Characterization of NOx-Ox relationships during daytime interchange of air masses over a mountain pass in the Mexico City megalopolis

Atmospheric Environment 177 (2018) 100–110

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Characterization of NOx-Ox relationships during daytime interchange of air masses over a mountain pass in the Mexico City megalopolis

T

J.S. García-Yeea, R. Torres-Jardóna, H. Barrera-Huertasa, T. Castroa, O. Peraltaa, M. Garcíaa, W. Gutiérreza, M. Roblesa, J.A. Torres-Jaramillob, A. Ortínez-Álvarezc,1, L.G. Ruiz-Suáreza,c,∗ a

Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad Universitaria, Ciudad de México, 04510, Mexico Instituto de Ciencias, Benemérita Universidad Autónoma de Puebla, 14 Sur y San Claudio, edif. 18, Ciudad Universitaria, Puebla, Puebla, 72570, México c Instituto Nacional de Ecología y Cambio Climático, Periférico sur 5000, Col. Cuicuilco, Ciudad de México, 04530, México b

A R T I C L E I N F O

A B S T R A C T

Keywords: Mexico City Megalopolis Photochemical age Ox-NOx relationship Air pollution transport

The role of the Tenango del Aire mountain pass, located southeast of the Mexico City Metropolitan Area (MCMA), in venting the city's air pollution has already been studied from a meteorological standpoint. To better understand the transport of gaseous air pollutants through the Tenango del Aire Pass (TAP), and its influence on the air quality of the MCMA, three mobile air quality monitoring units were deployed during a 31-day field campaign between February and March of 2011. Surface O3, NOx, and meteorological variables were continuously measured at the three sites. Vertical profiles of O3 and meteorological variables were also obtained at one of the sites using a tethered balloon. Days were classified as being under low pressure synoptic systems (LPS, 13 days), high pressure synoptic systems (HPS, 13 days), or as transition days (TR). The Mexican ozone standards at the Pass were not exceeded during LPS days, but were exceeded on almost all HPS days. A detailed analysis was performed using data from two typical days, one representative of LPS and the other of HPS. In both cases, morning vertical profiles of O3 showed a strong thermal inversion layer and near-surface O3 titration due to fresh NOx. In the LPS early morning, a single O3 layer of close to 45 ppb was observed from 150 to 700 magl. In the HPS early morning, 50 ppb was observed from 150 to 400 magl followed by a 400-mthick layer with up to 80 ppb. These layers were the source of the morning increase of O3, with a simultaneous sharp decrease of NOx and CO as the mixing layer started to rise. During the LPS day, a southerly wind dominated throughout most of the daytime, with surface O3 lower than 60 ppb. The same was observed for the well-mixed midday and afternoon vertical profiles. Under HPS, northerly winds transported photochemically active air masses from the MCMA all morning, as observed by a smoother increase of Ox and O3, reaching 110 ppb of O3. Just after midday, the wind shifted back, carrying high-O3 (100–110 ppb) aged air masses until sunset. In addition, the midday and afternoon vertical profiles showed wellmixed high-O3 (100–110 ppb) mixing ratios. Analysis of Ox-NOx correlations was performed for these peri-urban and MCMA sites. A parallel analysis for the nearest urban air quality monitoring station in the MCMA was also done. A comparison allowed us to distinguish between photochemically active (VOC sensitive) or aged parcels (NOx sensitive) arriving at the TAP. Separating the correlations into time groups associated with wind direction changes allowed us to better distinguish between local, MCMA, or regional influence. The results are relevant to air quality management in the Mexico City megalopolis.

1. Introduction It is well known that in a clean troposphere with only NO and NO2, the reactions controlling the concentrations of NO and NO2 are:

NO2 + hν → NO + O•

R.1

O• + O2 + M → O3 + M

R.2

∗ Corresponding author. Permanent address: Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad Universitaria, Ciudad de México, 04510, Mexico. E-mail address: [email protected] (L.G. Ruiz-Suárez). 1 Current address: Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad Universitaria, Ciudad de México, 04510, Mexico.

https://doi.org/10.1016/j.atmosenv.2017.11.017 Received 25 November 2016; Received in revised form 6 November 2017; Accepted 12 November 2017 Available online 22 November 2017 1352-2310/ © 2017 Elsevier Ltd. All rights reserved.

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O3 + NO → NO2 + O2

15 km WNW of and 220 m higher than Tenango del Aire, located on the slope of the eastern edge of the Sierra de Chichinautzin, facing the MCMA. During MCMA 2003 (Dunn et al., 2004; Velasco et al., 2007) and MILAGRO (Thornhill et al., 2008) it was used as a peri-urban observation site. Under conditions that drove MCMA plume towards the TAP, reported concentrations of VOC, SO2, CO, NOx, and ultra-fine PM in Santa Ana showed that this site is a receptor site of MCMA during the morning. When air came from the TAP, “clean air” was observed. Despite the importance of the TAP with regard to air mass exchange between the Mexico Basin and the Cuautla-Cuernavaca Valley, it is not clear how this dynamic works, and its impact on air quality over the MCMA has not been well defined. To clarify this situation, this study aims to, within the TAP: (1) characterize O3 and NOx levels, (2) identify the dynamics of transport and production of O3 and NO2, (3) determine the conditions when observed Ox is due to local photochemical activity or to vertical or regional transport of O3.

