We collected aerosol samples in four different airflows: two FT airflows and
two other airstreams potentially mixed with BL air. Samples collected at night
(PMT) are representative of the two FT airflows that prevail in
this region: the WES and the SAL. As already described, the WES flow from
North America across the North Atlantic at midlatitudes, with their southern
edge shifting to the subtropics in winter, and flow over Canada
reaching Izaña after circulation around the Azores High
in summer (see back-trajectories of the samples collected from 26 to
30 August – with “ddmmm” referring to ending sampling day – in
Fig. a). The SAL expands from North Africa to the Americas at
subtropical latitudes in summertime (see back-trajectories associated during
the study period in Fig. b), the season in which the Izaña
Observatory is mostly within this dusty airstream and the presence of the WES
is associated with southern shifts of the SAL.
Organic molecular tracers
In the next sections, organic speciation results are described.
Table shows the average concentrations of the 40 organic compounds
analysed in this study under the different scenarios (SAL and WES) for
PMT and PM2.5. In order to improve insight into the
origin and sources of some FT organic aerosols, correlations among the
organic groups and the major species are evaluated by means of the Pearson
correlation coefficient (r) (Table ); significance levels are
determined according to the p value (p). This coefficient is applied for
all correlation throughout the paper.
Levoglucosan
Levoglucosan (1,6-anhydro-β-D-glucopyranose) is emitted during biomass
burning as a consequence of the thermal alteration of cellulose and
hemi-cellulose present in vegetation . It is considered a
particle-phase marker for the identification of wood combustion due to its
source specific emission, but its atmospheric stability is still a matter of
discussion. Experiments carried out by and
showed that levoglucosan reacts with gas-phase hydroxyl
radicals (OH), especially under high relative humidity conditions. However,
studies performed by demonstrated no degradation of
levoglucosan under acidic conditions over a period of 10 days.
Levoglucosan daily values measured at Izaña within the FT and the BL were
< 1.5 ng m-3 for all samples, except on 28 August when
∼ 9 ng m-3 was measured. HYSPLIT back-trajectories and the
Atmospheric Infrared Sounder (AIRS) satellite images (NASA) indicate the
North American origin of the air masses, where several wildfires – such as
the Rim Fire – were affecting western USA 10 days before the air mass
started moving towards Izaña (detailed information not provided for the sake of
brevity). We detected a long-range transport biomass burning plume within the
FT from the fires originated in North America. Other studies performed at
Pico Mountain Observatory (Azores, 2225 m a.s.l.) have also detected the
impact of other biomass burning plumes by means of levoglucosan detection
. These results lend support to the atmospheric stability of
levoglucosan, under the specific atmospheric conditions of this long-range
transport. Due to the particular composition of this biomass burning event
(BBE), we will discuss this sample separately (Table ) and the
sample will not be included when describing the general composition of the
samples collected at night under westerlies conditions (FT-WES;
Table ). Levoglucosan concentration, measured at Izaña during the
BBE (9.3 ng m-3), is similar to the average levels detected in the
marine BL over the Azores in the North Atlantic (5.2 ng m-3) or at a
free tropospheric site in the European continent (7.8 ng m-3), but
much lower than those found at sites under the influence of local BB or
continental sites in winter (653–1290 ng m-3) .
Average concentration of the selected organic species for (i)
FT-PMT and BL-PM2.5 taking into account all samples, (ii)
FT-PMT and BL-PM2.5 collected within the Saharan Air
Layer (SAL), (iii) FT-PMT and BL-PM2.5 collected within
the westerlies (WES) without the FT-PMT biomass burning event and
(iv) FT-PMT biomass burning event (BBE).
