Atmospheric particulate matter (PM) was fractionated in six aerodynamic
sizes,
Important seasonal differences were observed at the rural site. In the
Multivariate curve resolution/alternating least squares showed that these
organic aerosols essentially originated from six source components. Four of
them reflected primary emissions related to either natural products, e.g.,
vegetation emissions and upwhirled soil dust, or anthropogenic
contributions, e.g., combustion products and compounds related to urban
lifestyle activities like vehicular exhaust and tobacco smoking. Two
secondary organic aerosol components were identified. They accumulated in the
smallest (
Toxicologically relevant information was also disclosed with the present approach. Thus, the strong predominance of biomass burning residues at the rural site during the cold period involved atmospheric concentrations of polycyclic aromatic hydrocarbons that were 3 times higher than at the urban sites and benzo[a]pyrene concentrations above legal recommendations.
Atmospheric aerosols are comprised of particles with diameters between a few nanometers and tens of micrometers (Seinfeld and Pandis, 2006) and a high diversity of chemical compounds. These complex mixtures have an influence on atmospheric visibility (Watson, 2002), climate forcing (Forster et al., 2007) and human health (Pope III et al., 2002; Brunekreef and Forsberg, 2005). A significant but variable aerosol fraction is comprised of organic material, e.g., between 20 and 90 % of the particulate matter (PM) (Kanakidou et al., 2005). This organic aerosol (OA) originates from primary and secondary sources (Donahue et al., 2009). At urban locations the primary organic aerosols (POA) are emitted from combustion sources, including heavy and light duty vehicles, wood smoke, cooking activities, industries, soil and road dust. At rural areas, biomass burning, including wood burning, is an important primary aerosol source (Fine et al., 2004; Puxbaum et al., 2007) together with soil dust particles (Simoneit et al., 2004). Saccharides, as constituents of soil dust and vegetation detritus that are present in residues generated after biomass combustion, can make up an important part of the POA fraction (Simoneit et al., 2004; Medeiros and Simoneit, 2007).
In the absence of these saccharides, most of the water-soluble fraction is thought to derive from secondary organic aerosols (SOA) that are composed of oxygenated compounds, such as dicarboxylic acids (Hallquist et al., 2009). Although these acids are emitted in small quantities from traffic and vegetation, they are mostly formed in the atmosphere after photochemical transformation of volatile and semivolatile organic compounds from non-fossil, e.g., vegetation, and fossil, e.g., fossil fuel combustion residues, origins (Heald et al., 2010; Kleindienst et al., 2012; Paulot et al., 2011). The importance of oxidized organic compounds is emphasized by their strong contribution (40–90 %) to the total organic fraction in the fine PM (Jimenez et al., 2009).
Important information on sources, fate and mutual interaction of gas phase and aerosol organics has been obtained from filtration of large volumes of ambient air and analysis (Schauer et al., 2007; Goldstein and Galbally, 2007; Bi et al., 2008; Fu et al., 2010). The constituents of the organic primary and secondary aerosols are distributed among the wide aerodynamic size range of the constituent particles (Aceves and Grimalt, 1993a; Kavouras and Stephanou, 2002). The collection and analysis of particles in different size ranges provides insight into the sources and fate of the OA, which is useful for characterizing the different types of aerosols that may be found in diverse environments, either urban or rural. The combination of particle size filter techniques with gas chromatography–mass spectrometry (GC-MS) allows in-depth speciation that is useful for reconstructing the emissions from different sources (Schauer et al., 2007; Alier et al., 2013).
This approach is used in the present manuscript to characterize the size distribution of organic aerosol from an urban background site in Barcelona and a rural site in the Pyrenees during warm and cold periods (2012 and 2013) and to identify the similarities and differences of the OA generated in these sites. The urban study area of Barcelona is characterized by one of the highest vehicle densities in Europe as well as a densely populated city center. Moreover, its geographical position (western Mediterranean basin) favors photochemical reactions and accumulation of secondary aerosols (Querol et al., 2009; Pey et al., 2009; Pérez et al., 2010; Pandolfi et al., 2014).
