Tropospheric and Stratospheric Pollution — Revision Notes

⚡ 30-Second Revision

Tropospheric Pollutants: — $SO_x$, $NO_x$, $CO$, Particulate Matter, VOCs, Ground-level $O_3$, PAN.
Tropospheric Effects: — Smog, Acid Rain, Respiratory issues, Plant damage.
Stratospheric Pollutants: — CFCs ($CF_2Cl_2$), Halons.
Stratospheric Effects: — Ozone layer depletion, Increased UV radiation, Skin cancer, Cataracts.
Ozone Depletion Reaction: — $Cl cdot + O_3 \rightarrow ClO cdot + O_2$; $ClO cdot + O \rightarrow Cl cdot + O_2$.
Acid Rain Precursors: — $SO_2 \rightarrow H_2SO_4$; $NO_x \rightarrow HNO_3$.
Good Ozone: — Stratospheric; Bad Ozone: Tropospheric.
Montreal Protocol: — Phased out ODS (CFCs).

2-Minute Revision

Tropospheric pollution affects the lowest atmospheric layer, causing issues like photochemical smog and acid rain. Key primary pollutants include sulfur oxides ($SO_x$), nitrogen oxides ($NO_x$), carbon monoxide ($CO$), and particulate matter, primarily from fossil fuel combustion.

These lead to secondary pollutants like ground-level ozone ($O_3$) and peroxyacetyl nitrate (PAN), which are harmful to human health (respiratory problems, eye irritation) and vegetation. Acid rain, formed from $SO_x$ and $NO_x$, damages ecosystems and infrastructure.

In contrast, stratospheric pollution focuses on the depletion of the ozone layer, a vital shield against harmful UV radiation. Chlorofluorocarbons (CFCs) and halons are the main culprits; they release chlorine and bromine radicals in the stratosphere, which catalytically destroy ozone molecules.

This depletion results in increased UV radiation reaching Earth, leading to higher risks of skin cancer, cataracts, and harm to marine life. Remember, stratospheric ozone is beneficial ('good ozone'), while tropospheric ozone is a harmful pollutant ('bad ozone').

International agreements like the Montreal Protocol aim to control ozone-depleting substances.

5-Minute Revision

Atmospheric pollution is categorized into tropospheric and stratospheric, each with distinct causes and consequences. Tropospheric pollution occurs in the lowest layer (0-12 km). Primary pollutants, directly emitted, include sulfur dioxide ($SO_2$) from burning coal, nitrogen oxides ($NO_x$) from vehicle engines, carbon monoxide ($CO$) from incomplete combustion, and particulate matter (dust, soot).

These can lead to secondary pollutants formed in the atmosphere. For example, ground-level ozone ($O_3$) and peroxyacetyl nitrate (PAN) are formed when $NO_x$ and volatile organic compounds (VOCs) react in sunlight, creating photochemical smog.

This smog causes respiratory issues, eye irritation, and plant damage. Another major issue is acid rain, where $SO_2$ and $NO_x$ react with atmospheric water to form sulfuric acid ($H_2SO_4$) and nitric acid ($HNO_3$), which then fall as acidic precipitation, harming forests, aquatic life, and buildings.

Stratospheric pollution primarily concerns the ozone layer (15-30 km in the stratosphere), which absorbs harmful UV radiation. Naturally, ozone is formed ($O_2 \xrightarrow{UV} O+O$; $O+O_2 \rightarrow O_3$) and destroyed ($O_3 \xrightarrow{UV} O_2+O$; $O_3+O \rightarrow 2O_2$) in a balanced cycle.

However, human-made ozone-depleting substances (ODS) like chlorofluorocarbons (CFCs) and halons disrupt this balance. CFCs, stable in the troposphere, migrate to the stratosphere where UV radiation breaks them down, releasing highly reactive chlorine atoms ($Cl cdot$).

