Chem. 253 – 2/25 Lecture

Announcements I Return HW 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)

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)

Sulfur, Aerosol, and Cloud Chemistry Review 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

Sulfur, Aerosol, and Cloud Chemistry Review of Main Concepts II Sulfur Chemistry –both natural and anthropogenic sources –mostly emitted as SO 2, but reduced S is also important –predominant pathway is oxidation to H 2 SO 4 –gas phase oxidation occurs through 2-step OH reaction –gas phase H 2 SO 4 production can lead to new particle formation (although mostly leads to growth of existing particles) –aqueous phase SO 2 oxidation adds mass to accumulation mode sized particles

Sulfur, Aerosol, and Cloud Chemistry Review of Main Concepts III Cloud/Precipitation Chemistry –Will review + add new material

Cloud/Precipitation Chemistry - Incorporation of Pollutants

Cloud Chemistry - Incorporation of Pollutants Main mechanisms - Nucleation of cloud droplets on aerosol particles - Scavenging of gases - Reactions within the droplet

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 (d < 50 nm) have more curved surfaces and are harder to nucleate

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  m)

Cloud Chemistry - Nucleation of Cloud Droplets aerosol size distribution (mass based) log d p (  m) log d p (  m) Nucleation efficiency hygroscopic aerosol Soot, soil

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 H 2 O/m 3 of air The higher the LWC, the lower the concentration (dilution effect) Cloud nucleation leads to heterogeneous cloud droplet composition – Ignored here for calculations

Cloud Chemistry Nucleation 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?

Cloud Chemistry - Scavenging of Gases Also Important for covering water chemistry (e.g. uptake of CO 2 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 K H = constant (at given T) and X = molecule of interest

Cloud Chemistry - Scavenging of Gases: “unreactive” gases When LWC and K H are relatively low, we can assume that P X is constant (good assumption for SO 2 and CO 2 ) Then [X] = K H ∙P X where P X comes from mixing ratio When K H is high (>1000 M/atm), conservation of mass must be considered (P X decreases as molecules are transferred from gas to liquid) We will only consider 2 cases (low K H case and 100% gas to water case)

Cloud Chemistry - Scavenging of Gases “unreactive” gases For compounds with high Henry’s law constants, a significant fraction of compound will dissolve in solution f A = K H RT(LWC) where f A = aqueous fraction (not used in assigned problems) When f A ~ 1, can use same method as for cloud nucleation From Seinfeld and Pandis (1998)

Cloud Chemistry - Scavenging of Gases: “reactive” gases Many of the gases considered are acidic and react further Example: Dissolution of SO 2 gas Reaction:Equation: SO 2 (g) + H 2 O(l) ↔ H 2 SO 3 (aq)K H = [H 2 SO 3 ]/P SO2 H 2 SO 3 (aq) ↔ H + + HSO 3 - K a1 = [H + ][HSO 3 - ]/[H 2 SO 3 (aq)] HSO 3 - ↔ H + + SO 3 2- K a2 = [H + ][SO 3 2- ]/[HSO 3 - ] Note: concentration of dissolved SO 2 = [S(IV)] = [H 2 SO 3 ] + [HSO 3 - ] + [SO 3 2- ] = [H 2 SO 3 ](1 + K a1 /[H + ] + K a1 K a2 /[H + ] 2 ) “Effective” Henry’s law constant = K H * = K H (1 + K a1 /[H + ] + K a1 K a2 /[H + ] 2 ) = function of pH

Cloud Chemistry Some Example Problems Example Problem (low K H case): What is the concentration of CH 3 OH in cloud water if the gas phase mixing ratio is 10 ppbv and a LWC of 0.2 g/m 3 ? 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 K H case): Determine the pH and aqueous NO 3 - concentration (in M) if air containing 1 ppbv HNO 3 enters a cloud with a pressure of 0.90 atm, a T = 293K, and a LWC of 0.50 g/m 3. Assume 100% scavenging.

Break for Group Activity

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 P x (pre-cloud) = P x (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)

Cloud Chemistry - What determines pH? It is complicated Strong acids (HNO 3 (g) and H 2 SO 4 (l)) provide [H + ], tempered by NH 3 and other bases Both SO 2 and CO 2 can add acidity through reaction of H 2 XO 3 with water In many senarios, including “background” locations, neither SO 2 nor CO 2 significantly contribute to pH

Cloud Chemistry - What determines pH? SourcepH 400 ppmv CO ppbv SO ppbv HNO 3 ; LWC = 0.5 g/m  g/m 3 NH 4 HSO 4 aerosol; LWC = 0.5 g/m Independent Senerios

Cloud Chemistry - A Modeled Example example 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)

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

Cloud Chemistry - Reactions in Clouds Reaction of S(IV) and H 2 O 2 - HSO H 2 O 2 → HSO H 2 O (acid catalyzed) - Rate = k[HSO 3 - ][H + ][H 2 O 2 ] - Rate = k’[H 2 O 2 ]P SO2 - Effectively pH independent

Cloud Chemistry - Reactions in Clouds Reaction of S(IV) and Ozone - Two main reactions: HSO O 3 → HSO O 2 moderately fast SO O 3 → SO O 2 fast reaction is faster at high pH because more S(IV) is present in reactive forms

Cloud Chemistry - Reactions in Clouds

Oxidation of S(IV) –H 2 O 2 is more important oxidant in acidic clouds –O 3 can be important in cleaner air –Bulk models underestimate O 3 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 (H 2 O 2 /O 3 ) at combined pH ~ 1000 rate ratio (H 2 O 2 /O 3 ) from independent reactions in two drops ~ 0.5

Cloud/Precipitation Chemistry - Incorporation of Pollutants

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 From Mosimann ETH Dissertation diffusion growth (top) to high degree of riming (bottom)

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

Chapter 4: Consequences of Polluted Air Effects Covered in Chapter 4 Include: Haze, Acid Precipitation, and Health Effects We will cover health effects when covering toxicology

Chapter 4: Consequences of 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 –NO 2 absorption and soot absorption contribute to a lesser extent

Chapter 4: Consequences of Polluted Air - Haze Light scattering is most mass efficient (most scattered light per g of aerosol) for d p ~ 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

Chapter 4: Consequences of Polluted Air – Acid Rain Main contributors are strong acids HNO 3 and H 2 SO 4 These species form slowly (e.g. relative to ozone), so worst places are downwind of major NO x and SO 2 sources Besides pollution sources, two other factors are important: –atmospheric neutralization –soil chemistry

Chapter 4: Consequences of Polluted Air – Acid Rain Neutralization by Atmospheric Bases –NH 3 (from fertilizers and animal excretions) –CaCO 3 (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

Chapter 4: Consequences of 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) ↔ Al OH - Many such metals are toxic to fish at higher concentrations