Introduction
VOCs play an important role in atmospheric chemistry. Their reactions with
hydroxyl (OH) radicals, ozone (O3), and nitrate (NO3) radicals
produce secondary gas- and particulate-phase species (Atkinson, 2000;
Hallquist et al., 2009). Many of the VOCs present in the atmosphere have
adverse effects on human health (Kampa end Castanas, 2008) ,with exposure to
compounds such as benzene, formaldehyde, and acetaldehyde having been related
to cancer (Flesca et al., 1999). The European Commission has established an
average annual limit of 1.5 ppb for benzene.
VOCs may originate from both natural and anthropogenic sources. Vegetation
(Kesselmeier and Staudt, 1999; Goldstein and Galbally, 2007), volcanoes
(Tassi et al., 2009, 2013), and marine emissions from the decay of organic
matter (Kettle and Adreade, 2000; Meskhidze and Nenes, 2006; Colomb et al.,
2008) are some of the natural sources that contribute significantly to the
global VOC budget. Anthropogenic emissions originate mainly from the use and
production of fossil fuels, industrial processes, and biofuel combustion.
Biomass burning also contributes substantially both at the regional and
global scales (Crutzen and Andreae, 1990; Karl et al., 2007; Koppmann, 2007).
Online measurement techniques with high resolution can provide valuable
information about the sources of atmospheric pollutants (Goldstein and
Schade, 2000; Millet et al., 2005; Slowik et al., 2010). The proton transfer reaction mass spectrometer (PTR-MS),
developed by Lindinger et al. (1998), can continuously measure the levels of
a range of VOCs (DeGouw and Warneke, 2007).
Several campaigns focusing on VOC levels have been conducted in the
Mediterranean Basin. Holzinger et al. (2005) reported secondary production of
methanol and acetone in biomass burning plumes over the eastern Mediterranean
during the MINOS campaign. PTR-MS measurements during summer in the
background site of Finokalia (Crete, Greece) (Salisbury et al., 2003)
indicated methanol, acetone, acetonitrile, benzene, and toluene
concentrations of 3.3–6.1, 2.9–4.5, 0.15–0.44, 0.13–0.38, and
0.04–0.08 ppb respectively. Davison et al. (2009) studied the concentration
and fluxes of biogenic VOCs in a Mediterranean ecosystem in western Italy
during May–June 2007. Median concentrations of 1.6–3.5 ppb for methanol,
0.4–1.3 ppb for acetaldehyde, 1.0–2.0 ppb for acetone, 0.1–0.14 ppb for
isoprene, and 0.2–0.3 ppb for the monoterpenes were reported. Seco et
al. (2011) compared the summer and winter VOC concentrations in a forest site
50 km away from Barcelona. Aromatic VOCs showed small variations between the
two periods, while biogenic VOCs (bVOCs) and oxygenated VOCs (oVOCs) were
elevated during summer, mainly due to higher physiological activity and
faster photochemistry.
In addition, real-time measurements have been reported for urban or
semi-urban environments in the Mediterranean. Filella and Penuelas (2006)
studied the sources and variations of VOCs by PTR-MS at a semi-urban site in
Barcelona. Aromatic species such as toluene and benzene had diurnal patterns
typical of traffic intensity. Their concentrations were higher during
December and March. Biogenic species such as isoprene and monoterpenes had
variable diurnal profiles during the four measurement periods. The isoprene
diurnal cycle suggested considerable contribution from anthropogenic
sources. Monoterpenes had higher concentrations at night, decreasing in the
morning until midday and increasing again in the evening. For some periods a
peak also occurred during morning hours (06:00–09:00 LT), which was
attributed to temperature-dependent emissions. Methanol, acetone, and
acetaldehyde levels were influenced by biogenic sources during summer. In a
study at two sites in the city of Athens, Rappenglück et al. (1998) monitored
VOCs continuously over a 30-day late-summer period in 1994 by gas
chromatography (GC). The C4–C12 hydrocarbons measured were
strongly related to traffic emissions.
Source apportionment analysis has been applied to VOC datasets in order to
quantify the contributions of the different VOC sources (Millet et al., 2005;
Brown et al., 2007; Vlasenko et al., 2009; Yuan et al., 2012). Slowik et
al. (2010) performed PMF analysis of a PTR-MS dataset as well as a unified
AMS/PTR-MS dataset over a 2-week period in Toronto Canada. PTR-MS data
included 10 mass-to-charge ratios (m/z). Factors related to traffic,
long-range transport, local oxidation, and other sources were reported.
Crippa et al. (2013) also performed PMF analysis on an AMS/PTR-MS unified
dataset for the city of Paris during the MEGAPOLI project (summer 2009 and
winter 2010 campaigns). The resulting common factors associated a large
percentage of the aromatic VOCs with the hydrocarbon-like organic aerosol
(HOA) during summer, while isoprene and the monoterpenes were mainly related
to the semi-volatile oxygenated organic aerosol (SV-OOA). For the winter
period wood burning also contributed to the levels of the above species. Yuan
et al. (2012) reported that reactions interfere with the PMF analysis of
VOCs, proposing that one source can yield several factors at different stages
of photochemical processing.
Despite the previous efforts, little is known concerning the current VOC
sources in urban areas of the eastern Mediterranean. Elevated PM
concentrations have been detected in many urban areas (Pikridas et al.,
2013), but the corresponding VOC emission and concentrations have not been
quantified. Enhanced sunlight intensity, higher temperature, and O3
concentrations during summer promote the emissions and oxidation of several
VOCs. The aim of this work is to provide insights about the current VOC
composition and origin in urban areas in the eastern Mediterranean during
summer and winter. Another objective of the present study is to assess the
impact of residential biomass burning for wintertime VOC levels in these
urban areas.
Experimental
Sampling sites
Summer measurements in Patras (population ∼ 300 000) were conducted during
11–26 June 2012 in the Institute of Chemical Engineering Sciences (ICE-HT),
located 8 km northeast of the city center (lat 38.298∘, long
21.809∘, elevation 100 m). The area is surrounded by low vegetation
and olive tree fields. The Athens (population ∼ 4 million) summer campaign was
conducted between 3 and 26 July 2012 at the Demokritos National Center for
Scientific Research in Athens (N.C.S.R.), which is located 8 km from the
city center (lat 37.995∘, long 23.816∘, elevation 280 m) at
the foothills of a mountain covered with pine vegetation. For the Athens
winter campaign, the National Observatory of Athens (N.O.A.) (lat
37.973∘, long 23.718∘, elevation 110 m) was selected due to
its proximity to the city center (< 1.5 km). The selected urban site is
located on top of a small hill away from major city roads. The Athens winter
campaign took place from 9 January to 6 February 2013. The locations of all
sampling sites are presented in Fig. 1.
Maps of Greece, Athens, and Patras together with the locations of
the measurement sites used during the three campaigns.
Experimental setup
Patras summer campaign. VOC concentrations were monitored by a
PTR-MS (PTR-QMS 500, Ionicon Analytik). A high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS, Aerodyne Research) was used
for the characterization of the aerosol composition. A multi-angle absorption
photometer (MAAP 5012, Thermo Scientific) monitored the black carbon (BC)
concentration. A series of commercially available gas monitors was used for
measuring nitrogen oxides (NOx), O3, and sulfur dioxide (SO2)
(API Teledyne, models T201, 400E, and 100EU respectively). The NOx
monitor uses a molybdenum oxide catalyst and chemiluminescence and its
measurements are known to be subject to interferences by gas-phase nitric
acid, alkyl nitrates, etc. (Dunlea et al., 2007). A scanning mobility
particle sizer (SMPS) provided the number size distributions (TSI, models
3080 and 3787). Meteorological variables and sunlight intensity were also
measured. The sampling was conducted at approximately 15 m above ground. All
gas species were sampled through Teflon (PTFE) tubing, while all particulate
species were sampled through 3/8 in. copper tubing.
