**3. Results**

NOx.

terms of industrial infrastructure, and one of the cities with the worst air quality problems in Mexico [18]. The MMA (see Figure 1) has been in violation of the 1-hr Mexican Air Quality Standard for O3 (0.11 ppm) since the establishment of its routine air quality monitoring system in 1993. For example, the 1-hr O3 standard was exceeded on 48 different days in 2011 [19]. Peak O3 concentrations can reach 170 ppb and typically occur at the downtown or western air quality stations [20]. Some studies report that the MMA is the fifth most polluted Latin American city in terms of O3 [21]. In addition to the large number of industrial facilities located inside or nearby the metropolitan area that contribute to the poor air quality, emissions from several gas-fired electric utilities and one of the six refineries that operate in the country (located less than 40 km to the east of downtown Monterrey) add to the anthropogenic burden imposed on the airshed. Despite the importance of the contribution of area and point sources to the total emissions inventory, the mobile sources represent approximately 75% of the total anthropo‐ genic emissions released in the MMA [20]. This is a result of a relatively high proportion of vehicles per inhabitant registered in this urban center (approximately one vehicle for every

The study used data collected by the *Sistema Integral de Monitoreo Ambiental* (SIMA; Integrated Environmental Monitoring System) of the MMA. At the time of the study, valid data from six operational routine air quality stations were available: Downtown, Southeast, Southwest, Northeast, Northwest, and North (see Figure 1). Data archived for the period of August 2012 to August 2013 were retrieved. The database contained hourly-average concentrations of CO, NO, NO2, and O3, as well as meteorological parameters (relative humidity, atmospheric pressure, dry bulb temperature, solar radiation [SR], wind speed [WS], and wind direction [WD]). These data go through a quality assurance/quality control process defined by SIMA in compliance with international standards. Additional air quality parameters monitored by the stations (mainly, SO2 and particulate matter with aerodynamic diameter less than or equal to 10 microns [PM10] and less than or equal to 2.5 microns [PM2.5]) were not used in this study, as

In addition to the above chemical parameters, NOy was monitored in the Downtown station from August 2012 to August 2013, using a NO-NOy (Thermo Scientific, Model 42i-Y) chemilu‐ minescence continuous sampling device. NOy is a chemical parameter that is not routinely measuredbyMexicanairqualitystations,as itisnot consideredacriteriapollutant.Thedecision to deploy the NO-NOy instrument atthe location ofthe Downtown station was based on spatial homogeneity studies performed in the past for the MMA [22], which indicate that pollutant levels observed in the Downtown station are representative of the MMA, with the exception of the Southeast region. The data collected by the NO-NOy device went through a validation process similar to that routinely conducted by SIMA on its own collected data. The main conditions to reject data included the following red flags: obstructed capillary tubing, low flow in the inlet, and concentrations outside the measurement range (i.e., above 200 ppbv). The data were thenconsolidatedandanalyzedonadaily basis, amonthly basis, andby seasons (Summer

two inhabitants).

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**2. Methods: Database and analysis tools**

the focus was on the relationship between O3 and NOx.

### **3.1. Descriptive statistics**

Table 1 summarizes the descriptive statistics of the chemical and meteorological parameters retrieved from the Downtown air quality monitoring station. The values presented are based on the daily means for the August 2012-August 2013 period. The climatology of the MMA is portrayed in these results: a hot semi-arid region characterized by extreme weather conditions [23]. The long-term average temperature of Monterrey is around 23°C, with temperatures that can reach 40°C in summer and below 0°C in winter. Average humidity is 62%; the rainy season is between August and October, directly related to the occurrences of hurricanes in the Gulf of Mexico (the average mean annual precipitation is around 600 mm). Due to its geographic location, the MMA is influenced by anti-cyclonic systems from the Gulf of Mexico [24], which in some instances result in high atmospheric stability in the region, thereby inhibiting the vertical mixing of pollutants.


