atmospheric ethene concentrations in mexico city: indications of strong diurnal and seasonal...
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Atmospheric Environment 39 (2005) 5219–5225
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Atmospheric ethene concentrations in Mexico City:Indications of strong diurnal and seasonal dependences
V. Altuzara, S.A. Tomasa,�, O. Zelaya-Angela,�,F. Sanchez-Sinencioa, J.L. Arriagab
aDepartamento de Fısica, Centro de Investigacion y de Estudios Avanzados del IPN, AP 14-740, Mexico 07360 DF, MexicobLaboratorio de Quımica de la Atmosfera, Instituto Mexicano del Petroleo, Eje Central Lazaro Cardenas 152, Mexico 07730 DF, Mexico
Received 25 March 2004; received in revised form 21 August 2004; accepted 28 September 2004
Abstract
Monitoring of atmospheric ethene was carried out in Mexico City with a 12C16O2-laser-based photoacoustic
spectrometer. We assessed the variation of the ethene content in atmospheric samples simultaneously collected in
stainless-steel containers at three stations of the local government’s air quality monitoring network in November 1999
and March 2000. The ethene levels in November were higher than those detected in March, reaching up to 36.9 ppbV.
In addition, continuous measurements of ethene were carried out in February 2001 for 1 week, with a time resolution of
1min. It allowed recording of real-time ethene levels of up to 68 ppbV. A comparison between ethene profiles recorded
on weekends and working days clearly shows a pronounced difference in concentration, which is almost three-fold
higher on working days.
r 2004 Elsevier Ltd. All rights reserved.
Keywords: Ethene; CO2-laser; Photoacoustic spectroscopy; Vehicle exhaust-emission
1. Introduction
Ethene is a gas of extreme importance in the fields of
plant physiology and the environmental sciences. In
plant physiology, this gaseous phytohormone plays a
crucial role in many physiological processes. For
instance, it is emitted by fruits and plants in response
to saline, thermal, mechanical, or chemical stresses, and
regulates processes such as ripening, growth, germina-
tion, and senescence (Abeles et al., 1992). The environ-
mental interest in this molecule is related to its
anthropogenic origin, namely, its production in combus-
tion processes that include fires, industry and mainly
e front matter r 2004 Elsevier Ltd. All rights reserve
mosenv.2004.10.002
ing authors. Fax: +5255 5061 3386.
esses: [email protected] (S.A. Tomas),
vestav.mx (O. Zelaya-Angel).
vehicle exhausts. As a consequence, ethene has been
considered to be a potential human carcinogen, since
this alkene is partly metabolized by mammals via ethene
oxide, a direct-acting alkylating agent that induces
cytogenetic alterations, mutations, and cancer. As
claimed by Tornqvist (1994), exposure to 10 ppbV of
ethene per hour during a week is expected to lead to a
lifetime risk of cancer death amounting to approxi-
mately 70 per 100,000. Since ethene is mostly detected at
ppbV levels in urban atmospheres, the development of
sensitive and selective detection systems that allow for its
detection has been strongly encouraged during the last
few years (Sigrist, 1994).
During the last decades, air pollution in Mexico
City has become the greatest concern. Mexico City,
a metropolis with nearly 20 million inhabitants, is
located on a high valley surrounded by mountains, at
d.
ARTICLE IN PRESSV. Altuzar et al. / Atmospheric Environment 39 (2005) 5219–52255220
approximately 2270m over the sea level. Such moun-
tains work as natural wind barriers, obstructing the
dispersion of air pollutants released by industry and
emitted by over 4 million cars. As a result, frequent
thermal inversion phenomena and severe smog events
were reported from the 1980s to the mid 1990s (Blake
and Rowland, 1995).