Athens summer campaign. The same instrumentation (PTR-MS and
HR-ToF-AMS) was used for the Athens summer campaign. BC concentration was
provided by an Aethalometer operating at 880 nm (Magee Scientific, AE31). An
SMPS measured the particulate number distributions (model 3022 TSI, custom
DMA). A PM2.5 cyclone and a silica dryer were installed prior to the
SMPS and the Aethalometer. The PTR-MS and AMS were measuring from inside the
mobile laboratory of the Laboratory of Air Quality Studies (LAQS) parked next
the N.C.S.R. station. Sampling for these instruments was conducted at 6 m
above ground. For the PTR-MS sampling 1/4 in. PTFE tubing was used. For the
AMS 3/8 in. copper tubing was selected. O3 and NOx concentrations
were measured by the Ministry of Environment, Energy and Climate Change at an
adjacent (300 m away) station.
Athens winter campaign. The instrumentation used was the same as in
the Patras summer campaign. Additionally, carbon monoxide (CO) and carbon
dioxide (CO2) were monitored (API Teledyne, models 300E and T360
respectively). A Horiba monitor (model APSA 365) was used for the SO2
measurements. Meteorological data were obtained by the National Observatory
of Athens. All sampling inlets were approximately 6 m above ground.
PTR-MS operation. In all three campaigns, H3O+ was used as
the reaction reagent for the PTR-MS. The drift tube was operated at a
pressure of 2.2–2.3 mbar and its voltage was 600 V. The inlet flow was
0.5 L min-1 and the inlet tube and reaction chamber were heated to
60∘ C. The residence time in the sampling lines prior to the PTR-MS
was 10 s for the Patras summer campaign, 12 s for the Athens summer
campaign, and 16 s for the Athens winter campaign. The corresponding ratio
of the electric field strength to the gas number density was approximately
126 Td (1 Td = 10-17 V cm2). Blanks were obtained at
regular intervals through an activated carbon filter (Supelpure HC, Supelco).
A Teflon filter was installed before the sampling inlet. Concentrations were
monitored in the multiple ion detection (MID) mode with a total cycle time of
10 s. The dwell times were in the range of 5–500 ms. A 200 ms dwell time
was used for most of the reported m/z values. For some of the higher
m/z values a dwell time of 500 ms was selected. The dwell times for
m/z 21, 30, 32, and 37 were 200, 5, 5, and 10 ms respectively in all three
campaigns.
Calibrations were performed once per week. The sensitivities for the reported
compounds for all campaigns were in the range of 4.7 to 24 ncps ppb-1.
These values did not change significantly (less than 40 %) during the
three deployments. The detection limits for the calibrated compounds based on
Karl et al. (2003) were in the range of 34 to 97 ppt. Humidity effects on
the sensitivities of the individual compounds were not considered during the
calibrations. A precision calibrator (Teledyne, model 702) was used for the
dilution of the VOC standard with VOC-free air (using a Supelco filter). This
calibrator type does not substantially change the RH of the air. The ratio of
m/z 37 to m/z 19 was 0.039 ± 0.009 during the Patras summer
campaign, 0.044 ± 0.012 during the Athens summer campaign, and
0.038 ± 0.008 during the Athens winter campaign. In all campaigns the
m/z 37 to m/z 19 ratio was always less than 0.06 (typically ranging
from 0.025 to 0.05). This ratio is considered low and stable; thus, no
corrections were applied for the H3O+(H2O) ion. The
concentrations of all compounds were normalized to the primary hydronium ion
signal. For compounds for which calibration was not possible, the
corresponding mixing ratios were estimated based on Eq. (3) of Taipale et
al. (2008). Concentrations are calculated based on
[VOC]=CFRH+/H3O+,
where CF is the calibration factor for each VOC and RH+ and
H3O+ are the counts per second for the corresponding VOC and the
primary ion respectively. The concentrations reported for the non calibrated
m/z's are calculated for a reaction rate of
k=2.0×10-9cm3s-1.
Table S1 in the Supplement summarizes the VOCs measured during the three
campaigns. Formaldehyde (m/z 31) was not included in this dataset due to
issues related to the humidity dependence of the measurements and its low
proton affinity (DeGouw and Warneke, 2007). Methanol was also excluded due to
the difficulty of obtaining methanol-free air for the background measurements
with the activated carbon filter. Acetaldehyde (m/z 45) was also excluded
from the dataset due to negative values, probably resulting from CO2
interferences (DeGouw and Warneke, 2007). Finally, acrolein (m/z 57) and
m/z 41 were not included due to spikes associated with butanol emissions
from the SMPS instruments. Details for the rest of the instrumentation used
in the campaigns can be found elsewhere (Kostenidou et al., 2015; Florou et
al., 2016).
PMF analysis
The PMF technique (Paatero and Tapper, 1994; Lanz et al., 2007) was used for
the deconvolution of the PTR-MS data into factors. The analysis was
performed using the PMF evaluation tool (Ulbrich et al., 2009) with Igor Pro
6.22A (Wavemetrics). The uncertainties were calculated based on Poisson ion-counting statistics (DeGouw et al., 2003) and the measured background
concentrations. The overall uncertainty is described by (DeGouw et al.,
2003; Slowik et al., 2010)
Δ(I-Ib)=Iτ+Ibτb,
where I is the signal of the main measurements, Ib is the signal
from the background measurements, Δ(I-Ib) is the overall
uncertainty, τ is the dwell time of the main sampling, and
τb is the dwell time of the background sampling. A total of
29 m/z values (Table S1) were used for the PMF analysis. Mixing ratios
(ppb) were used as input for the PMF model. Solutions with up to 10 factors
were examined with an fpeak ranging from -2.0 to 2.0 with a step
of 0.2. The averaging time used was 5 min. The optimum solution for each
measurement period was selected by evaluation of the model residuals, the
mass spectra composition, and the correlations of the factor time series with
other measured pollutants. The estimated concentrations by the PMF correlated
very well (R2 ranged from 0.994 to 0.999, Fig. S47) with the measured
values for all three campaigns.
Concentrations (5 min averages) of VOCs and other pollutants during
the Patras summer campaign.
Lower
Median
Average
Upper
quartile
quartile
VOCs
m/z
Concentration (ppb)
Acetonitrile
42
0.08
0.10
0.12
0.14
Formic acid, ethanol
47
0.40
0.86
0.89
1.35
Acetone
59
2.05
2.44
2.93
3.23
Acetic acid
61
1.16
1.88
2.15
2.90
Isoprene
69
0.32
0.85
1.01
1.46
MVK + MACR
71
0.16
0.24
0.30
0.38
MEK
73
0.18
0.25
0.30
0.42
Benzene
79
0.06
0.09
0.12
0.17
Toluene
93
0.10
0.19
0.28
0.43
Xylenes
107
0.09
0.15
0.25
0.37
Monoterpenes
137
0.09
0.19
0.33
0.50
Other species
Units
Black carbon (BC)
µg m-3
0.27
0.44
0.49
0.67
Sulfur dioxide (SO2)
ppb
0.98
1.15
1.25
1.41
Nitrogen oxides (NOx)
ppb
1.8
3.7
5.1
6.4
Ozone (O3)
ppb
41.2
49.2
48.7
56.4
VOC concentrations and diurnal profiles
Patras summer 2012
The overall measurement period can be divided into three sub-periods based on
the prevailing meteorology, namely 11–15, 16–22, and 23–26 June. During the
first period SW winds prevailed with an average temperature of
24.4 ∘C, a wind speed of 3 m s-1, and 58 % relative
humidity. The second period was characterized by higher temperatures
(28.4 ∘C) and stronger (5.3 m s-1) E-NE winds. The average
relative humidity was 29 %. The final period had SW winds mainly during
the day and E-NE winds during the night. The average temperature was
27.2 ∘C and the average relative humidity was 49 %. Average wind
speed decreased during this period to 2.4 m s-1. There was no
precipitation during the campaign. Additional information about the
meteorological conditions and solar radiation can be found in the Supplement
(Fig. S1). Back-trajectory FLEXPART (Stohl et al., 2005) and HYSPLIT analysis
(Draxler and Rolph, 2013) was performed (Kostenidou et al., 2015). The air
masses were influenced mostly by the marine environment during the first
period and by continental Greece during the second and third period.
