GIS applications in air pollution modeling
Kirti Bhandari,Prasad Rao, Anuradha Shukla
Scientists, Central Road Research Institute, Mathura Road, New Delhi.
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).
In India, the number of motor vehicles has grown from 0.3 million in 1951 to approximately 50 million in 2000, of which, two wheelers (mainly driven by two stroke engines) accounts for 70% of the total vehicular population. Two wheelers, combined with cars (four wheelers, excluding taxis) (personal mode of transportation) account for approximately four fifth of the total vehicular population. The problem has been further compounded by steady increase in urban population (from approximately 17% to 28% during 1951-2001; Sharma; 2001 and larger concentration of vehicles in these urban cities specially in four major metros namely, Delhi, Mumbai, Chennai and Kolkatta which account for more than 15% of the total vehicular population of the whole country, whereas, more than 40 other metropolitan cities (with human population more than 1million) accounted for 35% of the vehicular population of the country. Further, 25% of the total energy (of which 98% comes from oil) is consumed by road sector only. Vehicles in major metropolitan cities are estimated to account for 70% of CO, 50% of HC, 30-40% of NOx, 30%of SPM and 10% of SO2 of the total pollution load of these cities, of which two third is contributed by two wheelers alone. These high level of pollutants are mainly responsible for respiratory and other air pollution related ailments including lung cancer, asthma etc., which is significantly higher than the national average (CSE, 2001; CPCB, 2002)
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. At present, most of the widely used highway dispersion models are Gaussian based (Briggs et al., 2000; Baratt, 2000).
Fig 1. Area of Concern for Modellers
In India various Gaussian based line source models like CALINE 3 and 4, GM and HIWAY 2 are routinely used to predict the impact of vehicular pollution along the roads/highways. Most of these predictions or estimations are carried out as part of Environmental Impact Assessment (EIA) studies. The EIA notification of May 4, 1994 of Ministry of Environment and Forests, Government of India (MoEF, 1994) had made it mandatory for all new and existing highway/roads projects, part of EIA requirements, prediction estimates of vehicular pollutants along the highways/roads are routinely carried out by using various Gaussian based dispersion models. Based on the modeling exercise, Environmental Management Plan (EMP) is suggested so that the predicted air pollution level does not exceed the National Ambient Air Quality Standards (NAAQS). Although Central Pollution Control Board (CPCB), Delhi under the Ministry of Environment and Forests had issued necessary guidelines for air quality modeling (CPCB, 1998), but unfortunately they do not contain any reference/guidelines, with respect to line source models, resulting in use of different type of line source models. The experience so far has shown that the values of various input parameters to these models are often adopted from other countries without understanding their applicability in Indian context, resulting in inaccurate and unreliable predictions. Moreover, many times these models are used for predicting air pollution levels along the roads under the urban environmental conditions. Interpretation based on that modeling exercise should be drawn very carefully when as these Gaussian based dispersion models have been found to perform poorly under these conditions (Namdeo and Colls, 1996; Micallef and Colls, 1999; Briggs et al., 2000).
1 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
) 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.).
Various Gaussian based dispersion models, initially developed in West (particularly in USA) are extensively used in India without properly calibrating them for Indian climatic and traffic conditions. Moreover, various input parameters, used in these models are not accurately known, leading to incorrect or sometimes even unreliable predictions. Greatest inaccuracy in vehicular pollution modeling exercise in India occurs due to the considerations for improper emission factors used for different categories of vehicles. Emission factors expressed in terms of grams of pollutants per unit of distance traveled (in km) depend, on several factors including type of fuel, engine type, driving cycle, age of the vehicle, speed of vehicle, driving mode etc. Uncertainties and unreliability associated with the emission factors have already been discussed in detail and reported by various researchers (Kulhwein and Friedrich, 2000 and Vlieger et al., 2000).
Unfortunately in India, no serious efforts have been made to accurately determine the emission factors for different categories of in-use vehicles as a function of vehicle speed, engine technology, fuel quality and age of the vehicles. Various researchers had used emission factors, which were obtained from limited experimental data on chassis dynamometer under laboratory conditions or directly adopting emission factors which are applicable to European vehicles. While use of emission factors obtained from old generation vehicles grossly over predicts the emissions from the new generation Euro I, Euro II compliance vehicles presently plying on Indian roads, the use of emission factors developed for European vehicles to that of Indian vehicles grossly under predicts the emissions from these Indian vehicles. The problem is further compounded, as vehicles with a wide range of engine technology with the different quality of fuels are being used in these vehicles (CPCB; 2000a, 2000b). In India, vehicles as old as belonging to 1970’s and as new as Euro II and Euro III compliant vehicles can be found to be plying on the roads. The quality of fuel supplied in whole country is also not same. While, better quality fuel, comparable to Europe and other developed countries is being supplied in Delhi and few other major metros, the quality of fuel being supplied in other parts of the country is still poor. Thus, with different combination of vehicles (age wise and technology wise), with a wide range of fuel quality, finding reliable emission factor for different categories of vehicle, under Indian driving and road conditions with limited emission testing facilities is a task, which requires immediate attention. Further, with recent emphases on replacing old technology vehicles with the latest ones, and improvement in fuel quality for whole country, the existing facilities need to be upgraded keeping in tune with the latest developments that are taking place in the other parts of the world. Recently, CPCB (CPCB, 2000a; Sengupta, 2000) has suggested a set of emission factors for different categories of vehicles on the basis of year of manufacture and engine technology. However, it is still a long way before more reliable emission factors that reflect Indian traffic conditions are worked out.