Diurnal and seasonal variation of volatile organic compounds in the atmosphere of Monterrey, Mexico

Atmospheric Pollution Research 6 (2015) 1073e1081

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Original article

Diurnal and seasonal variation of volatile organic compounds in the atmosphere of Monterrey, Mexico
ndez b, H. Lizette Menchaca-Torre a, *, Roberto Mercado-Herna Alberto Mendoza-Domínguez a a

Department of Chemical Engineering, Monterrey Institute of Technology, Monterrey Campus, Eugenio Garza Sada 2501 S, Col. Tecnologico, Monterrey, Mexico, 64849 School of Biological Science, Autonomous University of Nuevo Leon, Pedro de Alba s/n, San Nicolas de los Garza, Mexico, 66450

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 January 2015 Received in revised form 11 June 2015 Accepted 11 June 2015 Available online 12 October 2015

Volatile organic compounds (VOCs) were characterized in Monterrey, the third largest city in Mexico. In total, 53 VOCs were characterized, of which 29 were analyzed. Three sampling campaigns were performed during the spring of 2011 and the spring and the fall of 2012, using 4-h time intervals from 6:00 am to 10:00 pm. Measurements were performed on the premises of a monitoring station located in downtown Monterrey. The highest concentrations were found in the fall; the two spring measurements exhibited no statistically significant differences. VOC concentrations exhibited a marked diurnal behavior with higher concentrations during the morning intervals. Solar radiation peaked during the noon interval, allowing for greater secondary pollutant production. VOCs reached their lowest levels in the 14:00e18:00 time interval. Correlation analysis found evidence of mobile sources, fugitive fuel emissions, and the use of solvents as possible sources of the majority of the compounds. Additionally, the VOCeO3eNOx relationship was studied. The results indicate that the atmosphere in Monterrey, Mexico is sensitive to the concentration of VOCs. Copyright © 2015 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved.

Keywords: Volatile organic compounds Air pollution Ozone isopleths Mexico

1. Introduction The presence of volatile organic compounds (VOCs) in the environment has been studied due to their high concentration in urban areas. Anthropogenic activities emit large quantities of VOCs into the atmosphere (Carter, 1994; Atkinson, 2000; Mugica et al., 2002, 2003; Mendoza et al., 2009). For example, the use of vehicles and fuel transportation generate emissions of benzene, toluene, and other gasoline components (Franco and Pacheco, 2015); solvent use emits toluene, hexane, octane, and nonane, among others (Friedrich and Obermeier, 1999; Na et al., 2003; € hrnschimmel et al., 2006; Mendoza et al., Mugica et al., 2003; Wo 2009); food production generates ethene, propene, butane, and €hrnschimmel et al., 2006). Trees and acetylene, among others (Wo other small plants are also sources of VOCs. Isoprene, emitted * Corresponding author. Tel.: þ52 (181) 81620470. E-mail addresses: [email protected] (H.L. Menchaca-Torre), roberto.
ndez), [email protected] [email protected] (R. Mercado-Herna (A. Mendoza-Domínguez). Peer review under responsibility of Turkish National Committee for Air Pollution Research and Control.

primarily by biogenic sources (Fuentes et al., 2000), and monoterpenes are the most reactive natural VOCs in the lower atmosphere (Fuentes et al., 2000; Guenther et al., 2000). VOCs have adverse effects on human health (WHO, 2000). For example, exposure to benzene has been linked to different types of leukemia (Lynge et al., 1997). Prolonged exposure to toluene, xylenes, trimethylbenzene, and styrenes can cause irritability, loss of appetite, nausea, and migraines (Sitting, 1991). High concentrations of VOCs cause depression of the central nervous system (Maroni et al., 1995) and inflammation of the respiratory system when inhaled (Mølhave, 1991). The atmospheric chemistry of VOCs is complex. Their oxidation reactions are dependent on the presence of sunlight. During the
day, VOCs interact with hydroxyl radicals ( OH) (Finalyson-Pitts and Pitts, 2000). At night the primary oxidizing agents are nitrogen oxides (NOx ¼ NO þ NO2). Ozone (O3) reacts with VOCs with or without solar radiation (Finalyson-Pitts and Pitts, 2000; Albaladejo et al., 2003). During their residence time in the atmosphere, VOCs produce nitrogenized organic compounds, such as PAN (peroxyacetyl nitrate), and are precursors of oxygenated VOCs (aldehydes and ketones) (Possanzini et al., 2002). Furthermore, their oxidation