R.3

Under these conditions, NO, NO2, and O3 are rapidly equilibrated during daylight. Within this system in equilibrium, the mixing ratios of NO, NO2, and O3 can change, but not the mixing ratios of NOx (NO + NO2) or Ox (O3 + NO2). However, in urban areas, where VOCs, CO, and NOx are abundant, photochemical oxidation of VOCs and CO produces HO2• and RO2• which oxidize NO to NO2 in a competitive reaction with R.3. After a relatively short period, organic radical oxidation dominates, O3 begins to accumulate, and NOx and Ox mixing ratios change. The correlation of observed Ox vs. NOx has been used to identify both the dominant chemical regime in a region and the role of local NOx emissions in the occurrence of high ozone levels (Clapp and Jenkin, 2001; Song et al., 2011). The slope resulting from the correlation can be an indicator of the primary contribution of NOx to Ox while its intercept can show the contribution to OX levels of background O3. The Mexico City Metropolitan Area (MCMA) continues to record high ozone levels above 150 ppb, mainly from February to June, despite efforts made by the government and society throughout the past 25 years (Sedema et al., 2011). It has been suggested that the complex terrain around the MCMA contributes to the persistence of high ozone levels (de Foy et al., 2008; Doran et al., 1998, 2007; Fast et al., 2007; Jauregui, 1988; Mena-Carrasco et al., 2009). The influence of complex terrain on O3 behavior has been described elsewhere for the lower Fraser Valley in British Columbia, Canada (McKendry et al., 1998). Long-range and mesoscale transport under the influence of thermal lows in complex terrain showing some similarities with central Mexico have been analyzed for the Iberian Peninsula (Gangoiti et al., 2001; Millán et al., 1991). Peri-urban monitoring sites help to understand regional/urban pollution exchange (Trainer et al., 2000). The long-range influence of the MCMA urban plume outside the Mexico Basin has been explored by modeling and by surface and aircraft measurements (Molina et al., 2010); only a few studies have investigated the short-range air mass exchange between the MCMA and the neighboring metropolitan areas (Garcia-Reynoso et al., 2009; Salcedo et al., 2012). Few others have pointed to the contribution of biomass burning to regional photochemical smog in central Mexico (Crounse et al., 2009; Yokelson et al., 2007). Previous studies (de Foy et al., 2008; de Foy et al., 2006; Doran and Zhong, 2000; Fast and Zhong, 1998; Jazcilevich et al., 2003), using different names for it, have analyzed the role of the TAP (Fig. 1). Etymologically, tenango originates from the Nahuatl (Aztec) language word for ‘fortress.’ Before the conquest, in Tenango del Aire there was an Aztec garrison that guarded the pass to the Aztec's capital city. The TAP provides a unique type of natural wind ventilation for the MCMA. It has an average altitude of 2392 masl and it is located between the foothills of the Sierra del Chichinautzin and the Sierra Nevada with a width of approximately 14 km. The distance between the towns of Tenango del Aire (TEN) and Ozumba (OZU) is approximately 17 km. The TAP connects the Mexico Basin with the Cuautla-Cuernavaca Valley, located in the State of Morelos (Fig. 1). Doran et al. (1998) and Doran and Zhong (2000) used measurements taken in Chalco (north of the TAP, Fig. 1) and modeling results, respectively, to identify the occurrence of wind flows from Chalco to the Cuautla-Cuernavaca Valley in the morning and their return in the evening. These authors suggested that clean air masses from the Cuautla-Cuernavaca Valley may enter the Mexico Basin. Jazcilevich et al. (2003) modeled the occurrence of a confluence of northerly wind with southerly wind from the TAP, which may increase pollution levels along the confluence line. The position of this line would depend on the relative intensity of the converging winds. Furthermore, in the MILAGRO study, Ruiz-Suárez et al. (2010) observed the passing of photochemically-aged air parcels rich in ozone at TEN (Fig. 1) when southerly winds were passing over this site. Also in MILAGRO, Melamed et al. (2009) measured the vertical column of NO2 at the same site, observing that above 1000 m, NO2 levels are higher than those registered at the surface when winds come from the south. Santa Ana is a site

2. Methods An intensive air quality monitoring campaign was carried out from February 6 to March 8, 2011 at three sites located at the mountain pass. Three mobile monitoring units were situated at the Amecameca (AME), Tenango del Aire (TEN), and Ozumba (OZU) sites (Fig. 1). The bestequipped monitoring station was deployed at AME (19° 07′ 48″ N 98° 47′ 12″ W, 2440 masl), in a peri-urban area surrounded by agricultural fields as well as poultry and cattle farms. This monitoring site is located 700 m west of Federal Highway 115 (Chalco-Cuautla), 10 m north of the minor Amecameca-Ayapango road, and 2 km west of downtown Amecameca (approx. 50,000 inhabitants), which is a local hub for commuters who work in the MCMA. Most of the commuters use old and loosely-regulated vans to reach AME from their peri-urban settlements. There, they switch to old and loosely-regulated diesel buses in order to reach the MCMA. The TEN monitoring station (19° 09′ 18″ N 98° 51′ 50″ W, 2380 masl) was mounted on the roof of a secondary school in the northern outskirts of Tenango del Aire (approx. 5000 inhabitants). The OZU monitoring site (19° 02′ 40″ N 98° 47′ 51″ W, 2355 masl) was located on the northern edge of Ozumba (approx. 27,000 inhabitants), about 100 m to the west of Federal Highway 115; it is surrounded by agricultural fields and a scattering of neighborhoods. In addition, data gathered at UAM-Iztapalapa (UIZ) (19° 21′ 29″ N 99° 4′ 19″ W, 2240 masl), which at that time was the nearest urban air quality monitoring station, located 31 km northwest of TEN, were used for comparison (Sedema, 2016). UIZ is characterized by area emissions from households, small-scale industry, and mobile sources, i.e. light and heavy vehicles, drawn by its proximity to the largest wholesale food market in the country. At all sites, O3 was measured by photometry using commercial instruments of different make following the EPA equivalent method according to 40 CFR Part 53. CO was measured by non-dispersive IR photometry, and NO, NO2, NOx by chemiluminescence, following 40 CFR Part 53. Meteorological parameters (T, P, RH, WS, WD) were monitored using standard cup/vane meteorological stations, except for the TAP where wind was measured using a Vaisala 2D Weather Station Model WXT520. AME was the only site equipped with a total solar radiation sensor (Li-Cor). Before the start of the campaign, all monitoring instruments were calibrated following standard procedures using EPA-certified gas mixtures. Vertical profiles of wind intensity and direction, temperature, and relative humidity were obtained three times per day using a tethered balloon on 15 of the 31 days of the campaign. Additionally, a modified ozonesonde (Science Pump Corporation) was used with the tethered balloon to obtain vertical profiles of O3. However, winds stronger than 10 m/s forced the balloon to be grounded.

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Fig. 1. AME, TEN, and OZU location within the TAP, as well as reference station UIZ in the MCMA. Source: ArcGis Desktop v.10.2.

3. Results and discussion

from the south and west at 700 hPa. At the TAP, the winds are southerly almost all day. Four of the days with mixed behavior were similar to O3South, and one was similar to O3-North. On February 10 (LPS), AME and TEN recorded SE-S-SW winds (from the Cuautla-Cuernavaca Valley) throughout most of the day (Fig. 3a and c). At AME, wind speed was 0.5 m/s at 07:00, increasing to 5–7 m/s throughout the morning. In TEN, wind speed started at 3 m/s, gradually increasing to 6 m/s. On March 3 (HPS), AME and TEN (Fig. 3b and d) recorded calm or weak winds from the S and SW between 07:00 and 09:00. At both sites after 09:00, the wind direction changed by 180°, blowing from the NE-N-NW (from the MCMA) with moderate intensities of 1–3 m/s. AME at 15:00 and TEN at 16:00 showed winds blowing again from the S-SW with intensities ranging from 2 to 5 m/s lasting until 18:00. OZU was very similar to AME under LPS and HPS. With regard to other meteorological parameters, the influence of LPS (Fig. 4a and c) kept ambient temperature below 20 °C, relative humidity above 40%, and the sky partly cloudy. By contrast, under HPS (Fig. 4b and d), the temperature reached 26 °C, relative humidity decreased to 10%, and the sky was clear. The initial rise of the mixing layer height (MLH) was indirectly estimated by the simultaneous and sharp drop of surface CO, NO, and NO2 combined with a very wellmarked inflexion point in the O3 trace. It occurred around 08:00 in HPS and around 08:30 in LPS (Figs. 5 and 7).

