CHEM173 Final Exam ( ) 2

Transcription

1 1. Stratospheric Ozone (20 pts: 5, 5, 5, 5) a) What is the name of the set of reactions that control O 3 formation and destruction in the stratosphere? b) Write the four reactions named in part (a). c) What three factors are needed to calculate the rate constants for the photolysis reactions in part (b)? Which of these factors varies most with altitude? Why? d) What is the typical concentration of ozone in the ozone layer? a) The Chapman cycle describes the four steps by which O 2 is photolyzed to produce O atoms and O 3, which can then be photolyzed to recover O 2. b) O 2 +hν O+O O+O 2 +M O 3 +M O 3 +hν O 2 +O O+O 3 O 2 +O 2 c) Photolysis rate constants are calculated from quantum yield, absorption cross-section, and photon flux. Photon flux varies most with altitude because light absorbed higher in the atmosphere does not penetrate to lower levels of the atmosphere, e.g. UV light is higher above the ozone layer. d) Ozone has a concentration of up to 5-10 ppmv in the ozone layer.

2 2. Ozone Hole (40 pts: 5, 15, 5, 5, 10) a) What is the name for the depletion of the stratospheric ozone layer at the Arctic and Antarctic poles? b) The following questions refer to the depletion of stratospheric ozone in part (a). i) What meteorological feature of the polar regions is needed to transport Cl species downward from above 25km? ii) What type of clouds form when stratospheric ice freezes below 80C? iii) What two species release Cl2 by reactions on stratospheric ice particles? iv) What reaction releases Cl from Cl2 in springtime? v) What role do Cl species play when they are in the ozone layer? c) What is the role of chlorofluorocarbons in the depletion of stratospheric ozone in part (a)? d) Write the mechanism responsible for destruction of ozone by NO. e) State the net reaction for the cycle below. What role does it play in the ozone hole? ClO + ClO + M k 1 ClO ( ) 2 + M j ( ClO) 2 + hν 2 Cl + ClOO ClOO + M k 3 Cl + O 2 + M k 2Cl + 2O 4 3 2ClO + 2O 2 Write expressions for the rates of destruction of Cl, ClOO, and (ClO) 2 in terms of the concentrations of Cl, ClOO, (ClO) 2, ClO, O 3, and M and the constants k 1, j 2, k 3, and k 4. a) The ozone hole describes the depletion of the stratospheric ozone layer at the Arctic and Antarctic poles. b-i) Polar vortex transports HCl, ClO and ClONO 2 produced by photolysis at 25km. b-ii) Stratospheric ice (<-80ºC) forms PSCs. b-iii) HCl and ClONO 2 make Cl 2 on PSC particles. b-iv) Cl 2 + sunlight makes Cl by photolysis b-v) Cl, ClO catalyzes O 3 destruction to O 2. c) NO 2 +O N+ O 2 ; NO+O 3 NO 2 +O 2 ; Net: O+O 3 2O 2 d) Chlorofluorocarbons (CFCs) provide a long-lived source of reactive Cl which forms HCl, ClO and ClONO 2 produced by photolysis at 25km. e) Net: 2O 3 3O 2, destroying ~75% of O 3 lost in the ozone hole. d[cl]/dt=k 3 [M][ClOO] +j 2 [(ClO) 2 ]-k 4 [Cl][O 3 ]; d[cloo]/dt=-k 3 [M][ClOO]+j 2 [(ClO) 2 ]; d[(clo) 2 ]/dt=k 3 [M][ClO] 2 -j 2 [(ClO) 2 ].

3 3. Composition of the Atmosphere (40 pts: 10, 15, 15) a) Name the five most abundant components of the atmosphere. Give the volumetric percentage of the top two most abundant components. b) For the following tropospheric constituents, convert from the units provided to volumetric percentage. In all cases, show your work. You should assume 1 atm and 273K, and you may use the approximation that there are 22.4 L/mole air at these conditions. If the exact answer requires long division, please approximate to one significant digit but do make sure that you get the correct order of magnitude. i) CO ppmv ii) O ppbv iii) OH 10 6 molec cm -3 c) For the three atmospheric constituents listed in part (b) above, state which one is long-lived (more than 1 year), which one is moderately long-lived (1 hr to 1 yr), and which one is short-lived (less than 1 hr). (Each of the three constituents in part (b) above should be used exactly once.) State the approximate scale of spatial variability for each of these three classes of atmospheric constituent. j) CO ppmv ii) O ppbv iii) OH 10 6 molec cm -3 a) N 2, O 2, Ar, H 2 O, CO 2. 78% N 2, 21% O 2. b-i) %. b-ii) %. b-iii) (4x10-12 )% ~ 4x10-14 ~ (10 6 molec/cm 3 )*(22.4 L/mole)*(10 3 cm 3 /L)/[(6.02x10 23 molec/mole). c-i) Long-lived with spatial scales of more than 1000 km. c-ii) Moderately long-lived with spatial scales of 1 km to 100 km. c-iii) Short-lived with spatial scales of 1 m to 100 m.