http://dx.doi.org/10.1016/j.apr.2015.06.004 1309-1042/Copyright © 2015 Turkish National Committee for Air Pollution Research and Control. Production and hosting by Elsevier B.V. All rights reserved.

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and chain breaking aids in secondary organic aerosol formation (Watson et al., 2001; Na et al., 2003) and plays an important role in € hrnschimmel et al., 2006). the production of atmospheric O3 (Wo VOC evolution in the atmosphere depends on reactivity, reaction mechanisms, as well as the presence of solar energy and the availability of other reagents (Carter, 1994; Atkinson, 2000; Derwent et al., 2010). VOCs reactivity is referenced to the amount of O3 produced in the atmosphere during the VOCs residence time. Alkenes and substituted aromatic compounds are the most reactive, followed by aldehydes (Derwent et al., 2010). In Mexico, the study of VOCs in the atmosphere has been limited (Mugica et al., 2003). However, some research was conducted in Mexico City (Mugica et al., 2001, 2003; Velasco et al., 2007; Bon et al., 2011) and in the north of the country in the city of Mexicali (Mendoza et al., 2009). The Monterrey Metropolitan Area (MMA), the third most populated city in Mexico, has a wide variety of industrial complexes and a 1.89 vehicular fleet. Thus, atmospheric pollution is a serious problem; for example, the high O3 concentrations exceeded the 0.11 ppm set by the Mexican Standard NOM020-SSA-1993 for 31 days between January and October 2014 (SIMA, 2015). Additionally, Gonzalez-Santiago et al. (2011) identified concentration levels of PM2.5 that were consistently higher than the 50 mg/m3 established in the standard NOM-025-SSA11993, with average concentrations of up to 124 mg/m3 in the Santa Catarina area, one of the municipalities conforming the MMA. However, no studies have been performed in Monterrey, although personnel with the Integral Environmental Monitoring System (SIMA) have planned to begin the quantification of benzene, toluene, and xylenes in the near future. Characterization and analysis of VOC concentrations is important to design and regulate strategies to reduce emissions and secondary pollutants production in the city. In addition, a decrease in VOCs will reduce the levels of particulate matter less than 2.5 mm in diameter (PM2.5), because an important component of some particles is secondary organic car~ oz et al., 2011). bon (Amador-Mun 2. Methodology 2.1. Sampling site Sampling was conducted in the city of Monterrey, Mexico, between May 2011 and April 2012. Monterrey is the capital of the state of Nuevo Leon and is located in the northeast section of the country. The MMA (Fig. 1), which includes nine municipalities, has a total surface area of 5346.80 km2, and a population of 3.93 million people. The city center has an average altitude of 540 meters (m), and mountains, which act as natural barriers, surround the city. The MMA has a variety of industrial complexes including production of glass, steel, cement, paper, among others. Due to the large demographic explosion, there are 1.89 million vehicles. All of the samples were collected in the central station of SIMA. The station is located in the southeastern area of the first block of Monterrey at an altitude of 556 m above sea level. Information on how the station was selected is described elsewhere (Menchaca et al., 2015). 2.2. Sampling Samples were taken at a single point in agreement with the supersites PM monitoring system of the U. S. EPA (1999). All samples were collected at the Obispado station (center) of the SIMA of the
n (Fig. 1). The station is located in the state government in Nuevo Leo southeast section of downtown Monterrey at an altitude of 556 m above sea level with the geographical coordinates: 25 400 3200 N, 100 200 1800 W. Three sampling campaigns were performed; each