Table 1 summarizes the average, median, and upper and lower quartile of the
concentrations for the some of the VOCs along with other gas and particulate
species. Acetone was the most abundant VOC with an average concentration of
2.9 ppb. Acetic acid had a mean concentration of 2.2 ppb. The m/z 69,
reported here as isoprene, had an average concentration of 1 ppb. Throughout
the campaign, the acetonitrile levels were on average 0.1 ppb with a flat
diurnal profile, suggesting that biomass burning was not an important VOC
source. Time series for the reported VOCs are presented in the Supplement
(Fig. S2). Maximum values for the aromatic compounds were in the range of
1 ppb for toluene and the xylenes, while benzene concentrations of up to
0.4 ppb were measured. Isoprene and the monoterpenes had peak concentrations
up to 3 and 1 ppb respectively at noon.
The average diurnal profiles for some of the VOCs measured are shown in
Fig. 2. The concentrations of isoprene, the monoterpenes, methyl vinyl ketone
(MVK), and methacrolein (MACR) were low during nighttime (Fig. 2a–c) and
increased in the afternoon with a maximum at 15:00–16:00 LT. Other
compounds of mainly biogenic origin such as methyl ethyl ketone (MEK)
(Fig. 2d) displayed a similar diurnal pattern, having two additional small
peaks at 09:00 and 22:00 LT, something that shows either the influence of
anthropogenic sources or the contribution of other molecules to the m/z 73
signal, reported here as MEK. Most of the biogenic VOCs had higher
concentrations during the second and third period. During these periods the
air masses reaching the site passed over continental Greece, where there are
mountains with forests.
Average VOC diurnal profiles during the Patras summer campaign.
Green lines present the average values. The median value is shown with blue.
The blue area corresponds to the interquartile range.
Aromatic VOCs, like benzene, toluene, and xylenes, displayed diurnal cycles
(Fig. 2e, f, g) characteristic of traffic emissions, with a peak during the
morning rush hour at 08:00 LT and one peak in the evening at 22:00 LT. This
evening peak can be justified by the local lifestyle, especially the closing
of the market at 21:00. Higher concentrations were observed during the first
period and during daytime for the third period (Fig. S2). This is consistent
with significant local aromatic sources. The toluene to benzene ratio for the
overall period was 2.5. During the three periods, the toluene to benzene
ratio was 2.3, 2.0, and 2.6 respectively. Most of the reported toluene to
benzene ratios in the literature are in the range of 1 or less (for aged air
masses) to 4 (associated with fresh emissions measured in tunnels) (Roberts
et al., 1984; Salisbury et al., 2003; Kristensson et al., 2004). Assuming no
influence from biomass burning sources (the acetonitrile levels were low),
this ratio can be considered as a photochemical clock (Roberts et al., 1984);
thus, the sampled air during the second period was on average more aged.
During this period the air masses sampled were less influenced by the city of
Patras and the surrounding areas.
Acetone's concentrations were marginally elevated during daytime from
13:00 to 22:00 LT (Fig. 2i). Several oVOCs had a diurnal pattern similar to
the biogenic VOCs. The diurnal profile of acetic acid (Fig. 2j) peaked at
approximately 17:00 LT. Higher values of the oVOCs were observed during the
second period (Supplement Sect. S2, Fig. S2), suggesting strong biogenic
influence.
NOx and BC concentrations displayed diurnal profiles (Figs. S3 and S4)
similar to those of the aromatic species, typical of traffic emissions.
Higher values were observed for both species during the first and third
periods, while for the second period lower values were measured due to the
strong E-NE winds. The average concentration for NOx was 5.1 ppb, while
for BC a mean value of 0.5 µg m-3 was measured. SO2
concentrations were elevated during the day. A peak during morning rush hour,
similar to the aromatic VOCs, was observed but no peak was evident during
evening rush hour. This can be explained by the low activity of heavy trucks
during these hours (22:00–23:00 LT). A mean value of 1.3 ppb was found for
SO2. Ozone concentrations were elevated during the second period
exceeding 60 ppb (8 h average) during the night of 17 to 18 July. These
elevated O3 levels were the result of long-range transport from
continental Greece and the Balkans. The importance of the long-range transport
can be seen in Fig. S3, showing the ozone concentrations during this windy
period. The maximum ozone concentration was actually observed a little before
the midnight of 18 June. This was actually the highest ozone level during the
measurement period. This was in contrast to the low levels of NOx and BC
observed during the same period. These results shown in Fig. S3 strongly
support the conclusion that the high levels of ozone were due to long-range
transport. Similar conclusions have been reached by Kouvarakis et al. (2002),
reporting measurements performed onboard a cruise ship traveling on a
regular basis in the area. They concluded that long-range transport is the
main factor contributing to high ozone levels in eastern Greece. During these
two days an average value of 55 ppb was measured. Diurnally averaged O3
concentrations for the overall measurement period started to increase at
09:00 LT, with a maximum of 58 ppb during 15:00–16:00 LT. During the night
(21:00–07:00 LT) O3 concentrations were in the range of 45 ppb.
Concentrations (5 min averages) of VOCs and other pollutants
during the Athens summer campaign.
Lower
Median
Average
Upper
quartile
quartile
VOCs
m/z
Concentration (ppb)
Acetonitrile
42
0.16
0.19
0.20
0.23
Formic acid, ethanol
47
1.12
1.51
1.52
1.87
Acetone
59
3.27
4.09
4.28
4.92
Acetic acid
61
1.53
2.04
2.17
2.65
Isoprene
69
0.46
0.72
0.73
0.93
MVK + MACR
71
0.20
0.32
0.35
0.47
MEK
73
0.32
0.45
0.50
0.59
Benzene
79
0.12
0.16
0.22
0.24
Toluene
93
0.48
0.62
0.81
0.82
Xylenes
107
0.35
0.51
0.67
0.77
Monoterpenes
137
0.54
0.88
0.92
1.22
Other species
Units
Black carbon (BC)
µg m-3
0.43
0.60
0.72
0.89
Nitrogen oxides (NOx)
ppb
5.1
5.6
7.3
7.5
Ozone (O3)
ppb
49.6
55.5
54.1
59.1
Athens summer 2012
The Athens summer campaign was characterized by a prolonged heat wave with
temperatures reaching up to 40 ∘C. The average temperature was
29.6 ∘C and the corresponding relative humidity was 40 %. Local
topography dictated the wind patterns, with the wind having mainly an E-SE
direction. The wind direction varied especially during the morning hours
(08:00–11:00 LT) with wind shifts bringing air masses from the N or E of
the site. The average wind speed was 1.7 m s-1 with very low values
during the night. No precipitation occurred throughout the campaign. More
information concerning the meteorological conditions is included in the
Supplement (Fig. S5).