The rural site, in a forested area of the Pyrenees, is exposed to biomass burning in the cold period for domestic heating. In other seasons, such as fall, biomass particles could be generated from biogenic waste combustion in fields and gardens (van Drooge and Pérez-Ballesta, 2009). Air quality in this rural site is not influenced by industrial activities and traffic intensity in the area is very low. Due to its geographical situation (surrounded by mountains), the site is prone to thermal inversion episodes, especially in the cold periods.
The study of the main sources contributing to the GC-amenable organic compounds of the atmospheric aerosol size fractions generated in cold and warm weather at these sites affords a combined physical–chemical description of the changes in organic constituents from the two most typical areas inhabited by humans. Multivariate curve resolution/alternating least squares (MCR-ALS) (Tauler et al., 1995; Tauler, 1995) has been used for source apportionment in the present study. This method, previously used for source apportionment of aquatic pollutants (Terrado et al., 2009) and urban OA (Alier et al., 2013), is based on an alternating linear least squares optimization under non-negativity constraints that generates source components with better physical sense than principal component analysis (PCA; Tauler et al., 2009). The database of the analyzed organic tracer compounds has been used as input for these calculations, allowing the identification of similarities and differences between locations. The results will be useful for gaining insight into the processes of aerosol formation and into the pervasiveness of different compounds in the human respiratory tract.
Filters were analyzed by solvent extraction and subsequent gas chromatography coupled to mass spectrometry. This method allows detection and quantification of a wide range of semivolatile organic compounds with different polarities. Seventy-two compounds have been detected in ambient air and emission sources, and therefore many of them have been used as molecular tracers of these sources and atmospheric processes (Alier et al., 2013).
A six stage (including back-up filter) Anderson cascade impactor was used to
collect atmospheric particles in different sizes (
Location of the urban background sampling site in Barcelona and the rural sampling site in La Pobla de Lillet in the Pyrenees.
The meteorological conditions were determined from local meteorological
stations. The samples were divided into those belonging to “warm” and
“cold” periods (see Table S1 in the Supplement). Temperature was the
meteorological parameter with the largest difference between the urban and
rural sites. In the former, the average temperatures were 20 and
4
Before sampling, all filters were baked at 450
Anhydrosaccharides, acids, polyols and nicotine were analyzed following
procedures similar to those described elsewhere (Medeiros and
Simoneit, 2007; van Drooge et al., 2012). Briefly, an aliquot of the extract
(25
Polycyclic aromatic hydrocarbons (PAHs), hopanes, n-alkanes and quinones were analyzed in the remaining extract,
which
was evaporated to almost dryness under a gentle nitrogen stream and
redissolved in 0.5 mL (9 : 1
The sample extracts were injected into a Thermo GC-MS (Thermo Trace GC Ultra
– DSQ II) equipped with a 60 m fused capillary column (HP-5MS,
0.25 mm
Table S2, in the Supplement, contains a list of quantified
molecular organic tracer compounds. Besides retention time comparison,
levoglucosan, mannosan,
PAHs were identified by retention time comparison of the peaks generated with
the following ions: phenanthrene (
In all cases the recoveries of the surrogate standards were higher than
70 %. Field blanks were between 1 and 30 % of the sample
concentrations. Reported data were corrected for blank levels. The limits of
quantification (LOQ) were calculated by dividing the lowest measured levels
in the standard calibration curves by the volumes of the analyzed sample
fraction. These were 0.02 ng m
In order to observe the similarities and differences between the studied locations, the experimental data were merged for evaluation with MCR-ALS. The joint data set was imported into MATLAB 7.4 (MathWorks, Natick, USA) for subsequent calculations using MATLAB PLS 5.8 Toolbox (Eigenvector Research Inc., Manson, WA, USA). The MCR-ALS method had been applied successfully in a previous study on urban organic aerosol. A detailed description of these results can be found in Alier et al. (2013). Briefly, the MCR-ALS method decomposes the data matrix using an alternating least squares algorithm under a set of constraints such as non-negativity, unimodality, closure, trilinearity or selectivity (Tauler et al., 1995; Tauler, 1995). The explained variance by the different components is similar to a PCA; however, it is not orthogonal as in PCA (Jolliffe, 2002). Since the natural sources in the environment are rarely orthogonal, the MCR-ALS method provides more realistic descriptions of the components than the orthogonal database decomposition methods.