**Table 1.** Descriptive statistics for parameters monitored at the Downtown air quality station of the MMA (annual means, medians, maximum values, and minimum values of the daily averages)

Details of the monthly variation of the daily averages of the chemical and meteorological parameters are depicted in Figure 2. O3 levels tend to be the highest during springtime, as well as during late summer and early fall. These periods correspond to the times when temperature and SR are high and wet precipitation is low, i.e., conditions that foster photochemical reactions. Another relevant feature presented in Figure 2 involves the inverse relationship observed between O3 and NOx. That is, during the cold months, when SR is low, NOx levels tend to be high due to inhibition of the reactions between VOCs and NOx that promote the conversion of NO to NO2, as well as suppression of the photo-dissociation of NO2 to produce O3. We need to bear in mind that NOx emissions are mainly in the form of NO. Thus, an accumulation of NO and NO2 is observed in winter. At other times, temperatures and SR increase, promoting photochemical reactions. A meteorological component influences the observed NOx concentrations: shallow mixing heights and low wind speeds during winter. The O3 seasonal boxplots indicate that even though fall is the second most important season with respect to the frequency of peak O3 levels, it is also the time when the average O3 is the lowest (Figure 3). This can be explained by the incidence of rain events associated with this season.

Ambient Level of NOx and NOy as Indicators of Photochemical Activity in an Urban Center http://dx.doi.org/10.5772/59752 291

on the daily means for the August 2012-August 2013 period. The climatology of the MMA is portrayed in these results: a hot semi-arid region characterized by extreme weather conditions [23]. The long-term average temperature of Monterrey is around 23°C, with temperatures that can reach 40°C in summer and below 0°C in winter. Average humidity is 62%; the rainy season is between August and October, directly related to the occurrences of hurricanes in the Gulf of Mexico (the average mean annual precipitation is around 600 mm). Due to its geographic location, the MMA is influenced by anti-cyclonic systems from the Gulf of Mexico [24], which in some instances result in high atmospheric stability in the region, thereby inhibiting the

**Species/Parameters Mean ± Std. Dev. Median Maximum Minimum**

) 0.24 ± 0.12 0.24 0.70 0.00

O3 (ppb) 25.7 ± 9.0 24.4 50.7 2.6 NO2 (ppb) 16.3 ± 5.9 15.3 39.2 3.3 NO (ppb) 6.5 ± 4.7 5.3 34.4 0.6 NOy (ppb) 40.0 ± 20.3 35.2 188.2 12.1 CO (ppm) 0.78 ± 0.26 0.75 1.78 0.21

Temperature (°C) 24.4 ± 5.2 25.8 32.5 9.5 Wind speed (km/h) 1.84 ± 0.47 1.84 3.21 0.71 Relative Humidity (%) 62.9 ± 16.8 64.17 98.9 13.5

**Table 1.** Descriptive statistics for parameters monitored at the Downtown air quality station of the MMA (annual

Details of the monthly variation of the daily averages of the chemical and meteorological parameters are depicted in Figure 2. O3 levels tend to be the highest during springtime, as well as during late summer and early fall. These periods correspond to the times when temperature and SR are high and wet precipitation is low, i.e., conditions that foster photochemical reactions. Another relevant feature presented in Figure 2 involves the inverse relationship observed between O3 and NOx. That is, during the cold months, when SR is low, NOx levels tend to be high due to inhibition of the reactions between VOCs and NOx that promote the conversion of NO to NO2, as well as suppression of the photo-dissociation of NO2 to produce O3. We need to bear in mind that NOx emissions are mainly in the form of NO. Thus, an accumulation of NO and NO2 is observed in winter. At other times, temperatures and SR increase, promoting photochemical reactions. A meteorological component influences the observed NOx concentrations: shallow mixing heights and low wind speeds during winter. The O3 seasonal boxplots indicate that even though fall is the second most important season with respect to the frequency of peak O3 levels, it is also the time when the average O3 is the lowest (Figure 3). This can be explained by the incidence of rain events associated with this

means, medians, maximum values, and minimum values of the daily averages)

vertical mixing of pollutants.

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Solar radiation (kW/m2

season.

