Q.175 Zoology ⭐ 360° Option Elimination Active

Vault ↗

NEET 2021 Zoology Question 175 Solution

Topic: Environmental Issues · Official NTA Booklet Code M1

Question

Dobson units are used to measure thickness of:

(1)CFCs
(2)Stratosphere
(3)Ozone
(4)Troposphere

✓

Verified Against NTA Final Answer Key

Option (3) : Ozone

📚 Environmental Issues Easy

📋 Given Parameters & Problem Setup

Unit of Measurement Dobson Units (DU)

Question Objective Identify which gas / atmospheric layer thickness is measured in Dobson units

⚡

The 30-Second Exam Hall Shortcut: ⚡ "Dobson Unit (DU) = Canonical Metric for Ozone Thickness"

⏱️ Save ~20 seconds

The thickness of ozone in a column of air from ground to top of atmosphere is standardly measured in Dobson Units (DU). 1 DU corresponds to 0.01 mm of pure ozone at standard temperature and pressure. Mark Ozone in 5 seconds!

1 Recall atmospheric measurement units: Dobson Unit (DU).

2 Connect directly to ozone: The thickness of the stratospheric ozone layer is expressed in Dobson Units.

3 Select Option (3).

🛡️

Why You Can 100% Trust This Shortcut: NCERT Biology · Environmental Issues (Ozone Depletion in the Stratosphere: The thickness of the ozone in a column of air from the ground to the top of the atmosphere is measured in terms of Dobson units (DU)).

📐

Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Definition and Physical Basis of Dobson Units (DU)

The concentration and total thickness of the stratospheric ozone ( $O_3$ ) layer in a vertical column extending from Earth's surface to outer space is measured in Dobson Units (DU), named after Gordon Dobson, who designed the first spectrophotometer to measure total column ozone. - One Dobson Unit represents a layer of pure ozone gas that would be $10\,\mu\text{m}$ ( $0.01\text{ mm}$ ) thick under standard temperature and pressure ( $0^\circ\text{C}$ and $1\text{ atm}$ ).

1\,\text{DU} = 0.01\,\text{mm of pure } O_3 \text{ at STP} = 2.69 \times 10^{16}\,\text{molecules of } O_3 /\text{cm}^2

Step 2

Significance in Detecting the Antarctic Ozone Hole

Normal stratospheric ozone thickness ranges between 300 to 400 DU. A drop below 220 DU over the Antarctic continent is conventionally defined as the seasonal "ozone hole" caused by chlorofluorocarbons (CFCs).

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 1

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. CFCs are ozone-depleting haloalkanes measured in parts per billion/trillion (ppb/ppt), not Dobson units.

⚠️

Student Fallacy (Why candidates pick this trap): Associating Dobson with CFCs because CFCs cause ozone depletion.

2 2

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. Stratospheric thickness is measured in kilometers (km) of altitude, not Dobson units.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing the gas (ozone) with the atmospheric layer in which it resides (stratosphere).

3 3

✓ 100% Correct

Verdict & Scientific Reason:

Correct. Standard scientific unit for total column ozone thickness is the Dobson Unit (DU).

💡

Why this is airtight: None. Verbatim recall of NCERT environmental definitions.

4 4

✗ Incorrect

Verdict & Scientific Reason:

Incorrect. The troposphere is the lower atmospheric layer (0–10 km altitude), measured in km.

⚠️

Student Fallacy (Why candidates pick this trap): Confusing bad ozone (tropospheric) with unit definition.

⚠️

NTA Trap Alert: Gas vs Atmospheric Layer Trap

Candidates sometimes choose "Stratosphere" because they recall that ozone is located in the stratosphere. Dobson units measure the thickness of the *ozone gas* column, not the thickness of the stratospheric layer itself.

🛡️ How to avoid the -1 mark: Remember: DU measures pure Ozone gas thickness.

📖

Core NEET Syllabus Standard

NEET / NCERT Biology · Environmental Issues Standard Benchmark

Core Scientific Principle Tested:

"Column ozone thickness is measured in Dobson Units (DU)."

Syllabus Focus: Ozone Depletion in the Stratosphere

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

Ready to attempt under real exam pressure?

Test your speed and accuracy in the authentic 200-minute timed NTA simulator.

Launch NEET 2021 CBT Test →

← Previous (Q.174) All Zoology Questions Grid Next (Q.176) →

Spotted a mistake in this solution? Report an error ([email protected])

⚖️ Legal & Fair Dealing Notice: Sourced from publicly released NEET 2021 question papers and NEET answer keys for private study, research, and non-commercial educational practice under Fair Dealing (Section 52(1)(a) of the Indian Copyright Act, 1957). Not affiliated with or endorsed by NTA or NMC.

NEET 2021 Zoology Question 175 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ "Dobson Unit (DU) = Canonical Metric for Ozone Thickness"

Step-by-Step Rigorous Workflow & Mathematical Proof

Definition and Physical Basis of Dobson Units (DU)

Significance in Detecting the Antarctic Ozone Hole

360° Option Elimination Engine Key Feature

NTA Trap Alert: Gas vs Atmospheric Layer Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

🔗 Related Questions — Environmental Issues

Stratospheric Ozone Measurement Units

Aquatic Pollution: Eutrophication and Algal Blooms

Control of Air Pollution: Electrostatic Precipitator

Aquatic Pollution: Biomagnification Phenomenon

Solve 4 NEET Questions a Day — One per Subject