3.1. Overview of synoptic conditions The campaign period of February 6 to March 8, 2011 was influenced by synoptic systems typical of the transition from the dry-cold to the dry-warm season in central Mexico. It was also the beginning of the ozone season in the MCMA (Jauregui, 2002). To study the impact of meteorology on pollutant levels at the TAP, the campaign days were grouped as low pressure systems (LPS, 13 days), high pressure systems (HPS, 13 days), and transition (TR, 5 days). February 10 (Fig. 2a) is an example of LPS synoptic conditions, while March 2 (Fig. 2b) represents HPS conditions (NOAA, 2016). To associate the observations made at the TAP with O3 patterns in the MCMA, the classification of de Foy et al. (2005) was used. HPS days correspond to O3-South days (accumulation of O3 in the southern part of the MCMA), which are meteorologically stable, with clear sky, high solar radiation, and low winds from the north and sometimes from the east at 700 hPa. At the TAP, the wind direction is southerly at night, shifting to northerly in the early morning, and back to southerly in the afternoon. LPS days correspond to O3-North days (accumulation of O3 in the northern part of the MCMA), which are also meteorologically stable days, but cloudier, with lower solar radiation and stronger winds 102

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Fig. 2. Synoptic surface pressure maps at 6:00 h. Local time for two typical days of the dominant synoptic conditions observed during February 6th to March 8th, 2011. Source: NOAA, 2016.

Fig. 3. Surface wind speed and direction five-minute averages at AME and TEN, from 07:00–19:00 h (local time) on 2/10/2011 (LPS) and 3/02/2011 (HPS).

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Fig. 4. Surface temperature, relative humidity, and total solar radiation five-minute averages at AME and TEN, from 07:00–19:00 h (local time) on 2/10/2011 (LPS) and 3/02/2011 (HPS).

Fig. 5. Water mixing ratio and CO five-minute averages at AME, from 07:00–19:00 h (local time) for 2/10/2011 (LPS) and 3/02/2011 (HPS).

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3.2. Overview of time series and basic statistical comparisons In this section, we analyze our results for this rural/peri-urban area in the city belt of central Mexico in terms of its compliance with Mexican ozone standards, WHO critical levels, and AOT40 critical levels for vegetation damage. AME, TEN, and OZU showed similar diurnal patterns of O3 concentration (Fig. 6a), indicating that these sites had the same incidence of synoptic and surface wind flows. The days with the highest O3 concentration were March 1 to 3 (HPS) and the days with lowest O3 were February 6, 8, and 10 (LPS). Table 1 shows a summary of basic statistics for O3, NO, NO2, and NOx. The 95 ppb O3 one-hour average established by the Mexican standard (SSA, 2014) was exceeded only on days under HPS influence. In addition, the 8 h moving average of 70 ppb O3 established by the Mexican Standard (SSA, 2014) was only exceeded under HPS (47, 31, and 9 times at AME, TEN, and OZU, respectively). The WHO guideline for ozone (WHO, 2006) of 65 ppb 8 h moving average was exceeded under both synoptic conditions. Both moving averages, the Mexican Standard and the WHO critical value, were exceeded more times at TAP sites than at the urban reference site. Similarly, the AOT40 critical level (CLRTAP, 2015) which recommends a cumulative O3 maximum of 3000 ppb-hour for three months of exposure by agricultural crops, was exceeded at AME and TEN by more than twofold during the month-long monitoring campaign (6841 ppbhour and 6304 ppb-hour, respectively) and by approximately 1.5 times (4283 ppb-hour) at OZU. This shows the high O3 levels to which both the population and the vegetation may be exposed at these rural and peri-urban sites within the city belt of central Mexico. The NOx (Fig. 6b) and NO2 (Fig. 6c) time series do not include OZU since the NOx monitor at that site failed during the campaign. AME showed behavior typical of urban sites, recording 19 days with hourly averages above 120 ppb between 07:00 and 10:00; only 10 of these 19 days were HPS. TEN showed much smaller NOx peaks. By comparison, the urban site UIZ recorded higher than 300 ppb regardless of synoptic system (Table 1). High levels of NOx detected at AME during late night and early morning could be attributed to the accumulation of emissions in the inversion layer from the nearby active federal highway, the small city of Amecameca, and night burning of crop residues.

Fig. 6. Hourly time series for O3, NO, and NO2 from February 6th to March 8th, 2011.

Table 1 Hourly averages for O3, NO, NO2 and NOx, as well as 8-h mobile average for O3. The TAP sites (AME, TEN and OZU) and urban Mexico City as reference site (UIZ) under LPS and HPS influence. February 6th to March 8th, 2011. Values expressed in ppb. O3

AME

TEN

OZU

HPS

LPS

HPS

LPS

HPS

LPS

HPS

Hourly average Hourly maximum Number of hours > 95 ppb Hourly average (SSA, 2014)

31 82 0

38 109 9

39 78 0

42 105 6

30 68 0

34 101 1

34 125 14

34 136 12

8-h Maximum mobile average Number of hours > 70 ppb 8-h mobile average (SSA, 2014) Number of hours > 65 ppb 8-h mobile average for O3. Pressure at 0.77 atm and temperature at 273 K in the center of Mexico. (100 μg/ m3, WHO, 2006)