**Figure 2.** Time series plots of monthly averages of the mean daily values of chemical and meteorological parameters observed at the Downtown station

#### **3.2. Spatial analysis**

The exploratory analysis was complemented by the construction of bivariate polar plots of NOx (see Figure 4) and wind roses (see Figure 5) for five of the six air quality monitoring stations, covering the main sub-regions of the MMA. The five selected stations correspond to typical upwind locations (Northeast and Southeast), Downtown Monterrey, and typical downwind locations (Northwest and Southwest), as illustrated in Figure 5. Seasonal variation is also presented in these plots.

The polar plots represent NOx concentration as a function of wind speed and direction, and they help identify the possible occurrence/prevalence of horizontal transport conditions that lead to high NOx events. Further away from the center of the plot, the wind speed is higher. The form of the polar plot can provide an indication of the wind direction and speed frequency; however, this information is better observed through the corresponding wind roses.

**Figure 3.** O3 boxplots by seasonal periods for 1-hr average values reported at the Downtown station

In general, Figure 4 indicates higher transport of NOx from the northern stations (this can be catalogued as "fresh" NOx), which then is consumed as the air masses traverse the urban core. The transport patterns appear to be similar in all seasons. Thus, the change in NOx by season appears to be more strongly linked to photochemical conditions (temperature and SR) than to wind patterns.

Using the same analysis as was used for NOx, Figure 6 depicts the behavior of O3 as a function of wind speed and direction exclusively at the Downtown station. In all cases, the higher concentrations begin to occur when the wind velocity was above 2 m/s, indicating the influence of transport and aging of air masses on the observed O3 concentrations and weak O3 production under stagnant conditions. Transport influences the levels of O3; consequently, the highest O3 levels correlate with the prevailing wind directions (wind blowing from the Northeast-East-Southeast and from the North-Northwest-West). The East-to-West air flow channels air masses through the long axis of the MMA, adding more precursors to the chemically aged mixtures as they traverse the urban core, thereby promoting O3 production.

**3.2. Spatial analysis**

292 Current Air Quality Issues

wind patterns.

is also presented in these plots.

The exploratory analysis was complemented by the construction of bivariate polar plots of NOx (see Figure 4) and wind roses (see Figure 5) for five of the six air quality monitoring stations, covering the main sub-regions of the MMA. The five selected stations correspond to typical upwind locations (Northeast and Southeast), Downtown Monterrey, and typical downwind locations (Northwest and Southwest), as illustrated in Figure 5. Seasonal variation

The polar plots represent NOx concentration as a function of wind speed and direction, and they help identify the possible occurrence/prevalence of horizontal transport conditions that lead to high NOx events. Further away from the center of the plot, the wind speed is higher. The form of the polar plot can provide an indication of the wind direction and speed frequency;

however, this information is better observed through the corresponding wind roses.

**Figure 3.** O3 boxplots by seasonal periods for 1-hr average values reported at the Downtown station

In general, Figure 4 indicates higher transport of NOx from the northern stations (this can be catalogued as "fresh" NOx), which then is consumed as the air masses traverse the urban core. The transport patterns appear to be similar in all seasons. Thus, the change in NOx by season appears to be more strongly linked to photochemical conditions (temperature and SR) than to

Using the same analysis as was used for NOx, Figure 6 depicts the behavior of O3 as a function of wind speed and direction exclusively at the Downtown station. In all cases, the higher concentrations begin to occur when the wind velocity was above 2 m/s, indicating the influence of transport and aging of air masses on the observed O3 concentrations and weak O3 production The frequency of high O3 concentrations related to a specific wind direction cannot be inferred from the polar plots, however. That analysis was conducted by constructing boxplots for 1-hr average O3 concentrations for the three main wind categories (classified based on direction from which the wind was blowing) and segregated by season (see Figure 7). The year around the peak O3 concentrations and the maximum 1-hr average concentrations occur when the wind blows from the Northeast-East-Southeast. O3 levels, on average, are lower when the wind blows from the North-Northwest-West, and they are the lowest when winds blow from the South-Southeast (with the exception of the summer, when this direction provides higher average concentrations than the North-Northwest-West direction).