consisted of seven non-consecutive days during a two-week period, on the following dates: May 28 to June 9, 2011; June 6e18 and October 13e25, 2012. Sampling was done in 4-h intervals from 6:00e22:00 h. In total, 28 samples were collected in each campaign. Samples were collected using Entech (USA) SUMMA® electropolished stainless steel containers (6 L) following the United States Environmental Protection Agency (US EPA) Analytical Method TO-14 (US EPA, 1999). The air intake into the container was controlled with a Restek (USA) Veriflo® SC423XL flow regulator with an approximate flow rate of 25 ml/min. The sampling train, consisting of a steel container and a flow controller, was assembled prior to each collection event. Leak tests were performed prior to sample collection. Once the sampling was completed, the container valves were shut and a stainless steel lid was placed on the container to prevent further intake or output. Samples were stored at ambient temperature prior to being sent to the laboratory for analysis. The containers as well as the calibrated flow regulators were provided by TestAmerica Inc. (Austin, TX, USA), and were prepared by the manufacturer following the procedures required by the TO-14 methodology (US EPA, 1999). Meteorological data and criteria pollutant (PM10, PM2.5, O3, NOx, etc) for the sampling campaigns were obtained from the SIMA data records for the Obispado Station. 2.3. Analytical method After collection, the samples were sent to the laboratory at TestAmerica, Inc. in Austin, Texas for analysis. The procedure established in the US EPA TO-14 method was followed. Prior to starting the analysis, the sample was preconcentrated using two Tenax traps that removed CO2 and moisture. After the second trap, the sample was passed to a cryofocalizer where it was heated to 60  C to desorb the analytes into the gas chromatograph/mass spectrometer (GC/MS) for separation and quantification. The Agilent GC/MS system (USA) is equipped a split/splitless injector with electronic pressure control to ensure gas flow through the temperature program (rises to 150  C at 8  C/min). The process uses software from Entech (USA). The separation was conducted in a single capillary column, 60 m long RTX-1, with an internal diameter of 0.32 mm, 1 m film thickness. Once the sample was separated, it passed into the MS for identification and quantitation. Calibration was done in accordance to the US EPA TO-14 method prior to every analysis, and calibration checks were done for each batch. The analytic laboratory reported quality assurance/quality control for every analyzed batch. The overall uncertainty of the method was 6%, and the method detection limit (mdl) was less than or equal to 0.2 ppbv for all pollutants, except for TNMOC that had a mdl of 2.6 mg/m3. 2.4. Statistical analysis Concentrations greater than the method detection limits were classified by season (spring and fall), by campaign (in chronological order: 1, 2, 3) and by time interval (6:00e10:00, 10:00e14:00, 14:00e18:00 and 18:00e22:00). Addinsoft XL-Stat 2013© software was used to calculate the mean, variance, and standard deviation. Additionally, a discriminant analysis was performed, using the same software, to observe differences between groups (a ¼ 0.05) at a tolerance of 0.0001. No validation or prediction models were used, and observations with missing data were eliminated. The dependent variable (y) was defined as the classification group, i.e., time interval, while the independent variables (x) were the data observed for each compound. The p-value of the Fisher's square distance was used to accept significant differences among the groups.

H.L. Menchaca-Torre et al. / Atmospheric Pollution Research 6 (2015) 1073e1081

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Fig. 1. Geographical location of (a) the Monterrey Metropolitan Area and (b) the sampling site. Adjusted from Menchaca et al. (2015).

2.5. Meteorological conditions

3. Results

The meteorological conditions during the sampling campaigns are presented in Table 1. Overall, the spring campaigns had similar conditions with higher average temperatures (29 ± 10  C) and wind speeds (2.4 m/s) than the fall campaign, 25 ± 6  C and 1.4 m/s, respectively. Average wind direction was predominantly from the east, northeast during spring and east, southeast during the fall.

3.1. Characterization of the VOCs A total of 84 samples were collected; 28 for each monitoring campaign. Chemical analyses were conducted for 53 species and for total non-methane organic compounds (TNMOCs) as listed in Table 1. Only 41 of the 53 species in the table had concentrations above the detection limits. Compounds with characterized

Table 1 Chemical compounds quantified in this studya. No.

