Rainout

1
Rainout

• We mentioned a few of things that may rainout
• CH3OOH (CH4 oxidation, low NOx)
• H2O2 (CO oxidation, low NOx)
• HNO3 (CH4 and RH oxidation, high NOx)
• If hydrocarbons convert to acids during oxidation
  and dissolve in water ? acid rain.

2
History of Acid Rain

• In the 19th century, Robert Angus Smith
  discovered high levels of acidity in rain
  falling over industrial regions.
• In 1950s and 1960s, biologists noticed a
  decline of fish populations in lakes of southern
  Norway and North America.
• Later found that acid rain also affects
  vegetation, materials, structures.

3
Acid Rain Over North America and Norway
4
Two Main Sources of Acid Rain

• SO2 from industry oxidized to H2SO4
• NO, NO2 from automobiles oxidized to HNO3

5
Aqueous Phase Chemical Equilibrium

• What happens to a species that dissolves in
  water?
• partly dissociates into ions (bonds toward water
  ions stronger than to its own atoms)
• What happens to water in the process?
• partially ionizes
• These are reversible reactions that reach
  equilibrium rapidly
• At equilibrium
• Keq equilibrium constant
• M mol L-1

6
Concentration of H2O(aq)

• H2O is very large 55.5 M ? virtually constant
• Incorporate H2O into Keq
• Keq w HOH- 1.0 x 10-14 M2 at 298 K
• Concentration of ions in pure water
• Each water molecule that dissociates produces 1
  H and 1 OH-
• Concentration of ions is much smaller than H2O
  ? water has small conductivity.

7
pH of Pure Water

• Definition of pH
• Pure water at 298 K
• acidic pH lt 7
• alkaline pH gt 7
• neutral pH 7

8
pH of Clean Rainwater

• Clean rainwater is not pure water. It
  equilibrates with CO2
• hydrolysis
• ionization bicarbonate ion
• further ionization carbonate ion

9
Equilibrium Between Gas and Aqueous Phase

• Equilibrium for first reaction
• H2CO3(aq) aqueous phase concentration in
  equilibrium with gas phase
• pCO2 partial pressure of gas phase species (atm)
• 1 atm 760 mm Hg 760 Torr 1013.25 mbar
  1.01325 x 105 Pa (N m-2)
• KH Henrys law constant (for dilute solutions)
• Soluble gases have large KH.

10
Henrys Law Constants for Atmospheric Gases
11
What Does H2CO3(aq) Depend On?

• Does H2CO3(aq) depend on amount of liquid water
  available? No.
• Does H2CO3(aq) depend on size of droplet? No.
• Does H2CO3(aq) depend on temperature? Yes.
• vant Hoff equation
• similar to Clausius-Clapeyron equation
• DH reaction enthalpy at constant T and P or heat
  of dissolution
• L heat of vaporization
• KH increases as T decreases ? gas more soluble at
  lower T (less energetic molecules on surface,
  less evaporation, more stays in solution).

12
Heat of Dissolution for Atmospheric Gases
13
CO2/H2O System

• Reactions in CO2/H2O system

14
CO2/H2O System cont.

• Total dissolved CO2

15
Effective Henrys Law Constant

• effective Henrys Law constant for CO2
• Is greater than or less than
  ?
• Always greater than ? Total amount of CO2
  dissolved always exceeds that predicted by
  Henrys Law for CO2 alone (although not by much).
  

16
Effective Henrys Law Constant cont.

• What does depend on?
• T, pH of solution (H)
• As pH increases (H decreases), does
  increase or decrease?
• increase
• As pH increases, does
  increase or decrease?
• increase

17
Effective Henrys Law Constant of CO2 as a
Function of pH
18
Calculate the pH of CO2/H2O System Approximate
Method

• Since KH is small (compared to 107/108), assume
  pCO2 constant.
• Since Keq w so small, assume it does not
  contribute to H.
• Since Keq 3 so small, assume CO32- 0, then
  every molecule of H2CO3 that dissociates produces
  1H and 1 HCO3-
• H HCO3-
• pH of pure rainwater

19
Calculate the pH of CO2/H2O System More Exact
Method

• Since KH is small, still assume pCO2 constant.
• electroneutrality concentrations of ions will
  adjust so that solution is electrically neutral
• (Each CO32- ion contributes charge of 2-. Total
  negative charge is concentration of ions x 2.)