Comparison of the O3 levels presented by the polar plots (Figure 6) for spring and fall indicates that the influence of transport is less important during fall than in spring, as wind speeds tend to be lower; high O3 events are mainly related to events when the speed is in the range of 2.0-3.5 m/s. In spring, O3 levels can remain high at wind speeds above 4 m/s.

**Figure 4.** Polar plot for 1-h average NOx concentrations at five different monitoring stations within the MMA (the radi‐ al dimension is an indication of increasing wind speed [m/s])

**Figure 5.** Wind roses at five different monitoring stations within the MMA

Figure 5. Wind roses at five different monitoring stations within the MMA

**Figure 6.** Polar plot for 1-h average O3 concentrations at five different monitoring stations within the MMA (the radial dimension is an indication of increasing wind speed [m/s])

#### **3.3. Regression analysis**

Figure 5. Wind roses at five different monitoring stations within the MMA

**Figure 5.** Wind roses at five different monitoring stations within the MMA

294 Current Air Quality Issues

Figure 2 gives an initial indication of a direct relationship between NO and NO2 levels and an inverse relationship between NOx and O3 levels. This was explored further by means of scatter plots (not presented for brevity), which were used to derive linear regression expressions among NOx constituents and between NOx and O3:

$$
\begin{bmatrix} \mathsf{NO}\_{\times} \end{bmatrix} = \mathsf{1.5} \begin{bmatrix} \mathsf{NO} \end{bmatrix} + \begin{bmatrix} \mathsf{13.0} \end{bmatrix} \tag{11}
$$

$$
\begin{bmatrix} \mathbf{O}\_{\mathbf{o}} \end{bmatrix} = \mathbf{-O}.\mathbf{3}\mathbf{\hat{\lambda}} \begin{bmatrix} \mathbf{NO}\_{\mathbf{x}} \end{bmatrix} + \begin{array}{c} \mathbf{34} \end{array} \tag{12}
$$

where [*i*] represents the molar concentration of species i. Here, the 1-hr average concentrations were used to derive equations (11) and (12). These expressions corroborate prior findings. The relevance of equation (12) is that it provides a measure of the expected, first order change in O3 provided by a change in NOx concentration (which can occur from a change in NO emissions). The results indicate that O3 would, in fact, increase as NOx tends to be reduced. This counterintuitive finding is a well-known result seen in urban areas dominated by a VOCsensitive regime [25]. Other studies that have used complex photochemical air quality models have reached similar conclusions. Downtown Monterrey appears to be dominated by a VOCsensitive regime in which VOC controls would work better for reducing O3 levels [20].

**Figure 7.** Boxplots of 1-hr averaged O3 concentrations observed at the Downtown station, classified by prevailing wind di‐ rection: D1 (wind blowing from 20 to 120°), D2 (wind blowing from 270 to 360°), and D3 (wind blowing from 180 to 150°)

#### **3.4. Analysis of NOy levels**

Up to this point, our analysis has focused on describing the relationship between NOx and O3 production based on the meteorological conditions that foster photochemical reactions. The generalized chemical mechanism that describes this relationship indicates that other reaction products can be of interest in describing the dynamics of O3 production. In particular, the levels of NOy provide additional information and serve as an indicator of the degree to which the air masses have been subjected to photochemical processing (aging), thereby providing informa‐ tion on O3 production.

Table 2 presents the results of the correlation analysis of different chemical and meteorological parameters with NOy. A correlation coefficient (*R*<sup>2</sup> ) of NO2 with NOy above 0.78 is noted for months in which O3 production is not the highest, and between 0.40 and 0.68 for the months with the highest O3 production. In general, a high percentage of NOy corresponds to NOx (Figure 2); thus, the response of one clearly depends on the other.

