Chem. 250

1
Chem. 250 11/18 Lecture
2
Announcements I

1. Exam 2 Results Average 73
2. New Homework Set (Text Ch. 4 25 Ch. 7 3, 5,
   6, 8, 10, 24, 25, 26, 35, 44 Furlough Questions)

Score Range N
90-92 2
80s 4
70s 5
60s 2
lt60 3
3
Announcements - II

• Topic Covering
• Cloud Chemistry
• Precipitation Chemistry
• Hydrologic Cycle
• Water Properties
• Water Composition
• Some of the above topics may be covered in the
  next lecture

4
Announcements - III

• Rough Drafts due today (one copy to me keep
  track of who receives other copies)
• Next Wednesday Furlough Day
• - I will give a couple of additional homework
  problems for you to do as practice toward
  understanding of concepts (not collected or
  graded)

5
Furlough Problems

1. A cloud is nucleated on an aerosol containing 8.0
   µg m-3 NH4HSO4 with 75 efficiency and reaches a
   LWC of 0.5 g m-3. The SO2 mixing ratio is
   present at 2.0 ppb. If P 0.8 atm and T 15ºC,
   calculate the pH of the cloud water. Calculate
   the pH independently for aerosol only acidity and
   SO2 only acidity and use only the source which
   adds the most acidity. Then determine the
   following H2SO3 (aq), HSO3-, SO42-
2. Derive the equation on slide 34.

6
Cloud Chemistry

• Rationale for Studying
• - Cloud reactions can be important (e.g.
  formation of H2SO4)
• - Precipitation composition depends on cloud
  composition
• - Provide introduction to aqueous chemistry

7
Cloud Chemistry- Incorporation of Pollutants
8
Cloud Chemistry- Incorporation of Pollutants

• Main mechanisms
• - Nucleation of cloud droplets on aerosol
  particles
• - Scavenging of gases
• - Reactions within the droplet

9
Cloud ChemistryNucleation of Cloud Droplets
(some review?)

• Cloud droplets can not form in the absence of
  aerosol particles unless RH 300.
• Cloud droplets nucleate on aerosol particles at
  RH of 100.1 to 101.
• Cloud droplets should nucleate when RH 100
  except that the vapor pressure over a curved
  surface is less than that over a flat surface
  (due to water surface tension)
• Smaller particles (d lt 50 nm) have more curved
  surfaces and are harder to nucleate

10
Cloud Chemistry- Nucleation of Cloud Droplets

• Nucleation more readily occurs with
• - Larger particles
• - Particles with more water soluble compounds
  (due to growth according to Raoults law)
• - Compounds that reduce surface tension
• - Smaller aerosol number concentrations (less
  competition for water so higher RH values)

11
Cloud Chemistry- Nucleation of Cloud Droplets

• The concentration of constituents incorporated
  from nucleation depends on the efficiency of
  nucleation and on the liquid water content (or
  LWC).
• LWC g liquid H2O/m3 of air
• The higher the LWC, the lower the concentration
  (dilution effect)
• Cloud nucleation leads to heterogeneous cloud
  droplet composition Ignored here for
  calculations

12
Cloud ChemistryNucleation Example Problems

• Why is a RH over 100 required for cloud droplet
  nucleation?
• Why is nucleation efficiency higher in less
  polluted regions?
• An ammonium bisulfate aerosol that has a
  concentration of 5.0 µg m-3 is nucleated with 50
  efficiency (by mass) in a cloud that has a LWC of
  0.40 g m-3. What is the molar concentration?
  What is the cloud pH?

13
Cloud Chemistry- Scavenging of Gases

• Also Important for covering water chemistry (e.g.
  uptake of CO2 by oceans)
• For unreactive gases, the transfer of gases to
  cloud droplets depends on the Henrys law
  constant (always)
• In special cases, transfer can depend on LWC (if
  high), or can be limited by diffusion
• Henrys Law

where KH constant (at given T) and X molecule
of interest
14
Cloud Chemistry- Scavenging of Gases
unreactive gases

• When LWC and KH are relatively low, we can assume
  that PX is constant
• Then X KHPX
• When KH is high (gt1000 M/atm), conservation of
  mass must be considered (PX decreases as
  molecules are transferred from gas to liquid)
• We will only consider 2 cases (low KH case and
  100 gas to water case)
• Example Problem (low KH case) What is the
  concentration of CH3OH in cloud water if the gas
  phase mixing ratio is 10 ppbv and a LWC of 0.2
  g/m3? The Henrys law constant is 290 M/atm (at
  given temp.). Assume an atmospheric pressure of
  0.9 atm and 20C.