Previously, we have used an infrared photoacoustic
spectroscopy to detect atmospheric ethene in Mexico
City. Air samples were simultaneously collected in
electropolished stainless-steel containers at three differ-
ent sites, and they were later analyzed in our laboratory
with a CO2-laser-based photoacoustic spectrometer
(Altuzar et al., 2001). We have also reported the
continuous (on-line) monitoring of ethene; such mea-
surement was recorded in a single sampling site in two
different seasons (Altuzar et al., 2003). Here, we
conducted measurement campaigns in order to analyze
the variation of the ethene content in atmospheric
samples collected at three sites in two different seasons.
In addition, we analyze in detail the ethene time profile
over a 1-week period, including a comparison between
profiles recorded in weekends and working days. The
analysis of the ethene time profile is complemented with
ozone and nitrogen oxides data.
2. Methodology
2.1. Experimental set-up
The photoacoustic (PA) effect is based upon the
conversion of electromagnetic energy into acoustic
energy (Sigrist, 1994; Harren and Reuss, 1997). A
system that is excited by the absorption of light and
that undergoes subsequent non-radiative de-excitation
processes transfers its electronic, vibrational, or rota-
tional energy into translational energy, producing a
temperature increase. When the absorption is modu-
lated, the temperature changes periodically, leading to
pressure (acoustic) oscillations that can be detected by a
sensitive microphone. The photoacoustic spectrometer
used in this work (Altuzar et al., 2003) consists of a
continuous wave 12C16O2 laser and a PA trace gas
detection system. The laser wavelength can be tuned at
around 80 lines in the 9–11 mm infrared region. The
intracavity laser power typically reaches 40 W at the
lines selected for measurement. The PA cell is placed
inside the cavity of the laser and is designed as a
cylindrical acoustic resonator, extended by two buffer
volumes, to amplify the generated sound wave due to
absorption. The volume of the acoustic resonator is
4.24 cm3, which implies that the resonator has an
exchange rate of 3.8 s when using a flow rate of 4 l h�1.
The laser beam is intensity-modulated by means of a
mechanical chopper locked at the resonance frequency
of the PA cell (1160Hz). The PA system was calibrated
using a certified gas mixture of ethene buffered in air
(1.2 ppmV ethene). The PA signal is recorded by means
of an electret microphone connected to a lock-in
amplifier.
The most general situation in PA gas detection
involves the measurement of multicomponent samples.
PA signals, normalized with the laser power to account
for power fluctuations, have thus to be measured on
several laser lines coinciding with absorption bands of
the compounds under study. A subsequent least-squares
analysis (Bernegger and Sigrist, 1990) provides the
concentration of the different gases constituting the
mixture. In the present work, we have restricted
ourselves to the detection of ethene. The 10P14
(l=949.48 cm�1) and 10P12 (l=951.19 cm�1) laser lines
were selected to determine the ethene concentration. At
these lines, the optical absorption coefficients for ethene
are 30.4 cm�1 atm�1 and 4.31 cm�1 atm�1, respectively.
The recording of the PA phase was not important
because CO2 and water vapor were eliminated from the
airflow by inserting KOH and CaCl2 scrubbers in
combination with a cold trap (Moeckli et al., 1998).
To avoid additional problems, Teflon tubing was used.
The time resolution is mainly imposed by the time
needed to position the grating on the selected laser lines.
It resulted in nearly 1min. The detection limit for ethene
was 100 pptV, as determined from a signal-to-noise ratio
equal to one when the PA cell is operated in a
continuous-flow mode. Estimating an average uncer-
tainty of 3% for the cell constant, 2% for the certified
gas concentration, 1% for the 4 l h�1 flux of molecules
transported through PA cell, and 1% of random error of
the measurement, one finds an overall uncertainty for
PAS data of about 7%.