The difference between NOy and NOx (known as NOz) includes the inorganic and organic nitrates derived from photochemical processing of the NOx-VOCs mixtures. The typical composition of NOz is approximately 50%-55% HNO3, 35%-40% PAN, 1%-5% HNO2, and a marginal contribution of other nitrates [26, 27]. During the cold months, the production of NOz can be inhibited by meteorological conditions that do not foster photochemical reactions. Thus, the relative contribution of NOx to NOy is the highest, and, therefore, the responses of both species are expected to show high correlation. When conditions favor O3 production, more NOz is produced, thereby limiting the contribution of NOx to the total NOy. Consequent‐ ly, the level of correlation drops as the nonlinear photochemistry and sinks of NOy increase in importance. Of interest is the fact that during spring, the season with the highest peak O3 1-hr concentrations, the correlation of NOx species with NOy is the lowest. A similar response is obtained between CO and NOy, which can be explained by the typical strong correlation



**Season Wind class**

parameters with NOy. A correlation coefficient (*R*<sup>2</sup>

(Figure 2); thus, the response of one clearly depends on the other.

**O3**

296 Current Air Quality Issues

**3.4. Analysis of NOy levels**

tion on O3 production.

> **Summer 2012 Fall 2012 Winter 2013 Spring 2013 Summer 2013 D1 D2 D3 D1 D2 D3 D1 D2 D3 D1 D2 D3 D1 D2 D3**

**Figure 7.** Boxplots of 1-hr averaged O3 concentrations observed at the Downtown station, classified by prevailing wind di‐ rection: D1 (wind blowing from 20 to 120°), D2 (wind blowing from 270 to 360°), and D3 (wind blowing from 180 to 150°)

Up to this point, our analysis has focused on describing the relationship between NOx and O3 production based on the meteorological conditions that foster photochemical reactions. The generalized chemical mechanism that describes this relationship indicates that other reaction products can be of interest in describing the dynamics of O3 production. In particular, the levels of NOy provide additional information and serve as an indicator of the degree to which the air masses have been subjected to photochemical processing (aging), thereby providing informa‐

Table 2 presents the results of the correlation analysis of different chemical and meteorological

months in which O3 production is not the highest, and between 0.40 and 0.68 for the months with the highest O3 production. In general, a high percentage of NOy corresponds to NOx

The difference between NOy and NOx (known as NOz) includes the inorganic and organic nitrates derived from photochemical processing of the NOx-VOCs mixtures. The typical composition of NOz is approximately 50%-55% HNO3, 35%-40% PAN, 1%-5% HNO2, and a marginal contribution of other nitrates [26, 27]. During the cold months, the production of NOz can be inhibited by meteorological conditions that do not foster photochemical reactions. Thus, the relative contribution of NOx to NOy is the highest, and, therefore, the responses of both species are expected to show high correlation. When conditions favor O3 production, more NOz is produced, thereby limiting the contribution of NOx to the total NOy. Consequent‐ ly, the level of correlation drops as the nonlinear photochemistry and sinks of NOy increase in importance. Of interest is the fact that during spring, the season with the highest peak O3 1-hr concentrations, the correlation of NOx species with NOy is the lowest. A similar response is obtained between CO and NOy, which can be explained by the typical strong correlation

) of NO2 with NOy above 0.78 is noted for

**Table 2.** Correlation coefficients (*R*<sup>2</sup> ) of parameters monitored by the SIMA downtown station with NOy (in parenthesis *p*-values at an α of 0.05)

The results of the correlation analysis suggest a relationship between NOx and NOy, but a correlation between NOy and O3 is less evident. Figure 2 shows two peaks in the annual cycle of the NOy monthly average concentration (constructed from daily NOy average concentra‐ tions). One peak occurs during the cold months and is associated with the high levels of NOx (low NOz component), while the second peak appears during spring, which is the period with highest photochemical activity (highest O3 levels); thus, as NOx is consumed, and its levels decrease and NOy levels are high, NOz must be contributing heavily to NOy. The peaks of NOy are associated with different conditions, so we looked in greater detail into the seasonal levels of NOy. Figure 8 depicts the 1-hr NOy concentrations by season. The highest levels (both of peak and season-average concentrations) occur in winter, followed by fall and spring (when O3 is the highest), with summer registering the lowest levels. The O3 levels presented in Figure 3 do not appear to track the NOy levels.

