Air Pollution

Meteorology Defined: Meteorology is the scientific study of the atmosphere.

Atmospheric Medium: The atmosphere serves as the medium through which all air pollutants are released.

  • Key Atmospheric Processes:

    • Wind Movement: Atmospheric processes, including the movement of air (wind), play a crucial role.

    • Heat Exchange: Processes like convection and radiation contribute to the exchange of heat.

  • Fate of Pollutants: The destiny of pollutants is determined by atmospheric processes, encompassing transport, dispersion, transformation, and removal.

  • Air Pollution Meteorology: This field focuses on understanding how these atmospheric processes impact the behavior of pollutants.

  • Temperature Inversion

    • In meteorology, an inversion, also known as a temperature inversion, is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to an inversion of the thermal lapse rate. Normally, air temperature decreases with an increase in altitude.

    • Temperature inversions are a result of other weather conditions in an area. They occur most often when a warm, less dense air mass moves over a dense, cold air mass. This cold air then pushes under the warmer air rising from the valley, creating the inversion.

Smog is an effect of temperature inversion, where warm air forms a layer over cold air.

Sources of Air Pollutants

Natural:

  • Dust blown by wind

  • Pollutants from wildfires and volcanoes

  • Volatile organics released by plants

  • Withdrawing groundwater

Human Sources (mostly in industrialized and/or urban areas):

  • Stationary sources

  • Mobile sources

Some Pollutants in the Atmosphere Combine to Form Other Pollutants

◼Primary pollutants

◼Secondary pollutants

Effects of Air Pollution

  • Acid Rain

  • Photochemical Smog

  • Ozone Layer Depletion

  • Global Warming (Climate Change)

Short-term Health Effects:

  • Headache

  • Coughing

  • Pneumonia

  • Bronchitis

  • Skin Irritation

Long-term Health Effects:

  • Central Nervous System Diseases

  • Cardiovascular Diseases

  • Respiratory Diseases

  • Impacts on Liver

  • Impacts on Reproductive System

ATMOSPHERIC DISPERSION

  • Factors affecting air pollutant dispersion include emission point characteristics, pollutant nature, meteorological conditions, and terrain effects.

Dispersion of Air Pollutants

  • Industrial effluents are discharged vertically, expanding and mixing with ambient air.

  • Horizontal air movement bends the plume downwind.

  • Rising plume is diluted by surrounding ambient air.

ATMOSPHERIC DISPERSION

  • Source Characteristics

    • Most industrial effluents are discharged vertically through a stack or duct.

    • The plume tends to expand and mix with ambient air as it leaves the discharge point.

    • Horizontal air movement bends the discharge plume downwind.

    • Dilution occurs as the effluent plume rises, bends, and moves horizontally.

    • Gaseous effluents are dispersed toward the ground as they mix with ambient air.

    • Plume rise is influenced by exit gas mass, exit velocity, and exit gas temperature.

    • Plume rise, along with stack height, is known as the effective stack height.

  • Downwind Distance

    • Greater distance from discharge to receptor increases air volume for dilution.

  • Wind Speed and Direction

    • Wind direction determines the contaminated gas stream's movement across terrain.

    • Increased wind speed decreases plume rise, potentially increasing ground-level concentration.

    • Higher wind velocity increases effluent plume dilution, lowering downwind concentrations.

  • Stability

    • More unstable atmosphere enhances dilution power.

    • Non-ground-based inversions limit vertical dilution.

  • Dispersion Modeling

    • Mathematical description of meteorological transport and dispersion.

    • Quantified in terms of source and meteorological parameters.

    • Yields concentration estimates for specific locations and times.

    • Requires parameters like wind direction, wind speed, atmospheric stability, stack height, stack diameter, exit gas temperature and velocity, and emission rate.

  • Point Source Gaussian Dispersion Model

    • Assumptions:

      • Uniform atmospheric stability in the layer of gas discharge.

      • Turbulent diffusion described by Gaussian or normal equation.

      • Gas stream released above ground level at stack height plus plume rise.

      • Dilution is inversely proportional to wind speed.

      • Pollutant reaching ground level is reflected back into the atmosphere.

EFFECT OF METEOROLOGY ON PLUME DISPERSION

Looping - strong lapse condition

Coning - weak lapse condition

Fanning - inversion condition

Lofting - inversion below, lapse aloft

Fumigation - lapse below, inversion aloft

Trapping - weak lapse below, inversion aloft

AIR POLLUTION PREVENTION

  • Air Pollution Prevention

    • Renewable fuel and clean energy production

      • The most basic solution for air pollution is to move away from fossil fuels, replacing them with alternative energies like solar, wind, and geothermal.

    • Energy conservation and efficiency

      • Producing clean energy is crucial. But equally important is to reduce our consumption of energy by adapting responsible habits and using more efficient devices.

    • Eco-friendly transportation

      • Shifting to electric vehicles and hydrogen vehicles and promoting shared mobility (i.e., carpooling, and public transports) could reduce air pollution.

    • Green building

      • From planning to demolition, green building aims to create environmentally responsible and resource-efficient structures to reduce their carbon footprint.

AIR POLLUTION CONTROL OF STATIONARY SOURCES

Particulate Matter Pollutants

  • Settling Chambers: Simple device collecting dust of size >10um.

  • Inertial Separator or Cyclone: Process achieved by a sudden change in the direction of gas flow. Dust particles spin in a circular path.

  • Electrostatic Precipitators: Passage of particle-laden gases between high-voltage discharge electrodes.

  • Bag House and Filter: Most often used device required to meet any furniture standards limiting the emission of respirable particles.

Gaseous Pollutants

  • Absorption: Polluted air is passed through a scrubber containing a suitable liquid absorbent.

  • Adsorption: Process where gases, vapours, or liquids are concentrated on a solid surface due to surface or chemical forces (physical adsorption and chemisorption).

  • Combustion: Method for the removal of Volatile Organic Compounds (VOCs), as they can be decomposed to Cl2 and H2O. Direct and indirect methods are used, with the catalytic method preferred for low-temperature processes.

  • Flue Gas Desulfurization (FGD): Technology used to remove sulfur dioxide from the exhaust flue gasses of fossil fuel power plants.

CONTROL OF AIR POLLUTION FROM MOBILE SOURCES

  • Positive crankcase ventilation valves: Direct crankcase blowby emissions into the engine.

  • Exhaust gas recirculation valves: Reduce NOx formation during fuel combustion.

  • Charcoal canisters: Sequester volatile hydrocarbons for later burning in the engine.

  • Catalytic converters: Designed to oxidize partially combusted hydrocarbons and CO to CO2.

Gasoline Engine:

  • 1 kg gasoline needs about 15 kg air for complete combustion.

  • Incomplete combustion results in emissions of carbon monoxide, unburned gasoline, and hydrocarbons.

Diesel Engine:

  • Operates at a higher air-to-fuel ratio.

  • Fuel is directly injected into the combustion chamber, and power output is controlled by injection rate.

  • No spark ignition; air is heated by compression.

  • Well-designed diesel engines emit less CO and hydrocarbons but have higher NOx emissions due to higher operating temperatures.