Chem. 253

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Chem. 253 2/25 Lecture
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Announcements I

• Return HW 1.3 Group Assignment
• Last Weeks Group Assignment
• most did reasonably well
• New HW assignment (1.5 posted on website)
• Next Wednesday
• Will have HW due
• No Group Assignment
• Exam 1
• Justin will take over (covering water chemistry)

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Announcements II

• Exam 1
• On all topics covered through today
• Will review topics to know at end of lecture
• Exam will be mix of short answer questions
  (multiple choice or fill in the blank) work out
  problems
• I may post an example exam (if I can find a
  relevant copy)
• Todays Lecture Topics Tropospheric Chemistry
• Finish up cloud chemistry (Chapter 3 and 4)
• Atmospheric Effects (Chapter 4)

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Sulfur, Aerosol, and Cloud ChemistryReview of
Main Concepts I

• Aerosols
• suspension of particles in a gas
• particle size range is related to formation and
  growth
• three main sizes (ultrafine mode new particles
  from gas phase, accumulation mode processed
  particles, and coarse mode from mechanical
  production)
• distributions are log normal and can be defined
  based on number, surface area, or mass
• four main chemical classes (sea-salt, soil dust,
  sulfate, and organic)
• both primary and secondary sources

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Sulfur, Aerosol, and Cloud ChemistryReview of
Main Concepts II

• Sulfur Chemistry
• both natural and anthropogenic sources
• mostly emitted as SO2, but reduced S is also
  important
• predominant pathway is oxidation to H2SO4
• gas phase oxidation occurs through 2-step OH
  reaction
• gas phase H2SO4 production can lead to new
  particle formation (although mostly leads to
  growth of existing particles)
• aqueous phase SO2 oxidation adds mass to
  accumulation mode sized particles

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Sulfur, Aerosol, and Cloud ChemistryReview of
Main Concepts III

• Cloud/Precipitation Chemistry
• Will review add new material

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Cloud/Precipitation Chemistry- Incorporation of
Pollutants
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Cloud Chemistry- Incorporation of Pollutants

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

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Cloud ChemistryNucleation of Cloud Droplets

• 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

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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)
• While larger particles are more efficient at
  nucleation, there are a lot more small particles,
  so number of droplets formed is dominated by the
  accumulation mode (100 nm lt d lt 2.5 mm)

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Cloud Chemistry- Nucleation of Cloud Droplets
aerosol size distribution (mass based)
0.01
10
1
0.1
log dp (mm)
100
Nucleation efficiency
Soot, soil
hygroscopic aerosol
0
10
1
0.1
0.01
log dp (mm)
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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

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Cloud ChemistryNucleation Example Problems

• Why is an RH over 100 required for cloud droplet
  nucleation?
• Why is nucleation efficiency higher in less
  polluted regions (for a given particle size)?
• 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?

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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 (if
  reacting very fast in droplets)
• Henrys Law

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

• When LWC and KH are relatively low, we can assume
  that PX is constant (good assumption for SO2 and
  CO2)
• Then X KHPX where PX comes from mixing
  ratio
• 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)

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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)
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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

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Cloud ChemistrySome Example Problems

• 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.
• Example problem (high KH case) Determine the pH
  and aqueous NO3- concentration (in M) if air
  containing 1 ppbv HNO3 enters a cloud with a
  pressure of 0.90 atm, a T 293K, and a LWC of
  0.50 g/m3. Assume 100 scavenging.

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Break for Group Activity
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Cloud Chemistry- Overview of Scavenging

• Gases scavenged are almost always in Henrys law
  equilibrium
• We will assume one of two cases occurs
• so little scavenging that Px(pre-cloud)
  Px(in-cloud)
• or 100 scavenging (complete transfer from gas
  phase to aqueous phase)
• Aerosol scavenging depends on size and type of
  particles (with typical lower end of around 100
  nm)

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Cloud Chemistry- What determines pH?