70 0

93 47

70 0

94 31

60 0

82 9

90 31

100 21

7

66

7

54

0

20

41

25

NO

13 154 15 39 28 178

22 251 20 58 42 290

2 43 7 45 8 88

5 55 14 55 19 95

– – – – – –

– – – – – –

34 238 34 72 63 303

35 279 35 98 64 333

NO2 NOx

average maximum average maximum average maximum

In this section, we provide a detailed analysis of the daytime profiles of Ox, O3, NO2, NO, and CO, along with wind direction, within the context of the prevailing synoptic conditions, in order to better understand the transport of reactive species in and out of the Basin of Mexico through the TAP. Under LPS conditions, AME and TEN recorded southerly winds and low Ox patterns all day (Fig. 7a and c). However, at AME, exclusively in the early morning, NO2 made a significant contribution to Ox. This is due to the intense commuter activity starting even before sunrise at AME. TEN, with fewer nearby emission sources, showed more rural behavior. At both sites at approximately 08:30, O3 jumped to approximately 40 ppb, with a simultaneous drop in CO (Fig. 5a), NO, and NO2 (Fig. 7a). NO and NO2 were very low at TEN as the 3 m/s wind intensity was sufficient to dilute low local emissions. This small jump restored O3 to its background level when the MLH started to rise and a layer with 40 ppb was found from 100 m to 800 m, as observed when the tethered balloon was hoisted at 07:00. (Fig. 8a). At both sites from 09:00 to 14:00, O3 increased slowly to no more than 60 ppb. From then until sunset, O3 remained constant, making the major contribution to Ox (Fig. 7b and c). It was well-mixed as high as the tethered balloon was able to reach (Fig. 8b and c). Only AME showed a small NO2 contribution to Ox under the influence of the nearby federal roads. This sustained and relatively low O3 concentration could be explained by the mixing of regional O3 from the Cuautla-Cuernavaca Valley with clean air from the Pacific Ocean, which is located at 250–300 km in a S-SW

UIZ

LPS

Hourly Hourly Hourly Hourly Hourly Hourly

3.3. Comparison of daily profiles by synoptic condition

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Fig. 7. Surface wind direction, NO, NO2, O3, and Ox, five-minute averages for at AME and TEN from 07:00–19:00 h (local time) on 2/10/2011 (LPS) and 3/2/2011 (HPS).

northerly at AME, O3 continued to accumulate until 14:30, reaching 120 ppb. Then, wind began to blow from the SSW. The increase in O3 observed prior to the change of direction coincides with a peak in CO and NO2 concentration without a corresponding increase in NO (Figs. 5b and 7b). This pulse could be associated with the transport of photochemically active air parcels with longer residence time over the MCMA, having accumulated O3 and its precursors. An additional source of NO2 could have been PAN formed over the MCMA and its thermal dissociation process while passing over AME. The change observed at 14:30 at AME also caused a 20 ppb decrease of O3. NO2 and CO also decreased. Shortly thereafter, O3 recovered and reached a maximum of 115 ppb around 16:00 without any change in precursors. TEN showed a behavior that was very similar regarding O3 and NO2, but the wind continued to blow from the NNW until 16:00, reaching 110 ppb O3. Then, the wind direction suddenly changed and began to originate from the SSE; O3 fell to 95 ppb. With southerly winds, O3 at AME remained at 110 ppb while at TEN it remained at 95 ppb until 18:00. During this period, parcels from the MCMA that pass over AME and TEN towards the Cuautla-Cuernavaca Valley may return photochemically aged through the TAP. After 18:00, the decrease in O3 is clear. At AME this decrease in O3 was accompanied by a strong increase in CO and NO2; O3 is titrated by rising NO emissions at the beginning of the evening local commuter rush hour (Figs. 5b and 7b). The vertical profile at 17:00 shows winds from the SSW with increasing intensity with altitude, O3 is at 100 ppb until 200 m where it jumps to 110 ppb up to 500 m with winds up to 12 m/s. The afternoon shift in the wind direction was previously simulated by Doran and Zhong (2000), who indicated that there is a tendency for a stronger jet to occur on days with larger temperature differences between the Basin of Mexico (MCMA) and the CVV, sending cold air towards the Basin of Mexico.

arc. Near the surface, wind intensity was 4–6 m/s as recorded by the AME and TEN stations, but at 300 magl, at noon, wind speed was 10 m/ s or greater, as shown by Fig. 8b, and the tethered balloon had to be grounded due to safety concerns. These wind speeds and the strong slope into the Balsas River Depression just south of the TAP make this hypothesis very plausible. At AME just before 19:00, O3 decreased sharply, and NO and NO2 increased, due to the start of the late commuter rush hour. Under HPS conditions, AME and TEN (Fig. 7b and d) also showed similar O3 patterns, starting the day with near zero O3 at 07:00. The MLH began to rise at 08:00. The change in the slope of the surface O3 profile at both sites was less pronounced and the contribution of NO2 to Ox was greater. In contrast to LPS, the wind speed at TEN was 2 m/s less and this may explain some accumulation of NOx. AME recorded surface E and SE winds instead of S winds, indicating that the site was more directly under the influence of Federal Highway 115 and the local commuter rush hour. However, the tethered balloon recorded slow winds changing direction in a NW-S-NW arc between 50 and 200 m; from there up to 800 m the winds were calm. Further up, wind intensity started to increase, reaching 6 m/s at 1000 m arriving from the NE to the SE. The O3 profile shows four layers. The surface layer was O3 depleted up to 100 m, where it reached 50 ppb and remained constant up to 400 m, where it reached another 400-m-thick layer with 80 ppb of O3. Between 800 m and 1000 m, O3 dropped to 60 ppb. At 09:00 the wind began to blow from the NE at AME and from the NW at TEN; the difference was due to a massif located between both sites and a funnel effect when the surface wind began to drain from the Basin of Mexico into the Cuautla-Cuernavaca Valley basin. Thereafter, AME and TEN showed a sustained increase of O3 due to the transport of photochemically active air parcels from the MCMA. Additionally, as the surface mixing layer started to rise, they would mix with the higher 80 ppb layer observed at 400 m. Later, the vertical profile at AME measured after 12:00 (Fig. 8b) confirmed that slow winds came from the northwest with 90 ppb of surface O3 and 120 ppb at 800 m. The MLH was out of reach of the tethered balloon. While the wind remained

3.4. Correlations between NOx and Ox An observation site on the periphery of an urban area will exhibit 106

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Fig. 8. Vertical profile of O3, wind direction and wind speed at AME (local time). On 2/10/2011 (LPS) and 3/03/2011 (HPS).

