**3.4 Sources of PAHs in semirural sites**

We used the Flu/(Flu+Pyr), Ant/(Ant+Phe), Ind/(Ind+Bghi), BaP/(BaP+Cry), BkF/Bghi and BaP/Bghi ratios and the majority of samples fell into the section identifying pyrogenic sources (fossil fuel, grass and garbage combustion) (Table 3). This is logical considering the

Presence of Polycyclic Aromatic Hydrocarbons

PC 2 is vegetation combustion.

Accumulative Variance

soils in semirural areas (Table 5).

loading on BaA and Cry, indicators of fuel combustion (Table 4).

(PAHs) in Semi-Rural Environment in Mexico City 229

represented a mixed combustion. PC 2 explained 38.8% of total variance and had heavier

For irrigation water, PC 1 explained 52.4% of total variance and had heavier loadings Acy, Ace, Flu, Ant, Cry, BbF, BaP, Ind, DaA and Bghi. These compounds are from vegetation, fuels and industrial combustion. PC1 is classified as mixture combustion. PC 2 explained with 19.3% of total variance and had only Phe, as derived of vegetation combustion. Thus,

Compounds Apple Cactus stem Irrigation water

Explain Variance (%) 31.83 26.47 54.50 38.84 52.42 19.37

Table 4. Principal component analysis on apple, cactus stem and irrigation water in rural

For 2009, second step of sampling we analyze apple and cactus stem as one matrix while Tlahuac and Milpa Alta soil samples with same criteria with intention to corroborate the sources of contamination founded in 2008 sampling in others matrix and to understand the movement of PAHs in soil. In crops we employ three PC that explained 73.4% of the total variance. For soil samples we employed four PC that explained 68.9% of total variance for

In crops, PC 1 explained 48.7% of the total variance and had heavier loadings on Acy, Ace, Flu, Ant, Fla, BbF and Bghi. The aromatic compounds are associated with vegetation and fuels combustion. Thus, PC 1 can represent contribution from mixed combustion. PC 2 explained 13.6% of the total variance and had heavier loading on BaA, derived of fuel combustion; PC 2 was deduced to represent fuel combustion. And PC 3 explained 10.9% of the total variance and had heavier loading on Phe and Cry, these derived of fuel

areas of Tlahuac y Milpa Alta (2008, first step). Note: --- Not detected

(%) 31.83 **58.30** 54.50 **93.34** 52.42 **71.79** 

Nap --- --- --- --- --- --- Acy 0,621 -0,596 -0,687 -0,697 **-0,715** 0,446 Ace **0,773** -0,483 **-0,701** -0,697 **-0,795** 0,572 Flu 0,283 **-0,803 0,829** -0,520 **-0,882** -0,434 Phe -0,330 -0,536 **0,924** -0,381 -0,309 **-0,810**  Ant -0,384 -0,382 -0,682 -0,697 **-0,773** -0,516 Fla **0,855** -0,002 0,347 -0,622 -0,593 -0,608 Pyr 0,550 0,143 **0,765** -0,633 -0,531 0,404 BaA **0,881** 0,169 0,624 **-0,724** -0,535 0,299 Cry **0,808** -0,104 -0,643 **-0,758 -0,715** 0,121 BbF 0,596 0,346 **0,901** 0,371 **-0,891** -0,327 BkF **0,722** 0,368 0,687 -0,621 -0,541 0,170 BaP 0,146 -0,471 **0,775** -0,618 **-0,855** -0,288 Ind -0,095 **-0,858 -0,734** -0,673 **-0,718** 0,470 DaA 0,123 **-0,889 -0,713** -0,695 **-0,809** 0,433 Bghi -0,264 -0,489 **0,875** -0,473 **-0,909** -0,094

Factor 1 Factor 2 Factor 1 Factor 2 Factor 1 Factor 2

presence of heavy machinery (Tlahuac) and traffic jam (Tlahuac and Milpa Alta) and the vegetation combustion (which for example is sometimes used to remove weeds) in the city. Further our results are consistent with reports in Mexico City of the influence of vehicular traffic and industrial activities on atmospheric contamination and contamination of other environmental compartments such as water, soil, crops and organisms (Marr et al., 2004).


Table 3. Diagnostic ratios for identification of contamination source in rural sites from Mexico City.