Compound

No.

Compound

No.

Compound

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

TNMOCs Ethane Ethene Propane Propylene Isobutane Acetylene Butane trans-2-Butene 1-Butene cis-2-Butene Cyclopentane Isopentane Pentane 1,3-Butadiene trans-2-Pentene 1-Pentene cis-2-Pentene

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

2,2-Dimethylbutane 2,3-Dimethylbutane Isoprene 2-Methylpentane 3-Methylpentane 1-Hexene Hexane Methylcyclopentane/2,4-Dimethylpentane Benzene Cyclohexane 2,3-Dimethylpentane 3-Methylhexane 2,2,4-Trimethylpentane Heptane Methylcyclohexane 2,3,4-Trimethylpentane Toluene 2-Methylheptane

37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

3-Methylheptane Octane Ethylbenzene m,p-Xylene Styrene o-Xylene Nonane Cumene Propylbenzene 2- and 4- Ethyltoluene 1,3,5-Trimethylbenzene 2-Ethyltoluene 1,2,4-Trimethylbenzene Decane 1,2,3-Trimethylbenzene 1,3-Diethylbenzene 1,4-Diethylbenzene Undecane

a

Compounds in bold had concentrations higher than the method detection limit for at least 60% of the intervals sampled.

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Table 2 VOC concentrations (ppbv ± standard deviation) in Monterrey, Mexico. Compound

This study Spring 2011

TNMOCsa Ethane Ethene Propane Propylene Isobutane Acetylene Butane 1-Butene Isopentane n-Pentane 1-Pentene 2,2-Dimethylbutane 2,3-Dimethylbutane Isoprene 2-Methylpentane 3-Methylpentane Methylcyclopentane Benzene 2-Methylhexane 3-Methylhexane 2,2,4-Trimethylpentane Hexane Heptane 2,3,4-Trimethylpentane Toluene Ethylbenzene m,p-Xylene o-Xylene

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

66 2.2 1.4 2.6 0.3 0.6 1.2 1.0 0.1 1.1 0.4 0.1 0.04 0.1 0.4 0.3 0.2 0.1 0.3 0.1 0.2 0.07 0.9 0.1 0.1 2.2 0.1 0.4 0.2

125 5.9 2.6 5.2 0.8 1.3 1.6 3.3 0.4 3.3 1.0 0.1 0.2 0.1 0.4 0.6 0.3 0.1 1.4 0.2 0.2 0.4 0.3 0.2 0.4 2.6 0.1 0.5 0.3

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

50 4.9 1.5 4.0 0.6 0.9 1.2 2.0 0.2 2.1 0.6 0.02 0.1 0.8 0.3 0.4 0.2 0.1 5.1 0.06 0.1 0.2 0.3 0.1 0.3 1.8 0.09 0.2 0.2

Fall 2012 180 10.1 5.4 12.8 2.3 3.2 4.2 8.0 0.9 5.9 2.0 0.3 0.3 0.4 0.3 1.6 0.9 0.4 0.9 0.4 0.2 0.4 0.9 0.3 0.4 4.5 0.4 2.0 0.8

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

100 5.9 3.1 7.4 1.4 1.9 2.3 5.0 0.5 3.6 1.9 0.2 0.2 0.2 0.1 1.0 0.5 0.2 0.5 0.2 0.1 0.2 0.6 0.1 0.2 3.3 0.4 2.2 1.0

Tan et al., 2011 (urban site)

Franco and Pacheco, 2015 (Industrial-high traffic site)

Velasco et al. 2007 (urban site)

NAb 20.1 21.28 11.47 5.85 NAb 19.54 2.94 NAb 10.59 0.82 NAb NAb NAb 0.72 NAb NAb NAb 3.55 NAb NAb NAb NAb NAb NAb 8.96 NAb NAb NAb