NOx-Ox data points between these two extremes will represent photochemically active air parcels, aging as the points move counterclockwise in this NOx-Ox plane. Photochemical indicators describe the regional ozone production's sensitivity to VOCs and NOx. One of these, the O3/NOy ratio, may be used as an indicator of the air mass' “chemical age” (Chameides et al., 2013; Salcedo et al., 2012). NOy = NOx + NOz, where NOz represents more oxidized species such as HNO3 and PAN among others. Low O3/ NOy ratios, observed in the morning when NOx ∼ NOy, indicate either ozone production conditions sensitive to VOC levels and/or the influence of fresh NOx emissions leading to strong O3 titration (Sillman and He, 2002) On the other hand, high O3/NOy ratios, mostly observed in the afternoon, indicate ozone production conditions sensitive to NOx and the presence of photochemically aged air, with most of the NOy in

urban or rural behavior depending mainly on the direction of the wind (Parrish et al., 1991). Correlations of short time averages for Ox against NOx measured in rural or peri-urban sites but separated by synoptic condition may allow one to: (a) distinguish local from regional contributions of O3 and NO2 based on the observed levels of Ox, and (b) indirectly identify the photochemical aging of the associated air parcels based on the Ox/NOx ratio (Mazzeo et al., 2005). In general, a NOx-Ox scatter plot with high values of NOx and low values of Ox evidences air parcels in which Ox is mainly made of NO2 from relatively fresh NOx emissions, where O3 is titrated by high NO levels. Conversely, low values of NOx with high values of Ox indicate the passing of aged air masses with a low NO2 content, in which O3 is the main component of Ox. These would be parcels with little replenishment of precursors, coming from the nearby rural areas. Finally, 107

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Fig. 9. Hourly Ox and NOx correlations for AME, TEN, and UIZ from February 6th to March 8th 2011.

reference to observations at a site within Mexico City. The division into intervals of 07:00–09:00, 09:01–14:00, and 14:01–19:00 corresponds to the intervals at which the wind direction changed under HPS at the TAP. Each graph includes a line showing an approximation of the theoretical O3/NOy sensitivity transition ratio limit from VOC-sensitivity to NOx-sensitivity previously obtained by Torres-Jardón et al. (2009) for the MCMA region. The purpose of including this line was to provide additional qualitative support for the interpretation of the scatter plots. First, we analyzed the correlations for UIZ (Fig. 9e and f), the distinctly urban site, in order to later highlight the differences between the peri-urban sites. At UIZ, the difference due to synoptic system change was more noticeable in NOx but less in O3. NOx exceeded 200 ppb under both systems in the 07:00–09:00 interval with low values of Ox mainly composed of NO2, indicating fresh parcels rich in precursors. Between 09:01 and 14:00, Ox concentration grew due to O3 accumulation as the observed parcels aged. Between 14:01 and 19:00, the points moved

the form of NOz, but afternoon low O3/NOy ratios will represent VOCsensitive conditions typical of urban-like areas with photochemically young air masses. The transition from VOC-sensitive conditions to NOxsensitive conditions is represented by a constant O3/NOy ratio obtained from region specific modeling studies. The connection between the O3/ NOy indicator and the NOx-Ox correlation plot is explained by the evident similarities between these two approaches. The assumption made in the present study, namely that of using the Ox-NOx correlation as a surrogate for the O3/NOy indicator, is supported by the well-known fact that NOx chemiluminescence analyzers also measure HNO3, PAN, and nitrogen-oxidized species other than NO2 (Dunlea et al., 2007) Thus, it is highly probable that afternoon NOx measurements (which can be represented as NOx*) can serve as a reasonable surrogate for NOy. Fig. 9 shows the correlations performed with hourly data for Ox and NOx for the 07:00–09:00, 09:01–14:00, and 14:01–19:00 intervals for AME, TEN, and UIZ, grouped by synoptic system. UIZ was included as a 108

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4. Conclusion

towards the Ox axis with variable values of NOx whose influence on Ox depended on the type of synoptic system. Under LPS, most Ox was made of O3. However, under HPS influence there was a greater contribution from NO2. In both synoptic conditions, most of the data points fell under VOC-sensitive conditions with photochemically young parcels, as expected for an urban site. Similarly, in AME under both synoptic systems between 07:00 and 09:00 (Fig. 9a and b) Ox was mainly composed of NO2. Consistent with AME's role as a local hub for commuters, NOx reached values similar to those observed at UIZ, especially under HPS. However, from 09:01 to 14:00 and under both synoptic systems, the expected counterclockwise aging transition vanished very quickly. As the surface inversion started to break, NOx was quickly diluted and residual O3 was driven down from higher layers. Also, O3 from both neighboring air basins may have been be driven through TAP. Under LPS, AME received relatively clean air mases with Ox lower than 80 ppb and low NOx from the CuautlaCuernavaca Valley in the south. By contrast, under HPS, AME received aged-Ox-rich and NOx-poor air masses from the Valley of Mexico in the north; when Ox reached 120 ppb it was mostly composed of O3. Afternoon Ox levels were lower during LPS but higher under HPS conditions and composed mainly of regional O3. (Ox, NOx) levels were consistent with the transition to NOx-sensitive conditions typical of smog receptor sites. TEN, between 07:00 and 09:00 under LPS (Fig. 9c) showed low levels of NOx and Ox as result of low local emissions further diluted by clean air arriving from the Pacific Ocean; this was the site/synoptic condition combination that was most rural during the campaign. However, TEN under HPS (Fig. 9d) showed some accumulation of NOx due to slower wind. Although it reached lower values, the Ox-NOx distribution resembled that which was observed at UIZ. In a manner, similar to the behavior shown under LPS, winds continued to arrive from the south from 09:01 to 14:00; Ox did not exceed 60 ppb, with very low NOx. Nevertheless, despite its smaller population and lower vehicle activity compared to AME, TEN under HPS showed the counterclockwise aging transition of photochemically active air masses. Ox reached 120 ppb and NOx reached 100 ppb. The complex terrain at the TAP may explain this difference between AME and TEN under HPS. Due to the funnel effect, TEN was direct receptor of photochemically active air masses from most of the MCMA while AME only received aged air masses from the eastern side of the MCMA (Chalco and Texcoco), moving south alongside the western slope of the Sierra Nevada. From 14:01 to 19:00 under both synoptic conditions, the behavior of TEN, AME, and UIZ was similar, showing aged parcels. Ox-NOx data points were grouped into a clustered band that displayed no or little sensitivity to NOx, resembling high O3/NOy ratios. This tendency increased from the urban UIZ under HPS, to TEN under LPS. These results for TEN qualitatively agree with those reported for Santa Ana during MCMA 2003 (Dunn et al., 2004; Velasco et al., 2007) and MILAGRO (Thornhill et al., 2008), when both sites were upwind or downwind from the MCMA. In both field campaigns, Aerodyne Mobile Laboratory used Santa Ana as a peri-urban observation site for few days. Surface air masses from the MCMA arrived from the west as they drained along the mountain barrier towards the TAP. Dunn et al. reported criteria gases, Velasco et al. reported VOCs, and Thornhill reported BC, SPAH, CO, CO2, and NOy. In Santa Ana, the morning to midday flow from the MCMA was also observed. Due to the complex terrain in that part of the Mexico Basin, the timing of wind shifts or concentration peaks will not precisely match those of TEN and AME. Small local rush hour peaks were also observed, which were also quickly diluted as the MLH started to rise. Aging air parcels also arrived in Santa Ana from the MCMA. High peaks of O3 were observed coming from the SE (TAP) but they did not last as long as those observed in the TAP.

