**3.2 Crops**

Our study focus in two crops of economical importance for Tlahuac and Milpa further the government has impulse with economical, technical and material resources for development of farmer. Cactus stem has high acceptation in population because is traditional food from prehispanic to present time while apple crop recently is accepted as attractive and rentable crop. Although there are several agriculture products that grow in these areas and around such as spinach, broccoli, amaranth, olive, ornamental flowers and etc (Grupo Produce, 2006).

Human exposure to PAHs is 88–98% connected with food (meat, seed, vegetable, fruit and others). PAHs can penetrate food indirectly (from air or water) and directly, e.g. during smoking. Once these compounds are released into the atmosphere they can be transported away from their emission sources over long distances and/or deposited to the terrestrial and aquatic environment through dry and wet deposition. A major issue associated with the emission of these compounds is the zone of influence, which determines whether the possible source has predominately local impacts or contributes to regional or global background levels (Rey-Salgueiro et al., 2008).

According to Rey-Salgueiro et al. (2008) the form of vegetable or fruit influenced the PAH concentration over the surface as they found that leaves and quasispherical fruits (grapes

Presence of Polycyclic Aromatic Hydrocarbons

(PAHs) in Semi-Rural Environment in Mexico City 221

Table 1. Comparison of PAH concentrations in vegetable samples. Note: \* Number of aromatic rings; --- No detected; 1. This survey, 2008; 2. This survey, 2009; 3. Zohair et al.,

2006; 4. Mo et al., 2008; 5. Li et al., 2008; 6. Tao et al., 2004

and tomatoes) had greater concentrations of PAHs (4 and 5 aromatic rings) than quasiconical fruits (pepper). Some authors have suggested that lower molecular weight PAHs which dominate in the atmosphere can easily penetrate the cuticle surface of the foliage, while the higher molecular weight PAHs, mainly associated with the atmospheric particulates, are only superficially deposited on plant foliage and are thus more easily washed away by rain.

Kluska (2003) established that the content of PAH in fruits and vegetables depends on pollution in the environment (mainly air pollution) and on area of contact: Apples usually contain 200–500 ng/kg, tomatoes 200 ng/ kg, spinach 6600 ng/kg, and cabbage (savoy) 20,400 ng/kg; this provides a basis for estimating a low potential biological impact associated with the levels found. García-Falcón et al. (2006) screened for the presence of PAHs in soil of rural areas and found thattotal PAHs were always lower than 13,000 ng/kg. As a conclusion, the selected plant foods will probably not cause adverse biological effects to take place. In Tianjin (China), at a PAH contaminated site, total PAHs of rice leaves from various growth stages ranged from 58,900 to 548,000 ng/kg with a mean value of 216,000 ng/kg (Tao et al., 2006).

In Table 1 we compared our results with other vegetables where individual concentrations of PAHs in high molecular weight were highest; we believe that these differences were due at the type of skin (waxes) and morphological structure of crop further there aren`t values for apples and cactus stem. Although our concentrations were variables in 2008 and 2009 probable to time and amount of rain, winds and dust storm, mainly.

Vegetation is reportedly an effective media for the entrapment of these and other compounds, mainly through atmospheric deposition. The green parts of vegetables or skin of fruits are provided with an epicuticular wax which acts as a sorbent for lipophylic contaminants (Ratola et al., 2011). It is necessary evaluate crops from semi-rural zones of Mexico City by periodically to assess good quality of products of organic contaminants such as aromatic hydrocarbons. Due some compounds has described as carcinogens at benzo(a)anthracene BaA, chrysene Cry, benzo(b)fluoranthene BbF, benzo(k)fluoranthene BkF, benzo(a)pyrene BaP, indeno(1,2,3-cd)pyrene Ind, dibenzo(ah)anthracene DaA and benzo(ghi)perylene Bghi.

In figure 3 shows dominant individual PAHs in apple were Ant, Fla, Pyr, BaP and BaA mainly in dry season and BaA, BkF, BaP, DaA in wet season. For cactus stem were DaA, Bghi, Pyr, Cry, BaA, Fla, BaP, Ind, Ant and Phe in dry season and BaA, Fla, DaA, Pyr, Bghi and Ant in wet season. In wet season in both crops were high concentrations of PAHs we supposed a diminished by effect of washing but the irregular rains in Mexico City (low content of water and spread out) only causes drop of suspended particles in atmosphere in long time of wet season

It is recognized that the low molecular weight PAHs (two and some three rings) are common in fresh fuels, but also in combustion activities and in some industrial emissions, indicating mostly petrogenic origins, four-ring PAHs (and also some three-ring) are linked with motorized traffic in general and diesel consumption in particular and the high molecular weight PAHs (five and six rings) denote the existence of heavy machine or industrial activities (Yin et al., 2008; Wang et al., 2009)