Acetone was the most abundant VOC with an average value of 4.3 ppb
(Table 2), significantly higher than in Patras. Throughout the campaign the
acetonitrile concentrations were on average 0.2 ppb with a flat diurnal
profile, suggesting the lack of biomass burning sources. Figure S6 includes
all the VOC time series for the Athens summer campaign. Toluene and the
xylenes peaked during the morning hours at concentrations in the range of
3 ppb, while benzene had typical maximum values around 1 ppb during these
hours. Isoprene and the monoterpenes peaked at noon with concentrations in
the range of 1 and 2 ppb respectively.
The average isoprene concentration was 0.7 ppb with a diurnal profile
(Fig. 3a) similar to Patras. Local sources significantly influenced the
monoterpene concentrations (Fig. 3b) with a peak during the morning hours
(from 06:30 to 08:30) followed by several hours of elevated levels with a
maximum at approximately 15:00–16:00 LT. This behavior is similar to that
reported by Filella and Penuelas (2006) for the Barcelona semi-urban site.
Nighttime stagnation conditions followed by morning N or NE winds caused
these elevated concentrations by transporting to the site nighttime emissions
from the adjacent pine forest. MVK and MACR (Fig. 3c) had a similar diurnal
cycle to that of isoprene, while MEK (Fig. 3d) peaked at 11:00–12:00 LT and
had one more peak at 22:00, something also seen in the Patras summer
campaign.
Average VOC diurnal profiles during the Athens summer campaign.
Green lines present the average values. The median value is shown with blue.
The blue area corresponds to the interquartile range.
In Athens, higher aromatic concentrations were observed compared to the
Patras summer campaign. These species had similar diurnal cycles (Fig. 3e,
f, g) characteristic of traffic emissions, with a peak during the morning
rush hour at 08:00–09:00 LT and one wide peak during the evening from 19:30
to 01:00 LT. The toluene to benzene ratio for the Athens summer campaign was
3.4, a value similar to that of fresh traffic emissions (Kristensson et al.,
2004). Rappenglück et al. (1998) reported a toluene to benzene ratio of 2.3
for the Demokritos site during August–September 1994. The correlation
coefficients between the aromatic species (benzene, toluene, xylenes, C9 and
C10 aromatics) were high (R2 ranging from 0.92 to 0.96), suggesting a
single source related to traffic emissions.
The diurnal profiles of most oVOCs suggest anthropogenic influence (Fig. 3i
and j). Acetone concentrations were elevated during the day with additional
peaks during morning rush hour and during night (24:00 LT). Acetic acid
concentrations were lower during the day with a peak at 22:00–23:00 LT and
elevated concentrations during night hours. Morning rush hour had a very
small effect on acetic acid's levels. This compound appears to be associated
mainly with regional sources.
The measured concentrations of acetonitrile, acetone, benzene, and toluene in
Patras and Athens were lower than summer measurements from Paris, Beijing,
Mexico City, Tokyo, Houston, London, and Mohali (Sinha et al., 2014).
Isoprene concentrations though were comparable with the findings from the
above cities.
NOx and BC concentrations displayed diurnal profiles similar to those of
the aromatic species, typical of traffic emissions (Figs. S7 and S8). The
average concentration of NOx was 7.3 ppb, while BC had an average value
of 0.7 µg m-3. Increased solar radiation along with the
availability of O3 precursors resulted in high O3 exceeding the EU
60 ppb 8 h limit in 4 days during the 13-day period. The O3 average
diurnal pattern was similar to that of Patras during the summer with a
maximum of 60 ppb during 15:00–16:00 LT. During the night concentrations
of 50 ppb were measured.
Concentrations (5 min averages) of VOCs and other pollutants
during the Athens winter campaign. Values are shown for the overall period
and for the biomass burning periods.
Overall measurement period
Biomass burning periods
Lower
Median
Average
Upper
Lower
Median
Average
Upper
quartile
quartile
quartile
quartile
VOCs
m/z
Concentration (ppb)
Acetonitrile
42
0.05
0.08
0.16
0.18
0.34
0.45
0.55
2.70
Formic acid, ethanol
47
0.95
1.33
1.80
2.01
2.31
3.13
3.69
4.57
Acetone
59
1.10
1.52
2.24
2.48
4.05
5.04
5.76
6.74
Acetic acid
61
0.95
1.38
2.11
2.42
2.65
4.02
5.26
6.22
Isoprene
69
0.41
0.60
1.05
1.07
1.92
2.56
3.25
3.91
MVK + MACR
71
0.13
0.21
0.41
0.43
0.79
1.05
1.35
1.58
MEK
73
0.28
0.41
0.59
0.67
1.00
1.29
1.52
1.82
Benzene
79
0.29
0.55
1.00
1.10
1.94
2.58
3.18
3.70
Toluene
93
0.94
1.34
2.34
2.72
4.23
5.49
6.25
7.06
Xylenes
107
0.43
0.83
1.69
1.94
3.32
4.28
5.09
5.68
Monoterpenes
137
0.15
0.23
0.43
0.46
0.87
1.22
1.37
1.71
Other species
Units
Black carbon (BC)
µg m-3
0.46
1.00
2.02
2.35
5.78
7.63
7.98
9.54
Sulfur dioxide (SO2)
ppb
0.7
1.1
1.6
1.9
1.1
1.8
2.3
3.0
Nitrogen oxides (NOx)
ppb
5.9
1.3
24.6
26.5
51.8
70.3
79.7
93.1
Ozone (O3)
ppb
8.7
25.9
21.6
32.6
1.8
2.7
3.5
4.3
Carbon monoxide (CO)
ppm
0
0.1
0.3
0.4
1.0
1.2
1.4
1.7
Carbon dioxide (CO2)
ppm
380
388
390
402
420
429
432
441
Athens winter 2013
Due to the recent economic crisis and the increasing prices of heating oil at
the time, a significant percentage of the population in Greece has been using
wood for residential heating during the winter. During the winter of
2012–2013, extreme episodes of atmospheric pollution occurred in Athens,
with hourly PM1 levels reaching values up to 140 µg m-3
(Florou et al., 2016). The average temperature, humidity, and wind speed
during the Athens winter campaign were 11.9 ∘C, 70 %, and
3.4 m s-1 respectively. The wind direction patterns favored mainly
S-SW winds and occasionally N-NE winds. Minimum night temperatures ranged
from 3.3 to 15 ∘C. Precipitation occurred during 11, 16, 18, 24, 25,
and 26 January. Meteorological data are included in the Supplement (Figs. S9
and S10).
Table 3 summarizes the average, median, and lower and upper quartiles of the
concentrations for some of the measured VOCs. The substantial difference
between the median and average values for most species is due to periods with
significant residential wood burning and correspondingly high concentrations.
Concentrations of acetone and acetic acid reached 15 ppb, while isoprene
levels were as high as 9 ppb. Acetone, acetic acid, and toluene had mixing
ratios with median values in the range of 1.5 ppb. Acetone levels were
significantly lower compared to the summer campaign. Formic acid/ethanol and
acetic acid had concentrations similar to the summer. The time series of
the measured VOCs is shown in Fig. S11 in the Supplement.
Most of the VOCs (Fig. 4) had higher average concentrations during the night
and during the morning rush hour (peak at 09:00 LT). Evening concentrations
started to increase at 19:00 LT. Very high concentration periods were
detected for most of the measured VOCs. Aromatics increased during the
morning rush hour together with formic acid and acetone. The toluene to
benzene ratio for the campaign was 2.1, suggesting the importance of other
sources in contrast to the summer ratio (3.4) when traffic dominated.
Average VOC diurnal profiles during the Athens winter campaign.
Green lines present the average values. The median value is shown with blue.
