Air Pollution: Movement Of Air In Troposphere And Stratosphere

Earth is the only planet in the solar system which supports life. The main reason behind this extraordinary feature of our planet is the fact that it is surrounded by a layer of gases called the atmosphere. But the quality of the atmosphere is getting deteriorated day by day due to a variety of polluting agents, which are both natural and man-made.

The atmosphere is a complex mixture of gases having nitrogen and oxygen as its main components. Based on the temperature profiles, the atmosphere can be classified into four basic regions. The troposphere is the first layer extending 9km at the poles to 17km at the equators. Most of the atmospheric mass is being constituted in this layer i.e. 80%. As the altitude increases, there is a decrease in temperature which results in decrement in water vapour pressure. The main cause of air pollution in the troposphere is turbulence at low altitudes, which causes pollutant dispersion. The planetary boundary layer (PBL) is directly affected by the incfluenced by its contact with the ground surface. A major gap in PBL depth can be seen ranging from 50 – 2000 meters due to pollutant dispersion at low altitudes within different climatic conditions. The second basic layer of the atmosphere is Stratosphere, which ranges from the troposphere to 50Km. This layer is more close to the ozone layer due to which there is a reverse trend of temperature in this layer as compared to the troposphere, i.e. temperature here increases with height. This layer is comparatively less turbulent due to a warmer temperature profile, and hence a less pollutant dispersion. This bottom zone of the Stratosphere is the main region for commercial air vehicles since the bottom zone of the Stratosphere provides the least drag due to reduced turbulence. The third layer of the atmosphere is Mesosphere. This layer ranges from 50Km – 80Km in altitude. Most of the meteoroids that collide with the earth are mostly absorbed and melted in this layer and are not able to cross this layer. The last layer of atmosphere to consider is the Thermosphere which extends over a range of 80Km- 350Km based on the rate of solar activity in a region. Temperatures in this layer could reach a peak value of 1500°C.

Base on the movement of air it can be further categorised as ambient air or outdoor environment and indoor environment. Out of all the layers of the atmosphere, the two layers which are more close to the earth’s surface i.e. troposphere and Stratosphere are more prevalent to air pollution. Pollutant mixing is a common phenomenon in the troposphere due to its increasing temperature profile. When these dispersed/mixed pollutants rise to the stratosphere, they become stable due to decreasing temperature profile of the Stratosphere. The major reaction in this layer is the conversion of oxygen gas to Ozone photochemically in the presence of sunlight. It is noticed that due to the rotation of the earth and uneven distribution of solar radiation, the air pollutants are not subjected to a single area and hence they are transboundary in nature. This accounts for the global transport of ambient air. Apart from the global movement of air, the regional movement of air is dominated by pressure zones. Air moves from a high pressurised area to low pressure area. The local movement of air is mainly due to the temperature differences between land and water. During the day, cleaner air moves from the sea towards land, which in turn is polluted by human activities. There is a reverse movement of the polluted air from land to sea in the evening which in turn also disturb the water ecosystem. Understanding this phenomenon is an important criterion for coping with air pollution in coastal areas.