Living organisms are known to reflect environmental conditions according to their sensitivity. This is called biomonitoring, or biological monitoring, using the most sensitive animal or plant species to the pollutants being monitored in their environment. Lichens and mosses are good examples for assessing air pollution  . Several approaches have been proposed depending on the observation scale considered . Thus, we distinguish the ecological scale by the biological response of individuals (bio-indication) or communities (bio-integration), from the geochemical scale by the accumulation of contaminant (bioaccumulation). The ecological approach seeks to assess air quality based on the presence/absence of key species in a specific survey (e.g. lichen species). The first scales set up to assess sulfur pollution are no longer appropriate in their present state since the changes in atmospheric contaminants in recent decades  . The sensitivity of species must be regularly updated, as already done by  towards dominant nitrogen contamination. The analysis of contaminants in an atmospheric deposition is facilitated by the bioaccumulation approach . However, the mechanisms related to the integration of pollutants by plants in Togo and their possible releases are not yet fully documented. For this reason, some plant species were chosen as model organisms in the monitoring of atmospheric contamination in the city of Lomé during the period 2017-2020.
2. Sampling of Plant Species
The analysis of the preponderant pollutants in the city of Lomé made it possible to identify sites or sources of pollution presenting high atmospheric concentrations of certain gases. These results coupled with those determining the species having links with these sources allowed to retain the samples of sources of pollutants presenting high values specific to a pollutant. These sites were sampled for plant species related to the source (Figure 1).
Seven (7) species plant with a very strong link to the anthropogenic pollution source were selected for the determination of elements at the Laboratory. They are: Alternantherarepens, Amaranthus sp, Eragrotistenella, Sennaoccidentalis, Ocimumcanum, Newbouldialaevis, Pithecellobiumdulce. These samples were kept cool in minigrip bags to avoid water loss and then sent to the laboratory for chemical analysis.
Figure 1. Plant species sampling sites.
3. Determination of Sulfur Dioxide (SO2) by the Ripper Method
- Principle of determination
SO2 or sulfur dioxide exists in 2 forms: free and combined. The free form is determined in acid medium by direct iodometric titration and the combined form by the difference between total and free sulfur dioxide. In this analysis, both forms were evaluated. The combined SO2 is hydrolyzed in alkaline medium.
- Procedure for the titration of the iodine solution
Put 25 ml of the iodine solution into a 250 ml Erlenmeyer flask and titrate with the sodium thiosulfate solution until discoloration. V ml the volume poured.
- Procedure for the determination of free SO2
The sample was ground with a molinex added with distilled water. The obtained substrate was poured into an Erlenmeyer of 250 ml. The solution obtained was filtered and the filtrate constitutes the plant extract. 15 ml of the plant extract was added with 1 ml of starch starch and about 3 ml of sulfuric acid to 1/3. This was titrated with 0.02 N iodine until a persistent blue-brown hue appeared 5 - 10 s. N the volume poured.
4. Procedure for the Determination of Total SO2
- Decombination of SO2
15 ml of the plant extract was poured into a 250 ml Erlenmeyer flask containing about 6 ml of 1 M NaOH. The whole was stoppered and shaken. After 10 min of rest of the solution, 1 ml of starch starch and 2 ml of sulfuric acid to 1/3 were added and titrated with 0.02 N iodine until a persistent blue-brown hue appeared after 5 to 10 s. Let N1 ml be the volume poured.
- Total decombination of SO2
In the previous mixture, 24 ml of NaOH added and stoppered then shake and wait 5 mn. 1 ml of starch starch and 3 ml of sulfuric acid to 1/3 were added. The whole was titrated with 0.02 N iodine until a persistent purple coloration appeared, i.e. N2 ml, the volume poured.
- Iodine titration method
[I2] = [Na2S2O3] × V × 1/50 = 0.002 × V (mol/l)
- Determination of SO2 content
Free [SO2] mg/l = 64.07 × [I2] × N × 1000/Vext or [Free SO2] mg/l = 42.71 × N [Total SO2] mg/l = 64.07 × [I2] × (N1 + N2) × 1000/Vext or [Total SO2] mg/l = 42.71 × (N1 + N2) [SO2C] = [SO2T] − [SO2L].
