Comprehensive Study Notes on Environmental Chemistry: Atmospheric Composition and Stratification

General Overview and Characteristics of the Atmosphere

  • Definition of Atmosphere: The invisible gaseous envelope extending from the Earth's surface up to an altitude of 500 km500\,\text{km} is defined as the atmosphere.
  • Mass Distribution:
    • Within the first 11 km11\,\text{km} from the surface, 75%75\% of the total atmospheric mass/air is concentrated.
    • Up to an altitude of 30 km30\,\text{km}, 99%99\% of the gaseous mixture of the atmosphere is present.
  • Vertical Extent: The atmosphere spans from the Earth's surface up to an altitude of 500 km500\,\text{km}.
  • Total Atmospheric Mass: The total mass of the gaseous substances forming the atmosphere is approximately 5.0×1015 ton5.0 \times 10^{15}\,\text{ton} or 5.0×1018 kg5.0 \times 10^{18}\,\text{kg}.
  • Atmospheric Pressure Variation with Altitude:
    • At sea level (45∘45^\circ latitude), standard atmospheric pressure is 1 atm1\,\text{atm} or 760 mm Hg760\,\text{mm Hg}.
    • At an altitude of 15 km15\,\text{km}, atmospheric pressure decreases to approximately 100 mm Hg100\,\text{mm Hg}.
    • At an altitude of 100 km100\,\text{km}, atmospheric pressure drops significantly to 3.0×10−7 atm3.0 \times 10^{-7}\,\text{atm}.
    • Atmospheric pressure per unit area corresponds to 1033 kg atm−11033\,\text{kg atm}^{-1} (or 1033 g/cm21033\,\text{g/cm}^2).

Chemical Composition and Partial Pressures

  • Percentage Composition of Atmospheric Gases:
    • Nitrogen (N2N_2): 78.09%78.09\% by volume.
    • Oxygen (O2O_2): 20.99%20.99\% by volume.
    • Argon (ArAr): 0.93%0.93\% by volume. Argon is the only noble gas present in significant quantity in the atmosphere.
    • Carbon Dioxide (CO2CO_2): 0.03%0.03\% by volume.
  • Average Earth Temperature: The present average surface temperature of the Earth is 15∘C15^\circ\text{C}.
  • Partial Pressure Calculation:
    • Formula for partial pressure: Partial Pressure=Total Pressure×Mole Fraction\text{Partial Pressure} = \text{Total Pressure} \times \text{Mole Fraction}
    • Partial pressure of Nitrogen (PN2P_{N_2}) at standard sea-level pressure (1 atm1\,\text{atm}):     PN2=1×78.69100=0.78 atmP_{N_2} = 1 \times \frac{78.69}{100} = 0.78\,\text{atm}

Stratification and Layers of the Atmosphere

  • Primary Layers Overview: The atmosphere is divided into four main thermal regions based on temperature gradient, pressure, and altitude:
    • Troposphere (Turbulent Zone): Extends from Earth's surface to 15 km15\,\text{km} (8 km8\,\text{km} at the poles). Pressure range is 760 mm Hg760\,\text{mm Hg} to 100 mm Hg100\,\text{mm Hg}. Temperature decreases to −55∘C-55^\circ\text{C} at 12 km12\,\text{km}.
    • Stratosphere (Calm Zone): Extends from 15 km15\,\text{km} to 50 km50\,\text{km}. Pressure decreases from 15 mm Hg15\,\text{mm Hg} to 5 mm Hg5\,\text{mm Hg}. Temperature increases to +2∘C+2^\circ\text{C} at 50 km50\,\text{km}.
    • Mesosphere (Coldest Zone): Extends from 50 km50\,\text{km} to 85 km85\,\text{km}. Pressure continues to decrease. Temperature decreases to −93∘C-93^\circ\text{C} at 83 km83\,\text{km}.
    • Thermosphere (Thermal Layer): Extends from 85 km85\,\text{km} to 500 km500\,\text{km}. Pressure drops continuously to extremely low values. Temperature increases to 1727∘C1727^\circ\text{C} (2000 K2000\,\text{K}).

