Nitro-monoaromatic hydrocarbons (NMAHs), such as nitrocatechols,
nitrophenols and nitrosalicylic acids, are important constituents of
atmospheric particulate matter (PM) water-soluble organic carbon (WSOC) and
humic-like substances (HULIS). Nitrated and oxygenated derivatives of
polycyclic aromatic hydrocarbons (NPAHs and OPAHs) are toxic and ubiquitous in
the ambient air; due to their light absorption properties, together with
NMAHs, they are part of aerosol brown carbon (BrC). We investigated the
winter concentrations of these substance classes in size-resolved PM from
two urban sites in central and southern Europe, i.e. Mainz (MZ), Germany, and
Thessaloniki (TK), Greece. The total concentration of 11 NMAHs (
Atmospheric humic-like substances (HULIS) represent a complex mixture of aliphatic and aromatic compounds with multiple functional groups, such as hydroxyl, carbonyl, carboxyl, nitro, nitrooxy, and sulfate groups (Havers et al., 1998; Graber and Rudich, 2006; Hallquist et al., 2009; Claeys et al., 2012). They are a major constituent of aerosol water-soluble organic carbon (WSOC), contributing between 9 % and 72 % to WSOC mass (Decesari et al., 2000; Graber and Rudich, 2006; Lin et al., 2010; Zheng et al., 2013). The distribution of HULIS molecular weights (MWs) is unimodal and ranges between 100 and 500 Da, with most of the compounds grouping around 200 Da (Graber and Rudich, 2006; Claeys et al., 2012; Song et al., 2018), unlike soil humic and fulvic acids with MW distributions extending well beyond 1000 Da. Due to the presence of light-absorbing polyconjugated and aromatic compounds (Duarte et al., 2005; Graber and Rudich, 2006; Claeys et al., 2012; Zheng et al., 2013), HULIS are an important constituent of aerosol water-soluble brown carbon (BrC; Laskin et al., 2015, and references therein). The intense light absorption of HULIS in the ultraviolet, violet, and blue visible regions, between 200 and 500 nm, can affect aerosol optical properties and atmospheric photochemical processes (Andreae and Gelencsér, 2006). Owing to the presence of highly polar polyfunctional material, HULIS have surface-active properties and can make aerosols act as cloud condensation nuclei (CCN). In the aerosol aqueous phase, HULIS can increase the solubility of hydrophobic organic compounds and change the reactivity and solubility of metal aerosols, owing to metal-complexation properties (Graber and Rudich, 2006). Finally, due to the presence of redox-active moieties, HULIS can catalyse electron transfer reactions and formation of reactive oxygen species (ROS), which could pose oxidative stress in humans upon inhalation (Verma et al., 2015).
Biomass burning (BB) is considered as one of the main sources of HULIS in the atmosphere (Lin et al., 2010; Claeys et al., 2012; Pavlovic and Hopke, 2012; Zheng et al., 2013) and an important source of aerosol nitroaromatic compounds (Claeys et al., 2012; Song et al., 2018). Recent studies found that nitro-monoaromatic hydrocarbons (NMAHs), such as 4-nitrocatechol (4-NC; MW: 155 Da) and isomeric methyl-nitrocatechols (MNCs; MW: 169 Da), are abundant constituents of particulate matter (PM) HULIS, originating from BB (Claeys et al., 2012; Song et al., 2018).
NMAHs are emitted into the atmosphere by primary and secondary processes.