The blue area corresponds to the interquartile range.
Isoprene and the monoterpenes had elevated concentrations during the morning
but also during the evening (18:00–23:00 LT) (Fig. 4a and b), mainly due to
biomass burning. MVK, MACR, and MEK were also produced by biomass burning
sources (Fig. 4c and d). Although these compounds dominate their specific
m/z values (m/z: 69, 71, 73) in typical rural environments, other
compounds may be present in these m/z values during the Athens winter
campaign. Karl et al. (2007) reported furan and other alkenes being detected
at m/z 69, crotonaldehyde and other alkenes for m/z 71, and methyl
propanal for m/z 73 during the burning of tropical biofuels. Akagi et
al. (2011) and Yokelson et al. (2013) reported furan emissions factors higher
than isoprene for some biomass fuel types. Based on reported emissions from
various biofuel types, furan contributes around 30 % of the m/z 69 in
most cases (Stockwell et al., 2015; Sarkar et al., 2016). For the Athens
winter campaign isoprene is believed to be the major component of m/z 69.
The VOC concentrations in Athens during winter are lower than those in
Kathmandu or Karachi (Sarkar et al., 2016) findings. Acetonitrile, acetone,
benzene and toluene levels are similar to those measured in London, Paris, and
Barcelona, while the isoprene concentration is higher and closer to the
Kathmandu and Karachi values.
NOx, BC, and CO had time series and average diurnal profiles (Figs. S12
and S13) that were similar to the majority of the
VOCs with peaks at morning rush hour and during the evening
(18:00–23:00 LT). Their median values were 11.3 ppb,
1.0 µg m-3, and 100 ppb respectively. SO2
concentrations were elevated mainly during the morning rush hour. During
nighttime, lower SO2 values were observed with some peaks around
23:00 LT that can be attributed to traffic and residential heating using
diesel fuel. A median value of 1.1 ppb was observed. O3 levels were
reduced during the biomass burning periods to almost zero. For periods with
stronger winds and low gas and particulate pollutant levels, the O3
concentrations reached 30 to 40 ppb. The average O3 diurnal pattern had
a peak at 15:00 LT and a median value of 25 ppb for the campaign.
Emission factors (g kg-1) for VOCs and other species during
biomass burning.
Emission Factors g kg-1 of fuel
Species
Residential
Savanna fires
Yucatán
Pine-forest
Coniferous
African grass
Alfalfa
Black
Ponderosa
heating
(Sinha et al.,
Mexico
understory
Canopy
(Stockwell
(Stockwell
spruce
pine
(This work)
2003)
(Yokelson
(Yokelson
(Yokelson
et al.,
et al.,
(Stockwell
(Stockwell
et al.,
et al.,
et al.,
2015)
2015)
et al.,
et al.,
2009)
2013)
2013)
2015)
2015)
CO2
1600
1700 ± 60
1641 ± 40
1668 ± 72
1670 ± 128
1565 ± 14
1352 ± 55
1724 ± 35
1594 ± 109
SO2
0.02 ± 0.02
0.43 ± 0.30
3.16 ± 2.02
1.06 ± 0.39
1.06 ± 0.41
0.95 ± 0.28
1.2 ± 0.6
0.93 ± 0.01
0.88 ± 0.27
NOx (as NO)
1.6 ± 0.2
3.3 ± 0.6
4.19 ± 3.33
2.55 ± 0.41
2.40 ± 1.47
2.10
3.41
3.42
2.97
CO
22.2 ± 2.8
68 ± 30
80.2 ± 19.4
72.2 ± 26
85.3 ± 38.3
22.6 ± 4.8
76.5 ± 7.3
46.5 ± 8.7
95.3 ± 27
Acetonitrile
0.01 ± 0.002
0.5
0.13 ± 0.09
0.14 ± 0.06
0.02 ± 0.02
0.67 ± 0.11
0.05 ± 0.02
0.20 ± 0.11
Formic acid
0.05 ± 0.01
0.62 ± 0.18
1.53 ± 1.11
0.09 ± 0.09
0.22 ± 0.18
0.06 ± 0.04
0.14 ± 0.16
0.33 ± 0.17
0.96 ± 0.45
Ethanol
0.16 ± 0.23
0.04 ± 0.02
Acetone
0.14 ± 0.02
1.10
0.35 ± 0.29
0.37 ± 0.2
0.09 ± 0.07
0.82 ± 0.11
0.72 ± 0.78
0.82 ± 0.49
Acetic acid
0.11 ± 0.04
2.4 ± 0.9
4.04 ± 3.13
1.33 ± 1.27
1.19 ± 0.98
0.72 ± 0.47
5.5 ± 6.9
1.6 ± 0.9
5.6 ± 3.5
Isoprene
0.09 ± 0.02
0.04 ± 0.02
0.07 ± 0.06
0.10 ± 0.09
0.02 ± 0.02
0.43 ± 0.03
0.44 ± 0.25
1.5 ± 1.0
Furan
0.20 ± 0.21
0.16 ± 0.07
0.06 ± 0.04
0.22 ± 0.12
0.12 ± 0.04
0.41 ± 0.30
MACR
0.04 ± 0.006
0.05 ± 0.04
0.08 ± 0.07
0.07 ± 0.07
0.30 ± 0.03
0.23 ± 0.12
0.61 ± 0.35
MVK
0.22 ± 0.17
0.30 ± 0.20
MEK
0.04 ± 0.006
0.62
0.12 ± 0.11
0.12 ± 0.06
0.02 ± 0.02
0.29 ± 0.06
0.06 ± 0.04
0.21 ± 0.14
Benzene
0.12 ± 0.01
0.18 ± 0.09
0.76
0.18 ± 0.17
0.62 ± 0.59
0.04 ± 0.03
0.53 ± 0.26
0.59 ± 0.25
1.29 ± 0.90
Toluene
0.26 ± 0.03
0.13 ± 0.10
0.14 ± 0.12
0.25 ± 0.12
0.03 ± 0.02
0.46 ± 0.06
2.4 ± 1.2
3.0 ± 2.1
Xylenes
0.26 ± 0.03
0.17∗
0.18∗
0.006 ± 0.005∗
0.15 ± 0.02∗
0.19 ± 0.09∗
0.18 ± 0.08∗
Monoterpenes
0.08 ± 0.02
0.25 ± 0.24
0.62 ± 0.46
0.005 ± 0.004
0.12 ± 0.02
0.10 ± 0.04
0.22 ± 0.16
* Sum of m-, p-, o-xylene, and
ethyl benzene.
Acenonitrile (m/z 42) and levoglucosan's tracer (m/z 60 of the AMS)
were used as markers in order to identify periods where wood burning had a
substantial contribution to the VOC and particulate levels. The concentration
of these two species (Sect. S4.1) had a relatively high correlation (R2=0.78). In both cases elevated concentrations were observed mainly during the
night but also during the morning, having a peak at 09:00 LT. These morning
peaks can be attributed to residential heating at these hours but can also be
a result of vertical mixing of the atmosphere. The acetonitrile's relative
increase is higher than that of the levoglucosan tracer (Fig. 5) during the
morning rush hour. Recently, Dunne et al. (2012) reported interferences at
PTR-MS m/z 42 under the H3O+ reaction when measuring in polluted
urban environments. These interferences were attributed to the 13C
isotopologues of C3H5+ and the product ion C3H6+
formed by reactions with NO+ and O2+ that exist in trace amounts
in the reagent gas (H3O+). This leads to the conclusion that even
though an increase due to biomass burning is apparent during the morning
hours (increase in levoglucosan's tracer from the AMS), a fraction of
m/z 42 concentration is due to species other than acetonitrile. During the
morning, traffic appears to contribute significantly to the m/z 42
increase.
