Environmemtal Chemistry Notes  For BS/MSC/MS/MPhil/Phd etc...

Environmental Chemistry

Environmental chemistry is traditionally defined as “the study of the sources, reactions, transport, effects and fates of chemical species, in water, soil, and air environments, and the effect of human activity on these”. This definition implies a major focus on the measurement of pollutants in the various environments. However, a more precise definition would be the study of chemistry in natural systems and how this chemistry changes when perturbed by anthropogenic activities and/or the release of chemicals into the environment which changes their natural background levels. 

Components of Environment: 

 There are four components of Environment  

1. Atmosphere  

2. Hydrosphere  

3. Lithosphere  

4. Biosphere 

Atmosphere: An invisible ocean of gases that protects our planet from harmful radiations is called the Atmosphere.  % of gases in the Atmosphere  

● Nitrogen 78% 

● Oxygen 20.1%  

●Argon 0.1% with a number of other trace gases making up the remaining 0.2%.  

These other trace gases include water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), the chlorofluorocarbons (CFCs), ozone (O3), and many others.  

Water vapor is the one atmospheric gas that is highly variable. It is also the one that is most mobile, since it can condense to form clouds and precipitate back to the ground in the form of solid or liquid water. Note that the pressure simply drops off with increasing altitude, steadily decreasing until you reach the vacuum of outer space. But the change in temperature with increasing altitude shows a much different profile, and it is this profile that defines the different regions of the atmosphere which are as follows. 

➔ Troposphere  

➔ Stratosphere  

➔ Mesosphere  

➔ Thermosphere 

Troposphere: The lowest layer of the atmosphere, the one closest to the surface of the Earth, is the troposphere. It is the region where we live and where weather and climate occur. The temperature in this layer decreases steadily up to an altitude of about 20 km. So the temperature of the air at sea level is higher than that of the air at higher altitudes. The warmer air rises and mixes with the air above it, while the colder air sinks and mixes into the air below it. This causes the atmosphere in the troposphere to become well mixed. 

The ratio of nitrogen to oxygen remains the same throughout the troposphere, the concentrations of both gases drop as the overall pressure drops according to the ideal gas law. As you climb a mountain, you will notice that the temperature gets cooler and you may have difficulty breathing as you ascend due to the drop in both temperature and pressure with altitude in the troposphere. If you were to climb Mt. Everest, you would likely need special equipment to survive the ascent because the concentration of oxygen decreases sufficiently that humans who have not become acclimated to low oxygen levels require a breathing device. The temperature in the troposphere begins to level off at higher altitudes and finally reaches a region of constant temperature called the tropopause at an altitude of about 10–21 km. Because of this constant temperature, there is very little mixing of the atmosphere in the tropopause. The atmospheric layer above the tropopause is the stratosphere. Stratosphere: The layer just above the troposphere is the Stratosphere. The intensity of UV radiation from the Sun is higher in the stratosphere, which causes the photolysis of oxygen, forming ozone. Both molecular oxygen and ozone in the stratosphere act to absorb most of the incoming UV radiation from the Sun, protecting the troposphere from its harmful effects. This absorption of UV radiation in the stratosphere causes an increase in temperature with altitude. So the stratosphere has warmer air at higher altitudes and colder air at lower altitudes, resulting in much less turbulent mixing than occurs in the troposphere and creating a generally calmer atmospheric region. Because of this, the lower stratosphere is where most aircrafts (Not aeroplane) like to fly, as the air convection is minimal, creating a smoother ride. Above the stratosphere, at an altitude of about 48–52 km, is another region where the temperature of the atmosphere is fairly constant, called the stratopause.  Above the stratopause, there is another region called Mesosphere. 

Mesosphere: In the Mesosphere temperature again decreases with altitude, creating atmospheric mixing similar to that in the troposphere. At an altitude of about 85 km, the temperature again becomes fairly constant at the mesopause. Above the mesopause the temperature increases rapidly at very low pressure, at altitudes from 100 to 700 km. This atmospheric region is called the Thermosphere, also known as the ionosphere. 

Thermosphere: It is made up of very high-energy molecules as well as high-energy radiation and high-energy atomic particles (protons, electrons, and neutrons) from the Sun. The high-energy molecules undergo dissociation to atoms after collision with the high-energy atomic particles. This dissociation is followed by chemiluminescent recombination reactions, which result in the emission of visible light. These chemiluminescent reactions are especially prevalent over the planet’s polar regions, where the charged high-energy atomic particles (electrons and protons) are attracted to the Earth’s magnetic poles. At the top of the thermosphere the temperature is again constant with increasing altitude in the thermopause.  

Exosphere: The thermopause is followed by the exosphere, which lies from 700 to 10000 km. The exosphere contains light atoms and molecules such as helium, hydrogen, carbon dioxide, and atomic oxygen, along with smaller concentrations of heavier species. The pressure in th-e exosphere is extremely low, since this region of the atmosphere is essentially mixing with outer space. Most of our low Earth-orbiting satellites are located at altitudes of 160–2000 km in the thermosphere and the exosphere. 

Biogeochemical cycles: 

Term biogeochemical tells us that biological geological and chemical factors are involved. 

