L9 - Water vapour, clouds and aqueous chemistry

Question 1

What happens to water vapor density in colder and warmer air?

Answer 1

Colder air: Low H₂O vapor density.

Warmer air: High H₂O vapor density.

Question 2

What does the Clausius-Clapeyron equation describe?

Answer 2

The relationship between the change in partial pressure of a gas with temperature and the latent heat of vaporization
𝐿, and the difference in specific volume between the vapor (𝑉𝑣) and liquid (𝑉𝑙)

Question 3

What is the saturated vapor pressure (E sat
(T))?

Answer 3

The maximum amount of vapor air can hold at equilibrium for a given temperature 𝑇

Question 4

How is relative humidity (RH) calculated?

Answer 4

RH= p / E sat(T)×100%
Where p is the partial pressure of water vapor and E sat
(T) is the saturated vapor pressure at temperature 𝑇

Question 5

What happens to RH when temperature decreases from 30°C to 20°C (given p=21.12mbar)?

Answer 5

At 30°C:
E sat (T)=42.28mbar,
RH=50%.

At 20°C:
E sat (T)=23.3mbar,
RH=90.7%

Question 6

What is the typical range of liquid water content (LWC) in clouds?

Answer 6

LWC is usually in the range of 0.1–0.5 g water m⁻³ air

Question 7

How is liquid water mixing ratio (WL) calculated?

Answer 7

WL=10 ^−6×LWC
(If LWC is in g m⁻³, assuming water density = 1 g cm⁻³).

Question 8

What does Henry’s Law describe?

Answer 8

The equilibrium concentration of a gas (
A) between the gas phase and aqueous phase:
[A aq]=H ApA

Where HA is the Henry’s Law coefficient, and pA is the partial pressure of
A (atm)

Question 9

What is the definition of pH?

Answer 9

pH=−log 10 [H +]
For pure water at 298K, pH = 7

Question 10

What is the value of
Kw (ionic product of water) at 298K?

Answer 10

Kw = [H +][OH − ]=1×10^−14 M 2

Question 11

What is the primary basic gas in the atmosphere?

Answer 11

Ammonia (NH 3 )

Question 12

How does ammonia interact with water?

Answer 12

NH 3 + H2O↔NH 3 ⋅ H2O↔NH 4+ +OH −

Question 13

What is the equilibrium expression for NH4+ concentration in cloud droplets?

Answer 13

[NH4+] = HNH3 Ka [H+] / Kw pNH3

Question 14

What is the proportion of
NH3 in cloud water if
LWC=0.1gm
−3, pH=5, and gaseous NH3 mixing ratio = 1 ppb?

Answer 14

The proportion is calculated using Henry’s law,
HNH3, and Ka values, assuming a dilute solution

Question 15

How does sulfur dioxide behave in water?

Answer 15

Similar to carbon dioxide, undergoing equilibria:
SO2 (g) + H2O <-> SO2 . H2O
SO2 . H2O <-> H+ + HSO3-
HSO3- <-> H+ + SO 2/3-

Question 16

What are the key constants for sulfur dioxide equilibria in water?

Answer 16

HSO2 : Solubility coefficient.

Ks1: Equilibrium constant for HSO3-

Ks2 : Equilibrium constant for SO23-

Question 17

What is the most important aqueous pathway for sulfate formation?

Answer 17

Through reaction with hydrogen peroxide (H2O2)
1. HSO3- + H2O2 -> SO2OOH- + H2O
2. SO2OOH- + H+ -> H2SO4

Question 18

How do reaction rates in the aqueous phase compare to gas phase oxidation?

Answer 18

Aqueous phase reaction rates depend on liquid water content (LWC) and can exceed gas phase rates in deeper clouds where LWC is higher

Question 19

How is the rate of an aqueous reaction expressed in terms of air?

Answer 19

RIa = -10^-6 x LWC x k [Aaq] [S(IV)]
Where RIa is in mol/L of air/s

Question 20

How does LWC affect reaction competitiveness?

Answer 20

Close to cloud base (LWC <0.1 g m -3), aqueous reactions are less competitive. In deeper clouds (LWC > 1g m -3), reactions are up to 100 times faster.