**3. Results and discussion**

#### **3.1 Meteorological characterization**

**Table 1** gives the basic statistics of meteorological variables during the three periods of study. On average, November 2003 was colder than the other sampling periods. In turn, March 2004 was drier and had the highest wind speeds. November 2004 was characterized by low wind speeds. During the first two sampling periods, the most frequent wind direction was from the NW-NE sectors. In November 2004, both XAL and MER sites were affected by E-SE winds

The diurnal variation of the meteorological parameters during the study is shown in **Figure 2**. The hourly data at each sampling period and site is an average of 14 days. In all three sampling periods and the two sites, early morning was characterized by low temperatures at 6 h, which increased consistently to a maximum at 16 h to then


*The Use of Stable Isotopes to Identify Carbon and Nitrogen Sources in Mexico City PM2.5… DOI: http://dx.doi.org/10.5772/intechopen.107914*

> **Table 1.**

*Basic statistics of meteorological variables during the three periods of study.*

#### **Figure 2.**

*Hourly average of temperature, relative humidity, wind speed, and wind direction at MER and XAL during November 2003 (blue squares), March 2004 (red dots), and November 2004 (black diamonds). Each symbol is the hourly average of 14 days at each sampling site from the Mexico City Meteorological Network (REDMET).*

decrease again. In turn, the most humid conditions were present at 6 h and decreased to a minimum of 30% at 16 h. Wind speeds were usually low during the morning hours and increased to a maximum around 18 h, which favors contaminant dispersion from the valley. At MER, low wind speeds were consistently slow until 14 h, in contrast to speeds at XAL where wind speed increased from 6 h to maximum speeds at 18 h. The lowest daily average wind speeds were significantly lower at XAL during November 2004. In general, westerly winds were present at MER between 22 h and the next day 2 h. On average, easterly winds were dominant throughout the rest of the day at both locations.

#### **3.2 PM2.5 concentrations**

Scatterplots of the daily PM2.5 concentrations collected with the MiniVol samplers were compared with the respective average daily PM10 concentrations from hourly measurements from the RAMA stations at XAL and MER (**Figure 3**). At MER, hourly measurements of the MiniVol PM2.5 data were also compared with RAMA PM2.5 data. The scatterplots show a high correlation (R<sup>2</sup> > 0.9) between PM2.5 from our MiniVols and the RAMA data and show that our daily samples are representative of the average daily PM2.5 concentrations [18]. The high correlation between PM2.5 and PM10 further suggests a common source or formation mechanism of these particles. López-Veneroni [14] showed similar correlations in concentrations and carbon isotope compositions for simultaneous PM2.5 and PM10 samples in Mexico City.

The average diurnal variations of PM10 at XAL and MER, and of PM2.5 at MER from the RAMA stations during the period of study are shown in **Figure 4**. These data provide insight into the time of the major particle accumulation throughout the day. Morning PM2.5 concentrations were around 30 mg/m3 and peaked to 70 μg/m3 at 10 h. Concentrations then decreased until 18 h and remained constant at 30 μg/m3 throughout the night and early morning. In contrast, the diurnal variation of PM10 showed two major peaks at 8 h and 18 h, which probably result in particle accumulation during traffic rush hours. During the afternoon, the increase in wind speeds dispersed these pollutants.

*The Use of Stable Isotopes to Identify Carbon and Nitrogen Sources in Mexico City PM2.5… DOI: http://dx.doi.org/10.5772/intechopen.107914*

#### **Figure 3.**

*Scatterplots of (a): PM2.5 samples collected daily by MiniVol samplers vs. 24 h averaged PM2.5 samples from Mexico City atmospheric monitoring automatic network (RAMA) at MER. (b): PM2.5 samples collected daily by MiniVol samplers vs. 24 h averaged PM10 from RAMA at MER. (c): PM2.5 samples collected daily by MiniVol samplers vs. 24 h averaged PM10 from RAMA at XAL.*

#### **Figure 4.**

*Average diurnal variations of PM10 concentrations and NOx concentrations at MER and XAL from Mexico City atmospheric monitoring automatic network (RAMA), and PM2.5 diurnal variations at MER from RAMA, during November 2003 (blue squares), March 2004 (red dots), and November 2004 (black diamonds).*

Average PM2.5 concentrations for the two sites and three sampling periods are given in **Table 2**. In November 2003 and May 2004, concentrations were similar in both sites. In November 2004, PM2.5 concentrations were significantly lower at MER relative to XAL, in accordance with the industrial activity of this site [9, 10] and low wind speeds, which preclude pollutant dispersion. Average concentrations were below 65 μg/m3 (the Mexican Health Standard Norm at the time of the study) at all sampling stations and dates (the current allowable maximum is 41 μg/m3 , [7]). In the two sites, PM2.5 concentrations were highest during weekdays (Monday to Friday) relative to the weekends (*t*-test, p < 0.01).