20
Calculate the pH of CO2/H2O System Third Method

• CO2 H2O H2CO3
• H2CO3 H HCO3- 2H CO3
• K1 4.3x10-7
• K2 5.3x10-11
• aC H HCO3-
• a fraction of concentration in the form of ions
• C concentration
• (1-a)C H2CO3
• 1-a fraction of concentration in the form of
  acid
• K1 HHCO3- / H2CO3 (aC)2 / (1 - a)C
  a2C/(1 - a)

21
Calculate the pH of CO2/H2O System Third Method
cont.

• CO2 solubility 0.759 liters_at_1atm/liter H2O
• CO2 Partial pressure Pp is 345 ppm
• n VPp/RT
• n 0.759 x 345x10-6 / (0.082 x 298) 1.07x10-5
  mol
• In 1 liter C 1.07 x 10-5 mol/liter
• 4.3 x 10-7 1.07 x 10-5 a2 / (1 - a)
• a2 0.04a - 0.040
• a 0.18
• H 0.18 x 1.07 x 10-51.93x10-5 M
• pH -log H 5.7

22
SO2/H2O System

• Reactions in SO2/H2O system
• bisulfite ion
• sulfite ion

23
SO2/H2O System cont.
Total dissolved sulfur
24
S(IV)

• Sulfur occurs in 5 oxidation states in the
  atmosphere.
• Chemical reactivity decreases with sulfur
  oxidation state.
• Water solubility increases with sulfur oxidation
  state.
• Essentially all dissolved species that come from
  SO2 are in oxidation state 4.

25
Sulfur Oxidation States
26
Sulfur Oxidation States cont.
27
Total Dissolved Sulfur

• Total dissolved sulfur
• Effective Henrys Law constant for SO2

28
Effective Henrys Law Constant of SO2 as a
Function of pH
increases by 7 orders of magnitude
with pH ? Acid-base equilibrium pulls more
material into solution. (Which material?
H2SO3(aq) does not depend on pH.)
29
If Assume Constant pSO2

• Open system unlimited LWC, unlimited SO2
• S(IV)Tot(aq) increases dramatically with pH

30
If Dont Assume Constant pSO2

• Closed system supply of SO2 limited ? Cannot
  assume pSO2 constant to calculate
  concentrations, but can calculate mole fractions
  as a function of pH

31
Mole Fractions cont.
32
S(IV) Mole Fractions as a Function of pH

• pH lt 2 S(IV) mainly in the form of H2SO3(aq)
• 3 lt pH lt 6 S(IV) mainly in the form of HSO3-
• pH gt 7 S(IV) mainly in the form of SO32-

33
How Does This Affect Aqueous Phase Reactions?

• Since concentrations depend on pH, reaction rates
  in solution will depend on pH.
• Why is this important?
• We still have not calculated the pH of the sulfur
  system with varying pSO2 (closed system).
• So far we have H2SO3(aq), HSO3- ,SO32-.
• We dont yet have the acid H2SO4.

34
Sulfuric Acid

• What oxidation state is H2SO4 in? 6
• We need to convert S(IV) to S(VI) via aqueous
  phase reactions.
• S(IV) reacts with many species in solution
• O3, H2O2, CH3OOH, O2, OH, NO2, HCHO, Mn, Fe,
• Of these, O3 and H2O2 are the most important for
  converting S(IV) to S(VI).

35
Aqueous Phase Reaction Rate

• S(IV) A(aq) ? S(VI) rate constant k in M-1
  s-1
• R k S(IV) A(aq) in M s-1 (mol L-1 s-1)

36
The S(IV)/O3(aq) System

• Reactions in the S(IV)/O3(aq) system

37
Rate Constant of the S(IV)/O3(aq) System as a
Function of pH

• An increase in pH results in an increase in
  equilibrium HSO3- and SO32- ? results in an
  increase in dS(IV)/dt.