Time series of (a) AMS levoglucosan tracer (m/z 60 of the AMS)
and (b) acetonitrile for the Athens winter campaign. Light brown indicates
periods where wood burning contributed substantially to the total VOC and
particulate levels. Also shown (c) are their average diurnal profiles.
A set of criteria was established in order to select periods during which
residential heating dominated the gas and particulate composition of ambient
air. These periods (Table S2) occurred during the nighttime
(18:00–06:00 LT) and were associated with acetonitrile concentrations
exceeding 0.25 ppb. During these periods NW winds dominated, bringing air
masses from the northern parts of Athens to the site (Fig. S10b). The average
concentrations for these periods are shown in Table 3. Excluding the above
biomass burning periods from the campaign dataset resulted in a decrease in
the average values of the measured species by 11–34 %. That is, the
average values decreased by 26 % for acetonitrile, 11 % for formic
acid and ethanol, 17 % for acetone, 16 % for acetic acid, 23 %
for isoprene, 25 % for MVK and MACR, 17 % for MEK, 23 % for
benzene, 18 % for toluene, 22 % for the xylenes, and 24 % for the
monoterpenes.
VOC sources
Patras summer 2012 – PMF
For the Patras PMF analysis of the PTR-MS data a four-factor solution was
selected. This choice is further discussed in the Supplement (Sect. S5.1 and
S5.2). An fpeak of zero was chosen based on the m/z composition
of the factors and the correlations of the factors to known external (BC and
NOx) and internal (aromatic VOCs, biogenic VOCs) tracers (Supplement,
Sect. S5.3). Solutions in the fpeak range from -0.4 to 0.2 have
only minor differences. Other solutions deriving from different
fpeak selections are also presented in the Supplement. The factors
obtained were attributed to biogenic emissions (factor bVOC), traffic
emissions (factor TRAF), oxygenated VOCs (factor OVOC), and oxygenated VOCs of
biogenic origin (factor b-OVOC).
Emission ratios for biomass burning due to residential heating.
Comparison with reported emission ratios from various biomass fuel types.
Emission ratios (Δspecies / ΔCO, ppb ppm-1)
Species
Residential
SW fuels
SE fuels
Pine spruce
African
Alfalfa
Black
Ponderosa
heating
(Warneke
(Warneke
(Warneke
grass
(Stockwell
spruce
pine
(This work)
et al.,
et al.,
et al.,
(Stockwell
et al.,
(Stockwell
(Stockwell
2011)
2011)
2011)
et al.,
2015)
et al.,
et al.,
2015)
2015)
2015)
SO2
0.4 ± 0.5
–
–
–
18.8 ± 5.4
6.4 ± 3.7
10 ± 2
4.6 ± 1.4
NOx
52.7 ± 9
–
–
–
88
42
70
32
Acetonitrile
0.32 ± 0.05
0.56
1.03
1.05
0.50 ± 0.39
6.0 ± 0.9
0.73 ± 0.17
1.4 ± 0.6
Formic acid
1.3 ± 0.3
0.77
1.08
1.0
1.65 ± 0.81
1.08 ± 1.29
4.1 ± 1.5
5.9 ± 1.1
Ethanol
Acetone
2.9 ± 0.5
0.84
1.93
1.94
1.19 ± 1.42
5.2 ± 0.5
8.9 ± 11
3.9 ± 1.1
Acetic acid
2.0 ± 1.0
4.84
13.61
8.19
13.2 ± 6.8
31 ± 40
15.6 ± 6.4
25.4 ± 8.1
Isoprene
1.6 ± 0.22
0.53
1.38
1.57
0.33 ± 0.27
2.3 ± 0.1
3.7 ± 1.6
6.2 ± 2.1
Furan
1.1 ± 0.76
1.2 ± 0.6
1.1 ± 0.3
1.6 ± 0.65
MVK + MACR
0.7 ± 0.1
0.43
1.08
1.32
1.11 ± 0.94
1.6 ± 0.3
1.9 ± 0.7
2.5 ± 0.7
MEK
0.7 ± 0.2
0.41
1.28
1.17
0.26 ± 0.22
1.5 ± 0.3
0.5 ± 0.3
1.8 ± 0.5
Benzene
1.8 ± 0.2
0.86
0.83
2.29
0.57 ± 0.3
2.5 ± 1.1
4.5 ± 1.2
4.6 ± 1.8
Toluene
3.3 ± 0.8
0.30
0.48
0.81
0.3 ± 0.2
1.8 ± 0.1
14.8 ± 5.5
9.0 ± 3.5
Xylenes
2.8 ± 0.6
0.19
0.35
0.60
0.07 ± 0.04
0.5 ± 0.04
1.0 ± 0.3
1.0 ± 0.4
Monoterpenes
0.7 ± 0.2
0.16
0.55
1.25
0.004 ± 0.002
0.33 ± 0.03
0.42 ± 0.13
0.46 ± 0.19
The bVOC factor included mainly isoprene (m/z 69) and the monoterpenes
(m/z 137) (Fig. 6). There were small contributions by m/z values 71, 81,
87, and 101. The factor average diurnal pattern has a peak at 15:00 LT. The
factor showed weak correlations with the biogenic oxygenated organic aerosol
(b-OOA) and the moderately oxygenated OA (M-OOA) (R2=0.20 and 0.22
respectively) as obtained by the PMF analysis of the AMS data (Kostenidou et
al., 2015). This weak correlation is due to the fact that the b-OOA has been at least partially produced away
from the measurement site. This highlights the challenges of combining gas-
and particulate-phase measurements during source apportionment applications.
Sunlight intensity also had a weak correlation (R2=0.24) with the bVOC
factor. The factor also showed weak correlation (R2=0.29) with ambient
temperature. These correlations improved to 0.28 and 0.35 respectively after
shifting the time series of the factor earlier by 2 h. This suggests that
some of the biogenic components included in this factor are transported
emissions to the site from nearby areas. Wind roses indicate that the higher
concentrations (above the 75th percentile) were coming from E-NE (Fig. S24).
Results of Patras summer campaign PMF analysis. The left
side shows the diurnal profiles of the factors. Green lines present the
average values. The median value is shown with blue. The blue area is the
interquartile range. The right side presents the m/z composition of each
factor as a fraction of signal for each m/z.
Factor TRAF included the aromatic species (m/z values 79, 93, 107, 121, and
135) as well as m/z values 43, 59, 61, and 69 (Fig. 6). Some contribution
by m/z values 71 and 73 was also observed. Its diurnal profile as expected
is similar to that of the aromatic species (Fig. 2) with one peak at
09:00 LT and one at 22:00 LT. The toluene to benzene ratio of the factor
was 2.6, i.e., the same as the ratio for the third period. Correlations
between this factor and BC were relatively high (R2=0.66). The two
hydrocarbon-like OA factors (HOA-1 and HOA-2) reported by Kostenidou et
al. (2015) showed R2 of 0.49 and 0.43 with this factor respectively. The
PM1 organonitrate calculated for the AMS measurements based on Farmer et
al. (2010) also correlates well (R2=0.59) with the TRAF factor. The
main source of this factor based on the diurnal profiles and the correlations
seen is traffic in the city of Patras and the surrounding areas. This is
consistent with the wind rose for this factor (Fig. S24), which points out
that the higher emissions are located to the W-SW of the site where the city
of Patras is located. Other sources, such as cooking emissions and ships,
might contribute to this factor to some extent.
