Effect of Air Pollutants on Vegetation in Tropical Climate: A Case Study of Delhi City

pollution trans-boundary environmental and a matter of global many High concentrations of air pollutants due to numerous anthropogenic activities influence the air quality. There are many books on this subject, but the one in front of you will probably help in filling the gaps existing in the area of air quality monitoring, modelling, exposure, health and control, and can be of great help to graduate students professionals and researchers. The book is divided in two volumes dealing with various monitoring techniques of air pollutants, their predictions and control. It also contains case studies describing the exposure and health implications of air pollutants on living biota in different countries across the globe.


Introduction
Urban air pollution is a serious problem in both developed and developing countries (Li, 2003). As a rapidly expanding centre of government, trade, commerce and industry, Delhi, the Indian capital, has been facing many air pollution related problems. The vehicular exhaust contributes significantly to the pollution load in Delhi. The plant species are severely affected by various pollutants emitted from different sources e.g. SO x , NO x and particulates. To maintain ecological balance in this fastly developing capital, there is an immediate necessity to assess the effect of the air pollutants on the plant speices so that the strategies can be formulated and implemented to protect the species. Plants remove air pollutants by three mechanisms: absorption by the leaves, deposition of particulates and aerosols over leaf surfaces, and fallout of particulates on the leeward side of the vegetation (Tewari, 1994;Rawat and Banerjee, 1996). As a result, the chlorophyll concentrations in the leaf which is responsible for the photosynthetic activity may decrease (Seyyednejad et al, 2011). Hence, the ability of the plant to tolerate the air pollution gets affected. Plantation of tolerant tree species will have a marked effect on varied aspects of the quality of the urban environment and the cleanliness of life in a city (Bamnia et al, 2011). The Anticipated Performance Index (API) has been used as an indicator to assess the capability of some of the predominant species present in Delhi. Further, the air pollution tolerance index (APTI) of the plants needs to be monitored and checked for the predominant species that are located in the city.

Topography of Delhi
Delhi is positioned with the Great Indian Desert (Thar Desert) of Rajasthan to the west and southwest, central hot plains to the south and gangetic plains of Uttar Pradesh/Uttaranchal to the east while cooler hilly regions to the north. It is situated at latitude 28º24'17" and 28º53'00" North; Longitude 76º45'30" and 77º21'30" (East) at elevation of 216 m above the mean sea level (msl). Delhi's climate is very hot in summer (April -July) and cold in winter (December -January). The average temperature can vary from 25º C to 45º C during the summer and 22º C to 5º C during the winter respectively. The topography of the city is manly urban plain having two main features -the Ridge and the river Yamuna. The Ridge refers to an area inhabited by extraordinary plants and fierce animals.

Air quality status in Delhi
The Air quality in Delhi is primarily affected by the vehicular exhausts, small scale industries, power plants and biomass burning. The relative contribution of various sources for the air pollution in Delhi has been shown (Table 1&2). It can be found that the vehicular exhausts contribute a significant amount to the air pollution in the city. The pollutants include SPM, RSPM, SOx, NOx and Benzene. It has also been reported that the benzene concentration has exceeded seven times that of the permissible limit during the winter season. In summers, the concentration of the pollutant has been below the permissible standards prescribed. A Source apportionment study to identify the sources of the benzene is to be carried out (DPCC, 2011

Vegetation covers in the city
The landscape of Delhi consists of a broad spectrum of environments ranging from the city forests to highly modified artificial landscapes in certain parks (Khera et al, 2009 Regional as well as global scale VOC emission, from vegetation, may dominate over anthropogenic sources of emission (Guenther et al., 1995). Out of the nine commonly occurring tree species in Delhi, VOC emissions were found in six species, namely, Eucalyptus sp., Ficus benghalensis, Ficus religiosa, Mangifera indica, Melia azedarach, and Syzygium jambolanum, at higher concentrations. Further it has also been observed that that these emissions are dependent up on sunlight and temperature (Tingey et al., 1979;Lamb et al., 1987;Padhy and Varsheney, 2005). www.intechopen.com

Quantifying the effects of air pollution using APTI and API
The effect of air pollution on the plants can be quantified using a parameter, air pollution tolerence index (APTI) (Singh and Rao, 1993). The APTI is a function of total chlorophyll content of the leaf, pH, relative water content and the ascorbic acid content. The APTI for a particular tree species is given in eq (1) APTI = (A (T+ P) + R)/10 (1) Where A= Ascorbic acid content in mg/g dry weight, T= Total Chlorophyll content in mg/g dry weight, P= pH of the Leaf extract, R = Relative water content (%) The plant species can be convinently grouped based on the APTI values ( Further, the parameters of the APTI have been correlated with the increasing concentration of the pollutants. It has been found that the relative water content , total chlorophyll are negatively correlated and ascorbic acid is positively correlated with all the pollutants. The pH values are negatively correlated in the case of NO x (  Table 5. Correlation between Air pollutants and Biochemical parameters (Govindaraju et al, 2011).
The APTI index shows the effect of the pollutants only on the biochemical parameters. In order to combat air pollution by planning the green belt development in a particular area, many socio-economic factors are to be considered. Hence the anticipated performance index has been used to determine the same (Govindaraju et al, 2011). The method contains a grading system where a tree species is graded based on various parameters. The API has been calculated for predominant tree species in Delhi. The table 6 shows the parameters used to grade the performance of a particular tree species. It includes the parameters like APTI along with the socio-economic parameters. Based on the current grading system, a tree can secure a maximum of 16 positive points. These points were scaled to a percentage system and based on the score obtained, the category has been assessed.  Table 7. Anticipated performance Index (API) for species (Govindaraju et al, 2011).
To determine the effect of air pollutants on the tree species in Delhi, the API values have been calculated. Since the data on the APTI values of the tree species considered are not available, the analysis is performed for both the extreme cases i.e for maximum APTI and for the minimum APTI. Table 8 and 9 shows the maximum and minimum values of the API obtained.
It can be shown from the table 9 that even though the APTI of a species is negligible (less than 9) , but still the API of a plant species are securing a reasonable grades. Out of the eight species, three are good, two are moderate, two are poor and one is very poor.  Further, it has been found that among all the pollutants, SPM has the maximum effect on the biochemical parameters; and NOx has the least effect. The pollutants can be arranged in the following order. SPM> RSPM> SO x > NO x (Govindaraju et al, 2011). To incorporate the effect of increase of the concentrations of these pollutants, an additional weightage of 1, 0.75, 0.5 and 0.25 may be added to the negative weightage, respectively. This additional weightage will be added only when the concentration of the polluants exceed the specified standards. Thus, the incorporation of the negative weightage in the calculation of the API for existing plant species can make it more practical if applied for the city like Delhi.

Conclusions
It can be concluded that due to the increasing pollution load on the plants, the vegetation in the city is under extreme stress. To ensure that the generated pollutant load is removed from the atmosphere, the quantification of the effect of the pollutants on the tree species should be made at regular intervals to ensure that they perform well under pollutant stresses. Further, the source apportionment studies have to be conducted to identify the pollutants like benzene which are exceeding the permissible standards during winter. Aditionally, the effect of the water contamination and other miscellaneous factors may add additional stress to the vegetation which needs to be considered to ensure that the species sustain in a polluted environment.