Acid Rain and Ozone Layer Depletion — Revision Notes

⚡ 30-Second Revision

Acid Rain pH — $< 5.6$ (Normal rain $approx 5.6$ due to $ext{CO}_2$)
Acid Rain Precursors — $ext{SO}_2$ (from S-containing fossil fuels), $ext{NO}_x$ (from high-temp combustion)
Acid Rain Reactions

- $ext{SO}_2 xrightarrow{\text{Oxidation}} \text{SO}_3 xrightarrow{\text{H}_2\text{O}} \text{H}_2\text{SO}_4$ - $ext{NO}_x xrightarrow{\text{Oxidation}} \text{NO}_2 xrightarrow{\text{OH} cdot, \text{H}_2\text{O}} \text{HNO}_3$ - Marble erosion: $ext{CaCO}_3(\text{s}) + \text{H}_2\text{SO}_4(\text{aq}) \to \text{CaSO}_4(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g})$

Ozone Layer — Stratosphere (10-50 km), absorbs UV-B/C.
Ozone Formation (Chapman) — $ext{O}_2 + \text{UV} \to 2\text{O}$; $ext{O} + \text{O}_2 + \text{M} \to \text{O}_3 + \text{M}$
Ozone Depleting Substances (ODS) — CFCs, Halons, $ext{CCl}_4$, $ext{CH}_3\text{Br}$ (contain $ext{Cl}$ or $ext{Br}$)
Ozone Depletion Catalytic Cycle

- $ext{CF}_2\text{Cl}_2 + \text{UV} \to \text{CF}_2\text{Cl} cdot + \text{Cl} cdot$ - $ext{Cl} cdot + \text{O}_3 \to \text{ClO} cdot + \text{O}_2$ - $ext{ClO} cdot + \text{O} \to \text{Cl} cdot + \text{O}_2$

Ozone Hole — Over Antarctica, facilitated by Polar Stratospheric Clouds (PSCs).
Effects of UV-B — Skin cancer, cataracts, immune suppression, ecosystem damage.
Solution — Montreal Protocol (phase-out ODS).

2-Minute Revision

Acid rain, characterized by a pH below 5.6, is primarily caused by the atmospheric conversion of sulfur dioxide ($ext{SO}_2$) and nitrogen oxides ($ext{NO}_x$) into sulfuric acid ($ext{H}_2\text{SO}_4$) and nitric acid ($ext{HNO}_3$).

These precursor gases originate mainly from the combustion of fossil fuels. The effects are widespread, including the corrosion of historical monuments (like marble, $ext{CaCO}_3$, reacting with $ext{H}_2\text{SO}_4$ to form soluble $ext{CaSO}_4$), acidification of lakes harming aquatic life, and damage to forests by leaching essential soil nutrients.

Ozone layer depletion concerns the thinning of the stratospheric ozone layer (10-50 km), which is crucial for absorbing harmful UV-B and UV-C radiation. Ozone ($ext{O}_3$) is naturally formed and destroyed via the Chapman cycle.

However, human-made Ozone Depleting Substances (ODS) like Chlorofluorocarbons (CFCs) release highly reactive chlorine ($ext{Cl} cdot$) and bromine ($ext{Br} cdot$) radicals in the stratosphere. These radicals catalytically destroy ozone molecules in a chain reaction, where one radical can destroy thousands of $ext{O}_3$ molecules.

The 'ozone hole' over Antarctica is a severe seasonal depletion, intensified by Polar Stratospheric Clouds (PSCs) that activate chlorine. Increased UV-B exposure due to depletion leads to higher risks of skin cancer, cataracts, and immune system suppression in humans, alongside ecosystem damage.

The Montreal Protocol is a successful international effort to phase out ODS.

5-Minute Revision

Let's consolidate the critical aspects of Acid Rain and Ozone Layer Depletion for NEET. Acid rain is any precipitation with a pH below 5.6, distinguishing it from normal rain (pH $approx$ 5.6) which is only slightly acidic due to dissolved $ext{CO}_2$.

The main culprits for acid rain are sulfur dioxide ($ext{SO}_2$) from burning sulfur-rich fossil fuels and nitrogen oxides ($ext{NO}_x$) from high-temperature combustion in vehicles and power plants.