NAb 23 ± 20 6.6 ± 5.4 9.5 ± 7 1.5 ± 2.5 3.7 ± 2 NAb 11 ± 7.3 0.9 ± 0.7 10.8 ± 8.4 4.1 ± 3 1.4 ± 0.7 0.1 ± 0.2 0.7 ± 0.8 1.5 ± 1.1 NAb NAb NAb 4.6 ± 3.4 NAb NAb NAb 0.5 ± 2.7 NAb NAb NAb NAb NAb NAb

NAb 17.26 ± 11.62 20.33 ± 10.75 127.59 ± 62.77 5.9 ± 3.17 18.37 ± 9.08 13.37 ± 7.64 50.09 ± 24.87 3.04 ± 1.81 17.02 ± 9.28 7.41 ± 4.43 0.48 ± 0.28 1.38 ± 1.52 1.05 ± 1.07 0.33 ± 0.27 5.17 ± 2.98 3.33 ± 1.88 1.41 ± 0.77 3.17 ± 1.75 1.98 ± 2.53 2.61 ± 3.45 0.1 5.17 ± 4.02 1.8 ± 2.725 1.26 ± 0.67 13.45 ± 9.33 1.62 ± 1.43 5.67 ± 5.96 2.08 ± 2.08

TNMOCs concentrations in mg/m3. Not available.

concentrations in at least 60% of the cases underwent statistical treatment. The summary of the concentrations of these compounds is presented in Table 2 along with results found in other cities for comparison. The most abundant compounds in the sampling campaigns were propane (3.7e12.8 ppbv), ethane (4.3e10.1 ppbv), butane (2.8e8.0 ppbv), and isopentane (2.9e5.9 ppbv). The high positive correlation (r ¼ 0.7e0.9) between the alkanes
exceptions. The high reactive ethene and acetylene showed the highest correlation while toluene and hexane the lowest one. During the fall, slightly lower negative correlations were determined (0.6) for ethane, propylene and butane and (0.4) for ethane and propane. Second, the decrease in average wind speed from 2.6 to 1.4 m/s from spring to fall caused the elevation of pollutant concentrations (Fig. 2). The levels of benzene, toluene, and the xylenes can be used to describe the atmospheric conditions due to their residence time differences; benzene (B) >> toluene (T) > xylenes (X) (Prinn et al., 1987; Carter, 1994; Na et al., 2003). T/B ratios between 1.5 and 3 imply vehicular emissions; higher or lower values denote aged and fresh emissions, respectively. When the X/B ratio is approximately 3.6, the emissions are considered to be recent; a lower ratio

150

Spring Fall

120 TNMOC (ppbv)

a b

119 4.3 2.8 3.7 0.7 1.3 2.0 2.8 0.4 2.9 0.9 0.2 0.1 0.2 0.6 0.6 0.4 0.3 0.5 0.3 0.3 0.2 0.7 0.2 0.2 3.0 0.2 0.7 0.3

Spring 2012

90

60

30

0 0.0

0.5

1.0

1.5

2.0 2.5 Windspeed (m/s)

3.0

3.5

Fig. 2. TNMOC trends regarding wind speed.

