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1.0 Introduction
Indian metropolitan cities like Delhi, Mumbai, Kolkata, etc. have high emission of air pollutants, which is degrading the ambient air quality day by day. The degradation of air quality is a major environmental problem that affects many urban and industrial sites and the surrounding regions worldwide. Air pollution can reach levels, where it significantly influences human health, diminishes crop yield, and destroys infrastructure and patrimony. The phenomena involved in air pollution are complex. Once emitted into the atmosphere, primary pollutants are transported by wind, turbulence and diffusion, which can undergo chemical reaction, change phase and finally are removed from the atmosphere by dry and wet deposition. Health and environmental impact of secondary pollutants, i.e., those formed in the atmosphere can be more severe than their emitted precursor.

The inventorying of sources of emission, local meteorology and air quality gives the status of air quality. A predictive model may then be prepared to enact the pollution control regulation to achieve ambient standards/ goals. However, a better, efficient and cost effective approach for monitoring the ambient air quality is to develop an air quality management system.

2.0 Air Quality Management System (AQMS)
AQMS can be defined as the regulation of the amount, location and time of pollutant emissions to achieve some clearly defined set of ambient air quality standards or goals. It includes the evaluation of various set of emission control schedule to determine consequences to air quality. It also includes the formulation of alternative emission control schedules to meet air quality goals subject to some other constraint, e.g., technological feasibility or minimum cost. AQMS is basically a strategy to overcome the problems of air pollution and is most effective towards continuous improvements of air quality, particularly when targeting regional problems. Figure 1 shows various components of an AQMS which may be implemented for achieving and maintaining high ambient air quality standards (Nevers, et al., 1977).


Figure 1: Air Quality Management System
3.0 Conventional Approach for Air monitoring
Various techniques recommended by the Central Pollution Control Board (CPCB) for air monitoring of almost all criteria pollutants are point monitoring. This may involve integrative sampling, grab sampling and passive sampling techniques. The integrative sampling involves drawing a known volume of air through a medium (filter or adsorbent) for a typical period (e.g., 24 hrs) followed by its subsequent analysis in laboratory. Grab sampling involves collecting a sample in a container (e.g., Tedlar bag or stainless steel canister). Then, the air sample is analyzed with appropriate instrumentation (e.g., chromatograph) with an appropriate detector for determination of airborne organic compounds. Passive sampling functions like integrative active sampling, except that a concentration gradient between the ambient air and a sorbent provide the driving forces for collection of the airborne contaminant rather than a pump driven sampler. Exposure times can vary from short periods (8 hrs) up to one month (e.g., sulphation plates, passive diffusion samplers).

4.0 Air Pollution modelling
Modelling of air pollution is generally based on various models, namely, Gaussian models, box models, narrow plume hypothesis, gradient transport models and trajectory models. Gaussian models are based on a set of empirical equations. A three-dimensional axis system with an origin on the ground is set up within the downwind, crosswind, and vertical directions. Pollutant concentrations crosswind and vertically are described by Gaussian distributions. Box models assume uniform mixing throughout the volume of a three-dimensional box. This formulation is usually applied to urban photochemical models.

Narrow plume hypothesis based models assume that the main contributors are located upwind. The pollutant’s concentrations are calculated using a vertical growth rate instead of uniform vertical mixing. Gradient transport models are applied when the turbulence is limited to scales that are small compared with the volume of pollutant emitted. They are more appropriate to line and area sources rather than point sources. Trajectory models are based on a vertical column that is moved downwind at the mean wind speed, with pollutants added as they are emitted at every location.

A numbers of air pollution modelling software are available for ambient air modelling and transport related air pollution modelling. These include ISCST 3 (Industrial Source Complex Short Term, version 3) which is widely used model for estimating near field concentrations of non-reactive pollutant. AERMOD is an American Meteorological Society/US Environmental Protection Agency Regulatory Model used for regulatory purposes. SCREEN 3 is a model for environmental professionals to conduct a preliminary screening analysis using conservative techniques for the determination of whether more refined analysis is required or not. CALINE- 4 model has been developed by California Department of Transportation (CALTRANS) and is the leader in the development of dispersion models for highways. HIGHWAY model is a short-term (one hour) line source dispersion model and was developed by EPA in 1975 in FORTAN. HIGHWAY-2 is the upgraded version of the model and gives more realistic concentration estimates due to upgrade dispersion algorithm (Benson, 1988; Cohn et. al., 1982; EPA, 2001).

5.0 Air Pollution Database for Delhi City
Delhi air monitoring is achieved by point monitoring approach, which involves various types of sampling techniques, like, integrative sampling, grab sampling, passive sampling, etc. Data obtained by monitoring is in the form of non- spatial form. Tata Energy Research Institute (TERI) has developed an interactive database system for air pollution data presentation. It includes compressive air pollution data generated at eighteen monitoring stations. Out of these, eleven are operated by TERI and seven by Central Pollution Control Board. These stations were spread all over Delhi and represent residential, commercial, and industrial areas. After conducting the monitoring over that location, TERI has proposed a user-interactive system for the presentation of pollutant level at different locations. TERI system is capable of providing output in the form of either as graph or as tables.

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