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Levels of Aromatic Hydrocarbons in the atmosphere of one urban site
located at the northeast of Mexico during winter 2013
RAMÍREZ-LARA, E1; FERNÁNDEZ-DELGADILLO, S1; CERÓN-BRETÓN J.G2; CERÓNBRETÓN, R.M2; GUEVARA-CARRIÓ, E2; ALDERETE-CHÁVEZ, A2; CARBALLO-PAT, C.G2;
ANGUEBES-FRANSESCHI, F2; PEVA-PAMPLONA, INRY2; ORTÍNEZ-ÁLVAREZ, J.A3; AND
LÓPEZ-CHUKEN, U1.
1
Universidad Autónoma de Nuevo León, Facultad de Ciencias Químicas. San Nicolás de los Garza,
Nuevo León, México.
2
Autonomous University of Carmen, Carmen City, Campeche.
3 National Institute of Ecology and Climatic Change (INECC), México,D.F. México.
MEXICO
[email protected] http://www.uanl.mx
Abstract: - Atmospheric levels of aromatic hydrocarbons: benzene, toluene, ethylbenzene and p-xylene
(BTEX), were measured in an urban site located in Monterrey Nuevo León, Mexico using an active sampling
during winter 2013. 1.5 h-samples were collected using carbon packed cartridges at 09:00 h, 12:00 h and 15:00
h, and then analyzed using gas chromatography with flame ionization detector (GC-FID). Criteria pollutants
(CO, O3, NO, NO2, NOx, and SO2) were measured by automatic analyzers and correlated with BTEX by a
Principal Component Analysis (PCA). The relative abundance of BTEX followed the order: benzene>
toluene> ethylbenzene>p-xylene with mean concentrations of 30.97 g/m3, 19.86g/m3, 5.22g/m3 and
4.52g/m3, respectively. All measured BTEX showed a clear diurnal pattern. The highest mean levels for
benzene were obtained during the afternoon, and toluene, ethylbenzene and p-xylene showed the highest levels
during the morning sampling. Levels of BTEX were higher when wind blew from NE, N and SW, in these
directions are located the municipalities of Apodaca, General Escobedo and Santa Catarina, respectively,
where important industries, high traffic avenues and many oil and gas service stations are located. It can be
suggested that all these sources could contributed to the BTEX concentrations found in this site during the
sampling period.
Key-Words: - BTEX, ozone, criteria pollutants, aromatic hydrocarbons, Nuevo Leon, Mexico
1 Introduction
establish emission reduction strategies for these
compounds. Volatile organic compounds (VOCs)
may represent a threat to human health. This group
of compounds comprises some toxic air pollutants
like carbonyls and aromatic hydrocarbons (BTEX).
Long-term exposures to these pollutants may result
in mutagenic and carcinogenic effects [2]. BTEX
and NOx are emitted from motor vehicle exhaust,
chemical and petrochemical industries, steel
production, and area sources. An additional source
for these compounds are the evaporative emissions
produced from storage tanks, transport pipelines, oil
and gas service stations and waste area [3,4].
Monterrey Metropolitan Area (MMA) is one of the
most important urban and industrialized area in
Mexico. Vehicular traffic, industrial, social, cultural,
commercial and economical activities are performed
in this area. The most popular municipality is
Monterrey City, which is the third largest city in
Urban and industrial regions have the presence of
photochemical smog which constitutes a problem of
increasing concern [1]. Photochemical smog is
originated from the photochemical reaction of NOx
(NO+NO2) and volatile organic compounds (VOCs)
in the presence of sunlight. These compounds are
ozone precursors, which is the most important
oxidant in the troposphere. Thus, in urban areas
where NOx and VOCs levels are high (due to
vehicular traffic and industrial emissions), ozone
tends to accumulate rapidly. Ozone affects human
health, is toxic to plants and it is associated with
corrosion in materials, urban structures and cultural
and historic inheritance.