Mean and standard deviation (
PAH are toxic components of fossil fuels and primary products of incomplete
combustion of organic materials (Iinuma et al., 2007; Rogge et al., 1993;
Schauer et al., 2007) and they were found in all samples. Significant
concentration differences were observed among particle size fractions and
sampling periods. Overall, more than 70 % of the sum of all quantified
PAH were present in the fraction
At the urban site, the average
The assignment of PAH to wood burning in this site is consistent with the high
concentrations of retene at the rural site of the present study in the cold
period (3.2
Relative composition of PAHs to
The relative composition of PAH is useful to discriminate between the
combustion processes at the rural and urban site. The isomeric ratios
fluoranthene vs. pyrene (fla
Isomeric PAH ratios for the different period (warm vs. cold) in the rural (R) and urban (U) sites. fla is fluoranthene; pyr is pyrene; baa is benz[a]anthracene; chry is chrysene; bap is benzo[a]pyrene; bep is benzo[e]pyrene; ip is indeno[123cd]pyrene; bgp is benzo[ghi]perylene.
Quinones, with toxic potential, are released into the atmosphere along with
PAH during incomplete combustion (Ramdahl, 1983a; Iinuma et al., 2007;
Valavanidis et al., 2006). Atmospheric transformation of PAHs can also
generate quinones through a reaction with atmospheric oxidants (Alam et al., 2014;
Atkinson and Arey, 2007). Quinones were found in all sample fractions but
more than 68 % of the sum of quinones were observed in the finest
In the cold period, the
These hydrocarbons are molecular markers of mineral oils, whose occurrence in
atmospheric samples can be related to unburned lubricating oil residues from
primary vehicle emissions (Rogge et al., 1993; Schauer et al., 2007). The
compounds selected for quantification were 17
C
At the rural site, during the warm period the n-alkanes were evenly
distributed among all particle sizes (Table S3), with predominance of
n-C
Relative composition of n-alkanes to
The odd-to-even n-alkane carbon preference index (CPI) is another indicator
of biogenic or combustion contributions, where CPI
This alkaloid is present in high concentrations in tobacco smoke. It is
mainly present in the gas phase due to its high volatility but it can also be
detected at trace levels on PM filter samples (Rogge et al., 1994; Bi et al.,
2005). In the area of Barcelona, this compound has recently been found in PM
representing anthropogenic contributions (Alier et al., 2013). In the present
study, nicotine was mostly found in the fraction
These monosaccharide anhydrides are generated by the thermal alteration of
cellulose and hemicellulose that are emitted in large quantities during
biomass burning (Simoneit, 2002; Fine et al., 2004). Levoglucosan and its
isomers, galactosan and mannosan, were found in all samples. Major
differences were found between sites and sampling periods (Table S3). These
compounds predominated in the fraction
At the urban site, sucrose was the most abundant saccharide (
In the cold period, the concentrations of saccharides and polyols in the
different fractions decreased by 1 order of magnitude, which is consistent
with the seasonal decrease of biological activities. An exception was
observed for the fraction
C
The C
Dehydroabietic acid, a resin acid, was found in all samples. The highest
average concentrations, 470 ng m
The formation mechanisms of these compounds is poorly understood. They are
emitted from various primary sources (mobile emission, meat cooking, etc.)
although photochemical processes have often been attributed to their
occurrence in atmospheric samples (Jang and McDow, 1997; Kerminen et al.,
2000; Heald et al., 2010; Sheesley et al., 2010; Paulot et al., 2011).