4-NC, MNCs, nitroguaiacols (NGs) and nitrosalicylic acids (NSAs) are
predominantly formed by secondary oxidation of lignin thermal decomposition
products (e.g. m-cresol, phenols, methoxyphenols, catechols, salicylic acid)
in the gas and aqueous phases (Iinuma et al., 2010; Kelly et al., 2010;
Kroflič et al., 2015; Frka et al., 2016; Teich et al., 2017; Finewax et
al., 2018; Xie et al., 2017; Wang et al., 2019). Therefore, the
yellow-coloured water-soluble 4-NC and MNCs have been proposed as suitable
tracers for highly oxidized secondary BB aerosols (Iinuma et al., 2010;
Kitanovski et al., 2012b; Kahnt et al., 2013; Caumo et al., 2016; Chow et
al., 2016). In the past decade, the ambient PM nitrocatechols (NCs) have
been measured in several studies worldwide, i.e. in Europe (Iinuma et al.,
2010; Zhang et al., 2010; Kitanovski et al., 2012b; Kahnt et al., 2013; Mohr
et al., 2013; Teich et al., 2014; Frka et al., 2016), South America (Claeys
et al., 2012; Caumo et al., 2016), North America (al-Naiema and Stone,
2017), Asia (Chow et al., 2016; Li et al., 2016; Wang et al., 2019), and
Australia (Iinuma et al., 2016). They represent a significant fraction of
the PM organic carbon (OC), e.g. 0.8 % in winter PM
Polycyclic aromatic hydrocarbons (PAHs) and their nitrated and oxygenated
derivatives (NPAHs and OPAHs), as well as hydroxy derivatives (OH-PAHs), are
ubiquitous in the atmosphere (Walgraeve et al., 2010; Lammel, 2015; Bandowe
and Meusel, 2017; Shahpoury et al., 2018). They are primarily emitted from the
incomplete combustion of fossil fuels (Zielinska et al., 2004; Karavalakis
et al., 2010; Pham et al., 2013; Inomata et al., 2015) and wood, coal, and
biomass burning (Ding et al., 2012; Shen et al., 2012, 2013a, b;
Huang et al., 2014; Vicente et al., 2016). The PAH derivatives are
secondarily formed by the reaction of parent PAHs with atmospheric oxidants
such as OH,
NMAHs, PAHs, NPAHs and OPAHs significantly contribute to the aerosol BrC due to their light absorption capacity in the ultraviolet (UV) and visible range (Mohr et al., 2013; Samburova et al., 2016; Teich et al., 2017; Xie et al., 2017; Huang et al., 2018). Determining the size-resolved mass distribution of the PM molecular tracers is important for assessing the particle emission sources, atmospheric transport, and health effects (Neusüss et al., 2000). In particular, there is limited knowledge about the size-resolved characteristics of NMAHs, NPAHs and OPAHs and their relation to atmospheric HULIS (Claeys et al., 2012; Song et al., 2018). Therefore, the aim of the present work is to fill this gap by studying the size-resolved PM from polluted urban air at two locations in central and southern Europe, i.e. Mainz (MZ), Germany, and Thessaloniki (TK), Greece, and to apply these data to determine the possible emission sources. These sites were selected to reflect the dominant emission sources in the study areas – while TK is a biomass burning hotspot in southeastern Europe (Saffari et al., 2013; Velali et al., 2019), MZ in central Europe is dominated by traffic emission and long-range transport (Winkler and Junge, 1972; Wesp et al., 2000; Dusek et al., 2006).
Size-segregated wintertime (season of 2015–2016) PM samples were collected at
MZ and TK. In this period, the emissions influencing the sample sites are
very different, and, in terms of temperature changes and synoptically, the
sampling period is characterized by southwesterly advection with moderate
winds at MZ and weak southerly or northeasterly winds at TK (Saffari et
al., 2013; Voliotis et al., 2017). At MZ (
All PM samples were collected using a five-stage high-volume cascade impactor
with effective cutoff diameters: 0.49, 0.95, 1.5, 3, and 7.2
Sampling details.
Extraction of the filter samples for NMAH analysis was done using a
validated procedure (Kitanovski et al., 2012b) with small modifications.