For these reasons, it is very important to know
the trends of ozone precursors, their spatial and
temporal distributions, and their sources, in order to
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Mexico, where important urban and industrial
activities are carried out. This city is located at
25°40’N and 100°18’ W at 537 masl and covering
an area of 580.5 km2. MMA is characterized by the
presence of important education and research
centers, business activities and industrial
development.
2.2 Sampling Method
Thirty six samples were collected from February 26
to March 13, 2013. Benzene, ethyl benzene, toluene
and p-xylene were measured in ambient air.
Samples of air were collected within glass tubes
containing 226-01 Anasorb CSC (SKC): length 70
mm; inner diameter 4.0 mm; outer diameter 6 mm
packed in the first section with 100 mg of active
carbon and 500 mg in the second one, separated by
a glass wool section (Method INSHT MTA/MA030/A92) [5]. The downstream end of the glass tube
was connected to a calibrated flow meter. Ambient
air samples were passed through the glass tubes at a
flow rate of 200 ml min-1 at 1.5-hour intervals (day,
midday and afternoon). Sampling was carried out
using a Universal XR pump model PCXR4 (SKC),
at three sampling periods: B1 (from 09:00 to 10:30
h), B2 (from 12:00 to 13:30 h) and B3 (from 15:00
to 16:30 h). During sampling, the Swagelok® fitting
was removed from the marked end, and a diffusion
cap was fitted to the end of the sampling tube. Prior
to the main study, several pilot experiments were
conducted to evaluate the suitability of the sampling
procedure intended for use in the main study. This
procedure included determining appropriate
sampling times. Sampler tubes were protected from
bad weather conditions by aluminum shelters. After
the exposure time, the adsorption tubes were labeled
and capped tightly with PTFE caps and transferred
to the laboratory in cold boxes. This procedure was
applied to both clean and sample tubes for storage
prior to use or analysis. Field blanks were
transported along with samplers to the field and
stored in the laboratory during the sampling period.
Samples were analyzed within three weeks after
sample collection at the Environmental Sciences
Laboratory in the Autonomous University of
Carmen City (UNACAR).
In the present study, we present a monitoring
campaign of 4 aromatic hydrocarbons (BTEX):
benzene, toluene, ethylbenzene and p-xylene in
ambient air of one site located in Monterrey city,
during winter 2013. BTEX were determined by
active sampling in sorbent tubes, extracted with
carbon
disulfide
and analyzed
by gas
chromatography with FID detector. A total of 36
samples were collected from February 26 to March
13, 2013. At the same time various meteorological
parameters (relative humidity, barometric pressure,
temperature, wind speed, wind direction and
precipitation) and criteria air pollutants (NO2, NO,
CO, SO2, O3, PM10 and PM 2.5) were measured by
an automatic station. The influence of BTEX and
NOx levels and the meteorological parameters were
statistically analyzed.
2 Methodology
2.1 Sampling Site Location
The specific sampling site was located in the
facilities of the Chemistry Faculty, Post grade
Division of the Universidad Autónoma de Nuevo
León in Monterrey city within the MMA (25° 44'
42'' N; 100° 15' 17'' W), at 500 masl (Fig. 1). This
site is located within an industrial, residential,
educational and commercial area where also there
are three avenues of high vehicular traffic.
2.3 Analytical Method
Samples were extracted with 1 ml of CS2 for each
section of the samplers tubes, shaking during 30 s to
assure a maximum desorption. Extracted samples were
analyzed using a TRACE GC Ultra gas
chromatograph (Thermoscientific) and one flame
ionization
detector
(FID;
Thermoscientific
Technologies, Inc) (Method INSHT MTA/MA030/A92) [5]. The analytical column used was a
capillary column (57 m, 0.32 mm i.d., 0.25 μm film
thickness). Operation of the instrument was controlled
using a Trace Chemstation data system. The oven
temperature program was initially set to 40 °C for 4
min, which was then increased at a rate of 5 °C/min up
to 100 °C, and was finally maintained for 10 min at
Figure 1. Sampling Site Location.