In the atmosphere, $ext{SO}_2$ oxidizes to $ext{SO}_3$ and then reacts with water to form sulfuric acid ($ext{H}_2\text{SO}_4$). Similarly, $ext{NO}_x$ reacts to form nitric acid ($ext{HNO}_3$). These strong acids then fall as wet or dry deposition.

The consequences are severe: aquatic ecosystems suffer from lowered pH and increased toxic metal leaching (e.g., aluminum), forests are damaged by nutrient leaching from soil and direct leaf damage, and structures made of marble ($ext{CaCO}_3$) corrode due to reactions like $ext{CaCO}_3 + \text{H}_2\text{SO}_4 \to \text{CaSO}_4 + \text{H}_2\text{O} + \text{CO}_2$, where soluble $ext{CaSO}_4$ washes away.

Moving to ozone layer depletion, the ozone layer is a vital shield in the stratosphere (10-50 km), absorbing harmful UV-B and UV-C radiation. Ozone ($ext{O}_3$) is naturally formed by $ext{O}_2 + \text{UV} \to 2\text{O}$ followed by $ext{O} + \text{O}_2 + \text{M} \to \text{O}_3 + \text{M}$, and naturally destroyed by $ext{O}_3 + \text{UV} \to \text{O}_2 + \text{O}$ or $ext{O} + \text{O}_3 \to 2\text{O}_2$.

This is the Chapman cycle. However, human-made Ozone Depleting Substances (ODS) like Chlorofluorocarbons (CFCs), halons, and methyl bromide, which contain chlorine or bromine, disrupt this balance. These stable compounds reach the stratosphere, where UV radiation breaks them down, releasing reactive radicals (e.

g., $ext{Cl} cdot$). A single chlorine radical can catalytically destroy thousands of ozone molecules through a two-step cycle: $ext{Cl} cdot + \text{O}_3 \to \text{ClO} cdot + \text{O}_2$, followed by $ext{ClO} cdot + \text{O} \to \text{Cl} cdot + \text{O}_2$.

The 'ozone hole' is a dramatic seasonal thinning over Antarctica, exacerbated by extremely cold temperatures and Polar Stratospheric Clouds (PSCs) that activate chlorine reservoir species. The increased UV-B radiation reaching Earth leads to higher rates of skin cancer, cataracts, and immune system suppression in humans, and also damages crops and marine ecosystems.

The Montreal Protocol is a globally successful treaty aimed at phasing out ODS to allow the ozone layer to recover.

Worked Mini-Example (Acid Rain): If a lake's pH drops from 6.5 to 4.5 due to acid rain, what is the change in $ext{H}^+$ ion concentration? A change of 2 pH units means the $ext{H}^+$ concentration has increased by $10^2 = 100$ times. So, a small pH change represents a significant change in acidity.

Worked Mini-Example (Ozone Depletion): A CFC molecule like $ext{CF}_2\text{Cl}_2$ is released. It travels to the stratosphere and is photolyzed: $ext{CF}_2\text{Cl}_2 + \text{UV} \to \text{CF}_2\text{Cl} cdot + \text{Cl} cdot$. The released $ext{Cl} cdot$ then starts the catalytic cycle, destroying ozone. This illustrates how stable compounds become dangerous in the upper atmosphere.

Prelims Revision Notes

Acid Rain

Definition — Precipitation with pH $< 5.6$. Normal rain pH $approx 5.6$ (due to $ext{CO}_2 \to \text{H}_2\text{CO}_3$).
Primary Precursors

* **Sulfur Dioxide ($ext{SO}_2$)**: From combustion of S-containing fossil fuels (coal, oil) in power plants, industries. * **Nitrogen Oxides ($ext{NO}_x$)**: From high-temperature combustion in vehicles, power plants (reaction of $ext{N}_2$ and $ext{O}_2$).

