CO₂ TO 90 KM
<1% DESERT/POLE
HIGH IN SUBTROPICS
Composition of Atmosphere
Permanent & Variable Gases (Table 8.1) · Water Vapour Blanket · Dust & Hygroscopic Nuclei · Constituent Roles (28 Micro-Realms)
The Three-Component Life-Support Envelope of Planet Earth (NCERT Chapter 8)
The atmosphere is composed of three fundamental constituents: Gases, Water Vapour, and Dust Particles. While 99% of the total mass of the atmosphere is confined to the height of 32 km from the Earth's surface, the proportion of gases changes in higher layers so that Oxygen is almost negligible at 120 km and $\text{CO}_2$ and Water Vapour are found only up to 90 km. Together, $\text{CO}_2$ (transparent to incoming solar radiation but opaque to outgoing terrestrial radiation), stratospheric Ozone ($\text{O}_3$ at 10–50 km), thermal-blanket Water Vapour (4% in warm-wet tropics to <1% in deserts/poles), and Hygroscopic Dust Nuclei regulate Earth's climate and hydrological cycle.
I. Permanent & Variable Gases, Altitude Cutoffs & NCERT Table 8.1
Three Core Constituents
- 1) Gases (Permanent & Variable)
- 2) Water Vapour (0% to 4% by volume)
- 3) Dust Particles / Aerosols (NCERT Ch 8)
99% Mass Within 32 km
- Gravity pulls air molecules toward surface
- 99% of total atmospheric mass is confined
- Within 32 km of Earth's surface (NCERT)
120 km vs. 90 km Cutoffs
- Exact NCERT Prelims altitude numbers!
- Oxygen is almost negligible at 120 km
- $\text{CO}_2$ & Water Vapour found only up to 90 km
Nitrogen & Oxygen (99%)
- Nitrogen ($\text{N}_2$): 78.08% by dry volume
- Oxygen ($\text{O}_2$): 20.95% by dry volume
- Permanent gases with constant ratio
Table 8.1 Gas Order Trap
- Argon ($0.93\%$) is 3rd most abundant (NOT $\text{CO}_2$!)
- $\text{CO}_2$ ($0.036\%\text{ / }0.04\%$) is 4th
- Followed by Neon, Helium, Krypton, Xenon, $\text{H}_2$
Homosphere vs. Heterosphere
<80 KM MIXED
>80 KM LAYERED
- Homosphere (0–80 km): Turbulent mixing keeps $\text{N}_2/\text{O}_2$ ratio uniform
- Heterosphere (>80 km): Layered by molecular weight ($\text{N}_2 \to \text{O} \to \text{He} \to \text{H}$)
3-Stage Atmospheric Origin
- 1) Solar wind stripped primordial $\text{H}_2\text{/He}$
- 2) Volcanic degassing released $\text{H}_2\text{O, CO}_2, \text{N}_2$
- 3) Cyanobacteria photosynthesis added $\text{O}_2$
II. Carbon Dioxide, Stratospheric Ozone & Water Vapour Mechanics (NCERT Core)
$\text{CO}_2$ Radiation Asymmetry
(TRANSPARENT)↑✘ LW TRAPPED
(OPAQUE)
- Meteorologically a very important gas
- TRANSPARENT to incoming solar radiation
- OPAQUE to outgoing terrestrial radiation!
Greenhouse Effect & Rise
- Absorbs part of terrestrial longwave heat
- Reflects some back towards Earth's surface
- Burning fossil fuels raises $\text{CO}_2$ volume (NCERT)
Ozone ($\text{O}_3$) at 10–50 km
- Found between 10 and 50 km above surface
- Acts as a filter & absorbs ultra-violet rays
- Prevents intense harmful UV reaching ground
Stratospheric vs. Surface $\text{O}_3$
UV SHIELD
TOXIC SMOG
- Peak concentration at 20–25 km (Dobson Units)
- Depleted by CFC chlorine radicals
- Ground-level $\text{O}_3$ is a secondary pollutant
Water Vapour Altitude Drop
- Variable gas in the atmosphere
- Decreases rapidly with altitude (NCERT)
- Over 90% is concentrated in lowest 5 km
4% Tropics vs. <1% Poles
4% OF AIR VOL
<1% OF AIR VOL
- Warm & wet tropics: 4% of air by volume
- Dry & cold areas (deserts/polar): <1%
- Decreases from Equator towards Poles (NCERT)!