The OVOC factor mainly included acetone. Other m/z values contributing to
this factor were 43, 47, and 61 (Fig. 6). A weak correlation between the
factor and the AMS's b-OOA (R2=0.29) was observed. Additionally, low
correlations were observed with particulate sulfate and nitrate (R2 of
0.23 and 0.18 respectively). The factor's diurnal pattern (Fig. 6) reveals
elevated concentrations during the night and a minimum at 15:00 LT. The
factor seems to be affected by mixing height changes and from long-range
transport.
The last factor (b-OVOC) included oxygenated molecules (acetic acid,
acetone,
and formic acid/ethanol). Other m/z values contributing to this factor were
m/z 43, 69, 71, and 73 (Fig. 6). This factor includes mainly products of
the oxidation of biogenic VOCs but also some primary VOCs. Approximately
70% of the measured isoprene during the Patras summer campaign was
included by the PMF analysis in the bVOC factor (see also Fig. S20 in the
Supplement). The remaining 30 % was included in other factors, with
15 % in the b-OVOC factor. This small amount of isoprene assigned to this
factor can be justified as direct biogenic emissions that have
originated far from the area close to the measurement site and have not yet
reacted, or it can be due to the uncertainties in the PMF analysis. The factor
was named b-OVOC (biogenic oxygenated VOCs) due to the formic acid, acetone,
and acetic acid that it includes. The b-OVOC diurnal cycle is similar to that
of the bVOC factor shifted by approximately 2 h. Based on this diurnal
cycle, the OVOC species that contribute to this factor are either products of
the oxidation of biogenic molecules or direct biogenic emissions. Ambient
temperature and sunlight intensity had a moderate correlation with the b-OVOC
with R2 of 0.35 and 0.32 respectively. A weak correlation (R2=0.27) of this factor was seen with the moderately oxygenated (M-OOA) by the
AMS and no correlation (R2=0.08) was observed with the b-OOA. As in
the bVOC factor, the greater concentrations of the b-OVOC factor are coming
from the E-NE (Fig. S24). This supports the explanation that the b-OVOC
factor is mostly oxidation products of local biogenic VOCs as well as
temperature-sensitive emissions.
The contribution of each factor to the various VOC species is presented in
Fig. 7. In total, 81 % of isoprene (m/z 69) is included in the biogenic factors
bVOC and b-OVOC. Monoterpene m/z values (137 and 81) were mainly assigned
by the PMF to the bVOC factor (73 and 63 % respectively) and at smaller
percentages to factor b-OVOC (9 and 6 % respectively). Factor TRAF
included the majority of the aromatic species (73–91 %). The rest of the
aromatic species are assigned by the PMF to other factors. This is probably
due to the combination of the low concentrations of most of the aromatics and
the uncertainties in the PMF analysis. These values also provide an estimate
of the uncertainty in these results. Patras summer PMF analysis demonstrates
the importance of biogenic sources to the VOC budget of urban areas during
summer.
Species percentage (%) attributed to the various PMF factors
for the Patras summer campaign.
Athens summer 2012 – PMF
After the evaluation of various solutions, five factors were selected for
this period with an fpeak equal to zero (Supplement, Sect. S6.1,
S6.2, S6.3). The factors were attributed to monoterpene species (factor
TERP), other biogenic emissions (factor bVOC), traffic emissions (factor
TRAF), and two oxygenated VOC factors (factor OVOC-1 and OVOC-2).
Results of Athens summer campaign PMF analysis. The left side shows
the diurnal profiles of the factors. Green lines present the average values.
The median value is shown with blue. The blue area is the interquartile
range. The right side presents the m/z composition of each factor as a
fraction of signal for each m/z.
For the Athens summer campaign, the local monoterpene emissions were
separated by the PMF from the rest of the biogenic emissions, creating a new
factor named TERP. The m/z values 137 and 81 were the main components of
this factor, and its diurnal profile (Fig. 8) has an early morning peak
similar to that of the monoterpenes in this site (Fig. 3). Other m/z values
contributing to this factor were 43, 47, 59, 61, 69, 71, 73, 75, 77, and 93
(Fig. 8). No correlations (R2 < 0.05) were found between this factor
and measured particulate species. These monoterpene emissions were due to
local pine vegetation and their concentrations were affected by local mixing
phenomena.
Factor bVOC was mainly composed of isoprene (m/z 69) and isoprene
hyperoxides (m/z 101). A fraction of m/z values 43, 47, 59, 71, 73,
75, 87, 93, and 121 was also included. Its diurnal profile (Fig. 8) peaked at
15:00 LT, similar to the bVOC factor for the Patras summer campaign. This
factor had a weak correlation (R2=0.15) with the very oxygenated OA
factor (V-OOA) obtained by the PMF analysis of the AMS data (Kostenidou et
al., 2015). No correlation existed with the AMS M-OOA factor.
Factor TRAF included the aromatic species (m/z values 79, 93, 107, 121, and
135). Also, m/z values 43, 47, 59, 61, and 73 contributed to this factor
(Fig. 8). The diurnal profile is similar to that of the aromatic compounds
(Fig. 3) with one peak at 09:00 and one at 22:00 LT. The toluene to benzene
ratio of the factor was 3.1, indicating relatively fresh traffic emissions.
Factor TRAF had moderate to high correlations with most of the species
emitted by transportation sources: NOx (R2=0.69), BC
(R2=0.45), AMS nitrate (R2=0.64) and AMS HOA-2 (R2=0.58).
The factor's correlation with AMS HOA-1 was surprisingly weak (R2=0.16).
This inconsistency is believed to derive from the inhomogeneity of the source
distribution around the sampling site along with the shifting wind directions
(Kostenidou et al., 2015). The highest concentrations (upper quartile) of
this factor were observed during periods when the wind came from N-NE
(Fig. S34). Particulate organonitrates (AMS) showed good correlation with
factor TRAF (R2=0.65).
Factor OVOC-1 was dominated by acetone. Other m/z values contributing to
this factor were 43, 47, and 61 (Fig. 8). No correlation (R2 < 0.08)
was found between this factor and other species. Its concentration had a
modest afternoon peak. This factor is considered a mix of contributions by
long-range transport and various anthropogenic and biogenic sources.
The OVOC-2 factor included mainly acetone (m/z 59) and acetic acid
(m/z 61). The m/z 43 and 47 also contributed (Fig. 8). The factors
concentrations were elevated during the night. The factor had correlations
with PM1 nitrate (R2=0.49), HOA-1, and HOA-2 (R2 equal to
0.18 and 0.22 respectively), as well as with V-OOA (R2=0.34) and PM1
sulfate (R2=0.32). Due to these correlations and its diurnal profile,
this factor is believed to be linked to a mix of urban sources as well as
long-range transport.
The fraction of some selected VOC species attributed to each factor is shown
in Fig. 9. The majority (58 %) of isoprene is assigned to factor bVOC
and smaller percentages to factors OVOC-1, TERP, and TRAF (14, 20, and 7 %
respectively). Monoterpenes, m/z values 137 and 81, are almost exclusively
included in factor TERP (84 and 76 % respectively). The majority (55 to
73 %) of the aromatic species (m/z values 79, 93, 121, and 135) are part
of factor TRAF.
Species percentage (%) attributed to the PMF factors for the
Athens summer campaign.
Athens winter 2013 – PMF
A five-factor solution and an fpeak equal to zero were selected for
the Athens winter campaign (Supplement, Sect. S7.1, S7.2, and S7.4). One
factor was related to emissions originating from biomass burning (BBVOC). A
factor for traffic emissions (factor TRAF) was identified, as well as one
factor that was attributed mainly to industrial sources (factor IND).