Average concentrations of total carbon, nitrate, and ammonium for each sampling period were also similar at MER and XAL. **Figure 5** shows that wind speed is



*The Use of Stable Isotopes to Identify Carbon and Nitrogen Sources in Mexico City PM2.5… DOI: http://dx.doi.org/10.5772/intechopen.107914*

#### **Figure 5.**

*Scatterplots of wind speed vs. PM2.5 concentrations, averaged NOx concentrations from Mexico City atmospheric monitoring automatic network (RAMA), % N in PM2.5, and % C in PM2.5, for samples collected at MER (closed symbols) and XAL (open symbols) during November 2003 (blue symbols), March 2004 (red symbols), and November 2004 (black symbols).*

important in determining the PM2.5 concentration and carbon and nitrogen composition. As wind speeds decrease, particle concentrations increase and suggest that the emitted (primary) or coalesced (secondary) particles increase when no mechanism disperses them. Likewise, the percentage of N and NO3**−** concentration in PM2.5 also increased at low wind speeds, in accordance to the gas-to-particle conversion of nitrogen compounds [4]. By contrast, the percentage of the total carbon is increased along with an increase in wind speeds and a decrease in particle concentrations, and this suggests that at high wind speeds low particle concentrations appear to be primarily composed of direct carbon emissions.

#### **3.3 Stable carbon and nitrogen isotopes of PM2.5**

**Figure 6** gives the frequency histograms of stable carbon and nitrogen isotopes in PM2.5 collected at MER and XAL during the three sampling campaigns. Averages and ranges are given in **Table 2**.

The majority of δ 13C values fell in the −27 to −25‰ range, with an important number of data points skewed to more positive values (−24 to −15‰). A few samples

**Figure 6.**

*Upper panel: histograms of δ13C-PM2.5 (‰) at MER, XAL, and all data. Lower panel: histograms of δ15N-PM2.5 (‰) at MER, XAL, and all data.*

at XAL were lighter than −28‰. In contrast, during November 2004 at MER, most δ 13C values were heavier than −22‰. This range of isotopic compositions for bulk carbon contrasts with data from a previous study in Mexico City, where most δ 13C in PM2.5 ranged between −26 and −24‰ [14]. According to the carbon isotope composition of the different potential sources in Mexico City, the predominant carbon source in PM2.5 is the emissions of fossil fuels [14]. The extreme isotopic values at XAL show

#### **Figure 7.**

*Upper panel: scatterplots of δ13C-PM2.5 vs. wind direction, wind speed, total carbon, and organic carbon at MER. Lower panel: scatterplots of δ13C-PM2.5 vs. wind direction, wind speed, total carbon, and organic carbon at XAL. Blue symbols denote samples collected during November 2003 and March 2004. Red symbols denote samples collected during November 2004.*

*The Use of Stable Isotopes to Identify Carbon and Nitrogen Sources in Mexico City PM2.5… DOI: http://dx.doi.org/10.5772/intechopen.107914*

that during March 2004, light hydrocarbons (such as methane and propane) were the predominant emissions. In contrast, the relatively enriched δ 13 C values at MER during November 2004 reflect the emission of particles of geological origin.

Scatterplots of δ 13C values vs. wind direction, wind speed, organic carbon, and total carbon are depicted in **Figure 7.** The enriched 13C values during November 2004 are associated with low-speed E-SE winds and suggest organic-rich, soil-derived carbon. In contrast, the carbon content of PM2.5 in the other two sampling periods at MER is related to emissions from fuel combustion. At XAL, the heaviest δ13C values are associated with low-speed winds and organic carbon-enriched particles. The 13C-depleted samples apparently originate from SE winds.

δ 15N-PM2.5 values spanned between –9.9 and +21.6‰ (**Table 2**)**.** Although the average δ15N composition between sites for a given sampling period was similar (except for March 2004), the frequency histogram of δ 15N in PM2.5 shows a different distribution. At XAL, most δ 15N values fell in the 2‰ bin, with over 50% of the

**Figure 8.** *Time series of δ15N-PM2.5 values at MER (upper panel) and XAL (lower panel).*

data points between −2 and 6‰. In contrast, the δ<sup>15</sup>Ν-PM2.5 distribution at MER had a wider distribution, with most values between 2 and 10‰. The time series of δ15N-PM2.5 at the two sites shows that at XAL, values were always lower than at


#### **Table 3.**

*δ15N values in PM2.5 and PM10 in megacities.*

*The Use of Stable Isotopes to Identify Carbon and Nitrogen Sources in Mexico City PM2.5… DOI: http://dx.doi.org/10.5772/intechopen.107914*

MER (**Figure 8**)**.** Furthermore, δ 15N for PM2.5 at XAL during the low wind speeds of November 2004, was nearly constant during the sampling period.

A comparison between δ15N for PM2.5 in Mexico City with those of other megacities is shown in **Table 3**. The average values for XAL and MER are similar to the averages of New Delhi [22] and several months in Shijiazhuang [12], but lighter than for Paris [11], Beijing [23], and the warm months of New Delhi [22] and Seoul [21]. These differences can be attributed to different sources of NOx emissions [13, 23].

Scatterplots of XAL and MER δ15N values of PM2.5 samples vs. wind speed and averaged air quality data are given in **Figure 9**. The figure shows that as windspeed increases, the nitrogen isotopic composition becomes 15N-depleted, while at low wind speeds, PM2.5 particles are isotopically enriched. In turn, δ 15N values are positively correlated with average daily NOx ambient concentrations. At low wind speeds, particles are dispersed and the isotopic composition should reflect primary emissions. This is consistent with low δ15N values of fossil fuels. In turn, the NOx- δ 15N-PM2.5 positive correlation shows that the gas-to-particle condensation fractionates the condensed nitrogen, leaving isotopically enriched N in the particles.

**Figure 9.** *Scatterplots of δ15N-PM2.5 values vs. NOx (upper panel) and wind speeds (lower panel) at MER and XAL.*