4.0

4.5

H.L. Menchaca-Torre et al. / Atmospheric Pollution Research 6 (2015) 1073e1081

indicates an aged atmosphere. In addition, the T/B ratio is used to describe the atmosphere as urban or industrial (>2e6) or near heavy traffic (~2) (Hartmann et al., 1997; Chen et al., 2001; Guo et al., 2004; Elbir et al., 2007; Leuchner and Rappenglück, 2010; An et al., 2011). The T/B ratios were 5.5, 5.4, and 5.2, for spring 2011, spring 2012, and fall 2012, respectively, and were ascribed to concentrations of aged masses of air from urban areas. The average X/B ratios were 1.1, 0.9, and 2.1 for spring 2011, spring 2012, and fall 2012, respectively, and reflected aged emissions. The results agree with the location of the sampling site downwind from the Monterrey industrial zone as well as from the Mexican Petroleum Company (PEMEX) refinery, located at Cadereyta, NL, 42 km to the east of the Obispado station. The medium to high positive correlation (~0.7e0.9) between all the compounds denoted a common source for toluene, benzene, and the xylenes. VOCs concentrations are compared to results in other cities in Table 2. The characterized information for every city depends on multiple factors such as meteorological conditions, type of fuels consumed and their composition, industrial activities, among others. VOCs concentrations in MMA were generally lower than those found in Bogota, Colombia (Franco and Pacheco, 2015), Fosham City, China (Tan et al., 2011) and Mexico City (Velasco et al., 2007). Bogota has twice the population of Monterrey; however, VOC concentrations in the MMA were between two and three times lower, for most compounds, than in the Colombian city. Ethane and 1-pentene had a more significant difference being five and eight times higher in concentration in Bogot
a. Fosham City, with a population of 5.6 million and a variety of industries, had pollutant concentrations between 5 and 10 times larger than those in Monterrey even with the similar number of inhabitants and industrial zones. Isoprene and n-pentane were the only reported VOCs with similar concentrations. Finally, when compared to Mexico City, MMA pollutant mixing ratios were significantly lower. The difference was greater for propane and butane, markers of LPG the preferred fuel for residential use in Mexico City; natural gas is used in Monterrey. 3.2. Diurnal variations Fig. 3 illustrates the average diurnal variation of TNMOCs, acetylene, and toluene, because they are representative of the behavior of VOCs. Isoprene, the only biogenic VOC characterized, is also included because the oscillations in its concentration differed from compounds emitted primarily by anthropogenic sources. In general, the VOCs exhibited a higher concentration in the 6:00e10:00 time interval, which decreased during the following two sampling intervals to reach a minimum between 14:00e18:00. At the end of the day, concentrations increased again. Isoprene was
n the exception, exhibiting an opposite pattern of behavior. Cero et al. (2012) reported the same variations for benzene, ethylbenzene, and p-xylene in their research in Monterrey during the summer of 2011. Individual results demonstrated similar concentrations between the morning intervals for TNMOCs, ethane, ethene, acetylene, and propane, but they were significantly higher (a ¼ 0.05) than the afternoon concentrations. Isoprene had higher concentrations in the 10:00e14:00 and the 14:00e18:00 time intervals, while isopentane and benzene did not have significant differences between the different time intervals. Other compounds exhibited mixed results with higher concentrations in the morning, in general. The higher concentrations of VOCs in the early morning hours (6:00e10:00) are due to slower removal mechanisms at night, which lead to a longer residence time (Atkinson, 2000); the low boundary in the morning evidenced by the inverse medium correlations (0.6) between VOCs and temperature during the

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interval. The influence of rush hour traffic was evidenced by VOC correlation analysis. Acetylene was correlated (>0.8) with other markers of gasoline, such as pentane and isopentane, which indicated that a part of the emissions came from mobile sources. Medium-to-high positive correlations (0.6e0.9) of all VOCs with acetylene are evidence that a fraction of the pollutants came from emissions from mobile sources, including isoprene, a typical marker of natural sources. Borbon et al. (2003) found that isoprene has a higher biogenic contributor when the availability is higher as we found in Monterrey. Additionally, they found that part of the alkanes 0.7, with acetylene), and industrial emissions (high positive correlations, 0.7e0.9, among the solvent marker compoundsdtoluene, ethylbenzene, and/or xylenes) in the various sampling intervals. Isoprene, the primary marker of biogenic sources, is emitted by the chloroplasts of plants (Guenther et al., 2000); therefore, its concentration depends on the availability of solar radiation and temperature. As a result, the concentration of isoprene varies inversely with respect to other VOCs. The minimum concentration of isoprene occurs in the first hours of the morning and its emission is concluded to be due, in part, to mobile sources, because it is correlated (0.7) with acetylene. In the 10:00e14:00 and 14:00e18:00 time intervals, the concentration of isoprene increases due to the emission of biogenic sources as evidenced by its positive correlation with temperature (r2 ¼ 0.6 and 0.9, respectively) and solar radiation (r2 ¼ 0.5 and 0.7, respectively). 3.3. VOCs and wind direction Variations in VOCs with respect to wind direction have been studied in areas with industrial activity (Srivastava et al., 2006; Badol et al., 2008). The diagrams in Fig. 4 illustrate the overall average concentrations of six VOCs at an azimuth of ±22.5 for all sampling campaigns. No winds were present in the north or west directions; the south and southwest directions exhibited 4% and 2% of the wind currents, respectively. East and southeast directions