5. Determination of Carbon and Estimation of CO2 and CO
5.1. Colorimetric Method
Principle of determination: The carbon of the organic matter is oxidized by a mixture of potassium dichromate and sulfuric acid. The blue-green Cr3+ ions formed during the oxidation are determined directly by colorimetry reaction and which are proportional to the equivalents of oxidized carbon. The colorimetric titration curve is made against a glucose solution of known carbon content. Aqueous solution of potassium dichromate and concentrated sulfuric acid were used.
5.2. Colorimetric Mode of Determination
A soil sample containing between 0.4 mg and 15 mg of carbon in a 50 ml Erlenmeyer flask was weighed. 5 ml of potassium dichromate and 7.5 ml of concentrated sulfuric acid were added. The whole was covered with a watch glass and then placed in the oven at 105˚C for three hours. After cooling, distilled water was added and made the volume to 40 ml. The whole was shaken and left to decant overnight. A 3500 rpm centrifugation for 10 minutes in a glass tube was done and then weighed with a spectrophotometer at 590 nm. Finally a calibration was done.
5.3. Expression of Results
The percentage of carbon is: C% = Lc/Pe × 0.115 With: Lc = Curve reading in ppm or mg/l, Pe = Weight of soil (g).
According to the equation C + O2 CO2; (12 g C gives 44 g CO2 and 28 g CO).
6. Determination of Nitrogen by the Kjeldhal Method
6.1 Principle of Dosage
The organic matter has been destroyed by oxidizing attack with sulfuric acid. The nitrogen in its various forms is converted into ammonium sulfate in the presence of a selenium catalyst.
6.2. Method of Extraction and Determination of Nitrogen
0.25 g of plant powder in a matras was weighed and then added with 50 mg of salicylic acid and 6 glass beads. The matron was shaken and 5 cc of concentrated sulfuric acid was added. The matras was stoppered with a funnel. The whole assembly was allowed to stand overnight. 125 mg of plant catalyst was added and stirred. The whole was left until the residue was perfectly white and then allowed to cool. The funnel in the neck of the matron was removed after rinsing it with a squirt. 40 ml of soda 12N was added. The whole was distilled. The distillate was then collected in 25 ml of boric acid in the presence of a turn indicator. The titration was done with N/10 sulfuric acid. 4 drops of indicator were added and titration with N/10 sulfuric acid until green to red was performed. The results were expressed as mg per ml.
7.1. At the Industry Level
At the industrial level, Alternantherarepens stored more CO2 with a value of 53.3911 mg/ml. On the other hand, Pithecellobiumdulce stored more CO with a value of 44.3619 mg/ml. However, NO2 and SO2 are revealed present in Pithecellobiumdulce species with low amounts (Figure 2).
Figure 2. Concentration of the different gases at the level of the species identified at the level of the industries.
7.2. At the Level of Landfills
At the level of landfills, Newbouldialaevis stored more CO2 with a value of 65.8508 mg/ml. On the other hand, Sennaoccidentalis stored more CO with a value of 51.6106 mg/ml. However, NO2 and SO2 were found to be present in Newbouldialaevis and Ocimumcanum with low amounts (Figure 3).
7.3. At the Road Level
At the level of roads, EragrostisTenella stored more CO2 with a value of 74.4092 mg/ml and CO with a value of 62.2654 mg/ml. However, NO2 and SO2 were found to be present in Amanranthus sp and EragrostisTenella but with low amounts (Figure 4).
Figure 3. Concentration of the different gases at the level of the species identified at the landfill.
Figure 4. Concentration of the different gases at the level of the species surveyed at the roadsides.
Table 1 and Table 2 present the detailed results of the different plant species analyzed in relation to the specific sources of air pollution. At the level of industries, the amount of CO2 in Alternantherarepens is very high with a value of 53.3911 mg/ml. On the other hand the amount of CO in Sennaoccidentalis is 44.3619 mg/ml/. At the level of Pithecellobiumdulce the quantity of SO2 and NO2 are evaluated respectively to 0.1588 mg/ml and 0.3696 mg/ml. At the level of the discharges, the quantity of CO2 in Newbouldialaevis is very high with a value of 65.8508 mg/ml. On the other hand the amount of CO in Senna occidentalis is 51.6106 mg/ml/. At the level of Newbouldialaevis the amount of
Table 1. Plant species at sites with high gas-specific values.
Table 2. Contents of plant species in different chemical elements considered.