2.2. Air sampling
Electropolished stainless-steel canisters were used to
collect air samples. Field campaigns were conducted in
collaboration with the Instituto Mexicano del Petroleo
(IMP). The campaigns took place from 22 to 26
November 1999 and from 20 to 24 March 2000. Uptake
of ambient air was achieved by the use of an automated
portable air pumping system, type VOCS, provided with
a mass flow controller. The equipment was placed at 3m
above the ground. Sampling was simultaneously per-
formed from 06:00 to 09:00 h at three different sites,
at a filling rate of 33ml min�1. The selection of the
sampling sites was made according to meteorological
conditions, especially wind speed and wind direction, as
well as to urban characteristics, such as industrial acti-
vity and traffic density. The sites selected were Xalostoc,
La Merced and Pedregal (Fig. 1). Xalostoc, located
in a northeastern suburb of Mexico City, presents
intense industrial activity, as well as a high density of
ARTICLE IN PRESS
440000
460000
480000
500000
520000
540000
2080000
2100000
2120000
2140000
2160000
2180000
2200000
2220000
Xalostoc
Merced
Pedregal
Cinvestav
N
S
W
UTMX, M
UT
MY
, M
E
Fig. 1. Map of the metropolitan area of Mexico City showing
the location of the sampling sites: (a) Xalostoc, (b) La Merced,
(c) Pedregal, and (d) CINVESTAV. UTMX and UTMY stand
for the longitude and latitude, respectively, of the Universal
Transverse Mercator grid.
20 21 22 23 240
10
20
30
40
50
60
March 2000
22 23 24 25 260
10
20
30
40
50
60
La Merced Pedregal
ethe
ne c
once
ntra
tion
[ppb
V]
November 1999
Xalostoc
Fig. 2. Ethene concentration measured in air samples collected
simultaneously in Xalostoc, La Merced, and Pedregal. Air
sampling was carried out in November 1999 and March 2000.
V. Altuzar et al. / Atmospheric Environment 39 (2005) 5219–5225 5221
heavy-duty automotive vehicles. La Merced, situated in
central Mexico City, near downtown, is characterized by
a strong commercial and administrative activity, with
intense flux of cars and small-to-medium-size trucks.
Pedregal is a southwestern wealthy residential area with
low traffic density.
Real-time monitoring of ethene was carried out at the
Department of Physics, Centro de Investigacion y de
Estudios Avanzados del Instituto Politecnico Nacional
(CINVESTAV-IPN), located in northern Mexico City
(see Fig. 1). This sampling site is situated in a green area
of about 4� 104 m2, about 100m away from an avenue
with heavy traffic density. Ambient (outdoor) air was
continuously pumped to the photoacoustic spectrometer
from 05:00 to 23:00 h, from 17 to 23 February 2001.
3. Results and discussion
3.1. Samples collected in canisters
The ethene concentrations determined in integrated
samples collected in November 1999 and March 2000
are shown in Fig. 2. Except for 1 day, the highest ethene
concentrations in March were detected in the industrial
site, Xalostoc, with a mean value of 36.9 ppbV. It
supports previous works reporting this place as one of
the most polluted suburbs of Mexico City (Edgerton
et al., 1999). In spite of a strong industrial activity,
which permanently contributes to air pollution, noctur-
nal background levels of around 5 ppbV imply that
ethene comes mostly from vehicle emissions. This result
is in agreement with studies concluding that urban
ethene is primarily produced by motor vehicles (Doskey
et al., 1992). The place with the second highest presence
of ethene was La Merced (near downtown), with a mean
value of 34.1 ppbV, whereas Pedregal showed a mean
concentration of 11.6 ppbV. Regarding the field cam-
paign carried out in November, a similar order was
observed, namely, the highest ethene content was found
in Xalostoc followed by La Merced and Pedregal; the
measured concentrations were 40.3, 37.0, and 18.7 ppbV,
respectively. The fact that the ethene levels detected in
November are higher than those registered in March is
most probably due to a lower initial height of the
atmospheric boundary layer (ABL), associated with a
lower ambient temperature. Particularly, the average
temperature measured in La Merced was 4.570.4 1C
ARTICLE IN PRESSV. Altuzar et al. / Atmospheric Environment 39 (2005) 5219–52255222
lower in November than in March, with a minimum
8.3 1C registered at 07:00 h and a maximum 19.5 1C
reached at 16:00 h.