Table 1. Comparison of PAH concentrations in vegetable samples. Note: \* Number of aromatic rings; --- No detected; 1. This survey, 2008; 2. This survey, 2009; 3. Zohair et al., 2006; 4. Mo et al., 2008; 5. Li et al., 2008; 6. Tao et al., 2004

and tomatoes) had greater concentrations of PAHs (4 and 5 aromatic rings) than quasiconical fruits (pepper). Some authors have suggested that lower molecular weight PAHs which dominate in the atmosphere can easily penetrate the cuticle surface of the foliage, while the higher molecular weight PAHs, mainly associated with the atmospheric particulates, are only superficially deposited on plant foliage and are thus more easily

Kluska (2003) established that the content of PAH in fruits and vegetables depends on pollution in the environment (mainly air pollution) and on area of contact: Apples usually contain 200–500 ng/kg, tomatoes 200 ng/ kg, spinach 6600 ng/kg, and cabbage (savoy) 20,400 ng/kg; this provides a basis for estimating a low potential biological impact associated with the levels found. García-Falcón et al. (2006) screened for the presence of PAHs in soil of rural areas and found thattotal PAHs were always lower than 13,000 ng/kg. As a conclusion, the selected plant foods will probably not cause adverse biological effects to take place. In Tianjin (China), at a PAH contaminated site, total PAHs of rice leaves from various growth stages ranged from 58,900 to 548,000 ng/kg with a mean value of 216,000

In Table 1 we compared our results with other vegetables where individual concentrations of PAHs in high molecular weight were highest; we believe that these differences were due at the type of skin (waxes) and morphological structure of crop further there aren`t values for apples and cactus stem. Although our concentrations were variables in 2008 and 2009

Vegetation is reportedly an effective media for the entrapment of these and other compounds, mainly through atmospheric deposition. The green parts of vegetables or skin of fruits are provided with an epicuticular wax which acts as a sorbent for lipophylic contaminants (Ratola et al., 2011). It is necessary evaluate crops from semi-rural zones of Mexico City by periodically to assess good quality of products of organic contaminants such as aromatic hydrocarbons. Due some compounds has described as carcinogens at benzo(a)anthracene BaA, chrysene Cry, benzo(b)fluoranthene BbF, benzo(k)fluoranthene BkF, benzo(a)pyrene BaP, indeno(1,2,3-cd)pyrene Ind, dibenzo(ah)anthracene DaA and

In figure 3 shows dominant individual PAHs in apple were Ant, Fla, Pyr, BaP and BaA mainly in dry season and BaA, BkF, BaP, DaA in wet season. For cactus stem were DaA, Bghi, Pyr, Cry, BaA, Fla, BaP, Ind, Ant and Phe in dry season and BaA, Fla, DaA, Pyr, Bghi and Ant in wet season. In wet season in both crops were high concentrations of PAHs we supposed a diminished by effect of washing but the irregular rains in Mexico City (low content of water and spread out) only causes drop of suspended particles in atmosphere in

It is recognized that the low molecular weight PAHs (two and some three rings) are common in fresh fuels, but also in combustion activities and in some industrial emissions, indicating mostly petrogenic origins, four-ring PAHs (and also some three-ring) are linked with motorized traffic in general and diesel consumption in particular and the high molecular weight PAHs (five and six rings) denote the existence of heavy machine or

probable to time and amount of rain, winds and dust storm, mainly.

washed away by rain.

ng/kg (Tao et al., 2006).

benzo(ghi)perylene Bghi.

long time of wet season

industrial activities (Yin et al., 2008; Wang et al., 2009)

Presence of Polycyclic Aromatic Hydrocarbons

(~45%, 22 % and 10 % respectively).

waxes.

0%

Milpa Alta (MA).

**3.3 Soils** 

Apple Tl Dry season

Apple MA Dry season

many times higher than the content of PAHs in soil or water.

Cactus MA Dry season

Fig. 4. Distribution of PAHs in crops according number of aromatic rings in Tlahuac (Tl) and

Finally vegetables and fruits obtained from a polluted environment may contain higher PAH concentrations than those obtained from nonpolluted environments. According to ATSDR (1995) the PAH content of plants and animals living on the land or in water can be

According to the results we found higher values of PAHs in crops than soil. For the case of crops, the individual PAHs were slightly lower in wet than dry season; in the rainy season the irregular intensity and duration of rainfall cleans the atmospheric of suspended particles. In figure 4 the results show greater values in apple from Tlahuac and cactus stem from Milpa Alta in the wet season. In general, the compounds > 3 aromatic rings were