38
Self-Limiting Reaction

• The strong increase in dS(IV)/dt with pH makes
  the reaction self-limiting. Why?
• Production of H2SO4 (acid) lowers the pH and
  slows further reaction.
• ? The reaction of S(IV) with O3(aq) is a source
  of cloud water acidification when pH gt 4 and an
  important sink of gas phase SO2 when pH gt8 (sea
  spray).

39
The S(IV)/H2O2(aq) System
H2O2(aq) is 6 orders of magnitude higher than
O3(aq) Reactions in the S(IV)/ H2O2(aq) system
40
The S(IV)/H2O2(aq) System cont.

• Steady state approximation on SO2OOH-

41
Rate Constant of the S(IV)/H2O2(aq) System as a
Function of pH

• As pH increases (H decreases), first (2) first
  becomes faster (denominator dominates), then (2)
  becomes slower (numerator dominates).
• ? The reaction of S(IV) with H2O2(aq) is a source
  of cloud water acidification when pH lt 4 and an
  important sink of gas phase SO2 when pH lt7
  (clouds).

42
pH of the Sulfur System with Varying pSO2 (Closed
System)

• SO2(g) becomes depleted ? less production of
  S(IV) ? less acidic?
• O3(aq) and H2O2(aq) become depleted ? less S(IV)
  ?S(VI) ? less acidic?
• NH3 becomes depleted ? less neutralization ? more
  acidic?
• less S(IV) ? pH lower (Figure 6.7) ? faster S(IV)
  ? S(VI) via H2O2(aq) ? more acidic

43
The Sulfur Cycle
44
pH of Sulfur System as a Function of Cloud Liquid
Water Content

• Assumptions
• 1. All sulfate is in the form of H2SO4
• 2. All the acid is dissolved and dissociated to
  ions (2H and SO4)
• 3. Liquid water content is 1.0 gr/m3 (very heavy
  cloud)
• Note Total water content (vaporliquid) is 30 gr
  per m3 at 25oC and 100 humidity
• For SO4 1.0 mg/m3 (typical for remote
  pacific area)
• H 2 (moles H per mole H2SO4) x10-6 (g SO4
  per m3 air)
• / 1.0 (g H2O per m3 air) / 98 (g H2SO4 per mole
  H2SO4)
• x 103 (g H2O per L H2O) 2.04x10-5 M
• pH -logH 4.7
• For a thin cloud (0.1 gr/m3)
• H 2.04x10-4 M
• pH 3.7

45
pH of Sulfur System as a Function of Cloud Liquid
Water Content cont.
46
The HNO3/H2O System
Reactions in HNO3/H2O system nitrate
ion very soluble dissociates
quickly
47
The HNO3/H2O System cont.
Total dissolved nitric acid Effective
Henrys law constant for HNO3
48
If Dont Assume Constant pHNO3
The Henrys law constant of HNO3 is very high. ?
Cannot assume pHNO3 constant to calculate
concentrations, but can calculate mole fractions
as a function of pH
49
HNO3 Mole Fractions as a Function of pH

• Since Keq2 is so high, Keq2 /H gtgt 1
• ?
• ? Dissolved nitric acid in clouds exists
  exclusively as nitrate.
• Aqueous fraction of nitric acid as a function of
  pH and cloud LWC

50
H2SO4(g) and NOX(g)

• We looked at the aqueous phase equilibrium of
  SO2/H2O and HNO3/H2O.
• But SO2 (g) ? H2SO4(g) and NOX (g) ? HNO3 (g).
• What about solubility of H2SO4(g) and NOX (g)?
• They are soluble, but not important contributors
  to acid rain.
• Rate of gas phase oxidation of SO2(g) to H2SO4(g)
  by OH
• 0.3-3 / hr
• Rate of gas phase oxidation of NO2(g) to HNO3(g)
  by OH 10 x faster.

51
The NH3/H2O System