FT-PMT
BL-PM2.5
FT-PMT
FT-PMT
BL-PM2.5
BL-PM2.5
FT-PMT
ALL
ALL
SAL
WES
SAL
WES
BBE
Levoglucosan
Levoglucosan, ng m-3
0.75
0.53
0.41
0.40
0.34
1.04
9.33
Dicarboxylic acids
Succinic, ng m-3
6.51
3.70
5.70
3.52
4.03
2.80
33.35
Glutaric, ng m-3
1.97
0.83
1.90
0.74
0.85
0.77
7.23
Adipic, ng m-3
1.43
0.69
1.53
0.62
0.72
0.60
1.71
Pimelic, ng m-3
0.83
0.35
0.92
0.37
0.33
0.39
0.17
Suberic, ng m-3
0.48
0.30
0.50
0.29
0.29
0.31
0.39
Azelaic, ng m-3
0.88
0.79
0.93
0.57
0.78
0.82
0.71
Malic, ng m-3
2.01
2.15
0.75
1.20
1.67
3.43
32.21
Phthalic, ng m-3
3.18
2.77
2.19
9.43
2.38
3.80
6.21
Saccharides
α-glucose, ng m-3
9.26
0.90
10.79
0.62
0.90
0.89
1.65
β-glucose, ng m-3
9.13
1.00
10.63
0.61
1.01
0.98
1.65
Fructose, ng m-3
2.02
1.01
2.23
0.95
1.10
0.76
0.69
Sucrose, ng m-3
2.72
0.47
3.18
0.27
0.18
1.26
0.01
Mannitol, ng m-3
0.35
0.12
0.40
0.08
0.12
0.12
0.07
n-Alkanes
nC24, ng m-3
0.72
1.63
0.75
0.48
1.87
0.97
1.01
nC25, ng m-3
0.93
2.93
0.95
0.40
3.26
2.05
2.09
nC26, ng m-3
0.60
0.64
0.65
0.26
0.69
0.49
0.54
nC27, ng m-3
0.89
0.95
0.98
0.24
0.94
0.98
0.76
nC28, ng m-3
0.37
0.32
0.39
0.17
0.38
0.17
0.36
nC29, ng m-3
1.18
0.42
1.34
0.25
0.48
0.25
0.63
nC30, ng m-3
0.45
0.14
0.51
0.07
0.16
0.10
0.18
nC31, ng m-3
1.55
0.31
1.77
0.37
0.33
0.25
0.29
nC32, ng m-3
0.39
0.10
0.45
0.05
0.11
0.06
0.08
nC33, ng m-3
0.48
0.10
0.55
0.06
0.12
0.06
0.13
nC34, ng m-3
0.29
0.05
0.34
0.02
0.05
0.04
0.01
Hopanes
Hopane, ng m-3
0.06
0.02
0.07
0.01
0.03
0.01
0.03
Norhopane, ng m-3
0.07
0.05
0.08
0.02
0.07
0.02
0.03
PAHs
B [a] A, pg m-3
1.48
1.48
1.58
0.80
1.61
1.13
1.37
Chr, pg m-3
4.27
4.37
4.63
1.92
5.12
2.39
3.38
B [b+j+k] F, pg m-3
3.67
5.74
4.20
0.66
6.69
3.21
1.13
B [e] P, pg m-3
1.36
1.94
1.50
0.47
2.22
1.20
0.83
B [a] P, pg m-3
0.78
1.21
0.89
0.18
1.25
1.08
0.29
In[123cd] P, pg m-3
1.47
2.33
1.65
0.46
2.18
2.74
0.56
B [ghi] Per, pg m-3
3.29
6.94
3.56
1.73
6.37
8.46
1.84
SOA PIN
cis-Pinonic, ng m-3
27.83
15.24
32.72
0.89
13.23
20.59
1.00
3-HGA, ng m-3
0.21
0.51
0.09
0.24
0.39
0.81
2.88
MBTCA, ng m-3
0.03
0.24
0.01
0.05
0.13
0.54
0.27
SOA ISO
2MGA, ng m-3
4.22
2.38
4.46
1.63
2.65
1.65
6.56
2MT-1, ng m-3
6.64
4.94
7.27
3.40
5.16
4.33
2.40
2MT-2, ng m-3
15.45
9.53
16.79
8.95
9.24
10.31
5.53
B[a]A: benz[a]anthracene; Chr: chrysene; B[b+j+k]F:
benzo[b+k]fluoranthene; B[e]P: benzo[e]pyrene; B[a]P: benzo[a]pyrene;
In[123cd]P: indeno[1,2,3-cd]pyrene; B[ghi]Per: benzo[ghi]perylene; 3-HGA:
3-hydroxyglutaric acid; MBTCA: 3-methyl-1,2,3-butanetricarboxylic acid; 2MGA:
2-methylglyceric
acid; 2MT-1: 2-methylthreitol; 2MT-2: 2-methylerythritol.
Dicarboxylic acids
Dicarboxylic acids can be emitted in small quantities from several natural
and anthropogenic primary sources such as vegetation, meat cooking and motor
exhaust emissions (), although atmospheric
photochemical transformation of volatile and semi-volatile organic compounds
is considered to be an important source for the presence of these aged
compounds in the atmosphere
(; ; ). This oxidative degradation
of VOCs by tropospheric oxidants may be responsible for the similar mean
∑ dicarboxylic acid concentrations within the FT
(SAL: 14.4 ng m-3; WES: 16.7 ng m-3) and the BL
(SAL: 11.1 ng m-3; WES: 12.9 ng m-3) at Izaña.
Succinic (suc) and phthalic (pth) acids were the most abundant dicarboxylic
acids (Table ) with FT and BL average values (suc: 6.5–3.7;
pth: 3.2–2.8 ng m-3 for the FT-BL) much lower than those found for
PM2.5 in the FT Mount Tai (suc: 30 ng m-3;
), similar to those observed in PMT in the
Himalayas (4276 m a.s.l.) (suc: 13.7; pth: 9.5 ng m-3;
), but higher than those detected
in the North Pacific for remote marine PMT (suc: 2.8; pth: 0.66 ng m-3;
). Malic acid within the BL (the third most abundant
polyacid at Izaña; Table ) might be photochemical in origin via
OH oxidation of the surrounding biogenic compounds transported by the daytime
upslope winds. The Izaña Observatory is surrounded downhill by a forest
ring – an important source of biogenic volatile organic compounds (BVOCs) –
which contributes significantly to the concentrations measured at Izaña.