Dicarboxylic acids were found in all samples (Table S3) with predominance in
the finest
The applied analytical methodology – the use of BSFTA
In some studies, the relative distributions of oxalic, malonic and succinic
acids have been observed to follow the same seasonal pattern (Kawamura and
Ikushima, 1993). Good correlations between the concentrations of oxalic,
malonic and succinic acids in urban aerosols have also been observed (Ho et
al., 2010), which has been explained by the transformation of longer into shorter
carbon chain acids (Kawamura and Ikushima, 1993). These antecedents suggest
that the changes observed for succinic acid in the present study could also
be reflected in the non-determined C
In all cases, the average concentrations of the aliphatic dicarboxylic acids
in the rural site were higher than those in the urban site (Table S3). The
average concentration of glutaric acid in the rural site of the present
study, 9.1–21 ng m
Azelaic acid is an oxidation product of unsaturated fatty acids having the
double bond at position C-9 from the carbonyl (Kawamura and Gagosian, 1987).
As observed in Table S3, both in the urban and rural samples and in the warm
and cold seasons, the concentrations of this dicarboxylic acid are higher
than those of other homologues of similar carbon chain length, e.g., C
The average total concentrations of aliphatic dicarboxylic acids in the rural
site were 130 and 210 ng m
Malic acid is a presumed product of the OH oxidation of succinic acid
(Kawamura and Ikushima, 1993) as a consequence of photochemical aging. This is
consistent with the observed distributions of dicarboxylic acids in the rural
site. The average concentration of malic acid in the warm period was
99 ng m
Phthalic acid esters are used as plasticizers in resins and polymers. They
can be released into the air by evaporation because they are not chemically
bonded. Higher phthalic acid concentrations have been observed in summer
because of the higher ambient temperatures (Ho et al., 2010). However, these
compounds may also originate from combustion (Kawamura and Kaplan, 1987) or
atmospheric oxidation of aromatic hydrocarbons (Kawamura and Ikushima,
1993; Kawamura and Yasui, 2005). The average phthalic acid concentration of
the rural site was much higher in winter (33 ng m
The average concentrations of terephthalic acid were always higher in the
cold period (180 and 170 ng m
Cis-pinonic acid, pinic acid, 3-hydroxyglutaric acid and
3-methyl-1,2,3-butanetricarboxylic acid are related to the
photochemical oxidation of biogenic volatile
The further generation oxidation products, 3-hydroxyglutaric acid and MBTCA,
were only found in the fraction
Oxidation of
C
Loading of the six components from MCR-ALS resolved profiles for the organic compound composition.
These concentrations in the warm period were much higher than those observed
in the cold period at the rural and urban sites, where the compound of
highest concentration, 2-methylerythritol, ranged between 5 and
10 ng m
MCR-ALS allowed the identification similarities and differences of the OA constituents in the sampled sites and periods. Six components were identified from the use of this multivariate method. These components covered 93 % of the total variance of the concentrations of these compounds and the score values (see loadings in Fig. 5a–f) provided a description of the contributions of the different potential OA sources (Fig. 6).
This was the dominant component and explains 40 % of the total variance.
It was composed of primary biomass burning tracers, such as
anhydrosaccharides, dehydroabietic acid, PAHs, quinones, C
The presence of levoglucosan and its isomers, galactosan and mannosan, in
this component is consistent with these contributions from biomass burning,
including biomass waste from fields and gardens, as well as wood. However,
the presence of dehydroabietic acid and retene indicates
contributions of pine wood combustion. At the rural site considered for
study, pine wood from the Scots pine (
Although the difference between biomass waste burning and wood combustion could not be resolved in the augmented data set (joint urban and rural data set), a separate MCR-ALS analysis of the rural data set did identify it in a six-component resolution (Fig. S1 in the Supplement). Levoglucosan and its isomers were present in the loadings of both components (Fig. S1.1), while in one of the components (red) dehydroabietic acid was less dominant and retene was missing. Retene was entirely present in the other (blue) component. The score values of these two components (Fig. S1.2) show that both components are represented in the fall samples (R_COLD_1.x), while only one (blue) is represented in the winter sample (R_COLD_2.x). These findings suggest that about 50 % of the biomass burning in the fall could be contributed by wood burning, probably from domestic heating, while another source, probably biomass waste burning, could contribute to the other 50 %. In agreement with the previous statements, in winter the biomass burning component was dominated by (pine) wood combustion.