1
$Cl cdot + O_3 \rightarrow ClO cdot + O_2$
2
$ClO cdot + O \rightarrow Cl cdot + O_2$

The regenerated $Cl cdot$ can destroy thousands of ozone molecules. This ozone depletion leads to increased UV radiation reaching Earth, causing skin cancer, cataracts, immune suppression, and damage to ecosystems. It's crucial to distinguish between 'good' stratospheric ozone and 'bad' tropospheric ozone. International efforts like the Montreal Protocol have been vital in phasing out ODS to allow the ozone layer to recover.

Prelims Revision Notes

Tropospheric and Stratospheric Pollution: NEET Revision Notes

I. Atmospheric Layers & Ozone:

Troposphere (0-12 km): — Lowest layer, weather occurs, 'bad ozone' (pollutant).
Stratosphere (12-50 km): — Contains ozone layer, 'good ozone' (protective).
Ozone ($O_3$): — Allotropic form of oxygen.

II. Tropospheric Pollution:

Primary Pollutants (Directly emitted):

* **Sulfur Oxides ($SO_x$):** $SO_2$ from fossil fuel combustion. Causes respiratory issues, acid rain. * **Nitrogen Oxides ($NO_x$):** $NO, NO_2$ from vehicle engines, power plants. Causes respiratory issues, photochemical smog, acid rain.

* **Carbon Monoxide ($CO$):** Incomplete combustion. Highly toxic, binds to hemoglobin (carboxyhemoglobin). * Particulate Matter (PM): Dust, smoke, soot. Respiratory issues, reduced visibility. * Volatile Organic Compounds (VOCs): Hydrocarbons from vehicles, solvents.

Precursors to smog.

Secondary Pollutants (Formed in atmosphere):

* **Ground-level Ozone ($O_3$):** Formed from $NO_x$ + VOCs + sunlight. Component of smog, respiratory irritant, plant damage. * Peroxyacetyl Nitrate (PAN): Component of photochemical smog. Eye irritant, plant damage. * Acid Rain: $H_2SO_4$ (from $SO_2$) and $HNO_3$ (from $NO_x$). Damages ecosystems, buildings.

Photochemical Smog: — $NO_x$ + VOCs + Sunlight $\rightarrow$ $O_3$, PAN, aldehydes. Occurs in warm, sunny climates.

III. Stratospheric Pollution (Ozone Layer Depletion):

Ozone Layer: — 15-30 km, absorbs harmful UV-B/UV-C radiation.
Natural Ozone Cycle (Chapman Cycle):

* Formation: $O_2 \xrightarrow{UV} O+O$; $O+O_2 \rightarrow O_3$. * Destruction: $O_3 \xrightarrow{UV} O_2+O$; $O_3+O \rightarrow 2O_2$.

Ozone Depleting Substances (ODS):

* Chlorofluorocarbons (CFCs): ($CF_2Cl_2$, $CFCl_3$) Refrigerants, aerosols. Stable, reach stratosphere. * Halons: Fire extinguishers. * Other: $CCl_4$, $CH_3CCl_3$.

**Mechanism of Ozone Depletion (by $Cl cdot$ from CFCs):**

1. $CF_2Cl_2 \xrightarrow{UV} Cl cdot + CF_2Cl cdot$ (Release of chlorine radical) 2. $Cl cdot + O_3 \rightarrow ClO cdot + O_2$ 3. $ClO cdot + O \rightarrow Cl cdot + O_2$ (Chlorine radical regenerated, catalytic cycle continues)

Effects of Ozone Depletion:

* Increased UV-B radiation reaching Earth. * Human Health: Skin cancer (melanoma, non-melanoma), cataracts, immune suppression. * Ecosystems: Damage to phytoplankton, reduced crop yields.

Control: — Montreal Protocol (1987) to phase out ODS.

Vyyuha Quick Recall

Sunlight Needs Vehicles Often Produce Smog (for Photochemical Smog components/formation): Sunlight + Nitrogen oxides ($NO_x$) + Volatile Organic Compounds (VOCs) $\rightarrow$ Ozone ($O_3$) + Peroxyacetyl Nitrate (PAN) + Smog