A scavenging model analysis around a large coal-fired power plant in New Delhi with a particular reference to the scavenging action of the monsoonal rains

There have been reports in the recent past about the problem of SO₂ pollution over India. Some of them have even stated that corrosion of the Acropolis in Athens is now matched by the corrosion of the Taj Mahal in Agra. Mathematical models and experimental analyses have been undertaken to address the problem of SO₂ pollution over selected Metropolitan cities in India.An important feature revealed from these model studies is that most urban air quality models in India grossly over predict ambient SO₂ levels during the monsoon period. This is because washout calculation are not featured in these models. In this paper we have tried to demonstrate the efficacy of the southwest monsoon rains to a scavenge a soluble pollutant like SO₂ from an urban environment. India in general does not face the acid rain threat yet. However, results of rainwater analyses show that low pH of precipitation does occur at isolated pockets downwind of major industries and power plants. In this paper we have determined the extent of acidic deposition in the near field of a large coal-fired power plant in Delhi. SO₂ concentration profiles, with and without washout calculations have been shown. Probable periods of the year and the areas within the meteroplition regions of Delhi which could be worst affected by acid impaction have been identified on the basis of the model simulations with mean climatological data. Model computations show that maximum pollution is brought in to the city from this power plant during the month of October. The hourly GLC often exceeds 1000 μg m^-3 which is quite a close to the federal standard of 1100 μg m^-3; likewise the acid deposition flux is greatest during the month of August and is of the order of 50 μg m^-2 s^-1 at a downwind distance of 1 km.The nature of the washout coefficient during the monsoon pre and post monsoon periods in relation to the relative importance of the atmospheric variables concerned has been investigated. The role of individual monsoonal showers to scavenge SO₂ has been discussed.This is perhaps the first work of its kind from India wherein representative washout coefficients have calculated and subsequently featured as washout effects in an urban air pollution model. Results show that this parameter could be as high as 16.03 × 10^-5 s^-1 during August and as low as 1.1 × 10^-6s^-1 in April.