25-08-2012, 04:53 PM
GIS applications in air pollution modeling
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Status of Vehicular Pollution in India
Motor vehicles have been closely identified with increasing air pollution levels in urban centers of the world (Mage et al, 1996; Mayer 1999) . Besides substantial CO2 emissions, significant quantities of CO, HC, NOx, SPM and other air toxins are emitted from these motor vehicles in the atmosphere, causing serious environmental and health impacts. Like many other parts of the world, air pollution from motor vehicles is one of the most serious and rapidly growing problem in urban centers of India (UNEP/WHO, 1992; CSE, 1996; CRRI, 1998). The problem of air pollution has assumed serious proportions in some of the major metropolitan cities of India and vehicular emissions have been identified as one of the major contributors in the deteriorating air quality in these urban centers (CPCB, 1999). Although, recently, improvement in air quality with reference to the criteria pollutants (viz. NOx, SO2, CO and HC) have been reported for some of the cities, the air pollution situation in most of the cities is still far from satisfactory (CPCB, 2000). The problem has further been compounded by the concentration of large number of vehicles and comparatively high motor vehicles to population ratios in these cities (CRRI, 1998).
Vehicular Pollution Modeling in India
In air pollution problems, the air quality models are used to predict concentrations of one or more species in space and time as related to the dependent variables. They form one of the most important components of an urban air quality management plan (Elsom, 1994, Longhurst et al., 2000). Modelling provides the ability to asses the current and future air quality in order to enable “informed” policy decisions to be made. Thus, air quality models play an important role in providing information for better and more efficient air quality management planning. An effective air quality management system must be able to provide the authorities with information about the current and likely future trends, throughout the area enabling them to make necessary assessments regarding the extent and type of the air pollution control management strategies to be followed.
The air quality models can be classified as point, area or line source models depending upon the source of pollutants, which it models. Line source models are used to simulate the dispersion of vehicular pollutants near highways or roads where vehicles continuously emit pollutants. Several models have been suggested to predict pollutant concentration near highways or roads treating them as line sources. Vehicular pollution modelling, in general, refers to carrying out air pollution prediction estimates due to vehicular activity in a region. In urban environment it has to be taken into consideration along with other types of sources viz. area and/or point sources (FIG. 1). Whereas, the highway dispersion models are generally used for analyzing the output of an existing or proposed highways/ roads at a distance of tens to hundreds of meters downwind. In this region, the effect of vehicular pollution and vehicular activity is considered to be primary consideration for air quality prediction analysis.
Inadequacies of vehicular pollution modelling
Various line source models (viz. CALINE 4, GM, HIWAY 2, etc.) generally require various input parameters pertaining to meteorology, traffic, road geometry land use pattern. Besides the basic Gaussian dispersion approach, each dispersion model differs with respect to the treatment of modified wind and turbulence due to vehicle wakes (thus dispersion parameters ?y and ?z) near the roads. Different models also take care of cases of oblique and parallel winds and treatment of hot exhaust plumes from vehicles in different ways. Adequacies, limitations, reliability and associated uncertainties of these dispersion models have already been discussed by various researchers (Hanna,1988 ;Scorer, 1998 etc.).