Briefly, a 1.5 cm
The NMAHs were determined using an Agilent 1200 Series high-performance liquid chromatography (HPLC) system (Agilent
Technologies, Waldbronn, Germany) coupled to an Agilent 6130B Series single
quadrupole mass spectrometer equipped with an electrospray ionization (ESI)
source. High-purity nitrogen was used as a nebulizer and drying gas. The
separation of the targeted analytes was done on an Atlantis T3 column (150 mm
Analytes targeted in this study.
Q1:
NPAHs and OPAHs were extracted from PM samples following a QuEChERS (quick, easy, cheap, effective, rugged, and safe) method with
slight modifications (Albinet et al., 2014; Shahpoury et al., 2018).
Briefly, two strips of each filter paper were placed inside a glass
centrifuge tube (Duran, Schott, Mainz, Germany) and spiked with a mixture of
internal standards containing 60 ng of 1-nitronaphthalene-d
The purified extracts containing the analytes were concentrated to 0.5 mL,
and the solvent was exchanged by adding 5 mL of ethyl acetate, concentrating
the solution to 0.5 mL, and the process was repeated three times. The sample
volumes were adjusted to 0.3 mL and transferred to 2 mL vials containing 0.4 mL glass inserts. All solvents used for NPAH and OPAH analysis were high-purity
(Suprasolv, GC-MS grade; Merck, Darmstadt, Germany). All glassware used for
analysis was pre-washed with lab-grade detergent, tap water and deionized
water and baked at 310
The samples were analysed using a Trace 1310 gas chromatograph (GC; Thermo
Scientific, Waltham, Massachusetts, USA) interfaced to a TSQ8000 Evo triple quadrupole
mass selective detector (MS/MS; Thermo Scientific). The analysis was
performed in negative chemical ionization with methane used as an ionization
gas (1.5 mL min
Field blanks (
From the 11 targeted NMAHs, 8 were consistently detected in size-segregated
PM from MZ and TK. 4-NG and DNOC were not detected in MZ samples, while
they were sporadically detected in the coarse PM (
In Table S3, one can easily notice the consistently higher (
Correlation analysis for NMAHs in MZ samples presents a different picture
(Table S7). Significant correlations (
NPAHs and OPAHs were studied in size-resolved PM at both the MZ and TK sites. At both
sites, particle-phase OPAHs were detected more frequently than NPAHs: seven
out of eight OPAHs targeted for analysis were detected in nearly all MZ and
TK samples (Table S3; Figs. S3 and S4 in the Supplement). In contrast, only 8 out of
17 targeted NPAHs were found in the PM samples, of which only
1-nitronaphthalene (1-NNAP), 9-nitroanthracene (9-NANT), 2-NFLT, and
7-nitrobenz(a)anthracene (7-NBAA) were detected in both MZ and TK samples.
Interestingly, 3-nitrophenanthrene (3-NPHE), 3-NFLT, and 1- and 2-NPYR were
only found in TK samples. This was not due to differences in individual LOQs
between the two sites (see Table S2). The mean concentrations of NPAHs in PM
were dominated by 9-NANT followed by 2-NFLT and 7-NBAA at both sites (Figs. 1 and 2, Table S3), with concentrations reaching to 225, 154, and 71 pg m
Mass size distributions (MSDs) of PM-bound NMAHs, NPAHs, OPAHs, WSOC,
HULIS, and ions in Mainz (Germany). The error bars represent standard
deviations.
Mass size distributions (MSDs) of PM-bound NMAHs, NPAHs, OPAHs, WSOC,
HULIS, and ions in Thessaloniki (Greece). The error bars represent standard
deviations.
Overall, all NPAHs and OPAHs showed considerably higher concentrations in TK than in
MZ samples.
NPAHs and OPAHs were predominant in the sub-micrometre PM fraction (PM
MSDs of NMAHs over the two sampling locations are given in Figs. 1 and 2.
