this is consistent
with previous studies showing that cyclone tracks and the resulting warm
conveyor belts linked to the export of pollutants tend to occur over Canada
(, ; ,
). This scenario is illustrated in Fig. b,
where the cyclone track is highlighted by a dotted red line and the
circulation by the red arrow. This is consistent with the seasonal evolution
of the omega vertical wind component, which shows the lowest (negative)
values in August and September in eastern Canada (domain 4 in
Fig. c and d), indicating upward movements of the
North American outflow. At Izaña, the westerlies occur with a minimum
frequency in July (12 days) and August (9 days; Fig. 1c1), the period in
which the observatory is frequently within the easterly Saharan Air Layer
(, , ).
Observe how the air masses from inner North America have a lower impact at
Izaña compared to at other periods (Fig. b3).
From October to December, the North Atlantic high shifts southward, expanding
over the south-eastern US (Fig. S1d). The Gulf inflow weakens and the westerly
wind band shifts progressively southward prompting the transport of air from
the central US to the North Atlantic (Fig. a4–b4).
An overall analysis indicates marked seasonality in the atmospheric
circulations with potential implications for the export and transatlantic
transport of major aerosol components. The westerlies
(Fig. a–b), including the North American outflow
(Fig. ), occur at lower (subtropical) latitudes in winter than in
summer (midlatitudes). This seasonal shift is also associated with the
upward transport of air, which is important in the central and eastern US in
March–May (domains 1 and 2 in Fig. c and d) and
shifts northward along eastern North America during spring and summer
(domains 2 to 4 in Fig. c and d), reaching maximum
intensity when the North American outflow occurs over eastern Canada in
August and September (domains 4 in Fig. d). This affects which
source regions of North America impact downwind North Atlantic free
troposphere: air masses from southern US are transported across the Atlantic
in winter (Fig. b1) from the central US in spring
(Fig. b2) and from Canada in summertime (Fig. b3). This
is consistent with the seasonal shift of cyclone tracks, westerlies and warm
conveyor belt described in previous studies (,
; , ). Of special
relevance is spring, when the westerly jet blows over SO2 source
regions coupled with upward movements that are able to transport aerosols emitted near
ground to altitude above the boundary layer (Fig. b).
Transatlantic transport of North American aerosols
We studied the seasonal variability of the sub-10 µm aerosol
components under westerly airflow conditions at Izaña and its connection to
the transatlantic transport from North America. The chemical composition of
126 samples of PM10 collected at Izaña (2008–2013) under westerly
airflow conditions were used (back-trajectories are plotted in Fig. S1a).
Under westerly airflow conditions, the time series of the aerosol components
typically show a low background level and sporadic peak episodes; for
example, nss-SO4= shows a background of
0.05–0.15 µg m-3 and peak events
0.5–1.5 µg m-3, organic matter increase from
0.01–0.2 µg m-3 background level to
1–2.5 µg m-3 peak events, whereas elemental carbon has a
background < 0.01 µg m-3 and peak events within the range
0.03–0.1 µg m-3. For each aerosol component, we determined
the seasonal evolution of the monthly 30th, 50th and 80th percentiles to be
representative of the background levels, central position of concentration
distribution and high-concentration episodes respectively. The 30th and 80th
percentile plots are presented in the Supplement (Sect. S5). The Median
Concentrations at Receptor (MCAR) plots were determined for the study of the
connection of peak events of aerosol components at Izaña with episodes of
North American aerosol export.
Mean surface concentrations of the (a) SO4=,
(b) NO3-, (c) NH3+, (d) EC and
(e) OC in North America obtained in previous studies by GEOS-CHEM
modelling and validated with the observations in the network IMPROVE
(, ,, Copyright by the
American Geophysical Union).
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(a) Median Concentration At Receptor (MCAR) plots and
(b) monthly median distribution for nss-SO4=,
NO3-, and NH4+ for the study period. Percentiles 50 and
80 are shown for NO3-. The MCAR plots maximum concentration tick
label includes concentration higher than this upper limit. The location of
Izaña is highlighted (black circle).
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Sulfate
Figure a1 shows the MCAR plot for nss-SO4=, whereas
Fig. b1 shows the value of the 50th percentile
(50thP) concentration
for each month at Izaña. The monthly 50thP of nss-SO4= shows high
levels from March to July (0.28–0.41 µg m-3), with a maximum
in March–May (0.33–0.41 µg m-3), and low levels from
September to February (0.06–0.24 µg m-3). The March to July
period can be considered the high-sulfate-concentration season, given that
both the monthly background (30thP) and median (50thP) levels are high in
this period (Fig. S5a).