Oxidation of these biogenic precursors may also provide important amounts of
C7–C9 dicarboxylic acids.
High concentrations of dicarboxylic acids have been reported in plumes from
BB (), which is in line with the observed values
for the long-range transport BBE (82 ng m-3). Concentrations of
succinic, glutaric and malic acids were high (∼ 33, ∼ 7 and
∼ 32 ng m-3, respectively; Table ), compared to the
rest of the period, most likely as a consequence of the lofted concentration
emitted in the open fire and long-range transport photochemical aging
processes.
Saccharides
Primary saccharides and polyols are tracer compounds of surface soils
(), related to plant tissue and
microorganisms. Glucose (α, β), fructose and sucrose are
important constituents of OM in soils , whereas mannitol
is related to airborne fungal spores . They are completely
water soluble, contributing to water-soluble organic carbon (WSOC) in
aerosols . Wind erosion and up-lifted soil dust emit these
compounds to the atmosphere .
The average concentration of the saccharides exhibits a marked difference
within the FT-PMT (23.5 ng m-3) and the BL-PM2.5
(3.5 ng m-3). Previous studies have shown that some organic compounds
are strongly particle-size-dependent (), with
special emphasis on sugars and sugar alcohols, which are present mostly in
very large particles (). This size segregation is
clearly seen for the PM2.5 samples collected within the BL, that
cuts off an important fraction of the coarse organic soil dust aerosol,
showing similar concentrations under SAL and WES influence (BL-SAL =
3.3 ng m-3; BL-WES = 4.0 ng m-3). A different scenario takes
place with the PMT samples, for which concentrations rise 1 order
of magnitude from SAL influence to clean conditions (FT-SAL =
27 ng m-3; FT-WES = 2.5 ng m-3) linked to Saharan dust
contribution. The average saccharide levels in the FT-SAL are higher than
those observed in a natural forest area in tropical India
(12.78 ng m-3; ) and a rural background in Norway
(10.4 ng m-3; ), but very similar to the average
concentrations measured in the FT over central China (28.1 ng m-3;
).
Glucose (α+β) was the predominant saccharide within the SAL, with
a mean FT concentration of ∼ 10 ng m-3 (Table ); both
isomers showed a statistically significant correlation (r=0.99,
p < 0.01) consistent with their relation in the soil
. Under the WES airflows, glucose (α+β),
sucrose and mannitol were slightly higher during the day (BL;
Table ), suggesting that there might be some soil contribution of
transported terrestrial OM by land breeze. This load is more evident in the
sucrose (FT-WES = 0.3 ng m-3; BL-WES = 1.3 ng m-3;
Table ), which is a predominant sugar in the phloem of plants
playing a key role in developing flowers and has been
suggested as a tracer for airborne pollen grains .
Pearson correlation coefficients matrix of the organic and inorganic
compounds within the free troposphere (PMT). Statistically
significant correlations (p value < 0.01) are highlighted. BBE was
excluded in this analysis.
Levoglucosan
Dicarboxylic acids
Saccharides
n-Alkanes
Hopanes
PAHs
SOA PIN
SOA ISO
Dust
Sea Salt
OM
EC
NO3-
NH4+
nss-SO4=
ss-SO4=
Levoglucosan
1.0
Dicarboxylic acids
0.0
1.0
Saccharides
0.0
0.4
1.0
n-Alkanes
0.1
0.5
0.5
1.0
Hopanes
0.0
0.6
0.7
0.8
1.0
PAHs
0.3
0.3
0.2
0.3
0.3
1.0
SOA PIN
-0.1
0.2
0.3
0.6
0.5
0.1
1.0
SOA ISO
0.2
0.3
0.2
0.3
0.3
0.6
0.4
1.0
Dust
0.0
0.7
0.6
0.7
0.9
0.2
0.7
0.2
1.0
Sea Salt
0.6
-0.1
-0.2
-0.1
-0.3
0.4
0.1
0.2
-0.2
1.0
OM
-0.1
0.3
0.3
0.7
0.8
0.2
0.8
0.4
0.9
0.0
1.0
EC
0.2
0.1
-0.1
-0.1
-0.2
0.7
-0.2
0.3
-0.3
0.3
-0.2
1.0
NO3-
0.0
0.4
0.7
0.8
0.8
0.5
0.6
0.4
0.8
-0.1
0.8
0.0
1.0
NH4+
0.1
0.1
0.2
0.2
0.3
0.6
-0.1
0.3
0.2
0.0
0.2
0.5
0.5
1.0
nss-SO4=
0.1
0.2
0.6
0.5
0.6
0.5
0.5
0.6
0.5
0.1
0.6
0.1
0.8
0.7
1.0
ss-SO4=
0.6
0.0
-0.2
0.0
-0.2
0.4
0.1
0.2
-0.2
1.0
0.0
0.3
0.0
0.0
0.0
1.0
OM: organic matter; EC: elemental carbon; nss-SO4=: non-sea-salt sulfate; ss-SO4=: sea salt sulfate.
n-Alkanes
n-Alkanes, or aliphatic hydrocarbons, are a result of biogenic and
anthropogenic emissions such as plant waxes and fossil fuel combustion
products (). In the present study n-alkanes from nC24 to nC34 were
quantified, with total n-alkane mean concentrations (∼ 8 ng m-3
in the FT and in the BL) much lower than those measured in the tropical
Indian summer (126 ng m-3; ), but
similar to those found in rural Spain during the warm period
(12 ng m-3; ).