Further, we employed some statistic tools such as principle components analysis (PCA) and extraction with different factor loadings indicated correlations of each pollutant species with each PC. Each PC was further evaluated and recognized by source markers or profiles as reasonable pollution sources according to Agarwal (2009), Wang et al., (2009) and Zhang et al. (2011).

In this investigation, PCA was performed for PAHs founded in soil samples using Statistica 16.0 software. Within principal components with values greater than 0.7 were retained. Two PCs were finally extracted and explained 58.3% of the total variance for apple case, 93.4% for cactus stem and 71.8% for irrigation water for 2008 (Table 4).

In 2008 for apple, PC 1 explained 31.8% of the total variance and had heavier loadings on Ace, Fla, BaA, Cry and BkF. The presence of Fla, BaA and Cry are typical tracers of traffic emission. Bkf and Bbf are also largely released by both gasoline and diesel engines (Wang et al., 2010). Thus, PC 1 can represent contribution from traffic emission. PC 2 explained 26.4% of the total variance and had heavier loading on Flu, Ind and DaA. As Ind and DaA are considered as predominant emissions of industrial and diesel combustion; PC 2 was deduced to represent industrial combustion (Agarwal, 2009). In cactus stem, PC 1 defined 54.5% and had heavier loadings on Ace, Flu, Phe, Pyr, BbF, BaP, Ind and DaA. The presence of 2 and 6 aromatic rings showed a vegetation and fossil fuel combustion, so PC1

presence of heavy machinery (Tlahuac) and traffic jam (Tlahuac and Milpa Alta) and the vegetation combustion (which for example is sometimes used to remove weeds) in the city. Further our results are consistent with reports in Mexico City of the influence of vehicular traffic and industrial activities on atmospheric contamination and contamination of other environmental compartments such as water, soil, crops and organisms (Marr et al., 2004).

Dry Probable source

MA: Pyrogenic

petrogenic

combustion

combustion

MA: Not detected

Tl: Vegetation combustion MA: Vegetation combustion and

MA: Pyrogenic and petrogenic

MA: pyrogenic and petrogenic

Tl: Traffic and vegetation

MA: Traffic and vegetation

Wet season

Tlahuac (Tl) Milpa Alta (MA)

Fla/(Fla+Pyr) 0.94 0.86 --- 0.88 Tl: Pyrogenic

Ant/(Ant+Phe) --- --- --- --- Tl: Not detected

BkF/Bghi 0.92 0.09 1.45 0.27 Tl: pyrogenic

Dry season

BaA/(BaA+Cry) 0.57 0.11 0.88 0.03 Tl: Pyrogenic and petrogenic

Table 3. Diagnostic ratios for identification of contamination source in rural sites from

Further, we employed some statistic tools such as principle components analysis (PCA) and extraction with different factor loadings indicated correlations of each pollutant species with each PC. Each PC was further evaluated and recognized by source markers or profiles as reasonable pollution sources according to Agarwal (2009), Wang et al.,

In this investigation, PCA was performed for PAHs founded in soil samples using Statistica 16.0 software. Within principal components with values greater than 0.7 were retained. Two PCs were finally extracted and explained 58.3% of the total variance for apple case, 93.4% for

In 2008 for apple, PC 1 explained 31.8% of the total variance and had heavier loadings on Ace, Fla, BaA, Cry and BkF. The presence of Fla, BaA and Cry are typical tracers of traffic emission. Bkf and Bbf are also largely released by both gasoline and diesel engines (Wang et al., 2010). Thus, PC 1 can represent contribution from traffic emission. PC 2 explained 26.4% of the total variance and had heavier loading on Flu, Ind and DaA. As Ind and DaA are considered as predominant emissions of industrial and diesel combustion; PC 2 was deduced to represent industrial combustion (Agarwal, 2009). In cactus stem, PC 1 defined 54.5% and had heavier loadings on Ace, Flu, Phe, Pyr, BbF, BaP, Ind and DaA. The presence of 2 and 6 aromatic rings showed a vegetation and fossil fuel combustion, so PC1

Wet season

Diagnostic ratios

Mexico City.

(2009) and Zhang et al. (2011).

season

Ind/(Ind+Bghi) 0.50 --- 0.68 0.23

BaP/Bghi 1.93 3.55 3.36 1.63

cactus stem and 71.8% for irrigation water for 2008 (Table 4).

represented a mixed combustion. PC 2 explained 38.8% of total variance and had heavier loading on BaA and Cry, indicators of fuel combustion (Table 4).