The MCAR plot represents the nss-SO4= concentration recorded at
Izaña (median value, i.e. 50thP) when the airflows (tracked by
back-trajectories) have passed by each pixel of the study domain
(Fig. a1). Regions with relatively high nss-SO4=
concentrations (0.3–0.5 µg m-3 in yellow to red scale)
compared to the background (< 0.1 µg m-3 in blue) are
connected to potential transport routes. The MCAR plot suggests that there
are two preferential transport paths. The first route points to the transport
of nss-SO4= from the north-eastern US at ∼ 40∘ N; this is
consistent with the high SO2 emissions (Fig. b) and high
nss-SO4= concentrations (Fig. a) typical of this region
(Fig. b) that are associated with coal burning power plants
(, ; ,
, ) and the North American outflow
(Fig. ). Because the North American outflow occurs over this
region (NE US) during a great part of the year (Fig. ), this is
probably the most important nss-SO4= export region to the Atlantic.
The correlated seasonal evolution of omega in this region (domain 2 in
Fig. c and d) and nss-SO4= at Izaña (Fig. b1)
indicate enhanced upward movements of air from March to July and enrich the
North Atlantic free troposphere in sulfate aerosols. Maximum
nss-SO4= occurs from March to May (Fig. b1), when upward
air movements associated with the Gulf inflow (Fig. ), cyclones
and the occurrence of the North American outflow over this
region enhances the export of regional pollutants (Figs. a
and a). A second transport pathway is associated with
transatlantic transport at higher latitudes (50∘ N) and anticyclonic
circulation around the Azores High (Fig. a1); this route is
associated with the occurrence of the North American outflow over the northern US
and Canada, from midsummer (August) to mid-autumn (November;
Fig. a1). Observe how the drop in the median (50thP: from 0.30 to
0.20 µg m-3, Fig. S5a2) and of the background (30thP: from
0.26 to 0.16 µg m-3; Fig. S5a3) nss-SO4=
concentrations from July to August is associated with the northward shift of
the main westerly stream and the North American outflow (Fig. a1).
From August on the westerly jet occurs at higher latitudes over Canada, in
regions with less nss-SO4= compared to NE US (Fig. a). As
a result less nss-SO4= is exported and transported across the
Atlantic.
(a) Median Concentration At Receptor (MCAR) plots
and (b) monthly median distribution for elemental carbon (EC) and
organic matter (OM) for the study period. Percentiles 50 and 80 are shown for
EC. The MCAR plots maximum concentration tick label includes concentration
higher than this upper limit. The location of Izaña is highlighted (black
circle).
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Nitrate
The MCAR plot and the monthly 50thP of NO3- at Izaña are plotted
in Fig. a2 and b2. Nitrate was present in extremely
low concentrations most of the time. Concentrations were
< 0.05 µg m-3 in 97 samples, and
> 0.1 µg m-3 in only 21 samples. For this reason the 50thP
was ∼ zero for most months (Fig. b2). High NO3-
concentration events (80thP > 0.1 µg m-3) were mostly
recorded in winter and early spring (January–April, Fig. b2),
when high NO3- was within the range
0.2–0.8 µg m-3. This is typical behaviour for ammonium
nitrate, which mostly forms under low temperature conditions, whereas gaseous
nitric acid prevails in warmer environments . The MCAR
plot shows transport of NO3- at low latitudes 30–35∘ N
(Fig. a2), which is consistent with the circulation of the
westerlies (e.g. Fig. a1 and b1) and the North American outflow in
winter months (e.g. Fig. a1), when most of the above-described
high NO3- events occur (Fig. b2). High concentrations of
nitrate in North America occur in the central northern region
(Fig. b), where conditions favourable for the formation of ammonium
nitrate occur (, ; ,
): (i) enough high concentrations of gas phase precursors
(NH3 linked to emissions in agriculture fields treated with fertilisers
and HNO3 due to oxidation of NOx linked to fossil fuel combustion)
and (ii) suitable thermodynamic conditions (rather low temperature and enough
high relative humidity). Our results suggest that nitrate export events from
North America may be associated with NW winds (e.g. as the main stream of the
westerlies in winter, Fig. a1) over this high-nitrate region
(Fig. b) followed by export at 35–30∘ N
(Fig. b1) under geopotential/pressure systems that should be
studied in future research. The MCAR plot also suggests a second transport
route similar to that observed for nss-SO4=, i.e. transatlantic
transport at high latitudes (50∘ N) and circulation around the
Azores High (Fig. b1) which is probably associated with the autumn
events (Fig. b2), when the North American outflow occurs over
Canada (Fig. a1). The nitrate concentrations we observe at Izaña
are similarly low to those registered by at the Pico free
troposphere site in the Azores linked to long-range transport from North
America. Because nitrate may experience negative artefacts during sampling
(, ; ,
), we cannot discard underestimations. Further online
nitrate measurements (e.g. using an Aerosol Chemical Speciation Monitor)
should be included in the long-term aerosol measurements programme.