Chemical Reactions

* $ext{SO}_2 xrightarrow{\text{Oxidation}} \text{SO}_3 xrightarrow{\text{H}_2\text{O}} \text{H}_2\text{SO}_4$ (Sulfuric Acid) * $ext{NO}_x xrightarrow{\text{Oxidation}} \text{NO}_2 xrightarrow{\text{OH} cdot, \text{H}_2\text{O}} \text{HNO}_3$ (Nitric Acid)

Effects

* Aquatic Life: Lowers pH of lakes/rivers, leaches $ext{Al}^{3+}$ from soil into water, toxic to fish, amphibians. * Forests: Damages leaves, leaches essential nutrients ($ext{Ca}^{2+}$, $ext{Mg}^{2+}$) from soil, mobilizes toxic metals ($ext{Al}^{3+}$), weakens trees.

* Buildings/Monuments: Corrodes metals, dissolves marble ($ext{CaCO}_3$). Reaction: $ext{CaCO}_3(\text{s}) + \text{H}_2\text{SO}_4(\text{aq}) \to \text{CaSO}_4(\text{aq}) + \text{H}_2\text{O}(\text{l}) + \text{CO}_2(\text{g})$.

$ext{CaSO}_4$ is soluble and washes away (stone leprosy).

Ozone Layer Depletion

Ozone Layer — Located in the stratosphere (10-50 km altitude). Composed of $ext{O}_3$ molecules.
Function — Absorbs harmful UV-B (280-315 nm) and UV-C (100-280 nm) radiation.
Chapman Cycle (Natural)

* Formation: $ext{O}_2 + \text{UV} (lambda < 240,\text{nm}) \to 2\text{O}$; $ext{O} + \text{O}_2 + \text{M} \to \text{O}_3 + \text{M}$ * Destruction: $ext{O}_3 + \text{UV} (lambda < 320,\text{nm}) \to \text{O}_2 + \text{O}$; $ext{O} + \text{O}_3 \to 2\text{O}_2$

Ozone Depleting Substances (ODS)

* Chlorofluorocarbons (CFCs): (e.g., $ext{CF}_2\text{Cl}_2$, $ext{CFCl}_3$) Refrigerants, propellants. Very stable. * Halons: Fire extinguishers (contain Bromine, more potent). * **Carbon Tetrachloride ($ext{CCl}_4$), Methyl Chloroform ($ext{CH}_3\text{CCl}_3$), Methyl Bromide ($ext{CH}_3\text{Br}$)**.

Mechanism of Depletion

* ODS release $ext{Cl} cdot$ or $ext{Br} cdot$ radicals in stratosphere via UV photolysis (e.g., $ext{CF}_2\text{Cl}_2 + \text{UV} \to \text{CF}_2\text{Cl} cdot + \text{Cl} cdot$). * Catalytic Cycle: 1. $ext{Cl} cdot + \text{O}_3 \to \text{ClO} cdot + \text{O}_2$ 2. $ext{ClO} cdot + \text{O} \to \text{Cl} cdot + \text{O}_2$ Net: $ext{O}_3 + \text{O} \to 2\text{O}_2$. $ext{Cl} cdot$ is regenerated, destroying many $ext{O}_3$.

Ozone Hole — Severe seasonal depletion over Antarctica.

* Role of Polar Stratospheric Clouds (PSCs): Provide surfaces for heterogeneous reactions, converting inactive chlorine reservoirs ($ext{HCl}$, $ext{ClONO}_2$) into reactive forms ($ext{Cl}_2$, $ext{HOCl}$), which are then photolyzed by sunlight in spring.

Effects of Increased UV-B

* Human Health: Skin cancer (melanoma, non-melanoma), cataracts, immune system suppression. * Ecosystems: Damage to phytoplankton, reduced crop yields.

Mitigation — Montreal Protocol (1987): International treaty to phase out ODS. HFCs are ODS alternatives (no $ext{Cl}$/$ext{Br}$), but are greenhouse gases.

Vyyuha Quick Recall

For Acid Rain Pollutants: Strong Noxious Acids. S for Sulfur oxides ($ext{SO}_x$), N for Nitrogen oxides ($ext{NO}_x$).

For Ozone Depleting Substances (ODS): Can Harm Cool Molecules. C for CFCs, H for Halons, C for Carbon tetrachloride, M for Methyl bromide.