• It is complicated
• Strong acids (HNO3(g) and H2SO4(l)) provide H,
  tempered by NH3 and other bases
• Both SO2 and CO2 can add acidity through reaction
  of H2XO3 with water
• In many senarios, including background
  locations, neither SO2 nor CO2 significantly
  contribute to pH

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Cloud Chemistry- What determines pH?
4 Independent Senerios
Source pH
400 ppmv CO2 5.61
1 ppbv SO2 5.38
1 ppbv HNO3 LWC 0.5 g/m3 4.09
5 mg/m3 NH4HSO4 aerosol LWC 0.5 g/m3 4.06
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Cloud Chemistry- A Modeled Exampleexample
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)
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Cloud Chemistry- Reactions in Clouds

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

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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

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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

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Cloud Chemistry- Reactions in Clouds
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Cloud Chemistry- Reactions in Clouds

• Oxidation of S(IV)
• H2O2 is more important oxidant in acidic clouds
• O3 can be important in cleaner air
• Bulk models underestimate O3 reaction
• Under certain conditions, reactions can be
  diffusion limited

drop 1 pH 4.0
drop 2 pH 6.0
pH of combined drop 4.30
rate ratio (H2O2/O3) at combined pH 1000
rate ratio (H2O2/O3) from independent reactions
in two drops 0.5
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Cloud/Precipitation Chemistry- Incorporation of
Pollutants
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Precipitation Chemistry
From Mosimann ETH Dissertation diffusion growth
(top) to high degree of riming (bottom)

• 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

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Precipitation Chemistry

• Precipitation Formation
• In addition to in-cloud transfer, pollutants can
  be incorporated from below cloud scavenging
• This tends to be best for aerosols by snow and
  gases by rain
• Precipitation pollutants are typically somewhat
  lower than low-level cloud concentrations

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Chapter 4 Consequencesof Polluted Air

• Effects Covered in Chapter 4 Include Haze, Acid
  Precipitation, and Health Effects
• We will cover health effects when covering
  toxicology

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Chapter 4 Consequencesof Polluted Air - Haze

• How do aerosols affect visibility and what
  factors contribute to reduce visibility?
• Loss of light transmission (as in spectroscopy)
  can occur due to scattering or absorption
• usually aerosol scattering is most important
• NO2 absorption and soot absorption contribute to
  a lesser extent

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Chapter 4 Consequencesof Polluted Air - Haze

• Light scattering is most mass efficient (most
  scattered light per g of aerosol) for dp l
• Thus accumulation or fine aerosol mass is good
  indicator for poor visibility
• High humidity also makes problem worse due to
  hygroscopic growth of aerosol particles
• Meteorological conditions trapping pollutants or
  contributing to photo-oxidation also make
  visibility worse

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Chapter 4 Consequencesof Polluted Air Acid
Rain

• Main contributors are strong acids HNO3 and H2SO4
• These species form slowly (e.g. relative to
  ozone), so worst places are downwind of major NOx
  and SO2 sources
• Besides pollution sources, two other factors are
  important
• atmospheric neutralization
• soil chemistry

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Chapter 4 Consequencesof Polluted Air Acid
Rain

• Neutralization by Atmospheric Bases
• NH3 (from fertilizers and animal excretions)
• CaCO3 (in soil dust)
• Soil Also Allows Run-off Neutralization
• Occurs in soils containing carbonates (limestone,
  marble, etc.)
• Acid Rain More Strongly Affects Soils with Weak
  Buffer Capacity
• Granite or quartz bedrock regions cant buffer
  acidic precipitation
• This results in acidic lakes

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Chapter 4 Consequencesof Polluted Air Acid
Rain

• Problems with Acidified Water and Soils
• Plant growth in lakes is reduced, which can
  affect whole ecosystem
• Additionally, Al and other metals are mobilized
  at lower pH due to shift in
• Al(OH)3(s) ? Al3 3OH-
• Many such metals are toxic to fish at higher
  concentrations