15
Cloud Chemistry- Scavenging of Gases
unreactive gases

• For compounds with high Henrys law constants, a
  significant fraction of compound will dissolve in
  solution
• fA 10-6KHRT(LWC) where fA aqueous fraction
  (not used in assigned problems)
• When fA 1, can use same method as for cloud
  nucleation

From Seinfeld and Pandis (1998)
16
Cloud Chemistry- Scavenging of Gases reactive
gases

• Many of the gases considered are acidic and react
  further
• Example Dissolution of SO2 gas
• Reaction Equation
• SO2(g) H2O(l) ? H2SO3(aq) KH H2SO3/PSO2
• H2SO3(aq) ? H HSO3- Ka1 HHSO3-/H2SO3(aq
  )
• HSO3- ? H SO32- Ka2 HSO32-/HSO3-
• Note concentration of dissolved SO2 S(IV)
• H2SO3 HSO3- SO32- H2SO3(1
  Ka1/H Ka1Ka2/H2)
• Effective Henrys law constant
• KH KH(1 Ka1/H Ka1Ka2/H2)
  function of pH

17
Cloud Chemistry- Scavenging of Gases reactive
gases

• For SO2 problems in homework, assume
• Little SO2 is depleted from gas phase (usually
  valid)
• (This means PSO2 and H2SO3 are constant)
• pH is just determined from SO2 (usually not
  valid)
• The third reaction can be ignored (dissociation
  of HSO3- doesnt affect pH)
• Dissolution of HNO3
• Because both KH and Ka are large, we can not
  assume little HNO3 is depleted from gas phase
• Better assumption is 100 transfer to aqueous
  phase

18
Cloud Chemistry- Scavenging of Gases reactive
gases

• Example problem
• Determine the pH and aqueous NO3- concentration
  (in M) if air containing 1 ppbv enters a cloud
  with a pressure of 0.90 atm, a T 293K, and a
  LWC of 0.50 g/m3. Assume 100 scavenging.

19
Cloud Chemistry- Combining two scavenging
methodsexample including ammonium bisulfate,
sulfur dioxide and carbon dioxide
Equilibrium pH where sum of anion charge sum of
cation charge
Calculation method is fairly complex (uses
systematic method)
20
Cloud Chemistry- Reactions in Clouds

• Cloud reactions are important for water soluble
  species because of higher concentrations in
  clouds
• Only sulfur chemistry covered here

21
Cloud Chemistry- Reactions in Clouds

• Reaction of S(IV) and H2O2
• - HSO3- H2O2 ? HSO4- H2O (acid catalyzed)
• - Rate kHSO3-HH2O2
• - Rate kH2O2PSO2
• - Effectively pH independent (despite what text
  says)

22
Cloud Chemistry- Reactions in Clouds

• Reaction of S(IV) and Ozone
• - Two main reactions
• HSO3- O3 ? HSO4- O2 moderately fast
• SO32- O3 ? SO42- O2 fast
• reaction is faster at high pH because more S(IV)
  is present in reactive forms

23
Cloud Chemistry- Reactions in Clouds
24
Precipitation Chemistry

• Precipitation Formation
• Cloud droplets are collected by collisions with
  rain droplets or snow crystals and transfer their
  contents
• Snow crystals also can form mainly through
  diffusion from water vapor and are very clean
• Below Cloud Scavenging
• Incorporation of gases or particles

25
Cloud/Precipitation ChemistrySome Questions

1. Which reactant for sulfur dioxide oxidation is
   likely to be most important if a cloud is
   nucleated on a soil dust aerosol? on an acidic
   sulfate aerosol?
2. Two snow events occur down-wind of a pollution
   source. In one case, the snow is mostly crystals
   formed from diffusional growth. In the other the
   snow grew by removing cloud droplets. How will
   the snow composition be different?

26
Water ChemistryHydrological Systems

• Most of water on Earth is in the ocean
• Much of the freshwater is inaccessible for use
• Groundwater is becoming an increasingly important
  resource

from Girard
27
Water ChemistryHydrological Systems

• The hydrologic cycle is the cycle by which water
  is distributed around the Earth
• Evaporation removes most of the non-volatile
  constituents of water
• For this reason, atmospheric source of many
  compounds are not large (although they are
  important atmospheric sinks)
• As with clouds, regions of heavier precipitation
  tend to have greater dilution of pollutants
• Water flowing through sediments can add or remove
  constituents

from Girard
28
Water ChemistryHydrological Systems
Data From Chem. 31
N
Sacramento Valley
Groundwater??
sediments
River Water??
granite
Tap Water West East Transect
29
Water ChemistryProperties of Water

• See Text for boiling point/melting point and heat
  capacity properties
• Temperature Density Relationship density
  maximum occurs at 4C and ice density is much
  lower than water density
• Note if density increases with depth, water is
  stable.

from Girard
Kotz et al., Chemistry and Chemical Reactivity
(6th Ed.)
30
Water ChemistryProperties of Water

• In the oceans, production of dense water that can
  sink occurs when warm water evaporates producing
  cool water with high salinity
• This only occurs in two areas (near Iceland and
  near Antarctica)
• The volume of deep water formed equals the volume
  of upwelling water

31
Water ChemistryWater Composition

• Salt Water
• - main ions are sodium (1.06) and chloride
  (1.9) with lower amounts of magnesium and
  sulfate
• - main compound affecting pH is HCO3- ion (a
  weak base)
• Fresh Water
• - main ions are HCO3-, Mg2, Ca2, Na, and Cl-
• - main source of major ions is dissolution of
  carbonates
• e.g. CaCO3(s) CO2(g) H2O(l) ? Ca2 2HCO3-