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100 °C. The FID temperature was set to 250 °C using a
hydrogen/air flame with constant flows of 35 ml min-1
and 350 ml min-1 for ultra-pure hydrogen and extradried air, respectively. The ultra-pure nitrogen carrier
(99.999%) gas flow rate was 1 ml min [6]. Four
BTEX that included benzene, p-xylene, ethylbenzene,
and toluene, were investigated.
(B2). Toluene (T) had the highest levels of
concentration during the mornings (B1), decreasing
during the midday (B2), and registering the lowest
values during the afternoon sampling (B3). Diurnal
variation and descriptive statistics can be observed
in Fig 2. Abundance of BTEX had the following
order: B > T> EBz> X with levels of concentration
of 30.97 g/m3, 19.86g/m3, 5.22g/m3 and
4.52g/m3, respectively.
-1
2.4 Monitoring of meteorological parameters and
criteria air pollutants
Sevral meteorological parameters were monitored in
the study site (wind direction, wind speed, relative
humidity, temperature, and barometric pressure) by a
portable meteorological station Davis Vantage Pro II
model. Wind roses were constructed for each day
using the software WRPLOT (from Lakes
Environmental) [7]. Air masses trajectories were
calculated 24 h back for the studied period using
HYSPLIT model from the NOAA (National Oceanic
Administration Agency, USA) in order to identify the
probable origin of the air masses.
Comparing the found results in this study with
those reported in other sites around the world (Table
1), it can be observed that B levels were similar to
those reported in Rome [11] and Mexico City [14]
but higher than those registered for Seoul, Korea
[13]. T concentrations were similar to those found in
Hannover, Germany [12] but lower than those
reported for Mexico City [14], Seoul, Korea [13]
and Rome, Italy [11]. Ebz levels were lower than
those reported for Rome, Italy [1] and Mexico City
[14] but higher than those registered in Tijuca,
Brazil [9] and Hannover, Germany [12]. In the case
of p-xylene, its levels were lower than those
reported in Rome, Italy [11], Mexico City [14] and
Seoul, Korea [13] but higher than those reported for
Hamburh, Germany [10].
Criteria Air Pollutants (O3, NO, NO2, NOx, CO
and SO2) were obtained from the Integrated System
of Environmental Monitoring of the MMA (SIMA),
specifically from the Northeast Station, located in
the Laboral Unity District in San Nicolás de los
Garza, N.L. at 25° 44’ 42 “N and 100° 15’ 17” W at
500 m above sea level, within an area with high
density of population. All criteria pollutants were
determined using automatic analyzers API
Teledyne.
2.5. Correlation and Principal Component
Analysis (PCA)
Spearman rank was applied to all data collected at
the sampling site. To assess the relationships
between concentrations of studied BTEX,
meteorological parameters and criteria air
pollutants, a factor analysis (Principal Component
Analysis) was applied using the software XLSTAT
[8].
Figure 2. Diurnal variation and descriptive statistics
for BTEX measured during summer 2013: B1
(09:00-10:30 h), B2 (12:00-13:30 h) and B3 (15:0016:30 h).
3 Results
3.1 Diurnal variation
Ethylbenzene (EBz) and p-xylene (X) showed the
same diurnal pattern registering the highest
concentrations during the morning (B1) decreasing
during the afternoon (B3) and showing the lowest
values during the midday (B2). Benzene (B)
showed the maximum concentrations during the
afternoon (B3) decreasing during the morning (B1)
and showing the lowest values during the midday
ISBN: 978-960-474-375-9
3.2 Meteorological influence
A wind rose analysis was used to determine the
prevailing conditions during the sampling campaign
and to identify the probable sources using the
Software WRPLOT View (Lakes Environmental).