For irrigation water, PC 1 explained 52.4% of total variance and had heavier loadings Acy, Ace, Flu, Ant, Cry, BbF, BaP, Ind, DaA and Bghi. These compounds are from vegetation, fuels and industrial combustion. PC1 is classified as mixture combustion. PC 2 explained with 19.3% of total variance and had only Phe, as derived of vegetation combustion. Thus, PC 2 is vegetation combustion.


Table 4. Principal component analysis on apple, cactus stem and irrigation water in rural areas of Tlahuac y Milpa Alta (2008, first step). Note: --- Not detected

For 2009, second step of sampling we analyze apple and cactus stem as one matrix while Tlahuac and Milpa Alta soil samples with same criteria with intention to corroborate the sources of contamination founded in 2008 sampling in others matrix and to understand the movement of PAHs in soil. In crops we employ three PC that explained 73.4% of the total variance. For soil samples we employed four PC that explained 68.9% of total variance for soils in semirural areas (Table 5).

In crops, PC 1 explained 48.7% of the total variance and had heavier loadings on Acy, Ace, Flu, Ant, Fla, BbF and Bghi. The aromatic compounds are associated with vegetation and fuels combustion. Thus, PC 1 can represent contribution from mixed combustion. PC 2 explained 13.6% of the total variance and had heavier loading on BaA, derived of fuel combustion; PC 2 was deduced to represent fuel combustion. And PC 3 explained 10.9% of the total variance and had heavier loading on Phe and Cry, these derived of fuel

Presence of Polycyclic Aromatic Hydrocarbons

than 2.5 μm (European Communities, 2001).

soil, crops and organisms.

these conservation terrains.

populations (Wallace et al., 2010).

allergens and viruses (Wallace et al., 2010).

(PAHs) in Semi-Rural Environment in Mexico City 231

Although these studies have indicated different levels of PAHs in some land uses of urban areas, research about PAH composition and sources in different land uses of urban environment is scarce; it is thus highly desired to have a better understanding about how

According to Amador-Muñoz et al (2011) the principal sources were diesel, natural gas and fuel combustion, biogenic emissions and organic matter pyrolysis where PAHs are associated with airborne particles in atmospheric media. Generally, between 80% and almost 100 % of PAHs with 5 rings or more (which are predominately particle-bound in the atmosphere) can be found associated with particles with an aerodynamic diameter of less

The presence of heavy machinery (Tlahuac) and vehicle traffic (Tlahuac and Milpa Alta) with vegetation combustion (which for example is sometimes used to remove weeds) are the sources of PAHs in the city. Further, our results are consistent with reports in Mexico City of the influence of vehicular traffic and industrial activities on atmospheric contamination and contamination of other environmental compartments such as water,

The presence of low molecular weight PAHs (2-3 aromatic rings) is indicative of petroleum (fossil fuel combustion) while high molecular weight PAHs (> 4 aromatic rings) are more likely to be derived from organic material combustion (Ma et al., 2005). There are likely to be multiple contamination sources in vegetables and garbage from

Lastly, recent studies indicate that POPs atmospheric depositions are main source of contamination in big cities derived fossil fuels (diesel, gasoline and natural gas), garbage and vegetation combustion (Rossini et al., 2005). For this reason, the atmospheric compartment must be constantly monitored by supervisory authority and considered by Mexican regulation which, until now, has provided limits for some pollutants such as suspended particles, ozone, carbon monoxide and dioxide, nitrogen oxides and sulphur

According geography and meteorology conditions play critical roles in the dilution and dispersion of air pollution from source locations, through vertical mixing and horizontal transport in Mexico City (Figure 7). Vertical mixing is facilitated by upward motion of warm air near the surface, to cold air above. In cases where this temperature profile is reversed and warm air lies above colder air at the surface, vertical motion is restricted. This sets up a temperature inversion, which is characterised by stable atmospheric conditions, and results in the accumulation of air pollution (November to March, mainly). Horizontal wind speeds are also reduced, limiting transport of pollutants downwind. The resulting poor air quality and concentrate of organic contaminants may lead to health problems in susceptible