This work completes the existing knowledge regarding the TAP's role in the air quality of the MCMA, which had previously been explored mostly from a meteorological standpoint. Following de Foy's classification, we have shown that O3-North events in the MCMA correspond to LPS synoptic conditions as observed at the TAP. Under LPS, clean or photochemically aged air, rarely exceeding 70 ppb of O3 is driven into the basin through the TAP during most of the day. On the other hand, de Foy's O3-South events correspond to HPS conditions as observed at the TAP. From mid-morning on those days, northerly winds drain photochemically active air mases from the Valley of Mexico to the Cuautla-Cuernavaca Valley carrying up to 100 ppb of O3. Just after midday, the wind shifts back, returning O3-rich (up to 110 ppb), wellmixed, photochemically-aged air mases until sunset. For the duration of these flows throughout the day, O3-South conditions imply a net flow of O3-rich air masses towards the MCMA. The use of the tethered balloon allowed us to assess the contribution of O3 from the upper layers of the lower troposphere. Under both systems, stronger under HPS, the early morning rise of O3 mixing ratios shows a contribution of richer O3 from the higher layers, simultaneous with a gradual drop in CO and NOx as the MLH starts to rise. The comparison of NOx-Ox correlations in the TAP with those observed at UIZ, classified by synoptic condition and time of day, allowed us to approximate the O3-VOC-NOx sensitivity and photochemical age of the parcels observed at the TAP. From 09:00 on, with the exception of TEN under HPS, the NOx-Ox data points observed at the TAP sharply cluster towards high Ox (O3) and low NOx (NOy), indicating weak local precursor contributions to ozone under both synoptic systems. The more urban-like NOx- Ox data point distribution observed at TEN under HPS, from 09:00 to 14:00, is due to the transport of photochemically active air masses from the upwind MCMA. For air quality policy management in the Mexico megalopolis, it is important to note that under HPS, the 1-h average (95 ppb) Mexican ozone standard was exceeded on 9 out of 13 days at the TAP. In addition, the equal-ranking moving 8-h average (70 ppb) Mexican ozone standard was exceeded for 47 out of 150 daytime hours at TEN, 31 h at AME, and 9 h at OZU. Acknowledgments We wish to thank Project CONACYT/SEMARNAT 2006/2301 and Project PAPPIT-UNAM IN118706. José García expresses his gratitude to CONACYT for his Ph.D. scholarship. We also thank the heads of: UAEM campus Amecameca, High School no. 273 5 de Febrero at Tenango del Aire, and Outpatient Clinic Nicolás Bravo Bicentenario at Ozumba for the space and support provided during the field campaign. Bertha Mar supported us with the maps along with David Cabrera. We also wish to acknowledge the contribution made by Patrick Weill, for his assistance with the English language. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.atmosenv.2017.11.017. References Chameides, W.L., Fehsenfeld, F., Sillman, S., Hübler, G., 1994. Evaluation of the relative contribution of VOCs and NOx to ozone formation in rural and urban areas. In: Fehsenfeld, F., Meagher, J., Cowling, E. (Eds.), Southern oxidant study; 1993 data analysis report. North Carolina State University, Raleigh, N.C, pp. 92. Clapp, L.J., Jenkin, M.E., 2001. Analysis of the relationship between ambient levels of O3, NO2 and NO as a function of NOx in the UK. Atmos. Environ. 35, 6391–6405. http:// dx.doi.org/10.1016/S1352-2310(01)00378-8. CLRTAP, 2015. Mapping Critical Levels for Vegetation, Chapter III of Manual on Methodologies and Criteria for Modelling and Mapping Critical Loads and Levels and Air Pollution Effects, Risks and Trends, UNECE Convention on Long-range

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J.S. García-Yee et al.