Water Vapour as a Blanket
- Absorbs parts of insolation & radiated heat
- Acts like a blanket keeping Earth moderate
- Contributes to stability & instability of air!
III. Dust Particles, Subtropical Concentration & Hygroscopic Nuclei (NCERT)
6 Sources of Dust Particles
- Sea salts, fine soil, smoke-soot, ash,
- Pollen, dust & disintegrated particles
- Of meteors (NCERT verbatim list)
Convectional Dust Transport
- Generally concentrated in lower layers
- Yet convectional air currents may
- Transport them to great heights (NCERT)
Subtropical Dust Peak (NCERT)
- #1 UPSC Prelims trap from NCERT Ch 8!
- Higher concentration in subtropical & temperate
- Due to dry winds (compared to equatorial/polar)
Hygroscopic Cloud Nuclei
- Dust & salt particles act as
- Hygroscopic nuclei around which
- Water vapour condenses to produce clouds!
Rayleigh & Mie Scattering
- Selective Rayleigh scattering ($\propto \lambda^{-4}$)
- Gas & fine dust scatter short blue wavelengths
- Produces blue skies & crimson sunrises/sunsets
Sulfate vs. Black Carbon
COOLS (ALBEDO)
WARMS & MELTS
- Volcanic $\text{SO}_2$ sulfate aerosols reflect sun (cooling)
- Black Carbon (soot) absorbs sunlight (warming)
- Darkens Himalayan glacier albedo!
PM2.5, Haze & Winter Smog
INVERSION HAZE
- Anthropogenic aerosols + crop/vehicular soot
- Trapped under winter radiation inversions
- Forms persistent Indo-Gangetic smog & fog
IV. Ecological, Biospheric & Climatic Importance of Constituents
Importance of Nitrogen
- Prevents spontaneous runaway fires by diluting $\text{O}_2$
- Fixed into nitrates ($\text{NO}_3^-$) by lightning & bacteria
- Essential building block of amino acids & DNA
Importance of Oxygen
- Essential for aerobic cellular respiration
- Required for combustion of fuels
- Drives chemical oxidation weathering of rocks
Importance of Carbon Dioxide
- Raw material for green plant photosynthesis
- Keeps Earth's mean temp at $+15^\circ\text{C}$ (vs $-18^\circ\text{C}$)
- Dissolves in rain to drive karst carbonation
Trace Greenhouse Gases
- Water Vapour is the #1 natural GHG by volume
- Methane ($\text{CH}_4$ — wetlands, paddies, livestock)
- Nitrous Oxide ($\text{N}_2\text{O}$) & fluorinated gases
Meteor Shield & Sound Medium
- Mesospheric air friction vaporizes meteors
- Provides elastic medium for sound waves
- And aerodynamic lift for birds & aircraft
Artificial Cloud Seeding
- Mimics natural hygroscopic dust/salt nuclei
- Silver Iodide ($\text{AgI}$), Potassium Iodide, Dry Ice
- Induces precipitation in supercooled clouds
Master Composition Matrix
- Completes Atmospheric Composition (NCERT Ch 8)
- Links Constituent Physics to Vertical Layers
- Transitions to Realm 63 (Structure of Atmosphere)
Core Distinctions
• Gas Abundance Order (Table 8.1): Nitrogen ($78.08\%$) > Oxygen ($20.95\%$) > Argon ($0.93\%$) > Carbon Dioxide ($0.036\%$) > Neon > Helium > Krypton > Xenon > Hydrogen. (Argon is 3rd, NOT $\text{CO}_2$!).
• Altitude Cutoffs: Oxygen is almost negligible at 120 km, whereas Carbon Dioxide and Water Vapour are found only up to 90 km from the surface.
• Water Vapour: Highest in the warm and wet tropics ($4\%$ of air by volume) and lowest in the dry deserts and cold polar regions ($<1\%$); decreases steadily from the equator towards the poles.
• Dust Particles: Higher concentration is found in subtropical and temperate regions due to dry winds in comparison to equatorial and polar regions (where heavy rain washes out dust at the equator, and snow/ice covers the poles!).
Carbon Dioxide ($\text{CO}_2$) is transparent to incoming shortwave solar radiation but opaque to outgoing longwave terrestrial radiation (causing the Greenhouse Effect). Ozone ($\text{O}_3$ at 10–50 km) acts as a shield that absorbs incoming Ultra-Violet (UV) rays from the sun.
Atmospheric Composition Matrix
NCERT Chapter 8 Master Reference