Measurements of ambient ethene in Mexico City have
been previously performed by other authors (Blake and
Rowland, 1995). These researchers determined by gas
chromatography ethene levels of 15.5 and 27.8 ppbV in
samples collected in canisters at 06:00 and 12:00 h,
respectively. Their sampling site was Zocalo, which is
located near La Merced. As will be shown below, levels
at 06:00 h are low mainly due to low vehicular activity.
Therefore, for comparison with our results, we have
considered the second datum. In our case, except for the
unusually high value obtained on 22 March (see Fig. 2),
the mean value in La Merced was 28.8 ppbV, in
agreement with the levels reported by Blake and
Rowland (1995).
Mugica et al. (1998) have also reported the ambient
ethene concentration in samples collected in tunnels and
crossroads in Mexico City. They found concentration
values in the interval from 0.49 to 4.06 ppbC%,
measured with a gas chromatograph. It should be
mentioned that in such articles, the abundance of each
hydrocarbon in the source profile is the ratio of each
concentration in ppbC to total non-methane organic
compounds in ppbC, the latter varying from 7.27 to
30.79 ppmC. The ethene concentration therefore ranges
from 17.81 to 625 ppbV. Using the CO2-laser PA
technique, Sigrist et al. have reported ethene levels of
the order of 20 ppbV in open-traffic urban areas (Meyer
4 6 8 10 12
0
10
20
30
40
50
60
70
ethe
ne c
once
ntra
tion
[ppb
V]
dayt
Fig. 3. Time profile of ethene recorded with a CO2-laser-based photo
(Tuesday 20 February, 2001).
and Sigrist, 1990) and of 250 ppbV inside tunnels
(Moeckli et al., 1996) in Switzerland. All these data
are in correspondence with the data obtained in our
work and confirm the reliability of our system.
3.2. Real time (on-line) measurements
Fig. 3 shows the real-time concentration profile of
ambient ethene measured at the facilities of CINVES-
TAV-IPN on a typical weekday (Tuesday 20 February
2001). In general, two broad peaks were observed in the
ethene concentration versus daytime plot. The first peak
increased steeply from a background level of approxi-
mately 5 ppbV after 05:00 h and reached a maximum
value of 68 ppbV at 08:20 h. Thereafter, the ethene
concentration decreased down to 3 ppbV, oscillating
around this level from 12:20 to 18:20 h. A second peak
with a lower intensity took place at 20:30 h. This profile
is in agreement with the measurements performed in
Mexico City with an open-path Fourier transformed
infrared spectroscopy (Grutter et al., 2003). The intense
vehicle emissions, the expansion of the ABL, as well as
the presence of compounds that react with ethene play
an important role on these patterns (Horie and
Moortgat, 1998). Moreover, the high emission factor
for ethene due to cold-start conditions of vehicles
strongly influences the high concentrations observed in
the morning rush-hours; this is accentuated by the
presence of an old and technologically heterogeneous
vehicular fleet (Schifter et al., 2000).
14 16 18 20 22 24ime [h]
acoustic spectrometer during a typical weekday in Mexico City
ARTICLE IN PRESSV. Altuzar et al. / Atmospheric Environment 39 (2005) 5219–5225 5223
The ABL is greatly influenced by the topography of
the city and the thermal forcing. Raga et al. (1999) and
Doran et al. (1998) studied the time evolution of the
ABL and found that it is stable until 200m above the
ground at 06:00 h and that it experiences an expansion
up to approximately 1 km at 11:00 h; then, the ABL
height increases rapidly, reaching between 2500 and
3000 m in the late afternoon. It is also known that the
ethene released into the troposphere can be removed by
reaction with hydroxyl radical. The production of this
alkene is enhanced by the topographic setting of Mexico
City and a relatively high insolation related to its
tropical latitude (191190N). Particularly, ozone exceeded
the maximum permissible exposure regulations on most
days in the mid 1990s (MARI, 1994). Atkinson (2000)
has estimated that the atmospheric ethene lifetime is 1.4
days, due to a 12-h daytime average OH radical
concentration of 2� 106 molecules cm�3. Other factor
contributing to ethene removal is associated with the
presence of high ozone levels. Thus, the reaction of NOx,
ozone, atomic oxygen, and hydroxyl radicals with
ethene contribute to the destruction of the latter
compound (Fenske et al., 2000).