During the sampling period there were some air
pollution episodes with high values of BTEX
concentrations. B showed high values in February
28 (B1: 71.12 gm-3) when winds blew from NE
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(80% and 4-6 ms-1), in March 11 (B2: 81.06 gm-3)
when winds blew from N (60% and 1-3 ms-1), and
finally, in March 5 (B3: 49,41 gm-3) when winds
blew from N (40% and 7-10 ms-1). T had the highest
levels in March 5 (B1: 123.75 gm-3) when winds
blew from SW (30% and 1-3 ms-1), in March 4 (B2:
40.253 gm-3) when winds blew from SW (40% and
1-3 ms-1), and finally, in February 26 (B3: 52.05
gm-3) when winds blew from NE 8405 and 1-3 ms1
). The highest concentrations values for Ebz were
found in March 5 (B1: 16.67 gm-3) when winds
blew from SW (30% and 1-3 ms-1), in March 4 (B2:
7.39 gm-3 and B3: 8.48 gm-3) when winds blew
from SW (40% and 1-3 ms-1).
traffic; and the biggest airport in this region,
respectively. At N of the sampling site it is located
the municipality of General Escobedo where an
important industrial zone is located. Santa Catarina
municipality is located at SW of the sampling site
where many industries, avenues with heavy traffic
and oil and gas storage stations are located. At NW
of the sampling site it is located Apodaca
municipalty where wood and food industry facilities
are located. All these sources could contribute to the
observed BTEX levels.
a) B3: 04/03/2013
Table 1. Levels of BTEX (μg/m3) in other sites
around the world and their comparison with
atmospheric concentrations found in this study.
Location
Tijuca,
Brazil [9]
Hamburg,
Germany
[10]
Rome,
Italy [11]
Hannover,
Germany)
[12]
Seoul,
Korea [13]
Mexico
City
[6]
This Study
B
T
Ebz
X
1.1
4.8
3.6
10.4
1.4
4.7
ND
0.88
35.5
99.7
17.6
54.6
4.27
24.0
2.99
7.64
5.1
32.03
48.2
34.4
7.8
16.1
16
14.0
30.97
19.86
5.22
4.52
b) B2: 04/03/2013
B: Benzene; T: Toluene; Ebz: Ethylbenzene; X: p-Xylene
P-Xylene levels (X) were higher when winds blew
from SW (30% and 1-3 ms-1) in March 5 during the
mornings (B1) showing a maximum value of 25.04
gm-3; when winds blew from NW (40% and 4-6
ms-1) in February 26 during the midday (B2)
showing a value of 7.137 gm-3; and finally, when
winds blew from NE (40% and 1-3 ms-1) in
February 26 during the afternoon (B3) with a
concentration value of 11.79gm-3. Fig. 3 – Fig. 5
show the wind roses for the air pollution episodes
occurring in February 26, March 4 and March 5.
Apodaca municipality is located at NE from the
sampling site. In this direction is located an
important industrial zone, many oil and gas service
stations, important avenues with high vehicular
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Figure 3. Meteorological influence on BTEX levels
during an air pollution episode occurred during
March 4.
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a) B2: 26/02/2013
a) B3: 05/03/2013
a) B3: 26/02/2013
b) B1: 05/03/2013
Figure 4. Meteorological influence on BTEX levels
during an air pollution episode occurred during
February 26.
Figure 5. Meteorological influence on BTEX levels
during an air pollution episode occurred during
March 5.
3.3. Toluene to Benzene ratio (T/B ratio)
T/B ratio has been commonly used as an indicator
of traffic emissions. B and T are constituents of
gasoline and are emitted into the atmosphere by
motor vehicle exhausts. The toluene content of
gasoline and motor vehicle exhaust is 3-4 times
higher than B content [14]. Values lower than 2-3
are characteristic of vehicular emissions in many
urban areas worldwide [14, 15], whereas, values
higher than 3 may indicate that BTEX levels could
be associated to industrial facilities and area sources
(evaporative emissions, painting, cooking process,
an others).
