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Chem. 250 – 10/7 Lecture Updated 10/30. Instructor: Roy Dixon My Website for Course: Announcements - I. Lecture Format 1. Powerpoint – overview + graphics + some concepts 2. Blackboard – mainly problem solving B. Topics I’m Covering

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Chem. 250 – 10/7 LectureUpdated 10/30

Instructor: Roy Dixon

My Website for Course:

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Announcements - I

  • Lecture Format

    1. Powerpoint – overview + graphics + some concepts

    2. Blackboard – mainly problem solving

    B. Topics I’m Covering

    1. Aerosol and Cloud Chemistry (10/7)

    2. Precipitation/Water Chemistry (11/18)

    3. Metals in the Environment (12/2)

    4. Toxicology (12/9)

    Note: - See syllabus for pages from text to read

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Announcements - II

  • Exam based on understanding of main concepts and ability to solve relevant problems

    Note: Today’s lecture is on material I understand well – so exam questions will have greater emphasis on lecture than on text (also text emphasis is out-dated)

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Atmospheric Aerosols

  • Assigned homework (due next Wed.):

    Chapter 2 (12, 14, 15, 17, 18, 19)

    Chapter 6 (44a,b, 45, 48)

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Atmospheric Aerosols

  • Rationale for Studying

    1. Important in biogeochemical cycles (e.g. S cycle)

    2. Direct Effects on Visibility and Climate

    3. Effects on Clouds and Precipitation Formation

    4. Effects on Human Health

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Atmospheric AerosolVisibility Effects

Aerosol particles reduce visibility by scattering light

View from my window on typical day

Picture on unusually clear day from CSUS internet site

View of mountains blocked by particle scattering

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Introduction – Climate Effects

Direct Effect of aerosols - aerosols scatter more light back to space, leading to cooling at the earth’s surface.Example: Star Fire, Aug., 2001

smoke region looks lighter due to light scattered back to space

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Atmospheric AerosolsClimate Effects

Example of clouds modified by ship exhaust

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Atmospheric AerosolsHealth Effects

High aerosol concentrations correlate with hospital visits

Brauer and Hisham-Hashim, ES&T, 32, 1998

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Atmospheric Aerosols –Size Matters

  • Many Properties of Aerosol Particles Depend on Their Size

  • Most Aerosols have Log-Normal Size Distributions

  • Common Types of Size Distributions

    • Number (number of particles of given size)

    • Mass (or Volume)

    • Surface Area

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Atmospheric Aerosols –Normal Distribution

  • Normal Distribution (not very common)

Mean diameter = 34 nm; Standard deviation (σ) = 5 nm

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Atmospheric Aerosols –Log Normal Distributions

  • Log normal distribution – appears as a normal distribution when x-axis is plotted on log scale

Geometric Mean Diameter = 23 nm; Geometric Standard Deviation (σ) = 1.8

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Atmospheric Aerosols –Calculation Example

  • How many 10 nm particles would have the same volume as 1 100 nm particles?

    • N*[p(10 nm)3/6] = 1*[p(100 nm)3/6]

    • N = (100/10)3 = 1000

  • How many 10 nm particles would have the same surface area as 1 100 nm particle?

    • N*[p(10 nm)2] = 1*[p(100 nm)2]

    • N = 100

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Atmospheric Aerosols –N and Mass Distributions

Same aerosol, number distribution is dominated by smaller particles, mass distribution is dominated by larger particles

For Number:

Geometric Mean Diameter = 23 nm; Geometric Standard Deviation (σ) = 1.8

For Mass:

Geometric mean = 65 nm

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Atmospheric Aerosols –Sources of Aerosols

  • Major Classes (Based on Composition)

    • Soil Dust (coarse particles)

    • Sea Salt (coarse particles)

    • Sulfate (fine particles)

    • Carbonaceous or Organic (fine particles)

  • Classes (Based on Sources)

    • Primary Sources

    • Secondary Sources (typically from oxidation of gaseous precursors)

      Note: particle “aging” and physical processes make distinction of particle classes more difficult

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Atmospheric Aerosols –Sizes of Various Aerosols

Surface Area Distribution

(3 modes)

(Whitby, 1978)

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Atmospheric Aerosols –Sulfate

  • Originates from Oxidation of SO2

    • Gas Phase Reaction:

      • SO2 + OH + O2→ SO3 + HO2 (2 steps)

      • and SO3 + H2O(g) → H2SO4 (g)

      • H2SO4 (g) → H2SO4 (s)

        Step 3 can occur through a) addition to existing particles (growth of particles) or b) formation of new particles (one of very few ways to form new particles via atmospheric reactions)

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Atmospheric Aerosols –Sulfate

  • Aqueous Phase Reactions

    • Reactions occur in cloud droplets

    • Specific reactions covered later

    • Reactions add sulfate only to particles big enough to nucleate cloud droplets (>50 nm)

  • Properties

    • Acidic, unless neutralized by NH3(g)

    • Water Soluble

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Atmospheric Aerosols –Carbonaceous

  • Primary Sources

    • Biomass combustion (forest fire smoke)

    • Inefficient Fossil Fuel Combustion

    • Mechanically Produced (e.g. from tires)

  • Secondary Sources (generally richer in O)

    • Photooxidation of gaseous precursors (e.g.