Detailed Profile of Atmospheric Layers

  • Troposphere (ক্ষুব্ধ মণ্ডল):

    • Definition & Boundary: The atmospheric layer extending from the Earth's surface up to 15 km15\,\text{km} altitude (8 km8\,\text{km} at polar regions).
    • Temperature Behavior: Temperature continuously decreases with altitude. The temperature lapse rate is positive (+ve) because temperature drops as height increases.
    • Lapse Rate Value: Temperature decreases at a rate of 7∘C7^\circ\text{C} per 1000 m1000\,\text{m} (1 km1\,\text{km}) increase in altitude.
    • Weather and Climate: Known as the turbulent zone (ক্ষুব্ধ মণ্ডল) because all major weather phenomena—including rainfall, lightning/thunderstorms, dew, fog, and storms—occur in this layer. It acts as the primary controller of Earth's weather and climate system.
    • Mass and Habitation: Contains 75%75\% of atmospheric components within the first 11 km11\,\text{km}. Humans live in the troposphere.
    • Aviation: Passenger aircraft fly within the troposphere.

  • Stratosphere (শান্ত মণ্ডল):

    • Definition & Boundary: The layer extending from 15 km15\,\text{km} to 50 km50\,\text{km} above the Earth's surface.
    • Temperature Behavior: Temperature increases with altitude, giving it a negative (-ve) lapse rate.
    • Calm Nature: Known as the calm zone (শান্ত মণ্ডল) because no natural weather disturbances or storms take place in this region.
    • Aviation & Satellites: Jet aircraft travel through the stratosphere due to the absence of weather disturbances. Artificial satellites and spacecraft operate in or travel through higher atmospheric boundaries above this layer.
    • Ozone Layer (O3O_3): Most atmospheric ozone is concentrated in this layer. The primary constituent of this layer is ozone gas (O3O_3).
    • Mechanism of Temperature Rise: The increase in temperature within the stratosphere is caused by the direct absorption of solar ultraviolet (UV) radiation by ozone molecules.

  • Mesosphere (মেসোস্ফিয়ার):

    • Definition & Boundary: The atmospheric layer extending from 50 km50\,\text{km} to 85 km85\,\text{km}.
    • Temperature Behavior: Temperature decreases as altitude increases, resulting in a positive (+ve) lapse rate.
    • Coldest Layer: Reaches the lowest temperature in the entire atmosphere (−93∘C-93^\circ\text{C} at 83 km83\,\text{km}), making it the coldest atmospheric layer.
    • Meteor Protection: Meteors entering the Earth's atmosphere burn up and are destroyed in the mesosphere due to friction.

  • Thermosphere / Thermal Layer (থার্মোস্ফিয়ার / তাপমण्डल):

    • Definition & Boundary: Extends from 85 km85\,\text{km} up to 500 km500\,\text{km}.
    • Temperature Behavior: Temperature increases dramatically up to 1727∘C1727^\circ\text{C} (2000 K2000\,\text{K}), corresponding to a negative (-ve) lapse rate.
    • Ionic Species: High-energy radiation causes ionization, creating species such as O2+O_2^+, O3+O_3^+, N2+N_2^+, and NO+NO^+.
    • Light Gases: Abundant in lighter gases, specifically Hydrogen (H2H_2) and Helium (HeHe).
    • Kennelly-Heaviside Layer & Ionosphere: The lower region of the thermosphere contains the Kennelly-Heaviside layer (ionosphere region).
    • Radio Wave Reflection: The ionised layer reflects radio waves transmitted from the Earth's surface, allowing long-distance radio communication by bouncing signals back to Earth.
    • Space Exploration: The International Space Station (ISS) orbits within the thermosphere.

Structural Classification: Homosphere vs. Heterosphere

  • Comparison Table:
    • Altitude Range:
    • Homosphere: Earth's surface to approximately 85 km85\,\text{km}.
    • Heterosphere: Above 85 km85\,\text{km} (from 85 km85\,\text{km} to 500 km500\,\text{km}).
    • Gaseous Composition:
    • Homosphere: Uniform composition containing 78.09% N278.09\%\,N_2, 20.94% O220.94\%\,O_2 (or 20.99% O220.99\%\,O_2), and minor trace gases in fixed proportions.
    • Heterosphere: Non-uniform composition organized in layers of atomic, molecular, and ionic species (O2+O_2^+, O3+O_3^+, N2+N_2^+, NO+NO^+, H2H_2, HeHe).
    • Thermal Convection:
    • Homosphere: Thermal convection occurs.
    • Heterosphere: Thermal convection does not occur.
    • Constituent Layers Included:
    • Homosphere: Comprises the Troposphere, Stratosphere, and Mesosphere.
    • Heterosphere: Comprises the Thermosphere and Exosphere.