CHEM 219 Final Exam Prep: 100
Atmospheric Chemistry MCQs with
Answers & Rationales | McGill University
1. Which of the following correctly lists the major permanent
gases of the homosphere in order of decreasing abundance
(by volume)?
A) N₂ > O₂ > Ar > CO₂
B) O₂ > N₂ > Ar > CO₂
C) N₂ > O₂ > CO₂ > Ar
D) Ar > N₂ > O₂ > CO₂
Answer: A | Rationale: Nitrogen (~78%) > Oxygen (~21%) >
Argon (~0.93%) > CO₂ (~0.042%) in the well-mixed
homosphere.
2. The boundary between the troposphere and the
stratosphere is called the:
A) Tropopause
B) Stratopause
C) Mesopause
D) Thermopause
Answer: A | Rationale: The tropopause marks the top of the
troposphere (~8–15 km) where temperature stops
decreasing with altitude.
,3. Which atmospheric layer contains approximately 80–90%
of the total atmospheric mass?
A) Stratosphere
B) Troposphere
C) Mesosphere
D) Thermosphere
Answer: B | Rationale: The troposphere holds most
atmospheric mass due to gravitational compression,
despite being the lowest layer.
4. Which region of the atmosphere is characterized by a
temperature inversion (temperature increasing with
altitude)?
A) Troposphere
B) Stratosphere
C) Mesosphere
D) Thermosphere
Answer: B | Rationale: Stratospheric temperature increases
with altitude due to UV absorption by ozone (O₃), creating a
stable layer.
5. The average residence time of water vapor (H₂O) in the
atmosphere is approximately:
A) 10 days
,B) 1 year
C) 10 years
D) 100 years
Answer: A | Rationale: Water vapor cycles rapidly (~9–10
days) through evaporation and precipitation.
6. Which of the following is NOT considered a variable gas in
the atmosphere?
A) Water vapor (H₂O)
B) Carbon dioxide (CO₂)
C) Ozone (O₃)
D) Nitrogen (N₂)
Answer: D | Rationale: N₂ is a permanent gas with constant
concentration; H₂O, CO₂, and O₃ are variable gases.
7. Pressure at sea level is ~1013 hPa. At the tropopause
(~12 km), the pressure is roughly:
A) 500 hPa
B) 300 hPa
C) 200 hPa
D) 100 hPa
Answer: C | Rationale: Pressure drops exponentially; at 12
km it is ~200–220 hPa (~20% of sea level).
, 8. The atmospheric "homopause" is the altitude above
which:
A) Temperature becomes constant
B) Gases no longer exhibit constant mixing ratios
C) Ozone concentration reaches its maximum
D) Water vapor begins to condense
Answer: B | Rationale: Above the homopause (~80–100 km),
molecular diffusion replaces turbulent mixing, so lighter
gases become enriched.
9. The main source of tropospheric hydroxyl radicals (•OH)
is:
A) Reaction of O(¹D) with H₂O
B) Reaction of ozone with water vapor
C) Photolysis of H₂O₂
D) Reaction of nitrogen oxides with water
Answer: A | Rationale: O(¹D) + H₂O → 2•OH; O(¹D) comes
from ozone photolysis at <310 nm.
10. The ideal gas law (PV = nRT) expressed as density (ρ) is:
A) ρ = MP/RT
B) ρ = PM/RT
C) ρ = RT/PM
D) ρ = P/MRT
Atmospheric Chemistry MCQs with
Answers & Rationales | McGill University
1. Which of the following correctly lists the major permanent
gases of the homosphere in order of decreasing abundance
(by volume)?
A) N₂ > O₂ > Ar > CO₂
B) O₂ > N₂ > Ar > CO₂
C) N₂ > O₂ > CO₂ > Ar
D) Ar > N₂ > O₂ > CO₂
Answer: A | Rationale: Nitrogen (~78%) > Oxygen (~21%) >
Argon (~0.93%) > CO₂ (~0.042%) in the well-mixed
homosphere.
2. The boundary between the troposphere and the
stratosphere is called the:
A) Tropopause
B) Stratopause
C) Mesopause
D) Thermopause
Answer: A | Rationale: The tropopause marks the top of the
troposphere (~8–15 km) where temperature stops
decreasing with altitude.
,3. Which atmospheric layer contains approximately 80–90%
of the total atmospheric mass?
A) Stratosphere
B) Troposphere
C) Mesosphere
D) Thermosphere
Answer: B | Rationale: The troposphere holds most
atmospheric mass due to gravitational compression,
despite being the lowest layer.
4. Which region of the atmosphere is characterized by a
temperature inversion (temperature increasing with
altitude)?
A) Troposphere
B) Stratosphere
C) Mesosphere
D) Thermosphere
Answer: B | Rationale: Stratospheric temperature increases
with altitude due to UV absorption by ozone (O₃), creating a
stable layer.
5. The average residence time of water vapor (H₂O) in the
atmosphere is approximately:
A) 10 days
,B) 1 year
C) 10 years
D) 100 years
Answer: A | Rationale: Water vapor cycles rapidly (~9–10
days) through evaporation and precipitation.
6. Which of the following is NOT considered a variable gas in
the atmosphere?
A) Water vapor (H₂O)
B) Carbon dioxide (CO₂)
C) Ozone (O₃)
D) Nitrogen (N₂)
Answer: D | Rationale: N₂ is a permanent gas with constant
concentration; H₂O, CO₂, and O₃ are variable gases.
7. Pressure at sea level is ~1013 hPa. At the tropopause
(~12 km), the pressure is roughly:
A) 500 hPa
B) 300 hPa
C) 200 hPa
D) 100 hPa
Answer: C | Rationale: Pressure drops exponentially; at 12
km it is ~200–220 hPa (~20% of sea level).
, 8. The atmospheric "homopause" is the altitude above
which:
A) Temperature becomes constant
B) Gases no longer exhibit constant mixing ratios
C) Ozone concentration reaches its maximum
D) Water vapor begins to condense
Answer: B | Rationale: Above the homopause (~80–100 km),
molecular diffusion replaces turbulent mixing, so lighter
gases become enriched.
9. The main source of tropospheric hydroxyl radicals (•OH)
is:
A) Reaction of O(¹D) with H₂O
B) Reaction of ozone with water vapor
C) Photolysis of H₂O₂
D) Reaction of nitrogen oxides with water
Answer: A | Rationale: O(¹D) + H₂O → 2•OH; O(¹D) comes
from ozone photolysis at <310 nm.
10. The ideal gas law (PV = nRT) expressed as density (ρ) is:
A) ρ = MP/RT
B) ρ = PM/RT
C) ρ = RT/PM
D) ρ = P/MRT