Information about the possible source may be provided by the carbon number
maximum (Cmax). In general, nC27, nC29 and nC31 are related to waxes from
terrestrial higher plants, whereas low-molecular-weight alkanes (C22–C25)
are more associated with combustion sources . At Izaña,
the most abundant n-alkanes within the FT-SAL were nC27, nC29 and nC31
(∼ 1.0 ng m-3, ∼ 1.4 ng m-3 and
∼ 1.8 ng m-3 respectively; Table ) reflecting a
vegetative source as previously described for Saharan dust samples measured
in the North Atlantic , whereas the BL nC24–nC25 presented
higher concentrations (Table ) linked to anthropogenic emissions
carried by the upslope winds. Another indicator that can be used to show the
source type is the carbon preference index (CPI =∑odd
n-alkanes / ∑even n-alkanes) with CPI > 1 related to biogenic
origin and CPI ≈ 1 to combustion processes (). In
this study, CPI values ranged from 0.9 to 6.3 with average values for the
FT-SAL (∼ 2) higher than those for the BL (∼ 1.7) and the FT-WES
(∼ 1.2), reflecting the greater influence of vegetation within the
FT-SAL and the predominance of combustion contribution within the BL and
FT-WES samples. Although the vegetative source dominates in the FT, there is
a statistically significant correlation between n-alkanes and NO3- (r=0.8, p < 0.01; Table ), mostly due to its anthropogenic
fraction (C24–C25).
Hopanes
Hopanes (17α(H),21β(H)-29-norhopane and
17α(H),21β(H)-hopane) are linked to mineral oil and related to
unburned lubricating residues from primary vehicle emissions
(). ∑ Hopanes mean
concentrations were 0.13 and 0.08 ng m-3 within the FT and BL
respectively, values much higher than the 7×10-4 ng m-3
measured by in remote Greenland (3200 m a.s.l.)
where anthropogenic emissions in the surrounding region are minimal. Under
the WES airflows, hopanes concentrations were slightly higher during the day,
suggesting an influence of pollution transported within the BL, related to
motorized vehicle emissions. Quantified hopane and norhopane showed a
statistically significant correlation (r=0.97, p < 0.01), implying
the same emission sources.
A statistically significant correlation is observed in the FT between hopanes
and NO3- (r=0.8, p < 0.01; Table ) suggesting that
the origin of most NO3- in the FT lies in on-road vehicle emissions
rather than industry. Anthropogenic sources of NOx (the major NO3-
precursor) include fossil fuel emitted from agriculture, power plants,
industry and transport. The latter accounts for almost 50 % of nitrogen
oxides emissions
(http://www.eea.europa.eu/data-and-maps/indicators/eea-32-nitrogen-oxides-nox-emissions-1/assessment.2010-08-19.0140149032-3),
with on-road transport in 2010 being the highest (25.2 Tg yr-1)
compared to non-road (10.1 Tg yr-1), shipping (16.2 Tg yr-1),
aviation (3.0 Tg yr-1) or rail (1.6 Tg yr-1) .
Polycyclic aromatic hydrocarbons
Polycyclic aromatic hydrocarbons (PAHs) are organic pollutants generated
during incomplete combustion of organic natural material (e.g. forest fires,
volcanic activity) and anthropogenic (e.g. fossil fuel combustion, coke
production) sources (). PAHs are composed of two or more fused aromatic rings
and some of them have carcinogenicity or genotoxicity and are potentially
endocrine disruptive, affecting human health. At Izaña mean values of the
total PAHs exhibited higher values in the BL (24 pg m-3) than in the
FT (16.3 pg m-3), reflecting the contribution of the upslope winds as
described for other organic compounds.
Similar PAH concentrations were previously found by , who
measured an average PAH concentration of 33.1 pg m-3 at Izaña. In
general, all individual PAHs decreased in concentration with the exception of
benz(a)anthracene which increased by a factor of 1.5 and 1.35 with respect to
the mean concentrations of the FT and BL correspondingly. Much higher
concentrations have been reported in other remote FT locations such as Mt
Tai (1534 m a.s.l.) where ∼ 9 ng m-3 were measured
. In the FT there is a statistically significant correlation
between PAHs and EC (r=0.7, p < 0.01; Table ), which
points to the incomplete combustion of fossil fuels .