Finally, two oxygenated VOC factors (OVOC-1 and OVOC-2) were found. The
measurement period that was selected for this PMF analysis did not include
the first day of the measurements of the Athens winter campaign. Including
the first day in the Athens winter campaign in the PMF analysis resulted in
the separation of the biomass burning source into two or more factors for
solutions with four or more factors (Supplement, Sect. S7.3). No distinct
variation was seen for the favored wind direction between those BBVOC
factors. Their time series
(Fig. S41), though similar, showed significant differences for the first day
of the campaign (9 and 10 January), when lower temperatures were recorded.
Factor BBVOC correlated strongly with the AMS m/z 60 (R2=0.92)
linked to levoglucosan and also with acetonitrile (R2=0.80). It also
had a high correlation (R2=0.84) with the AMS BBOA factor (Florou et
al., 2016) and with other species emitted by biomass burning sources such as
CO (R2=0.59) and BC (R2=0.63). The factor mainly included
acetone, isoprene, MVK and MACR, MEK, benzene, xylenes, C9 aromatics, and
monoterpenes (Fig. 10). Due to the acetonitrile presence along with the high
correlations obtained with known species of biomass burning origin (BBOA, AMS
m/z 60, CO, BC), this factor was considered to derive from emissions of
biomass burning. The diurnal profile (Fig. 10) of the factor is
characteristic of residential heating use: moderate use during the early morning
hours and high use during the night (from 18:00 to 24:00 LT). Wind roses
(Fig. S46) indicate that the higher concentrations for this factor were
coming from N-NE of the site.
Results of Athens winter campaign PMF analysis. The left side
shows the diurnal profiles of the factors. Green lines present the average
values. The median value is shown with blue. The blue area is the
interquartile range. The right side presents the m/z composition of each
factor as a fraction of signal for each m/z.
Factor TRAF has the same m/z values as in the summer campaigns and is
mainly linked to traffic. The aromatics are the main VOCs contributing to
this factor. Other m/z values that are present in considerable amounts are
43 and 59 (Fig. 10). The toluene to benzene ratio for this factor is 3.0,
similar to the value (3.1) of the same factor during the summer campaign in
Athens. Its diurnal profile includes one large peak corresponding to the
morning rush hour and a smaller peak at approximately 19:00 LT (Fig. 10).
The three factors related to traffic (factors named TRAF for the three
studied campaigns) had a consistent m/z spectrum with R2 values
ranging from 0.68 to 0.90.
Factor IND mainly includes toluene and acetone (Fig. 10). Its diurnal pattern
is relatively flat with slightly elevated concentrations during the night. A
weak minimum is observed at 09:00 LT. Due to the presence of toluene this
factor is considered of anthropogenic origin, probably related to industrial
activities far from the site.
Factor OVOC-1 includes mainly formic acid (m/z 47) and some isoprene. The
factor's diurnal profile (Fig. 10) was characterized by higher values during
the early morning hours, peaking at 08:00 LT. After a significant decline in
the morning it slowly increased during the rest of the day. No significant
correlations were found between this factor and other measured species.
Factor OVOC-1 could not be associated with any potential source. Wind roses
(Fig. S46) indicate that the higher concentrations were coming from the N-NE
(city center) and that this factor is probably influenced by anthropogenic
sources.
The last factor (OVOC-2) includes m/z 43 and acetic acid. The diurnal
profile had elevated values during the night hours and was slowly decreasing
through the day. It is probably associated with background concentrations of
several VOCs (Fig. 10).
For this measurement period, over 35 % of benzene, 14 % of the
xylenes, 13 % of the C9 aromatics, and 14 % of the C10 aromatics is
estimated to be due to residential biomass burning (Fig. 11). Additionally,
traditionally biogenic species such as m/z 69 and the monoterpenes were
originating mainly (46 and 36 % respectively) from residential biomass
burning during this winter period. The Athens winter PMF analysis highlights
the importance of biomass burning during winter for the VOC budget. Biomass
burning and traffic were the major sources for the majority of the VOCs that
were measured. However, biogenic sources had a limited contribution to
the VOC budget.
Species percentage (%) attributed to the PMF factors for the
Athens winter campaign for the five-factor solution.
PMF overview
Most of the urban source apportionment VOC studies separate the
anthropogenic VOCs into several categories related to vehicle exhaust
emissions, the evaporation of fuel, industrial solvents, diesel exhaust
emissions, etc. (Brown et al., 2007; Tolga et al., 2007; Badol et al., 2008;
Leuchner and Rappenglück, 2010; Yurdakul et al., 2013; Boynard et al., 2014;
Wang et al., 2014). In the present study the PMF analysis did not result in
such separations since the VOC portfolio did not include light alkanes, which
are usually included in datasets obtained by gas chromatographic (GC) techniques. The vehicular exhaust and LPG sources widely reported in the
literature include a large fraction of these species; thus, such a separation
was not favored by the PMF analysis.
Conclusions
Online measurements of VOCs and other gas and particulate species were
performed in background urban sites of Greece during the summer and winter of
2012 and 2013. For the summer campaigns in Patras and Athens, the isoprene
concentration was on average 1.0 and 0.7 ppb respectively. The corresponding
values for the monoterpenes were 0.3 and 0.9 ppb with the elevated Athens
concentrations originating mainly from local emissions. Typically, the
isoprene concentrations peaked during the day with values up to 2 ppb, while
during the night the concentrations were lower (< 0.5 ppb). Benzene,
toluene, and the xylenes average concentrations were 0.12, 0.28, and 0.25 ppb
for Patras and 0.22, 0.81, and 0.67 ppb for Athens respectively. These
differences are considered modest if the population difference (one order of
magnitude) is taken into account. During the morning rush hour aromatic
compounds such as toluene, peaked up to 5 ppb for Athens during the summer
and 1 ppb for Patras. Analogous concentrations were seen for the xylenes,
while benzene values during rush hour were in the range of 1 ppb for Athens
and 0.3 ppb for Patras.
During winter in Athens the average concentrations for isoprene and the
monoterpenes were similar to the summer values (1.1 ppb for isoprene and
0.4 ppb for the monoterpenes), indicating the importance of biomass burning
sources of these compounds. Benzene, toluene, and the xylenes concentrations
were elevated during winter (1.0, 2.3, and 1.7 ppb respectively) compared to
summer, something attributed to the lower mixing height and the presence of
additional sources. Concentrations up to 15 ppb for acetic acid, toluene, and
the xylenes were measured. For the majority of the measured species, elevated
concentrations were observed during night, signifying the importance of
biomass burning, which was enhanced by the lower mixing heights.
PMF analysis showed that traffic was the major aromatic VOC source in all
three campaigns. A traffic-related factor was identified including more than
60 % of benzene, 60 % of toluene, 70 % of the xylenes, and
70 % of the C9 aromatics. Especially during summer, traffic dominated the
above aromatic budget by contributing 73 % of benzene for Patras and
Athens and 80 and 62 % of toluene for Patras and Athens respectively. In
total, 85 % of the xylenes in Patras and 70 % in Athens were also
apportioned to traffic.
The recent economic crisis along with the higher prices of fossil fuels has
led to increased use of biomass burning for residential heating. Biomass
burning emission ratios (ERs) and emission factors (EFs) due to residential
heating were estimated. PMF analysis showed that the emissions of the
aromatics by biomass burning were comparable to traffic emissions.
Approximately 35% of benzene was due to biomass burning during the Athens
winter campaign. Additionally, during winter, the “traditional” biogenic
species, such as m/z 69, the monoterpenes, MVK, and MACR were originating
mainly from biomass burning processes. During summer these biogenic species
were almost exclusively linked to emissions from vegetation. During summer
several oVOCs such as acetic acid, formic acid, and ethanol were also linked
to biogenic emissions either directly or as secondary products.