Weinheimer, A.J., Shetter, R., Apel, E., Montzka, D.D., Knapp, D.J., Zheng, W., 2009. Assessing the regional impacts of Mexico City emissions on air quality and chemistry. Atmos. Chem. Phys. 9, 3731–3743. http://dx.doi.org/10.5194/acp-9-3731-2009. Millán, M.M., Artíñano, B., Alonso, L., Navazo, M., Castro, M., 1991. The effect of mesoscale flows on regional and long-range atmospheric transport in the western Mediterranean area. Atmos. Environ. Part A. General Top. 25, 949–963. http://dx. doi.org/10.1016/0960-1686(91)90137-V. Molina, L.T., Madronich, S., Gaffney, J.S., Apel, E., de Foy, B., Fast, J., Ferrare, R., Herndon, S., Jimenez, J.L., Lamb, B., Osornio-Vargas, A.R., Russell, P., Schauer, J.J., Stevens, P.S., Volkamer, R., Zavala, M., 2010. An overview of the MILAGRO 2006 Campaign: Mexico City emissions and their transport and transformation. Atmos. Chem. Phys. 8697–8760. http://dx.doi.org/10.5194/acp-10-8697-2010. NOAA, 2016. National Weather Service, Weather Prediction Center. WPC Surface Analysis Archive. Archive of North American Analyses: 2005 to present. Available at: http://www.wpc.ncep.noaa.gov/archives/web_pages/sfc/sfc_archive.php, Accessed date: 6 December 2017. Parrish, D.D., Trainer, M., Buhr, M.P., Watkins, B.A., Fehsenfeld, F.C., 1991. Carbon monoxide concentrations and their relation to concentrations of total reactive oxidized nitrogen at two rural U.S. sites. J. Geophys. Res. Atmos. 96, 9309–9320. http:// dx.doi.org/10.1029/91JD00047. Ruiz-Suárez, L.G., Torres-Jardón, R., Woernschimmel, H., Steinbrecher, R., Junkerman, W., Nigenda, E., García-Reynoso, A., Melamed, M., Jazcilevich, A., Mar-Morales, B.E., Grutter, M., Molina, L., 2010. In Depth Analysis of the Role of the Mountain Gap South of the Valley of Mexico on the Air Quality in Mexico City. European Geosciences Union, General Assembly 2010, Vienna, Austria EGU2010-14043-1, Available at: http://meetingorganizer.copernicus.org/EGU2010/EGU2010-14043-1. pdf, Accessed date: 6 December 2017. Salcedo, D., Castro, T., Ruiz-Suárez, L.G., García-Reynoso, A., Torres-Jardón, R., TorresJaramillo, A., Mar-Morales, B.E., Salcido, A., Celada, A.T., Carreón-Sierra, S., Martínez, A.P., Fentanes-Arriaga, O.A., Deustúa, E., Ramos-Villegas, R., RetamaHernández, A., Saavedra, M.I., Suárez-Lastra, M., 2012. Study of the regional air quality south of Mexico City (Morelos state). Sci. Total Environ. 414, 417–432. http://dx.doi.org/10.1016/j.scitotenv.2011.09.041. Sedema, 2016. Estaciones de monitoreo. Secretaría del Medio Ambiente, Gobierno de la Ciudad de México, Ciudad de México Available at: http://www.aire.cdmx.gob.mx/ default.php?opc=%27ZaBhnmI=&dc=%27ZA, Accessed date: 6 December 2017. Sedema, SMAGEM, Semarnat and Cofepris, 2011. Programa para mejorar la calidad del aire de la Zona Metropolitana del Valle de México 2011–2020. Secretaría del Medio Ambiente del Distrito Federal, Secretaría del Medio Ambiente del Estado de México, Secretaría del Medio Ambiente y Recursos Naturales and Comisión Federal para la Protección de Riesgos Sanitarios, Ciudad de México Available at: http://www.aire. cdmx.gob.mx/descargas/publicaciones/flippingbook/proaire-2011-2020-anexos/, Accessed date: 6 December 2017. Sillman, S., He, D., 2002. Some theoretical results concerning O3-NOx-VOC chemistry and NOx-VOC indicators. J. Geophys. Res. Atmos. 107 (D22). http://dx.doi.org/10. 1029/2001JD001123. ACH 26-1 to ACH 26-15. Song, F., Shin, J.Y., Jusino-Atresino, R., Gao, Y., 2011. Relationships among the springtime ground-level NOX, O-3 and NO3 in the vicinity of highways in the US East Coast. Atmos. Pollut. Res. 2, 374–383. http://dx.doi.org/10.5094/APR.2011.042. SSA, 2014. Norma Oficial Mexicana NOM-020-SSA1-2014, Salud Ambiental. Valor Límite Permisible para la Concentración de Ozono (O3) en el Aire Ambiente y Criterios para su Evaluación. Secretaría de Salud, Ciudad de México Available at: http://www.dof. gob.mx/nota_detalle.php?codigo=5356801&fecha=19/08/2014, Accessed date: 6 December 2017. Thornhill, D.A., Foy, B.d., Herndon, S.C., Onasch, T.B., Wood, E.C., Zavala, M., Molina, L.T., Gaffney, J.S., Marley, N.A., Marr, L.C., 2008. Spatial and temporal variability of particulate polycyclic aromatic hydrocarbons in Mexico City. Atmos. Chem. Phys. 8, 3093–3105. http://dx.doi.org/10.5194/acp-8-3093-2008. Torres-Jardón, R., García-Reynoso, J.A., Jazcilevich, A., Ruiz-Suárez, L.G., Keener, T.C., 2009. Assessment of the ozone-nitrogen oxide-volatile organic compound sensitivity of Mexico city through an indicator based approach: measurements and numerical simulations comparison. J. Air Waste Manag. Assoc. 59, 1155–1172. http://dx.doi. org/10.3155/1047-3289.59.10.1155. Trainer, M., Parrish, D.D., Goldan, P.D., Roberts, J., Fehsenfeld, F.C., 2000. Review of observation-based analysis of the regional factors influencing ozone concentrations. Atmos. Environ. 34, 2045–2061. http://dx.doi.org/10.1016/S1352-2310(99) 00459-8. Velasco, E., Lamb, B., Westberg, H., Allwine, E., Sosa, G., Arriaga-Colina, J.L., Jobson, B.T., Alexander, M.L., Prazeller, P., Knighton, W.B., 2007. Distribution, magnitudes, reactivities, ratios and diurnal patterns of volatile organic compounds in the Valley of Mexico during the MCMA 2002 & 2003 field campaigns. Atmos. Chem. Phys. 7, 329–353. http://dx.doi.org/10.5194/acp-7-329-2007. WHO, 2006. WHO Air Quality Guidelines for Particulate Matter, Ozone, Nitrogen Dioxide and Sulfur Dioxide. GLOBAL Update 2005. Summary of Risk Assessment. World Health Organization Available at: http://apps.who.int/iris/bitstream/10665/ 69477/1/WHO_SDE_PHE_OEH_06.02_eng.pdf, Accessed date: 6 December 2017. Yokelson, R.J., Urbanski, S.P., Atlas, E.L., Toohey, D.W., Alvarado, E.C., Crounse, J.D., Wennberg, P.O., Fisher, M.E., Wold, C.E., Campos, T.L., Adachi, K., Buseck, P.R., Hao, W.M., 2007. Emissions from forest fires near Mexico City. Atmos. Chem. Phys. 7, 5569–5584. http://dx.doi.org/10.5194/acp-7-5569-2007.