We then attempted to determine a clearer relationship between O3 and NOy by conducting a Principal Components Analysis (PCA) (see Table 3). The first principal component (PC1) has to do with O3 precursors in the form of primary emissions or intermediate products (NOx, NOy, and CO), in line with the results from the descriptive statistics. The second principal component (PC2) covers photochemical O3 production (O3 and solar radiation). The third principal component (PC3) refers to a combined meteorological effect that includes horizontal transport (wind direction) and temperature— an effect that was also observed in the descrip‐ tive statistics results. The first two principal components represent almost 60% of the variance.

**Figure 8.** Annual average distribution of NOy concentrations


**Table 3.** Principal components analysis results

Given these results, we conducted an additional assessment on the effects of meteorological parameters on O3 and NOy levels, which showed the effects of horizontal transport. O3, temperature, wind speed, and wind direction data were particularly correlated (see Table 4). In addition, complementary polar plots were constructed for NOy (Figure 9). The highest correlation between O3 and wind speed occurred during the winter of 2013. In this case, the slope of the linear regression was positive (i.e., as the wind speed increases, O3 also increases). In a similar way, the highest correlation between O3 and wind direction occurred during the winter of 2013. This indicates that transport is a major factor responsible for the average O3 levels observed in the winter in the MMA. The polar plot for NOy confirms that high levels of NOy during winter (which are, as discussed, mostly NOx) are associated with low wind speeds. This relationship between NOy and wind speed would indicate that NOx is not being processed and translated to high local production of O3 in the winter.

transport (wind direction) and temperature— an effect that was also observed in the descrip‐ tive statistics results. The first two principal components represent almost 60% of the variance.

**Component PC1 PC2 PC3** NOy **0.481** 0.055 –0.009 NO2 **0.387** 0.087 –0.123 NO **0.375** 0.029 0.151 O3 –0.226 **0.447** 0.075 CO **0.400** 0.110 0.081 Solar radiation –0.090 **0.497** 0.031 Temperature –0.197 0.370 **0.411** Wind velocity –0.288 0.276 0.084 Wind speed 0.175 0.339 **0.794**

Accumulated variance (%) 0.395 0.595 0.672

Given these results, we conducted an additional assessment on the effects of meteorological parameters on O3 and NOy levels, which showed the effects of horizontal transport. O3, temperature, wind speed, and wind direction data were particularly correlated (see Table 4). In addition, complementary polar plots were constructed for NOy (Figure 9). The highest correlation between O3 and wind speed occurred during the winter of 2013. In this case, the

**Figure 8.** Annual average distribution of NOy concentrations

298 Current Air Quality Issues

**Table 3.** Principal components analysis results


**Table 4.** O3 correlations with meteorological parameters (*p*-values at an α of 0.05)

An additional way to establish the influence of transport on the levels of air pollutants is to analyze the CO-to-NOy ratio. Values close to 10 are indicative of an influence of local sources, and ratios larger than 100 indicate the influence of regional (remote) sources [11]. Figure 10 depicts the CO-to-NOy ratios observed at the Downtown station as a function of the season. The average ratio tends to be the inverse with respect to the behavior of the average levels of NOy, although one main difference exists: the lowest ratios are observed during winter but also during spring, intermediate values are observed during the fall, and the highest average values are registered during the summer. In a few cases, the CO-to-NOy ratio reached values of 100 or more, and the 95th percentile, in general, was below 56. Thus, air pollution in the MMA during winter appears to be heavily influenced by low wind conditions (Figure 2 indicates the lowest average wind speeds during this season), and poor photochemical activity limits the levels of NOz and O3. The O3 levels that are registered are characteristic of background amounts that are transported to the urban center when wind speeds are sufficiently strong. The spring level is also heavily influenced by local emissions, but meteorological conditions do favor photochemical activity, resulting in production of the highest levels of O3. Fall would seem to be influenced partly by local emissions and partly by transport. Finally, summer is character‐ ized by the influence of more regional (and aged) emissions. During the first two months of the summer season, the ambient conditions tend to be characterized by high temperatures and deep planetary boundary layers that promote vertical mixing. In addition, conditions favor the mixing of these vertical columns with pollutants that undergo long-range transport.