During the detected North America wildfire event (28 August), PAH
concentration was 9.4 pg m-3, which is much lower than levels measured
in Thailand for PMT samples during BBEs in the dry season (1150 to
4140 pg m-3; ). The
concentrations of PAHs measured in the sample corresponding to the fire event
were no higher than those observed in the other samples, which may be due to
photochemical transformations of PAH in the atmosphere during long-range
transport.
Tracers of α-pinene oxidation (SOA PIN)
Vegetation emits large quantities of biogenic volatile organic compounds
(BVOCs) into the atmosphere compared to anthropogenic VOCs
(), particularly monoterpenes
and isoprene. The most abundant volatile monoterpene, emitted mainly by
coniferous trees (i.e. Pinus canariensis), is α-pinene
() and the
tracers related to its photochemical oxidation (SOA PIN) are cis-pinonic
acid, 3-hydroxyglutaric acid (3-HGA) and 3-methyl-1,2,3-butanetricarboxylic
acid (MBTCA) ().
SOA PIN organic tracers were not detected in all samples, with values in the
FT influenced by a few extreme points that increased their average
concentration. SOA PIN exhibited the lowest concentration in the FT-WES
(1.2 ng m-3) with a predominance of cis-pinonic acid
(Table ). Aircraft measurements in the FT over central China
recorded higher concentrations of 3-HGA (8.5 ng m-3) and
MBTCA (1.9 ng m-3) than those measured in the present study
(Table ). Further-generation oxidation products (3-HGA and MBTCA)
were higher in the BL (0.51 and 0.24 ng m-3 correspondingly;
Table ), with a statistically significant correlation (r=0.90,
p < 0.01) pointing to a same precursor. Monoterpenes react relatively
rapidly, with atmospheric lifetimes ranging from minutes to hours
, resulting in α-pinene emitted in the forest ring
that reacts along its upward transport to the observatory. Daytime emissions
of gaseous α-pinene at Izaña were measured by with
concentration in the range of 0.011–0.102 ppbv (mean: 0.028 ppbv),
supporting evidence of its origin being close to the observatory during the
day.
Scatter plot between total concentration of SOA ISO and total
concentration of SOA PIN within the FT (PMT collected during the
night). Two tendencies can be distinguished: tendency 1 (t1; circles) and
tendency 2 (t2; squares). Filled markers correspond to measurements within
the SAL and open markers to measurements within the WES.
Tracers of isoprene oxidation (SOA ISO)
It is estimated that about a half of the total global BVOCs emission is due
to isoprene (535 Tg yr-1; ), making it the largest BVOC emitted from land
vegetation . Isoprene emission is limited to a number of
species in the plant kingdom, contrary to many other BVOCs that are emitted
from most plants . Secondary products of isoprene oxidation
(SOA ISO) evaluated in the present study are 2-methylglyceric acid (2-MGA),
2-methylthreitol (2-MT1) and 2-methylerythritol (2-MT2)
().
Analogous FT concentrations of 2-methylthreitol and 2-methylerythritol were
measured in the present study under the SAL (7.3 and 16.8 ng m-3;
Table ) and over the central China FT (8 and 17 ng m-3;
), one of the most important source regions
of isoprene emission in the world during summertime .
Similar SOA ISO concentrations were found in the BL within the SAL and the
WES (∼ 17 and ∼ 16 ng m-3 respectively) revealing the
emission and subsequent ascending transport of biogenic or anthropogenic
compounds, as found in previous studies performed at Izaña, which observed
emissions of isoprene during daytime associated with anthropogenic compounds
. A statistically significant correlation among 2-MT1 and
2-MT2 was found for individual values (r=0.90, p < 0.01) as
previously observed in other studies (), but with a mass
concentration ratio of 2-MT1 vs. 2-MT2 (slope from linear regression = 2.3)
slightly lower than that found by . This statistically
significant correlation between the two diastereoisomers would seem to
indicate they formed through the same photo-oxidation process.
The highest concentration of SOA ISO was measured under the FT-SAL
(∼ 28 ng m-3), associated with Saharan dust, as was observed for
SOA PIN (∼ 33 ng m-3). However, global estimations of isoprene
and α-pinene emissions and sources show they are diverse and not
equally distributed around the globe (). The correlation between total concentration of SOA
ISO and total concentration of SOA PIN (Fig. ) exhibits two
distinct trends in the FT that might be associated with different global
sources of the precursor volatile compounds, although the trajectories of the
sampled air mass do not clearly distinguish between different origins. Some
species with high isoprene emission potential have been identified in central
and western Africa, but quantification of isoprene emissions are largely
unverified for West Africa . Several evaluations of isoprene
and α-pinene global emissions () confine the North Africa sources to a small belt
over the northern part of Morocco, Algeria and Tunisia, whereas Europe is a
potential source. Some episodes, for which SOA PIN and SOA ISO were measured,
do not have a trajectory over this African belt (based on the HYSPLIT model), suggesting that air masses from Europe can also incorporate gaseous
precursors and oxidized species previous to their passing over Africa and
Izaña in the Atlantic Ocean.