ISBN: 978-960-474-375-9
The range for the study site was between 0.0053 and
2.0438 being higher during the morning sampling
period as it can be observed in Table 2. These
values are in agreement with typical values of
vehicular emissions reported for other urban areas,
suggesting that this site was under the influence of
mobile sources.
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Table 2. Toluene/Benzene Ratios
Table 3. p-Xylene/Ethylbenzene Ratios
Toluene/Benzene
B-1 µg/m3 B-2 µg/m3 B-3 µg/m3
1.9733
1.7239
1.5735
0.3640
0.0439
0.2883
0.6260
1.1186
0.0908
0.4830
0.4496
1.0588
0.6689
1.5003
0.1095
2.0438
0.0053
0.0617
0.4242
0.5538
0.2963
0.7526
0.9160
0.6011
0.4048
0.5706
0.6803
0.1820
0.1688
0.3981
0.5793
0.5757
0.2843
0.6600
0.3236
0.3023
p-xylene/ethylbenzene ratio
B-1 µg/m3 B-2 µg/m3 B-3 µg/m3
3.4 p-Xylene to Ethylbenzene ratio (X/Ebz ratio)
3.5 Principal Component Analysis (PCA)
A PCA analysis was applied for the BTEX
concentrations (benzene, toluene, ethylbenzene and
p-xylene), meteorological parameters (temperature:
T, barometric pressure: P, relative humidity: RH,
wind direction: WD, and wind speed: WS). Figs. 68 show the PCA loadings obtained for the morning,
midday and afternoon sampling periods (B1, B2 and
B3, respectively) for winter 2013.
0.9763
1.2455
0.9218
1.8634
1.8660
1.5021
0.1223
1.5083
1.1637
0.4400
1.2494
1.2977
In Table 3 are shown X/Ebz ratios for this study.
This ratio is used commonly as indicator of the
photochemical age of the air masses. A ratio of 3.6:
1 of (X/Ebz) has been established as a typical
emission relation for these species [16, 17]. This
ratio is useful to determine the staying of pollutants
in the atmosphere, high values of this ratio means
that air masses have stayed a long time in the
atmosphere (old emissions) and low values of this
ratio indicate that air masses are recent (fresh
emissions). Kuntasal et al. [18] used a value of 3.8
for this ratio. Gasoline fresh emissions have shown
values between 3.8 and 4.4 for this ratio. In this
study, during winter 2013, the whole period
registered low values for this ratio (from 0.0299 to
1.8972), indicating that most of the air masses
correspond to “fresh emissions”. It can be suggested
that these fresh emissions correspond to vehicular
emission from mobile sources.
1.1015
0.3378
0.1329
0.9031
1.8972
0.1826
0.4133
0.4908
0.6957
0.2647
0.2128
0.5862
1.4190
0.0763
0.1004
1.2257
0.0300
0.0814
0.3205
0.6839
0.2397
0.4332
0.1469
0.1624
Fig. 6 shows the principal compound analysis
(PCA) among BTEX, meteorological parameters
and criteria pollutants during winter 2013 for the
morning sampling period (B1). A significant
positive correlation was found among CO, NOx,
NO2, and NO (Pearson correlation factor > 0.89)
indicating that probably these compounds had their
origin in common sources, probably vehicular
emissions. Ozone showed a significant negative
correlation with CO (-0.667), NO (-0.614), NO2 (0.662) and NOx (-0.535), indicating that all these
compounds were Ozone precursors. Ebz and X had
a non significant negative correlation with ozone
indicating that both compounds could contributed at
least partially to ozone formation in this site. All
BTEX showed a good correlation among each other
indicating that these compounds probably had the
same emission sources (local area sources).
Fig. 7 shows the principal component analysis
(PCA) among BTEX, meteorological parameters
and criteria pollutants during winter 2013 for the
midday sampling period (B2). NO, NO2, NOx, T,
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Ebz and X a significant positive correlation with CO
(0.881, 0.87, 0.665, 0.579, 0.5 and 0.865,
respectively) which is evidence that all these
pollutants could be originated from mobile sources.
