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Chem. 253 – 2/25 Lecture

Chem. 253 – 2/25 Lecture. Announcements I. Return HW 1.3 + Group Assignment Last Week’s 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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Chem. 253 – 2/25 Lecture

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  1. Chem. 253 – 2/25 Lecture

  2. Announcements I • Return HW 1.3 + Group Assignment • Last Week’s 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)

  3. 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) • Today’s Lecture Topics – Tropospheric Chemistry • Finish up cloud chemistry (Chapter 3 and 4) • Atmospheric Effects (Chapter 4)

  4. 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

  5. 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

  6. Sulfur, Aerosol, and Cloud ChemistryReview of Main Concepts III • Cloud/Precipitation Chemistry • Will review + add new material

  7. Cloud/Precipitation 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 • 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 < 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 Raoult’s 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 < d < 2.5 mm)

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

  12. 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

  13. 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?

  14. 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 Henry’s 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) • Henry’s Law: where KH = constant (at given T) and X = molecule of interest

  15. 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] = KH∙PX where PX comes from mixing ratio • When KH is high (>1000 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)

  16. Cloud Chemistry- Scavenging of Gases “unreactive” gases • For compounds with high Henry’s 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)

  17. 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 = [H+][HSO3-]/[H2SO3(aq)] HSO3- ↔ H+ + SO32- Ka2 = [H+][SO32-]/[HSO3-] Note: concentration of dissolved SO2 = [S(IV)] = [H2SO3] + [HSO3-] + [SO32-] = [H2SO3](1 + Ka1/[H+] + Ka1Ka2/[H+]2) “Effective” Henry’s law constant = KH* = KH(1 + Ka1/[H+] + Ka1Ka2/[H+]2) = function of pH

  18. 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 Henry’s law constant is 290 M/atm (at given temp.). Assume an atmospheric pressure of 0.9 atm and 20°C. • 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.

  19. Break for Group Activity

  20. Cloud Chemistry- Overview of Scavenging • Gases scavenged are almost always in Henry’s 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)

  21. 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

  22. Cloud Chemistry- What determines pH? 4 Independent Senerios

  23. 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)

  24. Cloud Chemistry- Reactions in Clouds • Cloud reactions are important for water soluble species because of higher concentrations in clouds • Only sulfur chemistry covered here

  25. Cloud Chemistry- Reactions in Clouds • Reaction of S(IV) and H2O2 - HSO3- + H2O2→ HSO4- + H2O (acid catalyzed) - Rate = k[HSO3-][H+][H2O2] - Rate = k’[H2O2]PSO2 - Effectively pH independent

  26. 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

  27. Cloud Chemistry- Reactions in Clouds

  28. 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

  29. Cloud/Precipitation Chemistry- Incorporation of Pollutants

  30. 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

  31. 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

  32. 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

  33. 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

  34. 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

  35. 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

  36. 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 can’t buffer acidic precipitation • This results in acidic lakes

  37. 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

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