NSAs (3-NSA and 5-NSA) and NCs (4-NC, 4-M-5-NC, 3-M-5-NC, and 3-M-4-NC)
showed unimodal distributions with MSDs generally peaking in the finest PM
fraction (PM
Nitrophenols (i.e. 4-NP, 2-M-4-NP, and 3-M-4-NP) showed bimodal distributions
with a dominant peak in the finest fraction (PM
The MMD of NMAHs was 0.10
NPAH and OPAH MSDs are shown in Figs. 1 and 2. On average, the MMDs of NPAHs were
0.06
In terms of the inter-site variability of the target substance MSDs, the
size fraction PM
Because of their water solubility, NMAHs are constituents of PM HULIS and
WSOC (Claeys et al., 2012; Teich et al., 2017). This substance class
contributed
Mean absolute concentrations and mass mixing ratios (in brackets)
of HULIS
Our reported NMAH contribution to HULIS mass is in good agreement with the
results of previous reports from urban sites in Europe (Kitanovski et al.,
2012b; Claeys et al., 2012) and Brazil (Caumo et al., 2016). Specifically,
Kitanovski et al. (2012b) found that NMAHs contributed 0.4 %–1.3 % to the OC
mass in winter PM
In Sect. 3.2.1, we emphasized the similar MSDs at both locations between
HULIS on one side and NCs and NSAs on the other. These two NMAH subclasses
on average contributed to
With mass mixing ratios of the order of 1 %, NMAHs are constituents of HULIS with limited significance by mass, but their relevance is more significant due to their optical properties (Mohr et al., 2013; Laskin et al., 2015; Teich et al., 2017; Xie et al., 2017). Teich et al. (2017) found that the mass contributions of total NMAHs (NPs and NSAs) to WSOC on average was five times lower than their contribution to the light absorption of the aqueous PM extract at 370 nm (Teich et al., 2017). This implies that even small fractions of chromophoric HULIS compounds such as NMAHs can have an excessive influence on the aerosol light absorption (Mohr et al., 2013; Teich et al., 2017) and the atmospheric photochemical processes, especially in polluted areas (Laskin et al., 2015; Teich et al., 2017).
We studied the composition and MSDs of NMAHs, NPAHs, and OPAHs in PM from urban locations in Germany and Greece, with some of the target substances (i.e. NSAs, MNCs, and MNPs) studied in size-resolved PM for the first time. At both locations, NCs were the most abundant NMAH species, and OPAHs were more abundant and more frequently detected than NPAHs. The total concentrations of the most abundant NMAHs, NCs, NPAHs, and OPAHs were up to 10 times higher in TK than in MZ. Correlation analysis of NMAHs revealed distinct features among the sites, suggesting mixed air masses influenced by fresh BB and fossil fuel combustion emissions at TK and aged advected air influenced by combustion emissions (i.e. BB and coal combustion) at MZ.
The MSDs of NMAHs, OPAHs, and NPAHs were rather similar, but they exhibited
temporal and spatial variations due to daily changes in atmospheric
conditions and different sources. In general, NCs, NSAs, OPAHs, and NPAHs
showed unimodal MSDs peaking in the finest PM fraction, PM
The dataset used in this paper is included in the Supplement, and further information is available from the corresponding authors (z.kitanovski@mpic.de; p.shahpoury@mpic.de).
The supplement related to this article is available online at:
GL and CS conceived the study. PS and AV conducted the air sampling and field measurements. ZK and PS did the chemical analysis of samples. ZK, PS, and GL did the data analysis. ZK, PS, and GL discussed the results and wrote the paper with input from all co-authors.
The authors declare that they have no conflict of interest.
We thank Eleni Papakosta (prefecture of Thessaloniki), Thorsten Hoffmann, and Anna Honcza (Max Planck Institute for Chemistry) for on-site and laboratory support. This research was supported by the Max Planck Society and the postgraduate programme “Environmental Chemistry and Pollution Control” of the Aristotle University of Thessaloniki.
The article processing charges for this open-access publication were covered by the Max Planck Society.
This paper was edited by Alexander Laskin and reviewed by two anonymous referees.