In earth science a biogeochemical cycle is a pathway by which a chemical substance moves through biotic (biosphere) and abiotic (lithosphere, atmosphere, hydrosphere) compartments of each. A cycle is a series of changes which come back to the starting point and can be repeated. 

A cycle is a series of changes which come back to the starting point and can be repeated. 

What sustains the life on earth? 

a. Solar energy. 

b. Recycling of matter. 

c. Gravity of earth. 

An ecosystem survives by a combination of energy flow and matter recycling. Abiotic components includes; 

 

Matter cycling in ecosystem: 

i. Nutrient cycle ii. Global recycling. 

Global recycling: Global cycling called biogeochemical cycles, are cycling of nutrients from abiotic reservoirs to biotic reservoirs. 

Nutrients are elements essential for growth and reproduction of organisms.  

Biogeochemical cycles move all nutrients through air, water, soil, rock, and living organisms over millions of years. Abiotic: Rocks, Water 

Biotic:  Carbon cycle, Nitrogen 

Green House Effect: 

The greenhouse effect is a process that occurs when energy from a planet's sun goes through its atmosphere and warms the planet's surface, but the atmosphere prevents the heat from returning directly to space, resulting in a warmer planet. Light arriving from our Sun passes through Earth's atmosphere and warms its surface. The warmed surface then radiates heat, which is absorbed by greenhouse gases such as carbon dioxide. Without the natural greenhouse effect, Earth's average temperature would be well below freezing. Current human-caused increases in greenhouse gases trap greater amounts of heat, causing the Earth to grow warmer over time.  

Water Pollution: Water pollution is the contamination of water sources by substances which make the water unusable for drinking, cooking, cleaning, swimming, and other 

activities. Pollutants include chemicals, trash, bacteria, and parasites. All forms of pollution eventually make their way to water. 

Biogeochemical Cycle:  

The term biogeochemical cycle tells us that ecological, geological and chemical factors involved. 

In each surface, a biogeochemical cycle is a pathway by which a chemical substance moves through biotic and abiotic compartment of earth. 

Cycle: A cycle is series of change which come to the starting point and can be repeated. What sustains the life on the earth? 

Solar energy. 

Recycling of matter. 

Gravity of earth. 

An ecosystem is survived by a combination of energy flow and matter recycling. 

Cycle for abiotic components: 

 

 

Matter cycling in ecosystem: 

Nutrient cycle: Global cycle called as biogeochemical cycle or cycling o nutrients from abiotic reservoirs to biotic reservoirs.  

Nutrients: Nutrients are the elements that are essential for the growth and reproduction of organism. 

Bio-geological cycle: Move all the nutrients through air, water, soil, rock and living organisms over millions of years. 

 

Water cycle: 

 There are strong forces of attraction between molecules of water. 

 Water is a liquid over wide range of temperature. 

 It takes a large amount energy for water to evaporate. 

 Liquid water dissolves a variety of compounds. 

 Water expands on freezing. 

 

The water cycle is the continuous movement of water on, above, and below the surface of the Earth. Outline of water cycle: 

Evaporation→ Surface water→ Water vapors→ Water absorbs heat energy from sun and turns into vapors from water bodies like oceans, seas, lakes, and rivers. 

These are the main sources of evaporation. 

Through the process of evaporation water comes from hydrosphere to sphere. As water evaporates, it reduces the temperature of bodies. 

 

Evaporation is the process by which water changes from a liquid to a gas.  The rate of evaporation depends on how dry the air is and the temperature difference between air and water. 

Transpiration is the process by which water evaporates from plants. 

Evaporation and transpiration are often combined into a single term called evapotranspiration. 

Condensation is the process by which water vapor in the air is changed into liquid water.  As air rises it cools. Cold air holds less water than warm air.  Thus, as the air cools, clouds form. 

Some clouds form during the day as the air temperature and evaporation increase.  Other clouds are associated with weather systems, such as a warm or cold front, in which warm air is forced over colder air. 

 

Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It happens when clouds can no longer hold water.  The water then falls to the earth because of gravity. 

A thunderstorm in the western United States. Thunderstorms form when air is forced upward at a very fast rate and therefore cools very quickly. 

 

Water infiltrates much faster and deeper into a sand compared to a clay.  This is because the pore spaces in the sand are much larger than in the clay and can therefore transport more water. 

Infiltration is the process by which water soaks into the ground.  Infiltration is faster on dry soils and in sandy soils compared to wet and clay soils. 

Surface runoff is precipitation or melted snow that runs off over the landscape. Runoff occurs when the rate of precipitation or snowmelt is greater than the rate of infiltration. 

 

Runoff water flows to sewers in some urban areas (then usually to surface water), to retention basins in other urban areas (then usually soaks into the ground), and to rivers, lakes, and wetlands in nonurban areas. 

 

Water that infiltrates the soil can either be taken up by plants, evaporate, be stored in the soil, or become ground water. 

 

from precipitation. 

Water in glaciers and icecaps is stored for a long time.  Some evaporates and glaciers slowly flow toward oceans and lakes. 97% of the earth’s water is in the oceans.  Of the remainder, 69% is in glaciers and icecaps, 30% in ground water, and 1% in lakes, rivers, and wetlands. Very little water is stored in the atmosphere.