Environmental Chemistry

5 min

Key facts on pollutants, acid rain, smog, and ozone depletion

Golden Rule of Atmospheric Pollution

Acid rain is caused by oxides of sulfur and nitrogen lowering rainwater pH below 5.6. Chlorofluorocarbons deplete stratospheric ozone by releasing active chlorine free radicals. Classical smog occurs in cool humid climates, whereas photochemical smog requires sunlight and nitrogen oxides.

Exam Relevance

Frequent direct questions on acid rain gases and pH cutoff in SSC CGL, CHSL, and Railways.
Smog types, BOD concepts, and Montreal Protocol are high-yield areas for CAPF and State PCS.
Radioactive hazards and heavy metal toxicity appear regularly in defence examinations.

Mnemonics & Shortcuts

PAN & Ozone = Photo Smog (Sunny days); Smoke & SO2 = Classical Smog (Cold winter).
Montreal protects Ozone (1987); Kyoto counters Global Warming Gases (1997).
High BOD means High Organic Pollution and severely Depleted Dissolved Oxygen.

Pollutants & Ecological Hazards

Hazard Type	Primary Agents	Critical Impact
Acid Rain	SO2 and NO2	pH < 5.6, corrodes marble (stone leprosy)
Photochemical Smog	NO2, O3, and PAN	Oxidizing haze, severe eye irritation
Classical Smog	SO2, smoke, and fog	Reducing nature, bronchial problems
Water Pollution	Organic waste (BOD)	Depleted DO; kills aquatic fauna below 4 ppm
Ozone Hole	CFCs and Halons	Excess UV-B reach; skin cancer, cataract

Visual Environmental Checkpoints

Acid Rain

(pH Below 5.6)

pH Below 5.6

H2SO4 and HNO3 dissolve in droplets to weather building facades.

Photo Smog

(Sunlight and PAN)

Sunlight and PAN

Solar UV rays interact with unburnt hydrocarbons to form toxic haze.

Ozone Layer

(Dobson Unit)

Dobson Unit

Stratospheric ozone thickness is measured globally in Dobson Units.

BOD Test

(Oxygen Demand)

Oxygen Demand

Clean drinking water has a BOD value of less than 5 ppm.

Exam Pitfalls & Traps

Tropospheric ozone shields from UV raysStratospheric ozone shields from UV rays

Stratospheric ozone is good and protective, but tropospheric ground-level ozone is a dangerous secondary pollutant and eye irritant.

Normal clean rainwater has neutral pH 7.0Normal clean rainwater has acidic pH 5.6

Due to dissolved atmospheric carbon dioxide forming weak carbonic acid, unpolluted rain naturally has a pH of approximately 5.6.

Classical smog is oxidizing in natureClassical smog is reducing in nature

Classical smog contains sulfur dioxide which is a reducing agent, while photochemical smog contains ozone and PAN which are strong oxidants.

Target Exam Quiz

Q1.Which of the following pollutants is primarily responsible for Classical (London) Smog?

Q2.What is the permissible upper limit of Biochemical Oxygen Demand (BOD) for clean drinking water?

Page 26 of 45