How atmospheric large eddies transport domestic cook stove emissions over slums: A Chennai city case study

摘要

The remit of mechanical engineering extends well beyond machines. Mechanical engineering theories have diverse applications – the transport and distribution of particulate matter into a turbulent boundary layer involves a systematic application of fluid mechanics. In this paper, we have used fluid mechanical models accompanied by experimental observations to study how turbulent eddies transport and distribute cook stove emissions over slums in the megacity Chennai - the Detroit of India. It is home to 8 million people and is a manufacturing hub with a propensity to release a variety of suspended particulate matter mixed with marine aerosols transported from the Bay of Bengal. Of these 8 million, about 2 million live in slums-these impoverished slum dwellers comprise of Chennai's huge unskilled work force. Our study revealed that they primarily use cow dung cakes and unseasoned wood to cook, releasing a huge amount of particulate matter up to radii of 10 microns (PM10). A scanning electron micro-graphy ascertained the drop size distribution ensuring that the majority of these particles were small enough for them to be easily transported by boundary layer eddies – the latter are simulated using the United Kingdom Met Office Large Eddy Simulation Model. It is observed that the large eddies are spread over alternating up and down draughts with maximum vertical velocity perturbations of the order of 1 ms~(-1), which is significantly higher than the Stokesian still air settling rates of suspended particles. Whilst remaining aloft, they act as cloud condensation nuclei and thence grow into cloud droplets. The large cloud covers over Chennai city can be linked to this latter effect. Chennai's hot and humid atmosphere ensures that these droplets collide within the cloud topped turbulent boundary layer to trigger a process of stochastic coalescence and subsequent rain showers.