By monitoring ethene on days with expected differ-
ences in the time profile of this pollutant, e.g. on
weekends and working days, the excellent sensitivity of
our instrument was tested. Ethene was measured during
the week from Saturday 17 to Friday 23 in February
2001, and in Fig. 4 the profile of two consecutive days
(Sunday 18 and Monday 19) was plotted. The highest
4 6 8 10 12
0
10
20
30
40
50
60
ethe
ne c
once
ntra
tion
[ppb
V]
dayt
Fig. 4. Comparison of the time profile of ethene recorded during two
value detected on Sunday reached 18 ppbV at around
12:30 h. On Monday morning, the maximum ethene
concentration was nearly three times higher, reaching
53 ppbV at 08:20 h. The difference in concentration is a
clear indication of the contrasting traffic density
observed during weekends and working days. Moreover,
the time lag between such maxima reflects a delay in the
start of activities of most people on Sunday. As seen
from the time lag of the second maximum, the activities
remain delayed for the whole day, although with a
smaller difference in the evening.
Fig. 5 presents the ethene profile registered at
CINVESTAV during the three consecutive days 17–19
(Saturday–Monday) of February 2001 For clarity, the
profiles recorded from February 20 to 23 are not
shown. The maximum concentrations were detected in
the range from 14 to 68 ppbV. As expected, the highest
ethene concentrations were invariably measured on
working days, with the global maximum taking place
on Tuesday. The mean dose of human exposure to
ethene was obtained by integrating the area under the
curve from 05:00 to 22:00 h, resulting in 13.7 and
7.8 ppbV on working days and weekends, respectively.
For comparison, the time profile of nitrogen oxides
(NOx) and ozone (O3) are also illustrated in Fig. 5.
These pollutants were monitored in La Merced by an
air-pollution-monitoring government network, the so-
called RAMA (Automatic Network for Atmospheric
Monitoring). No information about them is available
at the CINVESTAV-site. In spite of this, a good
14 16 18 20 22 24
ime [h]
Monday
Sunday
consecutive days in February 2001 (Sunday 18 and Monday 19).
ARTICLE IN PRESS
0 12 24 36 48 60 720
50
100
150
200
MondaySundaySaturday
ethe
ne, o
zone
and
NO
x [p
pbV
]
time [h]
C2H4
O3
NOx
Fig. 5. Time profile of ethene, nitrogen oxides, and ozone recorded in the winter of 2001, from 17 to 19 February. For clarity, the
profiles recorded from 20 to 23 February are not shown. Ethene was monitored at CINVESTAV, whereas the nitrogen oxides and
ozone data were recorded at La Merced by the Automatic Network for Atmospheric Monitoring.
V. Altuzar et al. / Atmospheric Environment 39 (2005) 5219–52255224
correlation between these profiles was found. Specifi-
cally, both ethene and NOx peaked at around the same
time. This result is expected because ethene and NOx are
strongly associated with vehicular emissions. On the
other hand, the maximum ozone concentrations were
measured between 13:00 and 15:00 h, showing the
photochemical nature of this compound.
4. Conclusions
In summary, infrared photoacoustic spectroscopy has
been successfully applied to the detection of atmospheric
ethene in Mexico City. The on-line monitoring of ethene
has revealed that levels of about 68 ppbV are frequently
reached in the morning. A mean dose of human
exposure to ethene of over 10 ppbV on working days
represents a potential risk for adverse human health
conditions, indicating an urgent need for additional
emissions control.
Acknowledgments
The authors thank the Instituto Mexicano del
Petroleo that partially financed the project IMP-FIES-
97-07-VI and to the RAMA authorities for allowing the
use of the meteorology database. This work was also
supported by CONACyT via the Project 33126-E.
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