Scatter plot between SOA ISO and nitrate within the SAL under FT
conditions (FT-PMT samples collected during the night). Three
tendencies can be distinguished: tendency 1 (t1; circles), tendency 2 (t2;
squares) and tendency 3 (t3; triangles). Westerlies were excluded when
calculating the regression coefficients as values were under the detection
limit.
previously reported high concentrations of SO4=,
NO3- and NH4+ for air masses arriving at Izaña from the
Atlantic coast of Morocco, eastern Algeria, northern Algeria and Tunisia.
Industrial states with sources of gaseous precursors (SOx, NOx and
NH3) of these aerosol compounds were identified. Although African
emissions seem to be responsible for the pollutants reaching the North
Atlantic FT, Europe may also contribute with amounts of pollutants that
should not be neglected as evidenced by the concentration of SOA PIN and SOA
ISO arriving at Izaña; this is supported by a previous study
in which it was suggested that carbonaceous, sulfate, and
nitrate particles – in aerosol plumes transported from North Africa over the
North Atlantic Ocean within the FT – were anthropogenic pollution from
Europe. These species may play a key role in secondary organic aerosol
formation, as some studies point to the influence of anthropogenic emission
on secondary organic aerosol formation (). SOA ISO
seems to depend heavily on the conditions (aerosol acidity, NOx
concentrations and pre-existing aerosol) used to oxidize isoprene
(). NOx concentration
determines the pathway (low NOx and high NOx) followed by the
isoprene oxidation, leading to different secondary organic species
(, a); the low-NOx pathway is
∼ 5 times more efficient than the high-NOx pathway
. Experiments carried out by evidence how
isoprene SOA yield varies depending on NOx concentration, increasing
from no injected NOx, to a plateau between 100 and 300 ppb NOx,
and decreasing at higher NOx concentrations.
Contribution of the eight analysed organic groups to the Izaña OM
composition within the FT and the BL under the SAL (FT-SAL and BL-SAL) and
the WES (FT-WES, BL-WES, BBE). Average total OM for each air mass is at the top.
FT-PMT samples were collected during the night (22:00–06:00 GMT) and
BL-PM2.5 samples were collected during the day (10:00–16:00 GMT).
We found that the relation between SOA ISO and NO3- within the FT-SAL
(Fig. ) presents three tendencies which might be associated with
the ratio isoprene : NOx in the source. The different correlations are
supported by the fact that the SOA ISO markers (2-MTs and 2-MGA) do not
exhibit the same temporal trend (r=0.4, p < 0.05 within the
FT-SAL), which has been suggested to be linked to the NOx concentration
influence on these SOA ISO marker formation pathways . The
high-NOx pathway leads to the reaction of isoprene peroxy radicals
(iRO2) with NO resulting in carbonyl and hydroxynitrate production
, whereas the low-NOx pathway leads to the reaction of
iRO2 with hydroperoxy radicals (HO2) resulting in hydroxy hydroperoxide
(iROOH), and carbonyl production to a lesser extent (). This has
implications for the abundance of the secondary organic markers from isoprene
photo-oxidation (2-MT and 2-MGA): high-NOx pathway results in the major
product 2-MGA and low-NOx pathway in major products 2-MTs
.
Statistically significant correlations in the FT between biogenic secondary
organic compounds and NO3- (r SOA PIN–NO3- = 0.6,
p < 0.01; Table ) and nss-SO4= (r SOA
ISO–nss-SO4==0.6, p < 0.01; Table ), point to its
formation from the oxidation of their gaseous precursors NOx and SO2
respectively. Dust transformation in the FT is also evidenced by its
statistically significant correlation with SOA PIN (r=0.7,
p < 0.01; Table ) in addition to both saccharides (r=0.6,
p < 0.01) and hopanes (r=0.9, p < 0.01) showing that natural
and anthropogenic substances might be mixed after aging processes.
FT-PMT (night) and BL-PM2.5 (day) loadings and
scores of the three components from MCR-ALS resolved profiles. Filled markers
correspond to FT-PMT and open markers to BL-PM2.5. Grey
lines separate the compounds belonging to the different organic groups:
dicarboxylic acids, SOA PIN, SOA ISO, levoglucosan, saccharides, n-alkanes,
hopanes and PAHs (from left to right).
Fraction determined of OM
Bulk organic carbon (OC) determined for every single day (thermo-optical
transmittance method) at Izaña during this study was within the range
0.01–2.20 µg m-3, which is in line with that found in other
FT studies (e.g. 1.4 µg m-3 at Qomolangma, Mt Everest,
4276 m a.s.l., by and 4 µg m-3 at the NW Pacific,
2–6.5 km column by ). FT-PMT OC under SAL
conditions (0.77 µg m-3) was higher than under WES
(0.52 µg m-3, including the BBE) events. Figure
shows the mass closure (sum of the organic species determined by speciation)
of bulk organic matter OM (determined as OC ⋅ 1.8); this mass closure
accounts for 2–100 % of the OM for every single day, depending on the
sample and the airflows.