The average
At the urban area, the presence of levoglucosan in the samples from the cold
period, with a moderate average value of 160 ng m
Carboxylic acids, high-molecular-weight n-alkanes with high CPI and sucrose
are the constituents of the component that represents biogenic primary
sources, e.g., plant tissue particles, at the urban area (20 % of the
total variance; Fig. 5b). This component is mainly present in the coarse
fractions between
Hopanes, nicotine, carboxylic acids, low-molecular-weight PAHs and small
contributions of dicarboxylic acids constitute a third component involving
12 % of the variance (Fig. 5c).This component is essentially found in the
fraction
Another component is grouping several secondary organic compounds of biogenic
origin such as malic acid, 3-hydroxyglutaric acid, MBTCA, pinic acid,
C
Adipic, cis-pinonic, phthalic and terephthalic acids and, to smaller extend,
high-molecular-weight n-alkanes group together in another component of
chemically modified organic aerosol products (9 % of variance; Fig. 5f).
The presence of the dicarboxylic acids and also phthalic acid suggests an
origin related to secondary aerosol formation. cis-Pinonic acid is also a
first-generation product of
This constituent essentially occurs in the size fractions between 0.5 and
1.5
The score values are highest at the urban site with the exception of the rural sample collected in the fall, when biomass waste burning was contributing substantially to the overall biomass combustion. In this period all PM fractions showed high score values and high concentrations of phthalic and terephthalic acid. The origin of these compounds during that event is not clear but could be related to thermal stripping during combustion. In previous studies, terephthalic acid was related to the combustion of plastic (Kawamura and Pavuluri, 2010) in the presence of 1,3,5-triphenylbenzene, a typical organic tracer for plastic combustion (Fu et al., 2010; Simoneit et al., 2005). However, this latter compound has not been found in the aerosols of the present study. Another source for phthalic acid could be combustion or atmospheric oxidation of aromatic hydrocarbons (Kawamura and Kaplan, 1987; Kawamura and Ikushima, 1993; Kawamura and Yasui, 2005), which is consistent with the high concentrations of these compounds in the cold period samples collected in the rural site under intense biomass combustion.
The analysis of the concentrations of 72 organic compounds present in six size fractions of urban and rural aerosols from Mediterranean areas has allowed the identification of the main organic aerosol source constituents and the description of their particle size distribution. The six main components identified exhibit strong particle size, seasonal and geographical dependences (Figs. 6 and 7).
Relative scores of the components (%) in rural and urban site in relation to the warm and cold period: (1) combustion POA, (2) vegetation POA, (3) urban POA, (4) aged SOA, (5) soil POA and (6) fresh SOA.
The main component identified in the present study is related to combustion
sources. It involves 40 % of the total variance and is essentially
represented in the aerosols collected in the cold period, with a dominating
presence in the smaller PM
In contrast, biomass burning is only a minor contributor to the organic
aerosol in the warm period. Then, the PM
In the present study, the third component (12 % of the variance)
constituted nicotine, hopanes and low-molecular-weight PAH, corresponding
to tobacco smoke and vehicular traffic emissions. This component was
essentially found at the urban particles in the smallest PM
According to these results, the organic composition of the smallest size
fraction (
The particles
Technical assistance from R. Chaler and D. Fanjul is acknowledged. The meteorological data of La Pobla de Lillet were supplied by Albert Fajula. This work was supported by the scientific research projects AERTRANS (CTQ2009-14777-C02-01) and TEAPARTICLE (CGL2011-29621). Edited by: S. A. Nizkorodov