Ozone correlated in a significant positive way with
NO, NO2 and NOx (0.549, 0.559 and 0.521,
respectively) being evidence that all these
compounds could be originated from photochemical
processes in the atmosphere. B did not show
significant correlations with the other BTEX,
indicating that this compound could be originated
from a different source. Good correlations among T
with Ebz and X, indicates that these hydrocarbons
could be originated from common sources, probably
vehicular emission due to they showed a good
correlation with CO (0.579, 0.5 and 0.865,
respectively).
Figure 6. PCA Analysis for the morning sampling
period (B1).
F2 (14.83 %)
Variables (axis F1 y F2: 59.24
1
%)
0.75
B
0.5
WDR
Ebz
0.25
0
T
O3 X
TOUT
NO2
CO
NONOX
RH
PRS
-0.25
SO2
-0.5
WSR
-0.75
PM10
-1
In Fig. 8 can be observed that during the afternoon
sampling period (B3) CO showed significant
positive correlations with NO (0.852), NO2 (0.851),
and NOx (0.602), indicating that these compounds
could be originated from vehicular emissions.
Ozone had negative significant correlations with CO
(-0.731), NO (-0.509), indicating that all these
compounds could be contributed to O3 formation in
the study site during this period. All BTEX had no
significant negative correlations with ozone,
indicating that at least in a partial way could
contribute to ozone formation. B had a good
correlation with Ebz (0.551), being evidence that
both hydrocarbons could have common sources. T
had good correlations with Ebz and X (0.698 and
0.953, respectively), this is evidence that these
compounds probably were originated from sources
in common (local area sources).
-1 -0.75-0.5-0.25 0 0.25 0.5 0.75 1
F1 (44.41 %)
Figure 7. PCA Analysis for the midday sampling
period (B2).
Variables (axis F1 y F2: 55.23
1
%)
X
0.75
T Ebz
WDR
F2 (20.11 %)
0.5
WSR
O3
0.25
0
PRS
-0.25
SO2
-0.5
B
TOUT
NOX
PM10
NO2
CO
NO
RH
-0.75
-1
Variables (axis F1 y F2: 66.36
1
%)
X PM10 TOUT
Ebz
F2 (31.23 %)
0.75
0.5
0.25
F1 (35.12 %)
T
WSR
WDR
B
-0.25
Figure 8. PCA Analysis for the midday sampling
period (B2).
NOX NO
CO
NO2
0
-0.5
-1 -0.75-0.5-0.25 0 0.25 0.5 0.75 1
4 Conclusion
RH
SO2
O3
PRS
Measured BTEX had the following relative
abundance: B> T> Ebz >X. Mean concentrations of
benzene are higher than those reported in Tijuca,
Brazil and Hamburg, Germany. BTEX levels were
similar than those reported for Mexico City and
Rome. All measured BTEX showed a clear diurnal
pattern. B had the highest concentrations during the
-0.75
-1
-1 -0.75-0.5-0.25 0 0.25 0.5 0.75 1
F1 (35.13 %)
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afternoon, and toluene, ethylbenzene and p-xylene
showed the highest levels during the morning
sampling. Levels of BTEX were higher when wind
blew from NE, N and from SW. In these directions
the municipalities of Apodaca, General Escobedo
and Santa Catarina are located, where there are
important industries, high traffic avenues and many
oil and gas service stations. It can be suggested that
all these sources could contributed to the BTEX
concentrations found in this site during the sampling
period. T/B ratios showed that BTEX were
originated from vehicular emissions and X/Ebz
ratios indicated that all BTEX emissions
corresponded to fresh emissions from mobile
sources. These results indicate that BTEX
concentrations were highly influenced by vehicular
emissions. PCA analysis showed that CO, NO, NOx
and NO2 played an important role in the
tropospheric ozone formation in the study site.
Environment, Economics and Technological
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