Concentrations of OM were much higher in the FT-SAL
(1.39 µg m-3) than under FT-WES (0.04 µg m-3
without the BBE; 0.94 µg m-3 including the BBE) conditions.
The selected tracers (levoglucosan, SOA ISO, SOA PIN, n-alkanes, saccharides,
dicarboxylic acids, hopanes and PAHs) represent the following on average:
15 % of the OM (Fig. ), under FT-SAL conditions
(when mean OM was 1.39 µg m-3). This fraction determined is mainly composed of SOA ISO (30 %), saccharides (27 %) and dicarboxylic acids (18 %).
84 % of the OM (Fig. ), in the FT-WES airflows without the BBE
(when mean OM was 0.04 µg m-3). This fraction determined is comprised
of dicarboxylic acids (44 %; mainly succinic and phthalic, indicating aged aerosols
after the long-range atmospheric transport) and SOA ISO (34 %), with a minor presence
of saccharides (8 %). Biogenic SOA represents an important fraction of the OM at
Izaña (∼ 40 %), as seen in other remote high-altitude regions .
3 % of the OM (Fig. ) in the FT-WES during the BBE (when mean
OM was 3.64 µg m-3). The fraction determined of OM for 28 August (3 %) contained 68 % of
dicarboxylic acids (mostly succinic and malic acids) and 8 % of the BB tracer levoglucosan. The
OM profile of this sample has the highest contribution of aged SOA (di-acids), formed during the long-range
atmospheric transport, and the lowest contributions of SOA PIN (4 %) and SOA ISO (12 %), which may indicate the further oxidation of these products under BBE air mass conditions.
64 % of the OM in the FT-WES including the BBE (when mean OM was 0.94 µg m-3).
This fraction determined is comprised mainly of dicarboxylic acids (50 %), and SOA ISO (28 %).
Time series of the total organic matter (OM; circle markers) and the
source contribution (SC; square markers) to the organic matter determined for
the FT-PMT (filled markets) and the BL-PM2.5 samples
(open markers). Sources: (a) biomass burning (BB),
(b) combustion (comb.) POA and (c) organic dust.
Differences in the fraction determined of the samples collected under the
FT-SAL and FT-WES influence – as observed in Fig. – is a result
of the method limitation, as the analysis of the samples by means of gas
chromatography mass spectrometry (GC-MS) covers a very small fraction (often
< 5 %; ) of the organic matter.
The OM composition determined within the BL (PM2.5; BL-FT: 9 % of
the OM; BL-WES: 77 % of the OM; Fig. ) remains almost the
same under both airflows, but with higher concentrations of SOA ISO and
n-alkanes under the SAL. However, due to the PM2.5 cut-off of the
collected particles, the influence of the dust-associated organic compound is
likely not well represented as these products are situated in the coarse
fraction of the PM.
Sources of OM
We used receptor modelling for apportionment of OM between the OA sources
traced by the species included in the speciation performed in this study.
This analysis is complementary to the mass closure performed above
(Fig. ). The data matrix was decomposed into two factors: loadings
(i.e. the relative amount of the chemical compounds in the source) and scores
(i.e. the relative contribution of the potential sources to the organic
aerosol) . The loading factors obtained in the MCR-ALS were
used to identify OA sources (Fig. a1–c1), whereas the
score factors were used as independent variables in the multi-linear
regression analysis (MLRA) to apportion the fraction determined of OM between
the identified sources. Three components (sources) were identified
(Fig. ), which accounted for 81 % of the total variance. MCR-ALS
method was also applied only to PMT samples to verify the influence of
PM2.5 in the final results; no significant differences were
observed as shown in Fig. S1 of the Supplement.
Biomass burning
The major component (accounting for 63 % of the total variance) is
associated with levoglucosan, dicarboxylic acids, phthalic acid, SOA PIN,
C24–C28 alkanes and hopanes, and PAHs to a lesser extent (Fig. a).
This factor represents biomass burning aerosols (BB). The peak event in the
score factor observed on 28 August (Fig. a2) is associated with the
episode of levoglucosan linked to the long-range transport of BB from North
America. SOA PIN indicates photochemical oxidation of biogenic volatile
organic compounds during the wild fire. The presence of short-chain
dicarboxylic acids, along with large amounts of malic acid, suggests the
effective oxidation of organic species to shorter di-acid chains during
long-range atmospheric transport. PAH contribution in this component is low,
despite potential emissions of PAHs during biomass burning. The low PAH
contributions may be the result of photochemical degradation during
long-range transport, which in turn could be related to the presence of
higher contributions of phthalic acid in this component. BB in the FT is
significantly correlated to the OM (r-FT = 0.40, p < 0.05;
Table S1) and EC (r-FT = 0.65, p < 0.01; Table S1) concentrations.