Ammonium
The MCAR plot and the monthly 50thP of NH4+ at Izaña are shown in
Fig. a3 and b3. Median (50thP) and 80thP
concentrations present a maximum in April–May (Fig. b3 and c1),
as nss-SO4=. The MCAR plot for NH4+ shows two main
transport pathways which resemble those of nss-SO4=: one transport
pathway from the north-eastern US at ∼ 40∘ N and a second
transport route pointing to the occurrence of the North American outflow by
Canada and subsequent transatlantic transport at high latitudes and
circulation around the Azores High (Fig. a3). Because of the
prevalent extremely low levels of nitrate, ammonium is attributed to
ammonium sulfate in most events. The transport (north-eastward export)
routes we observe for nss-SO4= and NH4+ in the eastern US
(Alabama, Tennessee and Virginia; Fig. a1 and a3) are similar to
those associated with the passage of spring cyclones and front in the region,
prompting the export of pollutants to the Atlantic .
Elemental carbon
Figure a1 and b1 shows the MCAR plot and the monthly
50thP and 80thP of EC. The analysis of the
monthly 50thP and 80thP values discloses two relevant
periods, associated to high (May–September) and low (January–April) EC
events. We associated this seasonal variability of the EC transported by the
westerlies to Izaña (i) with the spatial distribution of the EC source
regions in North America (Fig. d) and (ii) with the seasonal shift
of the westerlies (Fig. a1–a4) and the North American
outflow (Fig. ). The highest surface concentrations of EC are
estimated to occur in what we have called “EC-rich NE US regions”, which
include large urban areas placed 40–45∘ N south of the great lakes
to the Atlantic coast (e.g. Chicago, Detroit, New Jersey, Philadelphia and
New York; Fig. d), linked to fossil fuel combustion (mostly diesel
exhaust emissions and coal burning) according to .
The season of high EC concentrations at Izaña occurs from May to September,
when the westerlies shift northward from 40 to 55∘ N
(Fig. a) affecting the EC-rich NE US regions
(40–45∘ N, Chicago to New York, Fig. d). This seasonal
shift is associated with a rise of the upward air movements in the eastern US
(domains 2 and 3 in Fig. c and d), including the
EC-rich NE US regions (domain 3 in Fig. c and d),
that enhances the export of EC to the North Atlantic free troposphere in the
North American outflow. The highest median EC concentrations at Izaña are
observed in August and September (∼ 0.03 µg m-3;
Figs. b1, S5d1), when the eastward-propagating cyclones
over Canada prompt the export of pollutants from these EC-rich NE US
regions to the Gulf of Maine and the Atlantic , in a
scenario illustrated in Fig. b. The lowest values of omega in
eastern Canada (domain 4 in Fig. c and d) occur in
August and September, which indicates a great potential to lift boundary
layer air to the North Atlantic free troposphere. This interpretation is
consistent with the MCAR plot for EC, which shows a clear transport pathway
(at high latitudes, ∼ 50∘ N) from Canada and these EC-rich
NE US regions to the Gulf of Maine and then to the Atlantic with subsequent
circulation around the Azores High (Fig. a1); this EC transport
route is similar to the prevalent transport pathway of August and September
(Fig. b3).
Low EC concentrations at Izaña occur between January and April, when the
westerlies and the North American outflow occur at low latitudes
(< 40∘ N; Fig. a), to the south of the EC source regions
(40–45∘ N; Fig. d).
Figure shows the mean burnt fraction (%) of each
0.25∘ × 0.25∘ grid cell associated with fires, which occur mainly in
the south-eastern US in January–February (Fig. a). They then spread
northward from March on from the central US to southern Canada
(Fig. b) and over Canada and the NW US in June–September
(Fig. c) and then shift southward to the US (Fig. d).
Boreal fires prompt high EC concentrations in Canada in the NW and
north-central US (Washington, Oregon and Nevada; )
that can be exported to the Atlantic in the uplifting North
American outflow, potentially contributing to the EC records at Izaña in
August and September (Fig. b1).
