Earth’s Atmosphere: Protective Functions, Circulation, and Pollutants
Functions of the Atmosphere
- Protective Shield: The ozone layer in the stratosphere prevents most harmful ultraviolet (UV) radiation from reaching Earth’s surface. The atmosphere also protects against the impact of meteorites and other celestial bodies, which disintegrate upon entering its layers.
- Involvement in the Water Cycle: The atmosphere plays a crucial role in the water cycle. Water vapor condenses to form clouds, which then precipitate as rain or snow.
- Source of Life-Sustaining Gases: It contains essential gases required for life. Carbon dioxide (CO2) and oxygen (O2) are vital for processes like photosynthesis and respiration.
- Temperature Regulation: The atmosphere acts as a temperature regulator, maintaining an average temperature suitable for life. This is possible thanks to the greenhouse effect, which traps some of the solar heat that reaches Earth’s surface, preventing it from escaping back into space.
Global Atmospheric Circulation
Solar radiation heats the Earth’s surface unevenly, making equatorial regions warmer than polar regions. This temperature difference drives air circulation: warm, less dense equatorial air rises, while cold, denser polar air descends and flows towards the equator to replace the rising warm air. This would theoretically form two large convection cells, one in each hemisphere.
However, this simple model is not accurate for a rotating body like Earth. A physical phenomenon known as the Coriolis effect affects the circulation of winds and water. This effect is caused by the Earth’s counter-clockwise rotation. Since the rotational speed is slower at the poles than at the equator, moving air masses are deflected from their straight path, curving westward relative to the ground beneath them.
Atmospheric Moisture
Another factor that influences the movement of air masses is the amount of water vapor they contain. Two key measures are:
- Absolute Humidity: The amount of water vapor present in a given volume of air, expressed in grams per cubic meter (g/m³). There is a direct relationship between temperature and the amount of water vapor the air can hold.
- Relative Humidity: The amount of water vapor present in the air in relation to the maximum amount it could possibly hold at that temperature. It is expressed as a percentage: RH = (Total amount of water vapor / Maximum amount of water vapor) X 100.
When the air becomes saturated with moisture, it reaches its dew point, and the water vapor begins to condense. A relative humidity of 100% corresponds to the dew point. When the dew point is reached on colder surfaces, dew forms, or frost if the temperature is below 0°C.
Air Pollution
Air pollution is defined as a condition of the atmosphere where certain substances and/or energy reach levels above their normal environmental concentration, posing risks, damage, or disruption to people, ecosystems, and property.
Main Atmospheric Chemical Contaminants
Carbon Monoxide (CO)
- Source: Most natural CO is secondary, derived from the oxidation of methane (CH4). Other sources include oceans and forest fires. The primary anthropogenic source is incomplete combustion in vehicle engines. The rate of CO emitted by human activity is so high that it can exceed local natural elimination mechanisms.
- Effects: It is highly toxic to humans and higher animals and can cause death. It easily penetrates the pulmonary alveoli and combines with hemoglobin in the blood, preventing oxygen transport. This causes death by suffocation, even at low concentrations, and can lead to tragic accidents in confined spaces with poor combustion.
- Correction Methods: Exhaust systems can be fitted with thermal or catalytic reactors to complete the combustion process. In these reactors, CO is converted to CO2, and hydrocarbons are converted into CO2 and H2O.
Nitrogen Oxides (NOx)
While several different nitrogen oxides are known, only three are typically present in the atmosphere:
- Nitrous oxide (N2O): Emitted naturally and is not considered toxic, so it is not typically classified as an air pollutant.
- Nitrogen monoxide (NO): A poisonous, colorless, and odorless gas that comes mainly from natural sources and, to a lesser extent, anthropogenic ones.
- Nitrogen dioxide (NO2): A toxic, reddish-brown gas with a strong, suffocating odor, which originates almost entirely from anthropogenic sources.
Effects: The danger of NOx pollution to plants and animals is more potential than real at current atmospheric levels. For humans, NO2 presents the greatest toxicity, as it can affect the respiratory system and even be lethal. Regarding materials, it affects textile dyes and promotes corrosion in metallic structures.
Sulfur Dioxide (SO2)
Most of the sulfur oxides emitted into the atmosphere correspond to sulfur dioxide (SO2).
Effects: In the atmosphere, SO2 can photo-oxidize to form sulfuric acid (H2SO4), creating an acid mist known as sulfurous smog, which can be very harmful. Additionally, when H2SO4 dissolves in water droplets, it returns to the surface as precipitation, causing acid rain, which also produces considerable damage. SO2 causes lesions on the leaves of plants, which affects photosynthetic activity. Its effects on humans are primarily related to the respiratory system, although higher concentrations are required to cause damage than in plants. Sulfur dioxide can also affect mucous membranes, such as the eyes. This pollutant contributes to the cracking of paint and the deterioration of leather and paper. Lichens are very sensitive to SO2, even at very low concentrations, and are considered bioindicators of this type of pollution.