32
Water ChemistryWater Composition

• Dissolved solids
• Mass of material left after evaporating water
• Expressed in ppm
• Surrogate measure is electrical conductivity

33
Water ChemistryChemical Reactions

• Acid-Base Equilibria
• Dissociation of water (always important)
• H2O ? H OH-
• Carbon dioxide reactions
• 1) Acid-Base Reactions
• CO2 (g) ? CO2 (aq) KH 0.0338 M/atm
• CO2 (aq) H2O ? H HCO3- Ka1 4.45 x 10-7
• HCO3- ? H CO32- Ka2 4.7 x 10-11

34
Water Chemistry Chemical Reactions

• Acid-Base Properties continued
• Note If water is in contact with atmosphere,
  PCO2 fixed value, so CO2 independent of pH
• Other equations useful for solving water
  chemistry equations
• Mass balance T CO2 HCO3- CO32-
• where T total carbonate concentration
• Charge balance equation
• S(zicationi) S(zjanionj) zi charge of
  ion i

35
Water Chemistry Chemical Reactions

• Form of carbonate as a function of pH
• The fraction of carbonate species a present in a
  single form (e.g. HCO3-) can be calculated as
  follows

The right part to the equation can be derived
from equilibrium equations
When pH lt pKa1, CO2 is the dominant species, when
pKa1 lt pH lt pKa2, HCO3- is the dominant species,
and when pH gt pKa2, CO32- is the dominant species
36
Water Chemistry Chemical Reactions
80 of US surface water
37
Water Chemistry Chemical Reactions

• Second source of carbonate dissolution or
  weathering of carbonate rock/soil
• CaCO3(s) ? Ca2 CO32- Ksp 4.6 x 10-9
• This reactions normally must be considered with
  other reactions (because in most waters, the pH
  is such that HCO3- gtgt CO32-)
• Problems normally can be solved using 1) the
  systematic method or 2) simplifying assumptions

38
Water Chemistry Chemical Reactions

• Example of simplifying assumption
• Solubility of CaCO3 in pure water (no CO2
  present)
• Water with carbonate soils is usually in regime
  where a(HCO3-) gt a(CO2) gt a(CO32-), so a more
  representative reaction would result in HCO3-
• By combining CaCO3(s) ? Ca2 CO32-
• with H CO32- ? HCO3- and H2O ? H OH-
• The following is obtained CaCO3(s) H2O ? Ca2
  HCO3- OH- where K KspKw/Ka2 9.7 x 10-13
  Ca2 HCO3-OH-
• Assumption that Ca2 HCO3- OH- leads to
• Ca2 solubility 9.9 x 10-5 M pH 10.00
• Vs. solubility 6.8 x 10-5 M and pH 7.00
  considering solubility reaction only

39
Water Chemistry Chemical Reactions

• Simplifying assumption when both CaCO3 and CO2
  are present
• Combine simplified equation for CaCO3 solubility
  with first 2 CO2 reactions
• CaCO3(s) H2O ? Ca2 HCO3- OH-
• and CO2 (g) H2O ? H HCO3- (and H OH- ?
  H2O)
• Net reaction CaCO3(s) CO2 (g) H2O ? Ca2
  2HCO3-
• Notes 1) increased CO2 leads to increased
  solubility
• 2) 2Ca2 HCO3- expected

40
Water Chemistry Chemical Reactions
From Harris, Quantitative Chemical Analysis, 6th
Ed., 2003
Additional CO2 sources
Low Carbonate Soils
41
Water Chemistry Chemical Reactions

• Buffering capacity and alkalinity
• Through their reactions carbonate soils buffer
  water from the addition of acids.
• Alkalinity is a measure of the buffering capacity
  of water.
• Alkalinity mmol of acid that can be added to a
  1 L water sample before the pH ? 4.5.
• Alkalinity OH- 2CO32- HCO3-
  (approximate)
• Alkalinity OH- 2CO32- HCO3- H
  (better, but still approximate equation)

42
Water ChemistrySome Problems

1. What are first and second largest reservoirs of
   water on Earth?
2. What two ions are the most prevalent in
   sea-water?
3. What three ions are the most prevalent in fresh
   water?
4. How do the three major ions in fresh water
   generally get into fresh water?
5. How do the concentrations of major ions in rain
   water compare with fresh water?

43
Water ChemistrySome Problems - II

1. At 5C, the water hydrolysis equilibrium constant
   is 2.0 x 10-15. What is the pH of pure water (no
   CO2, no other sources of trace species)?
2. Determine the solubility of CaCO3 in water in
   equilibrium with 380 ppm CO2. What is the pH of
   the water? What is its alkalinity?
3. Coral is largely CaCO3. As PCO2 goes up, what
   will happen to the solubility of coral in the
   ocean? What should happen to the pH of the
   ocean? What will happen to Ca2?

44
Water ChemistryOne last problem

1. A water sample has a measured alkalinity of 0.4
   mM and a pH of 6.7. Determine the concentration
   of OH-, HCO3-, CO32-, and CO2.