Burned fraction (BF) of each 0.25∘ × 0.25∘
grid cell and vector wind at 850 mb averaged from 2008 to 2013 for
(a) January, (b) April, (c) August
and (d) November. The Global Fire Emissions Database Version 4
including small fires data (GFEDv4.1s; ,
) was downloaded from the Oak Ridge National Laboratory
Distributed Active Archive Centre (ORNL DAAC) for biogeochemical dynamics
(https://daac.ornl.gov/cgi-bin/dsviewer.pl?ds_id=1293).
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(a) Median Concentration At Receptor (MCAR) plots and (b) monthly median distribution for calcium (Ca), aluminium (Al) and
dust, and (c) major dust activity frequency (MDAF) for the study period: the
number of days with AI values > 1 divided by the total number of days with
available AI data in % (data source: http://disc.sci.gsfc.nasa.gov/). The
maximum concentration tick label of the MCAR plots includes concentration higher
than this upper limit. The location of Izaña is highlighted (black circle).
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(a) Median Concentration At Receptor (MCAR) plots and
(b) monthly median distribution for sea salt for the study period.
The MCAR plots maximum concentration tick label includes concentration higher
than this upper limit. The location of Izaña is highlighted (black circle).
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Organic matter
Figure a2 and b2 show the MCAR plot and the monthly
50thP of OM. This aerosol component shows very marked seasonal
evolution, with high levels from January to July, and a maximum from March to
May (Figs. b2, S5e1–e3). This is consistent with the MCAR
plot (Fig. a2), which shows a transport route from the south-eastern US
to Izaña at low latitudes (30–40∘ N), a common circulation of
winter and spring (Fig. a1–a2). From August to
December, OM concentrations transported by the westerlies to Izaña are low,
associated with the occurrence of the westerlies over North America at high
latitudes (Fig. a3–a4).
The seasonal evolution of OM is very different (almost the opposite) to that of
EC (Fig. a2 and b2). Air masses transported from the
south-eastern US to Izaña (January to April) are rich in OM and relatively
poor in EC (Fig. a2 and b2), whereas the air
transported from NE US to Izaña (typically from July to September) is poor
in OM and rich in EC (Fig. a2 and b2). This is
consistent with the spatial distribution of these aerosol species in the US
(Fig. d and e) and suggests that in the south-eastern US
there is a significant contribution to OM of sources that are not related to
combustion, but probably to biogenic emissions . Previous
studies estimated the contribution of biogenic SOA to OM to be within the range
50–60 % in the south-eastern US (,
; , ;
, ). Globally, about half of biogenic
volatile organic carbon emissions (BVOCs) are SOA precursors, i.e. isoprene
(∼ 50 %) and α-pinene (∼ 7 %) . Biogenic
emissions are among the principal sources of OM in the US, followed by three
combustion sources that also emit EC (wildfires, fossil fuels and biofuel)
. Particularly in the south-eastern US, BVOC emissions are mainly
isoprene (81 %) and monoterpenes (19 %) . A scenario
of biogenic emissions higher in the SE US than in the NE US is consistent with
the global distribution of the secondary organic aerosols, the concentrations
of which are usually higher near to the tropics than at midlatitudes
(, ; ,
). The importance of the spatial variability of the
OM and EC sources and the latitudinal shift of the westerlies over eastern
North America is illustrated from July to August, when a drop in OM
concentrations (0.85 to 0.20 µg m-3; Fig. b2) and
an increase in EC concentrations (0.005 to 0.03 µg m-3;
Fig. b1) is associated with the northern shift of the North
American outflow (43 to 50∘ N; Fig. a).
Mineral dust
Figure a and b show the MCAR plot and the monthly
50thP of calcium, aluminium and the associated bulk dust
concentrations. These aerosol components exhibit high concentrations from
February to May (Figs. b1–b3, S5f–h). The MCAR plot shows
a pattern of North American dust export at low latitudes
(∼ 35∘ N, through North Carolina) towards the north-east which,
once over the Atlantic, follow the anticyclonic circulation around the Azores
High to Izaña (Fig. a1–a3). We attribute these events to dust
emissions in a region that expands from SW Texas northward throughout the
High Plains, and subsequent dust export to the Atlantic. Figure c
shows the major dust activity frequency (MDAF) detected by satellite: dust
activity is observed in SW Texas (Chiguagua and Big Bend Desert) in February.
