L9 - Water vapour, clouds and aqueous chemistry Flashcards
What happens to water vapor density in colder and warmer air?
Colder air: Low H₂O vapor density.
Warmer air: High H₂O vapor density.
What does the Clausius-Clapeyron equation describe?
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)
What is the saturated vapor pressure (E sat
(T))?
The maximum amount of vapor air can hold at equilibrium for a given temperature 𝑇
How is relative humidity (RH) calculated?
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
What happens to RH when temperature decreases from 30°C to 20°C (given p=21.12mbar)?
At 30°C:
E sat (T)=42.28mbar,
RH=50%.
At 20°C:
E sat (T)=23.3mbar,
RH=90.7%
What is the typical range of liquid water content (LWC) in clouds?
LWC is usually in the range of 0.1–0.5 g water m⁻³ air
How is liquid water mixing ratio (WL) calculated?
WL=10 ^−6×LWC
(If LWC is in g m⁻³, assuming water density = 1 g cm⁻³).
What does Henry’s Law describe?
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)
What is the definition of pH?
pH=−log 10 [H +]
For pure water at 298K, pH = 7
What is the value of
Kw (ionic product of water) at 298K?
Kw = [H +][OH − ]=1×10^−14 M 2
What is the primary basic gas in the atmosphere?
Ammonia (NH 3 )
How does ammonia interact with water?
NH 3 + H2O↔NH 3 ⋅ H2O↔NH 4+ +OH −
What is the equilibrium expression for NH4+ concentration in cloud droplets?
[NH4+] = HNH3 Ka [H+] / Kw pNH3
What is the proportion of
NH3 in cloud water if
LWC=0.1gm
−3, pH=5, and gaseous NH3 mixing ratio = 1 ppb?
The proportion is calculated using Henry’s law,
HNH3, and Ka values, assuming a dilute solution
How does sulfur dioxide behave in water?
Similar to carbon dioxide, undergoing equilibria:
SO2 (g) + H2O <-> SO2 . H2O
SO2 . H2O <-> H+ + HSO3-
HSO3- <-> H+ + SO 2/3-
What are the key constants for sulfur dioxide equilibria in water?
HSO2 : Solubility coefficient.
Ks1: Equilibrium constant for HSO3-
Ks2 : Equilibrium constant for SO23-
What is the most important aqueous pathway for sulfate formation?
Through reaction with hydrogen peroxide (H2O2)
1. HSO3- + H2O2 -> SO2OOH- + H2O
2. SO2OOH- + H+ -> H2SO4
How do reaction rates in the aqueous phase compare to gas phase oxidation?
Aqueous phase reaction rates depend on liquid water content (LWC) and can exceed gas phase rates in deeper clouds where LWC is higher
How is the rate of an aqueous reaction expressed in terms of air?
RIa = -10^-6 x LWC x k [Aaq] [S(IV)]
Where RIa is in mol/L of air/s
How does LWC affect reaction competitiveness?
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.
