Table of Contents
Introduction
Air is essential for survival of humankind, consecutively this valuable asset must be kept clean since it is the elixir of life. However, in this developing world, with booming population, this fundamental asset is getting destroyed by human actions. Reasons for air contamination are both natural (e.g. volcanic eruption, dust storms, etc.) and artificial (e.g. power plants, vehicles, etc.). Air pollution results in production of perilous issues starting from respiratory issues, coughing, to sudden death.
Structure of Atmosphere
The atmosphere comprises of a complex mixture of gases, comprising majorly nitrogen and oxygen. Based on the ranging in temperature, the atmosphere is classified as four basic regions. Troposphere being the primary layer extending 9km at the poles to 17km at the equators. A vast majority of the climatic mass is established in this layer i.e. 80%. As the elevation increments, there is a decrease in temperature, which results decrement of water vapor pressure. The primary driver of air pollution in troposphere is turbulences at low heights, which causes pollutant dispersion.
The planetary boundary layer (PBL) is therefore directly affected by the influenced by its contact with the ground surface. A major gap in PBL depth could be observed ranging from 50 – 2000 meters, due to pollutant dispersion at low elevations within different climatic conditions. The second basic layer of atmosphere is Stratosphere, which ranges from troposphere to 50Km. This layer is all more near ozone layer because of which there is an invert pattern of temperature in this layer when contrasted with troposphere, i.e. temperature here increases with height. This layer is relatively less violent because of a hotter temperature profile, and consequently a less pollutant dispersion takes place here. The bottom zone of stratosphere is the main region for commercial air vehicles since bottom zone of Stratosphere provides minimal drag due to reduced turbulence.
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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 Thermosphere, which extends over a range of 80Km- 350Km based on the rate of solar activity in a region. Much of the X-ray and UV radiation from the Sun is absorbed in the thermosphere. Temperatures in this layer could reach a peak value of 1500°C.
Transport of Air
Based on the movement of air it can be categorized as ambient air or outdoor environment, and indoor environment. Out of all the four layers of atmosphere, the two layers which are more close to earth’s surface i.e. troposphere and stratosphere are increasingly pervasive to air contamination. Pollutant mixing is a common phenomenon in troposphere due to its expanding temperature profile. At the point when these dispersed/mixed contaminations ascend to the stratosphere, they become stable because of diminishing temperature profile of stratosphere. The major reaction in this layer is the conversion of oxygen gas to Ozone photo chemically in the presence of sunlight. It has been observed, that because of revolution of earth and uneven dissemination of sun based radiation, the air pollutants are not subjected to a single area and hence they are trans-boundary 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-pressurized area to low pressure area. The local movement of air nearby coastal areas is primarily because the temperature contrasts between land and water. During the day, cleaner air moves from 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.
In addition, indoor air quality significantly affects human wellbeing. For example, designing of a ventilation framework in a structure should improve its temperature, humidity and ventilation conditions. Care must be taken while designing such frameworks, keeping in mind that it should not hinder with life of people that will reside there, neither depriving them of the quality air available to them. The pressure difference is caused by temperature differences in different parts of the building. Such systems are called HVAC (Heating, Ventilation, and Air Conditioning). Natural ventilation should be preferred over mechanical ventilation in such buildings.
Air Pollutants
Air pollution is the presence of a substance (i.e. pollutant) in noticeable amount that is dangerous to human and the environment. Air pollutants may be natural (e.g. volcano eruption) or artificial (e.g. factories). Pollutants can be any of the three forms i.e. solid, liquid or gaseous. The human health impact caused by air pollution may include difficulty in breathing, wheezing, respiratory and cardiac conditions.
Some of the major pollutants include, Nitrogen Dioxide, which is a reddish-brown toxic gas with a characteristic sharp, biting odor. Major source of Nitrogen Dioxide gas is fuel combustion. Particulate matter (PM) is also a major kind of pollutant. It refers to fine particles (solid or liquid) with diameter of 10 micrometers or less, which can be either man made or natural.
Natural particulates constitute those from volcanoes, dust storms, forest and grassland fires, living vegetation, and sea spray. The burning of fossil fuels in vehicles, power plants, construction and various industrial processes produces artificial particulates. Large (PM10) cause problem in respiration as they produce cloggy throat and nose. Fine particles (PM2.5) make their way deeper in the bloodstream and affect many organs. Ground level Ozone is another polluting agent, which is produced by a photochemical reaction driven by sunlight. The concentration of ozone is maximum throughout the day. Concentration of ozone decreases at night since there is no sunlight.