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Fig. 3. Overall concentration variability of (a) TNMOCs, (b) acetylene, (c) isoprene, and (d) toluene. The graph shows the mean (=), median (strip) and atypical points (*).

Acetylene 6 NW

12 NE

E

SW

W

SE

E

SW

W

SW

SE

o-Xylene

N

NW

NE

1.5 NE

1 E

0

SE S

W

N

NW

2.5

SW

SE S

Toluene 5

E

0

S

N

NW

NE 0.3

0

Benzene

N

NW

NE

S

W

0.6

6

0

2

N

NW

3 W

Isoprene

Butane

N

NE 0.75

E

0

SW

SE

W

E

0

SW

S Fig. 4. VOC concentration and wind direction diagrams (ppbv).

SE S

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both had 17% of the wind currents while northeast and northwest directions were the predominant with 26% and 32%, respectively. Concentration changes of most compounds were similar to that of acetylene denoting that a proportion of them are emitted by mobile sources. Xylenes and toluene have common sources when the wind has a southeast direction. Additionally, toluene polygon suggests mobile sources, as one of the probable emitters given the polygon is similar to that of acetylene, but with more pronounced increase in concentrations in the southwest and southeast wind directions, evidence of additional sources. Benzene had a concentration polygon similar to acetylene, evidence that mobile sources is one of its main emitters. However, its elevated concentration during northeast winds denotes more than one source since acetylene does not show the same behavior. Aliphatic compounds, such as ethane, propane, and butane, exhibited graphs similar to the acetylene polygon. However, the rate of emission of aliphatic compounds was greater that than of acetylene in some wind directions; i. e. north west/south west (NW/SW) ratio for acetylene was ~2 while NW/SW for butane was ~3, denoting mobile sources and at least an additional emitter. Some VOCs, such as ethylbenzene and xylenes, had completely different diagrams, with the highest concentrations when winds came from the southeast. Isoprene presented similar concentrations for all wind directions, except for the northwest, when the maximum values were reached. The polygon described by isoprene reaches the medium concentration of the compound in the NE direction, as with acetylene, denoting that a fraction is emitted by mobile sources. However, the variations in the figure reveal that the primary source of the compound is not the same as that of the rest of the VOCs. By analyzing the correlation of isoprene with other compounds, it was concluded that it is primarily emitted by biogenic sources. The observation of the variation in concentrations of VOCs with respect to the wind direction provides evidence to strengthen the definition of the emission sources based on correlation analysis and ratios between compounds. 3.4. TNMOCseNOxeO3 Strict environmental standards are used to control the concentration of O3 in the atmosphere. Compliance with these standards depends on controlling its precursors, NOx and VOCs; therefore, it is necessary to limit the emission of these pollutants into the atmosphere. However, due to the complexity of the reactions that produce O3, knowledge of the interrelationships is vital for designing effective mitigation strategies. In areas with high VOC/NOx ratios, that is, with high concentrations of VOCs and low concentrations of NOx, the concentrations of O3 tend to decrease when the NOx concentrations or the concentrations of both precursors decrease. In this regime, because O3 formation is sensitive to NOx, it does not usually respond positively to the control of VOC emissions. Conversely, when an atmosphere is sensitive to the VOC concentrations, O3 concentration decreases when VOC concentrations decrease, keeping NOx constant. Fig. 5 presents TNMOCeNOxeO3 relationship present during the monitoring program in the two seasons of the year that were sampled. Observed O3 and NOx concentrations were higher during the spring, while those of TNMOCs were higher in the fall. Analyzing the graphics, during spring two high O3 concentration areas are observed. The first is between 250 and 350 ppbC of TNMOCs and between 0.03 and 0.05 ppbv of NOx, while the second has concentrations >600 ppbC of TNMOCs and 0.02 ppbv of NOx. The chemistry is dominated by TNMOCs, because a decrease of 30% in both cases would represent a decrease of 62.5% and 50% in the first and second areas, respectively, while a similar decrease of NOx would provide little or no change in O3 concentrations. During the