      a-pinene to pinonic acid)

    • Other (cloud, aerosol reactions)

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Atmospheric Aerosols –Carbonaceous - Composition

Rogge et al., ES&T, 1993; Los Angeles Samples

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Atmospheric Aerosols –“Aging” of Aerosols

  • Sea-salt and soil dust particles

    • Acids affect particle composition

    • Examples:

      • CaCO3(s) + 2HNO3(g) → Ca(NO3)2(s) + CO2(g) + H2O(g)

      • 2NaCl(s) + H2SO4(aq)→ Na2SO2 + 2HCl(g)

  • Fine particles

    • Neutralization of sulfuric acid

      • H2SO4(aq) + 2NH3(g) → (NH4)2SO4(s)

    • Oxidation/Nitration of Organic Compounds

    • Aggregation/Growth of particles

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Atmospheric Aerosols –Presence of Water

  • At relative humidity (RH) less than 100%, many aerosol particles exist at concentrated solutions

  • Concentration of solute is related to RH through Raoult’s law (provided particles are large enough):

Where: PH2O = the vapor pressure of water, P•H2O= the saturated vapor pressure of water; PH2O/ P•H2O = RH

XH2O = the mole fraction of water in the solution

i = number of species following dissociation (e.g. for NaCl, i = 2)

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Atmospheric Aerosols –Removal of Aerosols

  • Dry deposition particles

    • Most important for coarse particles (D>1 μm)

    • Settling rate larger for larger particles

    • Calculation in book (terminal velocity type) is misleading because mixing in boundary layer is fast (~ hours)

    • Very small particles (<30 nm) can be removed efficiently to surfaces because they have faster diffusion rates

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Atmospheric Aerosols –Removal of Aerosols

  • Wet Deposition

    • Removal in precipitation processes

    • Major pathway for fine particles but inefficient for particles with D<50 nm

    • In-cloud scavenging (1) nucleation of cloud droplets on aerosol particles and 2) formation of precipitation from cloud droplets)

    • Below-cloud scavenging

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Cloud Chemistry- Incorporation of Pollutants

for larger particles

Removal in Precipitation

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Atmospheric Aerosols –Some Example Problems/Questions

  • If SO2 gas is present at 10 ppbv at P = 0.9 atm and T = 20°C, what would be the mass concentration (in μg m-3) of a resultant sulfuric acid aerosol? What if it is converted to ammonium sulfate?

    • n/V = C = P/RT= (10 x 10-9)(0.9 atm)/(0.0821 L atm/mol K)(293K)

    • C = (3.74 x 10-10 mol/L)(1 mol ammonium sulfate/1 mole SO2)*

      (132 g/mol)(106mmol/mol)(1000 L/m3) = 49 mg/m3

  • The element sodium would be expected to exist primarily in which sized particles?

    • Coarse particles. This is because Na would be expected to mostly originate from sea-salt, which is predominantly in the course mode (since sea-salt has a high % Na and few other sources of Na are significant).

  • As an organic aerosol ages, what should happened to the ratio of the mass of carbon to mass of organics?

    • As an organic aerosol ages, it becomes oxidized so that O makes up a more significant fraction of the mass. This will decrease the ratio of the mass of C to mass organics.

  • Under what conditions will aerosol particles become more acidic or less acidic as they age?

    • Aerosol particles will become more acidic if there are significant sources of SO2 (which oxidizes to sulfuric acid). They will become neutralized (more basic) if there is a significant source of ammonia gas.

  • Which sulfur dioxide oxidation process leads to ultra-fine (D<50 nm) particles?

    • Gas phase oxidation (Aqueous phase oxidation requires large enough particles for nucleation of cloud droplets).

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Atmospheric AerosolsSome Example Problems/Questions – cont.

  • What is the mole fraction of ammonium sulfate in an aerosol particle present at a RH of 95%?

    At 95% RH, PH2O/P̽H2O = 0.95 = XH2O (mole fraction of water)

    0.95 = nH2O/(nH2O + 3nAS) note: nAS = moles ammonium sulfate)

    0.95nH2O + 2.85nAS = nH2O

    0.05nH2O = 2.85nAS

    nH2O/nAS = 57 or nAS/nH2O = 0.0175

    XAS = nAS/(nAS + nH2O) = (nAS/nH2O)/[(nAS/nH2O) + 1]

    XAS = 0.0175/(0.0175 + 1) = 0.0172 (or 1.7 % by mole)

  • What is the mass fraction of water in the above aerosol particle?

    mass ratio = mAS/mH2O = (nAS/nH2O)(132/18) = 0.128

    % H2O = mH2O*100/(mH2O + mAS)

    = (mH2O/mAS)*100/[(mH2O/mAS) + 1] = 89%

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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

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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

  • Incorporation into precipitation

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Cloud Chemistry- Incorporation of Pollutants

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Cloud Chemistry- Nucleation 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 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 Raoult’s law)

    - Compounds that reduce surface tension

    - Smaller aerosol number concentrations (less competition for water so higher RH values)

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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 concentration of liquid water in the cloud (also called liquid water content or LWC).

  • The higher the LWC, the lower the concentration (dilution effect)

  • Cloud nucleation leads to heterogeneous cloud droplet composition – Ignored here for calculations