Apple Tl Wet season

Apple MA Wet season

Cactus MA Wet season

6 rings 5 rings 4 rings 3 rings 2 rings

20%

40%

60%

80%

100%

(PAHs) in Semi-Rural Environment in Mexico City 223

of apple crop in dry season only Tlahuac has naphthalene and high proportion (~15 %) of compound of six aromatic rings than Milpa Alta (~5 %) while Milpa has high proportion of three, four and five aromatic rings (~30 %, 50 % and 25 % respectively) than Tlahuac; in wet season Tlahuac showed high proportion of four aromatic rings (~60 %) than Milpa Alta (~25 %) and Milpa Alta showed high proportion of three, five and six aromatic rings (~18 %, 52 % and 9 % respectively) than Tlahuac (~ 10 %, 25 % and 5 %). For cactus stem showed differences in four, five and six aromatic rings in dry (~35 %, 30 % and 18 %) and wet season

The differences among crops were due waxes composition of skin where cactus stem has a high concentration of PAHs than in apple. In cactus stem has elevated waxes than apple skin and favour accumulation of particles or contaminants due PAHs are affinity a fat and

Fig. 3. Values of PAHs for semi-rural terrains from Mexico City (Tlahuac and Milpa Alta).

There routes of PAHs in plants such as take up from soil via the roots or from air via the foliage; uptake rates are dependent on the concentration, solubility, and molecular weight of the PAH and on the plant species (ATSDR, 1995). Some plants have been used to monitor atmospheric deposition of PAHs, example, mosses, lichens and vegetables. In some studies, the atmospheric PAHs such as indeno(l,2,3-c,d)pyrene, fluoranthene, and benzo(a)pyrene are deposited on foliage (leaves and flowers). In general, the atmospheric deposition on leaves often greatly exceeds uptake from soil by roots as a route of PAH accumulation.

In figure 4 we appreciated the average of compounds for 2008-2009 for apple and cactus stem where aromatic compounds were variable in both season. In general dominant compounds for crops were four and five aromatic rings follow by three and six aromatic rings. These occurrence of compounds coincide with before describe by similar studies.

The differences of aromatic compounds are influenced by particularly conditions of each area; in Tlahuac (Tl) coexist mines of construction materials as sand and gravel with diesel machinery and transit of gasoline vehicles, garbage and weed combustion. While Milpa Alta (MA) there gas station, transit of gasoline vehicle, garbage and weed combustion. These sources and particular environmental conditions can explain occurrence of PAHs. For case

Fig. 3. Values of PAHs for semi-rural terrains from Mexico City (Tlahuac and Milpa Alta).

accumulation.

There routes of PAHs in plants such as take up from soil via the roots or from air via the foliage; uptake rates are dependent on the concentration, solubility, and molecular weight of the PAH and on the plant species (ATSDR, 1995). Some plants have been used to monitor atmospheric deposition of PAHs, example, mosses, lichens and vegetables. In some studies, the atmospheric PAHs such as indeno(l,2,3-c,d)pyrene, fluoranthene, and benzo(a)pyrene are deposited on foliage (leaves and flowers). In general, the atmospheric deposition on leaves often greatly exceeds uptake from soil by roots as a route of PAH

In figure 4 we appreciated the average of compounds for 2008-2009 for apple and cactus stem where aromatic compounds were variable in both season. In general dominant compounds for crops were four and five aromatic rings follow by three and six aromatic rings. These occurrence of compounds coincide with before describe by similar studies.

The differences of aromatic compounds are influenced by particularly conditions of each area; in Tlahuac (Tl) coexist mines of construction materials as sand and gravel with diesel machinery and transit of gasoline vehicles, garbage and weed combustion. While Milpa Alta (MA) there gas station, transit of gasoline vehicle, garbage and weed combustion. These sources and particular environmental conditions can explain occurrence of PAHs. For case of apple crop in dry season only Tlahuac has naphthalene and high proportion (~15 %) of compound of six aromatic rings than Milpa Alta (~5 %) while Milpa has high proportion of three, four and five aromatic rings (~30 %, 50 % and 25 % respectively) than Tlahuac; in wet season Tlahuac showed high proportion of four aromatic rings (~60 %) than Milpa Alta (~25 %) and Milpa Alta showed high proportion of three, five and six aromatic rings (~18 %, 52 % and 9 % respectively) than Tlahuac (~ 10 %, 25 % and 5 %). For cactus stem showed differences in four, five and six aromatic rings in dry (~35 %, 30 % and 18 %) and wet season (~45%, 22 % and 10 % respectively).

The differences among crops were due waxes composition of skin where cactus stem has a high concentration of PAHs than in apple. In cactus stem has elevated waxes than apple skin and favour accumulation of particles or contaminants due PAHs are affinity a fat and waxes.

Fig. 4. Distribution of PAHs in crops according number of aromatic rings in Tlahuac (Tl) and Milpa Alta (MA).

Finally vegetables and fruits obtained from a polluted environment may contain higher PAH concentrations than those obtained from nonpolluted environments. According to ATSDR (1995) the PAH content of plants and animals living on the land or in water can be many times higher than the content of PAHs in soil or water.