Transboundary Air Pollution. Available at: http://www.rivm.nl/media/ documenten/cce/manual/Final%20%20new%20Chapter%203%20v4%20(Oct %202017).pdf, Accessed date: 6 December 2017. Crounse, J.D., DeCarlo, P.F., Blake, D.R., Emmons, K., Campos, T.L., Apel, E.C., Clarke, A.D., Weinheimer, A.J., McCabe, D.C., Yokelson, R.J., Jimenez, J.L., Wennberg, P.O., 2009. Biomass burning and urban air pollution over the Central Mexican Plateau. Atmos. Chem. Phys. 9, 4929–4944. http://dx.doi.org/10.5194/acp-9-4929-2009. de Foy, B., Caetano, E., Magaña, V., Zitácuaro, A., Cárdenas, B., Retama, A., Ramos, R., Molina, L.T., Molina, M.J., 2005. Mexico City basin wind circulation during the MCMA-2003 field campaign. Atmos. Chem. Phys. 5, 2267–2288. http://dx.doi.org/ 10.5194/acp-5-2267-2005. de Foy, B., Fast, J.D., Paech, S.J., Phillips, D., Walters, J.T., Coulter, R.L., Martin, T.J., Pekour, M.S., Shaw, W.J., Kastendeuch, P.P., Marley, N.A., Retama, A., Molina, L.T., 2008. Basin-scale wind transport during the MILAGRO field campaign and comparison to climatology using cluster analysis. Atmos. Chem. Phys. 1209–1224. http://dx. doi.org/10.5194/acp-8-1209-2008. de Foy, B., Varela, J.R., Molina, L.T., Molina, M.J., 2006. Rapid ventilation of the Mexico City basin and regional fate of the urban plume. Atmos. Chem. Phys. 6, 2321–2335. http://dx.doi.org/10.5194/acp-6-2321-2006. Doran, J.C., Barnard, J.C., Arnott, W.P., Cary, R., Coulter, R., Fast, J.D., Kassianov, E.I., Kleinman, L., Laulainen, N.S., Martin, T., 2007. The T1-T2 study: evolution of aerosol properties downwind of Mexico City. Atmos. Chem. Phys. 7, 1585–1598. http://dx. doi.org/10.5194/acp-7-1585-2007. Doran, J.C., Bian, X., de Wekker, S.F.J., Edgerton, S., Fast, J.D., Hubbe, J.M., Shaw, W.J., Whiteman, C.D., Abbott, S., King, C., Leach, J., Mulhearn, M., Russell, C., Templeman, B., Wolfe, D., Archuleta, J., Elliott, S., Fernandez, A., Langley, D., Lee, J.T., Porch, W., Tellier, L., Chow, J., Watson, J.G., Coulter, R.L., Martin, T.J., Shannon, J.D., White, R., Martinez, D., Martinez, J.L., Mora, V., Sosa, G., Mercado, G., Pena, J.L., Salas, R., Petty, R., 1998. The IMADA-AVER boundary layer experiment in the Mexico City Area. Bull. Am. Meteorol. Soc. 79, 2497–2508. http://dx.doi. org/10.1175/1520-0477(1998)079<2497:TIABLE>2.0.CO;2. Doran, J.C., Zhong, S., 2000. Thermally driven gap winds into the Mexico City basin. J. Appl. Meteorol. 39, 1330–1340. https://doi.org/10.1175/1520-0450(2000) 039%3C1330:TDGWIT%3E2.0.CO;2. Dunlea, E.J., Herndon, S.C., Nelson, D.D., Volkamer, R.M., San Martini, F., Sheehy, P.M., Zahniser, M.S., Shorter, J.H., Wormhoudt, J.C., Lamb, B.K., 2007. Evaluation of nitrogen dioxide chemiluminescence monitors in a polluted urban environment. Atmos. Chem. Phys. 7, 2691–2704. http://dx.doi.org/10.5194/acp-7-2691-2007. Dunn, M.J., Jiménez, J.L., Baumgardner, D., Castro, T., McMurry, P.H., Smith, J.N., 2004. Measurements of Mexico City nanoparticle size distributions: observations of new particle formation and growth. Geophys. Res. Lett. 31. http://dx.doi.org/10.1029/ 2004GL019483. Fast, J.D., de Foy, B., Rosas, F.A., Caetano, E., Carmichael, G., Emmons, L., McKenna, D., Mena, M., Skamarock, W., Tie, X., Coulter, R.L., Barnard, J.C., Wiedinmyer, C., Madronich, S., 2007. A meteorological overview of the MILAGRO field campaigns. Atmos. Chem. Phys. 7, 2233–2257. http://dx.doi.org/10.5194/acp-7-2233-2007. Fast, J.D., Zhong, S.Y., 1998. Meteorological factors associated with inhomogeneous ozone concentrations within the Mexico City basin. J. Geophys. Res. Atmos. 103, 18927–18946. http://dx.doi.org/10.1029/98JD01725. Gangoiti, G., Millán, M.M., Salvador, R., Mantilla, E., 2001. Long-range transport and recirculation of pollutants in the western Mediterranean during the project regional cycles of air pollution in the West-Central Mediterranean Area. Atmos. Environ. 35, 6267–6276. https://doi.org/10.1016/S1352-2310(01)00440-X. Garcia-Reynoso, A., Jazcilevich, A., Ruiz-Suarez, L.G., Torres-Jardon, R., Lastra, M.S., Juarez, N.A.R., 2009. Ozone weekend effect analysis in Mexico City. Atmosfera 22, 281–297. ISSN 2395-8812. Available at: http://www.revistascca.unam.mx/atm/ index.php/atm/article/view/8627, Accessed date: 6 December 2017. Jauregui, E., 1988. Local wind and air pollution interaction in the Mexico basin. Atmosfera 1, 131–140. ISSN 2395-8812. Available at: http://www.revistascca.unam. mx/atm/index.php/atm/article/view/25944, Accessed date: 6 December 2017. Jauregui, E., 2002. The climate of the Mexico City Air Basin: its effects on the formation and transport of pollutants. In: Fenn, Mark E., Bauer, L.I. de, Hernandez-Tejeda, Tomas (Eds.), Ecological studies. Urban Air Pollution and Forest, vol. 56. Springer, New York, pp. 86–117. Chapter 5. https://doi.org/10.1007/978-0-387-22520-3_5. Jazcilevich, A., Garcia, A., Ruiz-Suarez, L., 2003. A study of air flow patterns affecting pollutant concentrations in the Central Region of Mexico. Atmos. Environ. 37, 183–193. http://dx.doi.org/10.1016/S1352-2310(02)00893-2. Melamed, M.L., Basaldud, R., Steinbrecher, R., Emeis, S., Ruiz-Suárez, L.G., Grutter, M., 2009. Detection of pollution transport events southeast of Mexico City using groundbased visible spectroscopy measurements of nitrogen dioxide. Atmos. Chem. Phys. 9, 4827–4840. http://dx.doi.org/10.5194/acp-9-4827-2009. Mazzeo, N.A., Venegas, L.E., Choren, H., 2005. Analysis of NO, NO2,O3 and NOx concentrations measured at a green area of Buenos Aires City during wintertime. Atmos. Environ. 39, 3055–3068. http://dx.doi.org/10.1016/j.atmosenv.2005.01.029. McKendry, I.G., Steyn, D.G., Banta, R.M., Strapp, W., Anlauf, K., Pottier, J., 1998. Daytime photochemical pollutant transport over a Tributary Valley Lake in Southwestern British Columbia. J. Appl. Meteorol. 37, 393–404. http://dx.doi.org/ 10.1175/1520-0450(1998)037%3c0393:DPPTOA%3e2.0.CO;2. Mena-Carrasco, M., Carmichael, G.R., Campbell, J.E., Zimmerman, D., Tang, Y., Adhikary, B., D'Allura, A., Molina, L.T., Zavala, M., Garcia, A., Flocke, F., Campos, T.,

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