SO2 is 0.2101 mg/ml and NO2 in Ocimumcanum is 0.2744 mg/ml.
At the of level of roads, the quantities of CO2 and CO in EragrostisTenella are very high with values respectively equal to 74.4092 mg/ml and 62.2654 mg/ml. On the other hand, the amount of NO2 in Amaranthus sp is 0.2304 mg/ml and that of SO2 in EragrostisTenella is 0.1691 mg/ml (Table 1 and Table 2).
Density of the road traffic, landfills and industries are the source of atmospheric pollution worldwide. Gases produced by anthropogenic sources are in turn captured by some plant species in nature. Among the main gases measured at the level of industries, landfills and roads in the city of Lomé, it was revealed a high concentration of these gases at the level of certain plant species. This is the case of   in the USA.
At the industrial level, it was found that Alternantherarepens stored more carbon dioxide (CO2) followed by Pithecellobiumdulce which stored more carbon monoxide (CO). However, SO2 and NO2 are stored with low concentrations. In Togo, in the city of Lome, industries are among the sources of pollution where carbon dioxide and carbon monoxide are the most released gases in nature. At the level of these industries the most frequent and widespread species are Alternantherarepens and Pithecellobiumdulce. These species undergo enormous pressures with respect to the gases that are released into the atmosphere. The presence of these gases in these species really shows that in these industrial areas, the atmosphere is polluted. In these industrial areas, sulfur dioxide and nitrogen dioxide are recorded at the level of these same plant species. Indeed according to several authors NO2 and SO2 are gases which should not be in the atmosphere. The presence of these gases confirms that the industries constitute sources of anthropic pollutions which are to be taken into account. These same studies were carried out on lichens, mosses, tobacco      . Indeed, most of these studies cited above have confirmed that lichens and mosses are very sensitive species that can be used to measure the degree of pollution of the atmosphere in a given area.
At the level of landfills, Newbouldialaevis and Sennaoccidentalis are recognized as species that have more accumulated carbon dioxide and carbon monoxide. In contrast the concentration of NO2 in Ocimumcanum low as well as the concentration of sulfur dioxide in Newbouldialaevis. At road level the concentration of carbon dioxide is higher in EragrostisTenella and that of carbon monoxide is higher in Amaranthussp.
The comparison of the levels of these gases in the species shows spatial variations of these gases in the city of Lomé. Unfortunately in Togo, several plant species are used extensively by urban, semi-urban and rural populations. And among these species we can cite Ocimumcanum, Newbouldialaevis, Senna occidenttalis, Alternantherarepens, Eragrostistenella, Pithecellobiumdulce. According to the  WHO (2005) values, the accumulation of pollutants in plants can cause carcinogenic diseases in human organism.
Thus, our study on biomonitoring in the city of Togo, carried out from the observation of its effects on these sensitive plants, confirms the influence of urban road traffic density, anthropic activities and climatic conditions. Our results, which are in agreement with ozone data measured continuously by AIRLOR on some urban and suburban sites in France, complete the knowledge of the situation of an important regional agglomeration with respect to these atmospheric pollutants.
The results showed that at the industrial level the concentration of CO2 in Alternantherarepens is high with a value of 53.3911 mg/ml. On the other hand, the quantity of CO in Sennaoccidentalis is 44.3619 mg/ml. At the level of Pithecellobiumdulce, the quantity of SO2 and NO2 are evaluated respectively to 0.1588 mg/ml and 0.3696 mg/ml. At the level of the dumps, the amount of CO2 in Newbouldialaevis is very high with a value of 65.8508 mg/ml. On the other hand, the amount of CO in Sennaoccidentalis is 51.6106 mg/ml. At the level of Newbouldialaevis the amount of SO2 is 0.2101 mg/ml and NO2 in Ocimumcanum is 0.2744 mg/ml. At the level of roads, the quantities of CO2 and CO in EragrostisTenella are very high with values respectively equal to 74.4092 mg/ml and 62.2654 mg/ml. On the other hand, the amount of NO2 in Amaranthus sp is 0.2304 mg/ml and that of SO2 in EragrostisTenella is 0.1691 mg/ml. The use of a plant bioindicator sensitive to pollutants, allowed concluding that the air of the city of Lome is polluted. The concentration of carbon dioxide and carbon monoxide is much more evident in return the health of plant species is threatened.
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