**Figure 9.** Polar plot for NOy at the Downtown monitoring station

**Figure 10.** Seasonal variation of CO-to-NOy ratios (the red line represents a value of 10 for the ratio)

#### **3.5. Weekend-weekday effect**

that are transported to the urban center when wind speeds are sufficiently strong. The spring level is also heavily influenced by local emissions, but meteorological conditions do favor photochemical activity, resulting in production of the highest levels of O3. Fall would seem to be influenced partly by local emissions and partly by transport. Finally, summer is character‐ ized by the influence of more regional (and aged) emissions. During the first two months of the summer season, the ambient conditions tend to be characterized by high temperatures and deep planetary boundary layers that promote vertical mixing. In addition, conditions favor the mixing of these vertical columns with pollutants that undergo long-range transport.

**Figure 9.** Polar plot for NOy at the Downtown monitoring station

300 Current Air Quality Issues

**Figure 10.** Seasonal variation of CO-to-NOy ratios (the red line represents a value of 10 for the ratio)

A final aspect of interest in this study was to define if a weekend-weekday effect could be established based on NOx and NOy readings. Previous studies have demonstrated that O3 levels in urban centers can be higher during weekends [28, 29], a condition that might be thought of as counterintuitive, as emissions from anthropogenic sources tend to be lower during weekends. An explanation for the occurrence of this weekend-weekday effect can be obtained by analyzing the photochemical mechanism for the production of O3. Urban centers tend to be VOC-sensitive; that is, NOx is in excess and, thus, processes that remove NO2 from the reactive mixtures are promoted. For example, NO2 efficiently scavenges HO• radicals through reaction 9, producing HNO3, which is a direct sink for NOx. This limits the reaction of HO• radicals with VOCs, which, in turn, controls the production pathways for O3. During weekends, the ratio at which VOCs and NOx are emitted can change, preferentially reducing the emissions of NOx. If this happens, less NOx is available in the atmosphere, reaction 9 becomes less important, more HO• radicals become available to react with the VOCs, and, thus, O3 tends to increase. Figure 11 illustrates the distribution of NOy concentrations by day of the week. At a first glance, NOy values tend to be quite constant throughout the day. However, Tuesdays and Wednesdays present the lowest peak values, while average levels on Sunday tend to be lower than during the rest of the week. If the average NOx-to-NOy ratio is examined, values tend to be lower during the weekends (Figure 12). The NOx-to-NOy ratio provides information on the level of photochemical processing of the observed air parcels. Ratios closer to 1 would indicate that most of the NOy is in the form of NOx, and thus, it could imply the presence of fresh emissions or the transport of unprocessed emissions. As the ratio drops, the relative contribution of NOz increases, indicating that an aged (photochemically processed) air parcel is being observed.

**Figure 11.** Boxplot for NOy levels by day of the week

**Figure 12.** Boxplot for the NOx-to-NOy ratio by day of the week

Finally, we conducted an Analysis of Variance (ANOVA) test for the NOx-to-NOy ratio by season (see Table 5). For the hot months (spring and summer), lower ratios were observed during weekends compared to weekdays. In fact, the lowest average ratio was obtained during spring, the season with the highest average and peak O3 concentrations. In contrast, the winter of 2013 showed a marginally higher ratio during the weekends than on the weekdays, but the difference was not statistically significant. This could be the result of an intensive use of fuels during the whole week in response to the low temperatures. For the fall of 2012, this statistical test also indicates no difference between weekdays and weekends. Fall, as the results from previous sections suggest, is influenced by days with high O3 production and days with very low O3 levels as a result of the transition from a period with high SR and temperature to the regional rainy season.


**Table 5.** Results of the ANOVA test for the NOx-to-NOy ratio (*p*-values are at an α of 0.05)