Many studies highlight a distinct increase in concentrations of pollution during temperature inversions in Los Angeles, London, Tokyo and others important cities. This seasonal variation also coincides with that of temperature inversions, which are also most frequent in the winter and spring and lead to the accumulation of not just air pollutants, but also

oxides especially considering the high vehicular units, industrial zones and landfills.

different land uses affect PAH distribution in urban soils (Liu et al., 2010).

combustion. In soils, PC 1 explained 35.8% of total variance and had principal compounds on Ace, Ant, Fla, Pyr, BaA, BbF, BaP and Bghi. These ranges of compounds are representative of vegetation, fossil fuel and industrial combustion (Agarwal, 2009). PC 2 explained 15% with BkF derived of diesel combustion, PC 3 explained with 9.5% on Cry derived fuel combustion and PC 4 explained 8.4% with DaA associated with industrial combustion (Zhang et al., 2011).


Table 5. Principal component analysis on crops (apple and cactus stem), and irrigation water in rural zones of Tláhuac y Milpa Alta (Step second).

Biomass burning and wildfire are important sources of organic contaminants (PAHs) at a global level. Motor vehicle emission in urban areas where population densities are much higher were found to be high, contribution of PAHs from motor vehicles going to air, dust, water, crops and human exposure; the risk is much higher than in rural areas (Shen et al., 2011). Although recent decades have shown a trend in some big cities of decreasing PAH concentrations due to emission control measures introduced in some countries.

The urban area comprises a wide range of different land uses such as traffic, industry, business, residence, garden and public green space, implying different patterns of human activities and their possible impacts on soil quality. Some work has demonstrated that specific land uses in the urban environment always showed higher PAH concentrations than other land uses. For example, soils collected at the roadside or in busy streets in Shanghai, Dalian and New Orleans all showed much higher levels of PAHs than those collected from parks and residential areas. Haugland et al. (2008) and Jiao et al. (2009) studied PAHs in urban soils from Bergen, Norway and Tianjin, China, respectively, and soils from both cities showed much higher PAHs in the industrial area than other areas.

combustion. In soils, PC 1 explained 35.8% of total variance and had principal compounds on Ace, Ant, Fla, Pyr, BaA, BbF, BaP and Bghi. These ranges of compounds are representative of vegetation, fossil fuel and industrial combustion (Agarwal, 2009). PC 2 explained 15% with BkF derived of diesel combustion, PC 3 explained with 9.5% on Cry derived fuel combustion and PC 4 explained 8.4% with DaA associated with industrial

> Nap 0,095 -0,519 0,152 0 0 0 0 Acy **-0,980** -0,115 0,023 0 0 0 0 Ace **-0,972** -0,125 0,024 **0,947** -0,047 0,203 -0,026 Flu **-0,915** -0,154 0,026 0 0,511 -0,028 0,365 Phe -0,109 -0,134 **-0,884** 0,682 0 0 0 Ant **-0,824** 0,435 0,233 **0,970** -0,211 0,008 -0,000 Fla **-0,966** -0,070 0,020 **0,912** 0,337 0,045 0,060 Pyr -0,302 0,651 0,286 **0,849** 0,407 0,033 0,257 BaA -0,390 **0,729** -0,177 **0,736** -0,323 0,363 -0,236 Cry -0,382 -0,037 **-0,833** 0,123 0,246 **-0,814** -0,467 BbF **-0,981** -0,112 0,022 **0,975** -0,066 -0,182 -0,069 BkF **-0,890** -0,197 0,102 0,675 **-0,696** -0,046 -0,065 BaP -0,127 0,381 -0,214 **0,717** -0,474 -0,395 -0,044 Ind -0,664 -0,390 0,094 0,584 -0,022 0,327 -0,316 DaA -0,102 0,580 -0,133 0,080 -0,496 -0,306 **0,715**  Bghi **-0,903** 0,022 -0,003 **0,763** 0,499 -0,122 0,004

Varianza explicada (%) 48.77 13.61 10.95 35.86 15.07 9.58 8.46 Varianza acumulativa (%) 48.77 62.48 **73.43** 35.86 50.93 60.51 **68.97**  Table 5. Principal component analysis on crops (apple and cactus stem), and irrigation water

Biomass burning and wildfire are important sources of organic contaminants (PAHs) at a global level. Motor vehicle emission in urban areas where population densities are much higher were found to be high, contribution of PAHs from motor vehicles going to air, dust, water, crops and human exposure; the risk is much higher than in rural areas (Shen et al., 2011). Although recent decades have shown a trend in some big cities of decreasing PAH