Ozone is a powerful oxidizing agent that reacts with various substances present in atmosphere to form toxic compounds. Ozone accounts for many respiratory problems like lung inflammation, breathing difficulties, etc. The recommended air quality standard for ozone is 100mg/m^3 by WHO. The partial combustion of fossil fuels in vehicles produce carbon monoxide, which is an another major polluting agent. The WHO air quality standard for CO is 30mg/m^3. Apart from these there are many more pollutants like carbon dioxide (CO2), chlorofluorocarbons (CFCs), ammonia (NH3), peroxyacetylnitrate (PAN), persistent organic pollutants (POPs), toxic metals (lead, cadmium, copper), etc. whose usage is regularly checked by government to control their overshooting usage.
Emission Estimation and Control
Emission is the amount of pollutant released by a in the air. Air Pollution sources are summed up into four sorts, i.e. point source (e.g. single heater), line source (e.g. vehicles on street), area source (e.g. timberland fire), and volume source (e.g. oil refineries). Further, they could be classified as stationary or mobile. There are five fundamental techniques used for estimation of presence of a pollutant in the air. Technique for direct estimation is done utilizing stationary or mobile sensors. Strategy for mass equalization is conducted by figuring contrast between the inflow and outflow in a system.
Mathematical modelling of chemical and physical processes includes method of process modelling. Method of emission factor modelling is ratio of emission and pollutant source intensity. Method of expert judgment incorporates estimation by experience and information. Out of these five recorded strategies, strategy for direct estimation is generally utilized and gives most accurate results, while at the same time being most expensive, whereas the technique of expert judgment is cheapest and is least utilized since it is a theoretical concept and isn’t essentially dependable.
The strategies to control air pollution include prevention, i.e. dealing with the use of clean and green fuels, and energy. Secondly, there are regulatory monitoring solutions set up by government that monitor, record and analyze the air quality of a region. A typical market arrangement incorporates exchange of carbon credits among different nations to accomplish a worldwide-sustained development. Air can be purified using some of the equipment for commercial use including cyclone, scrubber, baghouse and electrostatic precipitator.
In cyclone, a mixture of suspended gas particles is fed tangentially to the inlet at high pressure and velocity, that creates a spiral flow movement and the suspended particles are forced towards the wall due to centrifugal force generated, the particles that collide with the wall move downwards due to spiral flow and are collected at the bottom. In scrubber, a scrubbing liquid (usually water) comes into contact with a gas steam having dust particles. The greater the contact of gas and liquid steams is there, the higher is the efficiency of scrubber.
In baghouse filter, air containing dust particles enters the baghouse, which contains bags that act as filter, very high efficiencies can be achieved using the baghouse filter ranging up to ninety nine percent, and hence it can be used to separate very fine pollutants. In electrostatic precipitator, electrostatic forces are used to separate dust particles from exhaust gas. The contaminated gas is made to flow through the passage formed by discharge and collecting electrode, as a result, the airborne particle receives a negative charge and the charged particles are attracted to the grounded or positively charged electrodes when they pass through ionized field between the electrodes.
Air Quality Modelling
Air Quality Index (AQI) represents level of pollution in the air, which comprises of six levels ranging from good (0 AQI) to hazardous (500 AQI). The overall air pollution index for a site or region is determined by the concentration of five highest polluting agents: O3, PM2.5, PM10, NO2 and SO2. Air quality is modelled by air dispersion models, which include mathematical equations describing physical and chemical processes of air and its pollutants, which are then solved by numerical methods on modern computers. These models can be used to estimate and predict the concentration of air pollutants emitted from various sources. Government agencies use these models to make sure that the new industrial plants that are being set up in an area are in accordance with the relevant air quality standards. These models also find a use in emergency planning and cases of accidental release of chemicals.
Dispersion models vary depending on the mathematics used to develop the model. Some examples of dispersion models include, The Weather Research and Forecasting (WRF) Model, which is a next-generation mesoscale numerical weather prediction system, Global Forecasting System (GFS) in which real-time data is updated every 3 hours, The Community Multi-scale Air Quality (CMAQ) modelling system. Countries like UK and Europe use these models for determination of air pollution, and to analyze cloud-scale interactions.
Conclusion
Decrease of air toxins is important to adjust in a sounded way of life. The fundamental purpose for the investigation of air contamination is to avoid it and to counter different issues. Henceforth, the investigation of air contamination is of prime significance. The Central Pollution Control Board (CPCB) of India, was set up in 1974 to counter the increasing impacts of pollution. To tackle the issue of air pollution, it is important to comprehend the air related issues and search forward for ways to solve it.