Through March to May the activity expands northward across the High Plains
(western Texas to Nebraska); other dust sources with lower potential for
impact on the North Atlantic are also observed in the western US (Great Basin,
Mojave Desert and Colorado Plateau). The High Plains are among the major dust
sources in North America, and these sources are considered anthropogenic
(linked to agriculture), with maximum activity between February and May
. Dust emissions and eastward mobilisation are associated with
the intense westerly winds linked to eastward-moving cyclones, which also
prompt the upward transport of dust to several kilometres above the ground, according to
; this scenario is illustrated in Fig. a3, showing
that the associated air mass track is consistent with the dust export and
transatlantic transport route to Izaña observed in our analysis. During
these spring events high dust concentrations (100s of µg m-3)
are lifted to altitudes 6–12 km a.s.l. over southern US .
Upward transport of dust is also associated with convective activity in
the central US, Colorado and Oklahoma, in May–June . The
correlation we found between the seasonal evolution of omega in domain 1
(Fig. c and d) and dust at Izaña (Fig. b3)
supports the idea that from February to May westerly winds and the uplifting
of air in the High Plains enrich the North American outflow and the westerly
jet in the in dust aerosols.
Sea salt
Sea salt concentrations at Izaña are extremely low, with monthly
50thP values 0.07 to 0.22 µg m-3 between December
and May (Fig. ). These low concentrations are typical of free
troposphere sites; in fact, sea salt at Izaña (average = 0.25 µg m-3, median 0.16 µg m-3) is about
2 orders of magnitude lower than in the marine boundary layer of the Canary
Islands (average ∼ 11 µg m-3, ,
). The extremely low concentrations of this marine aerosol
at Izaña supports our interpretations; i.e. the aerosols transported by the
westerlies to Izaña are mostly linked to emissions and upward transport in
continental regions of North America, and not over the ocean.
Contribution of each aerosol specie to bulk PM10 in samples
collected at Izaña under westerly airflow conditions; data classified from
the highest to the lowest levels. (a) Considering major components
except dust. (b) Considering all major components including mineral
dust.
![]()
Mass closure of aerosols
Figure shows the contribution of each species to bulk PM10
aerosol mass in samples collected in the westerlies at Izaña. Data are
classified from the highest to the lowest levels. We considered two
approaches: including and excluding mineral dust. When mineral dust is not
included (Fig. a), the most important contributors to bulk
PM10 are by far nss-SO4= and OM. In the 1st–50th percentile
range for the sum of aerosol components (a proxy of background levels) –
0.05 to 1.0 µg m-3 – the most important contributors are
nss-SO4= (0.19 µg m-3 on average, accounting for
38 % of the sum of chemical species) and OM (0.14 µg m-3,
30 %). In the 75st–95th percentile range (a proxy of high load of aerosol
events) – ∼ 2.0 to 3.6 µg m-3 – the most important
contributors are OM (1.43 µg m-3, 57 %) and
nss-SO4= (0.48 µg m-3, 19 %). These results are
consistent with previous studies that did not include dust, such as
(, ), who focused on
the composition of aerosols in the background boundary layer of US, and
, who studied the aerosols transported from North America to
the North Atlantic free troposphere at Pico observatory in the Azores.
When mineral dust is considered, it becomes the most important contributor to
sub-10 µm aerosol mass (Fig. b). In the 1st–50th
percentile range (a proxy of background levels), bulk PM10=0.15–2.54 µg m-3, the most important contributors to bulk
aerosol mass are dust (0.78 µg m-3 on average, accounting for
53 % of bulk mass), nss-SO4= (0.21 µg m-3, 14 %),
OM (0.27 µg m-3, 18 %) and NH4+
(0.07 µg m-3, 5 %). In the 75th–95th percentile range (a
proxy of aerosol events), bulk PM10= 3.9–8.9 µg m-3.
The most important contributors to bulk aerosol mass are dust
(2.8 µg m-3, 56 %) and OM (1.23 µg m-3,
24 %) followed by nss-SO4= (0.47 µg m-3, 9 %) and
NH4+ (0.1 µg m-3, 2 %). The lack of previous
studies on transatlantic transport of North American dust make the comparison
with previous data difficult; it should be highlighted that
detected this transatlantic transport in June (out of the
seasonal maximum). Environmental studies have shown that the conversion of
natural lands to agriculture and pasturage fields has had a number of impacts
in North America (, ;
, ). Moreover, research has predicted that
this change of land use may increase (, ;
, ), and this suggests an enhancement
of dust impacts in downwind regions.
Our overall results provide evidence that dust and organic matter are the most
abundant aerosols transported from North America to the North Atlantic free
troposphere.