# Environmental Chemistry/Introduction

Environmental chemistry is a branch of chemistry. It is the study of chemical processes occurring in the environment which are impacted by humankind’s activities.

## Environment

The word environment means “surrounding”. Environment means the surroundings of living beings from all sides and that affect their lives.  Environment can be defined as one’s own surroundings including all of the living (biotic) and non-living (abiotic) factors that act on organism, population, or ecological community and influence its survival and development

### Abiotic factors

* Non-living components that influence population size and the environment
* Physical conditions and non-living resources that affect living organisms in terms of growth and reproduction
* E.g. temperature, light intensity, carbon dioxide levels, pH of water and soil
Abiotic factors' effects on population:
* Seasonal temperature variations affect when plants flower, when animals breed, when seeds germinate and when animals hibernate.
* Many types of plants grow better when they are fully exposed to sunlight
* For instance, the pH of the soil can have an effect on the types of plants which can grow in it.

### Biotic factors

* A living component that affects the population of another organism, or the environment
* E.g. a prey is a biotic factor since it’s a living thing that could affect the population of another organism (more prey = increase in predator population and vice versa)
Biotic factors' effects on population:
* Pathogens* are likely to cause a decrease in population
* Humans building cities and factories and disposing of waste into the water is affecting the environment.

### Segments of the Environment

1. Lithosphere: the earth crust* consisting of the soil and rocks. The soil is made up of organic, inorganic matter and water. They are responsible for the fertility of the soil and hence its productivity.
2. Hydrosphere: is the water in all states (solid, liquid and gas). This comprises of all the surface and ground water such as seas, oceans, lakes, streams, glaciers, polar ice caps, and the water locked in minerals below earth crust.
3. Biosphere: parts of Earth where life exists. Life does not exist outside this zone.
4. Atmosphere: layer of gases surrounding the planet. Oxygen is used by most organisms for respiration; carbon dioxide is used by plants for photosynthesis.

## Structure of the Atmosphere

The atmosphere is broadly divided into four major zones
* Troposphere
* Stratosphere
* Mesosphere
* Thermosphere

### Lapse rate

* Change of temperature with height is called lapse rate.
* Decrease of temperature with height is called positive lapse rate (troposphere)
* Transition from positive to negative lapse rate at the tropopause marks what is called the temperature inversion.

### Troposphere

* It is the region nearest to the ground
* Contains 70 % of atmospheric mass
* Temperature decreases with height
* 15 C at sea level to -56.5 C at 11,000 m
* Contains 99% of the water vapor in the atmosphere
* Tropopause is the layer between troposphere and stratosphere

### Stratosphere

* The region above troposphere
* Warmer layers higher and cooler layers closer to the Earth
* The increase in temperature is due to ozone absorbing UV rays from the sun
* Ozone is present in the stratosphere

### Mesosphere

* Region above stratosphere
* Temperature decreases with height
* Menopause is the layer between Mesosphere and Thermosphere, at which is the coldest temperature (about -100 C)

### Thermosphere

* It is the region above the Mesosphere
* Temperature increases with increasing height
* Atmospheric gases (oxygen and nitrogen) in this region absorb solar radiation and undergo ionization (lose electrons).
* O2+, O+, N+
* During night times, when UV radiation is not present, these species again combines with electrons to make neutral species.
* The region from 50-100 km is called ionosphere due to presence of positive ions and electrons

## Biogeochemical Cycles
### Water cycle

* Evaporation: The Sun heats up the water from oceans, lakes and rivers and water changes into water vapor
* Condensation: As the water vapor rises up into the air, it starts cooling down and forms tiny water droplets. These water droplets come together to form clouds. This process is called condensation.
* Precipitation: When the clouds start getting heavy and cannot hold the water droplets anymore, they fall back to the earth in the form of rain, hail or snow. This process is called precipitation.
Some of the water that falls on the earth seeps into the ground. This water is available to us in the form of groundwater. The remaining water falls back into oceans, lakes, rivers and seas. This process is called collection. Then, the sun starts heating up this water once again. This circulation of water is called water cycle.
* Transpiration: Plants also lose water in the form of water vapor from their leaves into the air by the process of transpiration

### Nitrogen cycle

The nitrogen cycle describes how nitrogen moves between plants, animals, bacteria, the atmosphere (the air), and soil in the ground. In order for nitrogen to be used by different life forms, it must change into different states. Nitrogen in the atmosphere, or air, is N2. Other important states of nitrogen include Nitrates (N03), Nitrites (NO2), and Ammonium (NH4).
* Fixation: is a process by which nitrogen (N2) in the Earths atmosphere is converted into ammonia (NH4+) by bacteria
* Nitrification: is a process by which ammonium gets changed into nitrates (NO3−) by bacteria. Nitrate, a form of nitrogen that we can use (the plants absorb them and moves through the community by the food chain). The nitrogen is in the form of proteins, amino acids etc in the plants which will than be reassembled in other animals when the plants are eaten
* Ammonification: When a plant or animal dies, decomposers like fungi and bacteria turn the nitrogen back into ammonium so it can reenter the nitrogen cycle.
* Denitrification: Extra nitrogen in the soil gets put back out into the air. There are special bacteria that perform this task as well.