Combustion POA
The second component (accounting for 21 % of the total variance) is
associated with long-chain dicarboxylic acids, SOA ISO, C24–C29 n-alkanes
and PAHs (Fig. b). This factor is related to the primary organic
aerosols linked to combustion sources. This is the component that best
represents the variability of PM2.5 samples collected during
daylight, when the BL may reach Izaña under the slope wind regime. High
loadings of suberic (C8) and azelaic acids (C9) indicate the presence of
oxidized compounds in the early stage of photochemical transformation
processes, such as the ozonolysis of oleic acid . Organic
species supporting the anthropogenic contribution are lower-molecular-weight
n-alkanes (C24–C25) and PAHs (from incomplete combustion processes). FT
aerosol is also described by this component with the exception of
low-molecular-weight alkanes (C24–C25), which is the main feature of the BL
samples. This component is representative of the measured EC for all samples,
and representative for the BL, as shown by its statistically significant
correlation (r-All = 0.36 and r-BL = 0.71, p < 0.05; Table S1).
Organic dust
The third component, accounting for 16 % of the total variance, is
comprised of short-chain dicarboxylic acids, SOA ISO, saccharides, C26–C34
alkanes, hopanes, and PAHs to a lesser extent (Fig. c). This
component, identified as organic dust, is associated with soil re-suspension
as evidenced by the saccharides and mannitol high loadings. A major presence
of the soil OM-related compounds, those related to fungi and terrestrial
higher plants (C27, C29 and C31), suggests fresh and primary OA.
Notwithstanding, glutaric, adipic and pimelic acids indicate oxidation
products, suggesting the aging of the samples. As a consequence of this
aging, natural and anthropogenic markers are mixed in this component. The
biogenic influence is indicated by the presence of soil-related markers and
oxidation products from isoprene (2MGA, 2MT-1 and 2MT-2), whereas the
anthropogenic influence is well defined by the presence of hopanes (primary
vehicle emissions) and high molecular weight PAH (products of incomplete
combustion). The scores of this component display the highest statistically
significant correlation with dust (r-All = 0.84, p < 0.01;
Table S1) and OM (r-All = 0.64, p < 0.01; Table S1) concentrations
for all samples. Although this component is not relevant for the
BL-PM2.5 samples – because part of the compounds are present in
the larger particle size fractions – the correlation with dust (r-BL =
0.73, p < 0.01; Table S1) and OM (r-BL = 0.75, p < 0.01;
Table S1) are statistically significant due to the mixing of dust with the
anthropogenic compounds.
Contribution of the identified organic aerosol sources to the total
organic matter within the FT and the BL under the SAL (FT-SAL and BL-SAL) and
the WES (FT-WES, BL-WES, BBE). Average total OM for each air mass is at top.
FT-PMT samples were collected during the night (22:00–06:00 GMT) and
BL-PM2.5 samples were collected during the day (10:00–16:00 GMT).
Source apportionment of OM in the SAL and the westerlies
The source apportionment of OM was performed by the multi-linear regression
technique described above. The difference between the bulk OM (determined by
thermo-optical method) and the sum of the organic species (determined with
speciation) was labelled as undetermined fraction. Figure shows
the time series of the daily contribution of each source to the OM determined
and Fig. shows the average source contribution to total OM in the
aerosol samples collected in the different airstreams. The statistically
significant correlation between the sum of the three components scores and
the OM within the FT (OM r-FT = 0.63, p < 0.05; Table S1) indicates
that the identified sources might describe not only the fraction determined
of the OM but also the total OM. This significant correlation is not seen for
the BL (OM r-BL = 0.33, p < 0.05; Table S1), where there could be
additional sources.
In the FT-SAL airflow, most OM was undetermined (∼ 85,
Fig. ). The three identified sources, i.e. organic dust,
combustion POA and biomass burning, accounts for 8, 6 and 1 % of the bulk
OM, respectively (62, 34 and 4 % of the OM determined, respectively). The
presence of biogenic SOA products mixed with combustion POA was also found in
previous studies which suggested that biogenic SOA formation may be more
efficient in polluted atmospheres (,
and references therein).
In the FT-WES, the undetermined fraction accounts for ∼ 36 % of the
OM (Fig. ). The contribution of the three identified sources,
i.e. organic dust, combustion POA and biomass burning, is 22, 19 and 23 %
of the bulk OM, respectively (28, 23 and 49 % of the OM determined,
respectively). proposed biomass burning as a source of
saccharides in the OA and found that some saccharides resist
degradation in the atmosphere over a period of 10 days, being able to be
transported over long distances; this may be the source of the organic
fraction of dust we detected in the FT-WES.
For the BL samples, the dust-related component is not well represented
(Fig. ), because the coarse fraction was not sampled here. On the
other hand, combustion POA explains 6 and 41 % (Table S2) of the bulk OM
for the SAL and WES, respectively. Background regional fires also affect the
BL as described by the BB component, which represent 2 and 36 % (Table S2)
of the bulk OM for the SAL and WES, respectively.