The urban area comprises a wide range of different land uses such as traffic, industry, business, residence, garden and public green space, implying different patterns of human activities and their possible impacts on soil quality. Some work has demonstrated that specific land uses in the urban environment always showed higher PAH concentrations than other land uses. For example, soils collected at the roadside or in busy streets in Shanghai, Dalian and New Orleans all showed much higher levels of PAHs than those collected from parks and residential areas. Haugland et al. (2008) and Jiao et al. (2009) studied PAHs in urban soils from Bergen, Norway and Tianjin, China, respectively, and soils from both cities showed much higher PAHs in the industrial area than other areas.

concentrations due to emission control measures introduced in some countries.

in rural zones of Tláhuac y Milpa Alta (Step second).

Factor 1 Factor 2 Factor 3 Factor 1 Factor 2 Factor 3 Factor 4

Compounds Apple and Cactus stem Soils

combustion (Zhang et al., 2011).

Although these studies have indicated different levels of PAHs in some land uses of urban areas, research about PAH composition and sources in different land uses of urban environment is scarce; it is thus highly desired to have a better understanding about how different land uses affect PAH distribution in urban soils (Liu et al., 2010).

According to Amador-Muñoz et al (2011) the principal sources were diesel, natural gas and fuel combustion, biogenic emissions and organic matter pyrolysis where PAHs are associated with airborne particles in atmospheric media. Generally, between 80% and almost 100 % of PAHs with 5 rings or more (which are predominately particle-bound in the atmosphere) can be found associated with particles with an aerodynamic diameter of less than 2.5 μm (European Communities, 2001).

The presence of heavy machinery (Tlahuac) and vehicle traffic (Tlahuac and Milpa Alta) with vegetation combustion (which for example is sometimes used to remove weeds) are the sources of PAHs in the city. Further, our results are consistent with reports in Mexico City of the influence of vehicular traffic and industrial activities on atmospheric contamination and contamination of other environmental compartments such as water, soil, crops and organisms.

The presence of low molecular weight PAHs (2-3 aromatic rings) is indicative of petroleum (fossil fuel combustion) while high molecular weight PAHs (> 4 aromatic rings) are more likely to be derived from organic material combustion (Ma et al., 2005). There are likely to be multiple contamination sources in vegetables and garbage from these conservation terrains.

Lastly, recent studies indicate that POPs atmospheric depositions are main source of contamination in big cities derived fossil fuels (diesel, gasoline and natural gas), garbage and vegetation combustion (Rossini et al., 2005). For this reason, the atmospheric compartment must be constantly monitored by supervisory authority and considered by Mexican regulation which, until now, has provided limits for some pollutants such as suspended particles, ozone, carbon monoxide and dioxide, nitrogen oxides and sulphur oxides especially considering the high vehicular units, industrial zones and landfills.

According geography and meteorology conditions play critical roles in the dilution and dispersion of air pollution from source locations, through vertical mixing and horizontal transport in Mexico City (Figure 7). Vertical mixing is facilitated by upward motion of warm air near the surface, to cold air above. In cases where this temperature profile is reversed and warm air lies above colder air at the surface, vertical motion is restricted. This sets up a temperature inversion, which is characterised by stable atmospheric conditions, and results in the accumulation of air pollution (November to March, mainly). Horizontal wind speeds are also reduced, limiting transport of pollutants downwind. The resulting poor air quality and concentrate of organic contaminants may lead to health problems in susceptible populations (Wallace et al., 2010).

Many studies highlight a distinct increase in concentrations of pollution during temperature inversions in Los Angeles, London, Tokyo and others important cities. This seasonal variation also coincides with that of temperature inversions, which are also most frequent in the winter and spring and lead to the accumulation of not just air pollutants, but also allergens and viruses (Wallace et al., 2010).

Presence of Polycyclic Aromatic Hydrocarbons

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Fig. 7. Scheme contamination for PAHs in Mexico City, while in spring, summer and autumn the behaviour of contaminants change according to environmental conditions.

Thus we found a good description of contamination sources for our studied matrix and the movement of PAHs in the semirural environment for Mexico City. With better data, the Mexican authorities can take more informed decisions in the management of natural resources, legislation and politics, for better control of contaminants and pollution in general.
