1. Chem. 250 – 11/18 Lecture

2. Announcements I A.Exam 2 Results Average = 73 B.New Homework Set (Text Ch. 4: 25; Ch. 7: 3, 5, 6, 8, 10, 24, 25, 26, 35, 44 + Furlough Questions) Score Range N 90-922 80s4 70s5 60s2 <603

3. Announcements - II C.Topic Covering 1.Cloud Chemistry 2.Precipitation Chemistry 3.Hydrologic Cycle 4.Water Properties 5.Water Composition 6.Some of the above topics may be covered in the next lecture

4. Announcements - III D.Rough Drafts due today (one copy to me + keep track of who receives other copies) E.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 NH 4 HSO 4 with 75% efficiency and reaches a LWC of 0.5 g m -3. The SO 2 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 SO 2 only acidity and use only the source which adds the most acidity. Then determine the following: [H 2 SO 3 (aq)], [HSO 3 - ], [SO 4 2- ] 2.Derive the equation on slide 34.

6. Cloud Chemistry Rationale for Studying - Cloud reactions can be important (e.g. formation of H 2 SO 4 ) - Precipitation composition depends on cloud composition - Provide introduction to aqueous chemistry

7. Cloud Chemistry - Incorporation of Pollutants

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

9. Cloud Chemistry Nucleation 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 < 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)

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 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

12. Cloud Chemistry Nucleation 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 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 Henry’s Law: where K H = constant (at given T) and X = molecule of interest

14. Cloud Chemistry - Scavenging of Gases: “unreactive” gases When LWC and K H are relatively low, we can assume that P X is constant Then [X] = K H ∙P X 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) 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.

15. 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 = 10 -6 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)

16. 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

17. Cloud Chemistry - Scavenging of Gases: “reactive” gases For SO 2 problems in homework, assume: –Little SO 2 is depleted from gas phase (usually valid) (This means P SO2 and [H 2 SO 3 ] are constant) –pH is just determined from SO 2 (usually not valid) –The third reaction can be ignored (dissociation of HSO 3 - doesn’t affect pH) Dissolution of HNO 3 –Because both K H and K a are large, we can not assume little HNO 3 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 NO 3 - 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/m 3. Assume 100% scavenging.

19. Cloud Chemistry - Combining two scavenging methods 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)

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 H 2 O 2 - HSO 3 - + H 2 O 2 → HSO 4 - + 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 (despite what text says)

22. Cloud Chemistry - Reactions in Clouds Reaction of S(IV) and Ozone - Two main reactions: HSO 3 - + O 3 → HSO 4 - + O 2 moderately fast SO 3 2- + O 3 → SO 4 2- + O 2 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 Chemistry Some 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 Chemistry Hydrological 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 Chemistry Hydrological 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 Chemistry Hydrological Systems Sacramento Valley Tap Water West East Transect sediments granite Groundwater?? River Water?? Data From Chem. 31 N

29. Water Chemistry Properties of Water See Text for boiling point/melting point and heat capacity properties Temperature – Density Relationship: density maximum occurs at 4°C 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” (6 th Ed.)

30. Water Chemistry Properties 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 Chemistry Water 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 HCO 3 - ion (a weak base) Fresh Water - main ions are HCO 3 -, Mg 2+, Ca 2+, Na +, and Cl - - main source of major ions is dissolution of carbonates e.g. CaCO 3 (s) + CO 2 (g) + H 2 O(l) ↔ Ca 2+ + 2HCO 3 -

32. Water Chemistry Water Composition Dissolved solids –Mass of material left after evaporating water –Expressed in ppm –Surrogate measure is electrical conductivity

33. Water Chemistry Chemical Reactions Acid-Base Equilibria Dissociation of water (always important) H 2 O ↔ H + + OH - Carbon dioxide reactions: 1) Acid-Base Reactions CO 2 (g) ↔ CO 2 (aq) K H = 0.0338 M/atm CO 2 (aq) + H 2 O ↔ H + + HCO 3 - K a1 = 4.45 x 10 -7 HCO 3 - ↔ H + + CO 3 2- K a2 = 4.7 x 10 -11

34. Water Chemistry Chemical Reactions Acid-Base Properties – continued Note: If water is in contact with atmosphere, P CO2 = fixed value, so [CO 2 ] = independent of pH Other equations useful for solving water chemistry equations: Mass balance: T = [CO 2 ] + [HCO 3 - ] + [CO 3 2- ] where T = total carbonate concentration Charge balance equation: Σ(z i *[cation] i ) = Σ(z j *[anion] j ) z i = charge of ion i

35. Water Chemistry Chemical Reactions Form of carbonate as a function of pH The fraction of carbonate species α present in a single form (e.g. HCO 3 - ) can be calculated as follows: The right part to the equation can be derived from equilibrium equations When pH pK a2, CO 3 2- 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 CaCO 3 (s) ↔ Ca 2+ + CO 3 2- K sp = 4.6 x 10 -9 This reactions normally must be considered with other reactions (because in most waters, the pH is such that [HCO 3 - ] >> [CO 3 2- ]) 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 CaCO 3 in pure water (no CO 2 present) Water with carbonate soils is usually in regime where α(HCO 3 - ) > α(CO 2 ) > α(CO 3 2- ), so a more representative reaction would result in HCO 3 - By combining CaCO 3 (s) ↔ Ca 2+ + CO 3 2- with H + + CO 3 2- ↔ HCO 3 - and H 2 O ↔ H + + OH - The following is obtained: CaCO 3 (s) + H 2 O ↔ Ca 2+ + HCO 3 - + OH - where: K = K sp K w /K a2 = 9.7 x 10 -13 = [Ca 2+ ][HCO 3 - ][OH - ] Assumption that [Ca 2+ ] = [HCO 3 - ] = [OH - ] leads to [Ca 2+ ] = 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 CaCO 3 and CO 2 are present Combine simplified equation for CaCO 3 solubility with first 2 CO 2 reactions: CaCO 3 (s) + H 2 O ↔ Ca 2+ + HCO 3 - + OH - and CO 2 (g) + H 2 O ↔ H + + HCO 3 - (and H + + OH - ↔ H 2 O) Net reaction: CaCO 3 (s) + CO 2 (g) + H 2 O ↔ Ca 2+ + 2HCO 3 - Notes: 1) increased CO 2 leads to increased solubility 2) 2[Ca 2+ ] = [HCO 3 - ] expected

40. Water Chemistry Chemical Reactions From Harris, Quantitative Chemical Analysis, 6 th Ed., 2003 Low Carbonate Soils Additional CO 2 sources

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 - ] + 2[CO 3 2- ] + [HCO 3 - ] (approximate) Alkalinity = [OH - ] + 2[CO 3 2- ] + [HCO 3 - ] – [H + ] (better, but still approximate equation)

42. Water Chemistry Some 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 Chemistry Some Problems - II 1.At 5°C, the water hydrolysis equilibrium constant is 2.0 x 10 -15. What is the pH of pure water (no CO 2, no other sources of trace species)? 2.Determine the solubility of CaCO 3 in water in equilibrium with 380 ppm CO 2. What is the pH of the water? What is its alkalinity? 3.Coral is largely CaCO 3. As P CO2 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 [Ca 2+ ]?

44. Water Chemistry One 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 - ], [HCO 3 - ], [CO 3 2- ], and [CO 2 ].