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Fig. 5. TNMOCeNOxeO3 relationships for (a) spring and (b) fall.

fall, two instances of high concentrations of O3 were observed, and in both, a decrease of NOx and TNMOCs was required to reach the minimum values observed. The sensitivity of the contamination dominated by TNMOCs during the spring agrees with the results modeled for the MMA by Sierra et al. (2013) for two days in the summer. TNMOCeNOxeO3 relationship during different sampling intervals is presented in Fig. 6. Because of the low O3 mixing ratio, Fig. 6a has a different ozone scale than the rest of the panels. The first morning time interval (Fig. 6a) had the lowest O3 concentrations since O3 is consumed during the night in oxidation reactions (Finalyson-Pitts and Pitts, 2000), and there is little secondary production during the interval because of the limited sunlight during the interval. The 14:00e18:00 time interval (Fig. 6c) exhibited the highest concentration of O3, and lower concentrations of NOx and TNMOCs, resulting from O3 production during the previous interval when sunlight peaks. Low to mid concentrations of O3 were observed in the second (Fig. 6b) and fourth (Fig. 6d) intervals because of the increase and decrease of sunlight received respectively. Ozone response to changes in TNMOC or NOx emissions shows mixed results for sensitivity in the 10:00e14:00 and 14:00e18:00 time intervals; some areas of the figure are governed by one of the pollutants while other need a combined decrease of TNMOC and NOx to decrease O3. During the performed research. i. TNMOC and 28 VOCs were characterized in Monterrey, Mexico. ii. Diurnal patterns were observed with concentrations being higher in the early morning iii. Correlation analysis and concentration-wind direction observation denote mobile sources, fugitive fuel emissions and solvent use as possible anthropogenic sources.

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Fig. 6. TNMOCeNOxeO3 relationships for different sampling time intervals: (a) 6:00e10:00, (b) 10:00e14:00, (c) 14:00e18:00, and (d) 18:00e22:00.

iv. Isoprene analysis found a fraction of the compound is produced by mobile sources during rush hours. v. TNMOCeNOxeO3 analysis evidenced a complex atmospheric chemistry. Further reactivity and ozone forming potential analysis is required to establish sensitivity. 4. Conclusions The concentrations of 29 VOCs were analyzed at a sampling point in the city of Monterrey, Mexico. The diurnal variations of the compounds were characterized by high concentrations in the morning and low concentrations at night. The lowest concentrations were reached during the 14:00 and 18:00 time interval, when photolysis reached the maximum. The seasonal variations showed a higher concentration for VOCs during the fall due to a lower vertical transport because of decreased temperature and the blockage of air masses caused by lower wind speeds. The univariate and bivariate analysis of the compounds gives evidence of mobile sources, fuel leaks, and the use of solvents as emission sources. However, a more detailed study is required to establish the origin of a larger number of compounds, as well as the relative importance of each of these sources. Finally, the analysis of the TNMOCeNOxeO3 relationship showed that VOCs are compounds with a greater influence on the chemistry of the MMA atmosphere. Greater control of the VOCs is necessary in order to improve the contamination indexes. The study of VOC emission sources, as well as their influence on the production of O3 in the atmosphere, will provide information with which to make the best decisions regarding the emission of VOCs. Conflict of interest There is no conflict of interest.

Acknowledgments To the National Science and Technology Council (CONACYT) for the grants that made this project possible. To Monterrey Institute of Technology for the support given to this research through CAT-186. To the Integral Environmental Monitoring System (SIMA) for their support and sharing their sampling facilities with us. References
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