4 4. Chemical Families and Cycles (40 pts: 10, 5, 10, 10, 5, 5) a) What is NO x? How does NO x differ from NO y? b) What is the most important type of anthropogenic source of NO x? c) What is the name for the pathways, interfaces and reservoirs pictured in the diagram below? d) Name the main atmospheric reservoirs of sulfur. For each reservoir, name the chemical compound, write its chemical formula, and identify the oxidation state of sulfur. e) Which reservoir is responsible for the removal of most of the reactive sulfur? What is the process (or processes) responsible for this removal? f) If the atmosphere contains 0.3 Tg(S) of SO 2 and the total removal rate is 120 Tg(S) per year, what is the residence time of SO 2 in the troposphere? a) NO x is the chemical family consisting of nitrogen oxides NO, NO 2 and NO 3. Mostly NO and NO 2 is okay too. NO y includes precursors and products that can be easily converted to or from NO x, such as PAN, HONO. b) Fossil fuel burning. c) The diagram shows the pathways by which and reservoirs through which compounds of nitrogen are transferred among the atmosphere, the hydrosphere (including the oceans), the biosphere (including plants), the cryosphere, and the geosphere (including soil), which is called the Nitrogen cycle. d) The six atmospheric reservoirs of sulfur are SO 2 (sulfur dioxide, +4), SO 4 2- (sulfate, +6), H 2 S (hydrogen sulfide, -2), COS (carbonyl sulfide, -2), DMS (dimethyl sulfide, -2), and CS 2 (carbon disulfide, -2). e) Most of the reactive sulfur is removed by wet deposition or precipitation as SO f) τ SO2 =M/F=0.3/120= a or 22 hr.

5 5. Smog (40 pts: 5, 5, 10, 10, 10, 10) a) Define air pollution. Give an example of a pollutant and identify what makes it a pollutant in the atmosphere. b) What happened in the demonstration in class with an orange peel? Which kind of smog has similar chemistry? Which detrimental effect of smog was visible in the demonstration? c) What is the main chemical component in the smog particles in London (in 1952)? What is the main chemical component in the smog particles in Los Angeles (today)? d) How is OH related to O 3? Write the most important reactions. e) Write the two limiting steps for the reaction(s) that control the production of ozone in the troposphere. Use these two reactions to write the expression for the production of tropospheric ozone (do not include null cycles). f) What radical acts as an atmospheric detergent in the troposphere? State why it is called a detergent. Name four pollutants that are oxidized by it. a) Air pollution is any atmospheric condition in which substances are present at concentrations high enough above their normal ambient levels to produce a measurable effect on man, animals, vegetation, or materials. b) The orange demonstration used high O 3 to cause VOC oxidation of the terpenes from the orange; the products of the VOC oxidation have low volatility so they condense and grow particles large enough to visibly scatter light. The orange demonstration is very similar to photochemical smog, in which high O 3 results in VOC oxidation, the products of which condense and grow particles large enough to visibly scatter light. Smog is a severe pollution episode, usually characterized by severe detrimental effects on visibility, health, and crops. In the orange demonstration, light scattering by particles illustrated the effect of reduced visibility. c) London fog results from SO 2 emitted by combustion of coal which is oxidized on particles to form sulfate. Los Angeles photochemical smog involves gas phase reactions with NO x and VOCs to produce high O 3, and high amounts of organics (SOA) and nitrates in particles. d) O 3 + hv O 2 + O( 1 D) (or O*) O( 1 D) + H 2 O 2OH O( 3 P) +O 2 O 3 e) HO 2 +NO -> NO 2 + OH RO 2 +NO -> NO 2 + RO f) The hydroxyl radical OH is the atmospheric detergent because it causes gas-phase oxidation of pollutants: e.g. SO 2 + OH H 2 SO 4 + NO 2 + OH HNO 3 + CH 4 + OH CO + CO 2 + NH 3 + OH NO +

6 6. Tropospheric Ozone (40 pts: 10, 5, 25) Figure Caption: Peak ozone mixing ratio (in ppb) in the urban plume of New York, on the fourth day of ozone-episode simulations [Sillman et al., 1990]. a) Use the figure above for ozone in New York. i) What is an ozone isopleth? ii) What is the ozone mixing ratio that corresponds to hydrocarbon emission factor of 1 and NO x emission factor of 2? iii) How will the ozone mixing ratio change if the NO x emission factor increases from 2 to 3? iv) At the NO x emission factor of 2, mark on the plot the hydrocarbon emission factor range for which ozone is sensitive to hydrocarbon emissions. b) What is a VOC? What is the difference between a hydrocarbon and a VOC? Name a compound that is both a hydrocarbon and a VOC. c) CO oxidation in the atmosphere can produce ozone. i) What is the important atmospheric oxidant in the troposphere that oxidizes CO? What other atmospheric pollutant is needed for CO oxidation to produce more ozone than is lost? ii) What are the 5 reactions that summarize CO oxidation when the oxidant and the pollutant named in (c-i) are present? Include the role of the pollutant and give the net reaction. Will this set of reactions happen at night? Why or why not? iii) What happens to the ozone production and to the pollutant when the concentrations of the pollutant are very high? a-i) An ozone isopleth is a line showing constant ozone concentration. a-ii) 120 ppb. a-iii) It will decrease by 40 ppb to 80 ppb. a-iv) From hydrocarbon emission factor of 0 to 1.5 at NO x emission factor of 1. b) VOC refers to volatile organic compounds. Hydrocarbons include molecules containing H and C, but some long-chained alkanes are not volatile, so they are not VOCs; VOCs can include O, S, and other atoms so not all are hydrocarbons. A compound that is both a VOC and a hydrocarbon is α-pinene (also CH 4, ethane, benzene). c-i) OH; NO x ; no, sunlight is also needed. c-ii) CO+OH CO 2 +H; H+O 2 +M HO 2 ; HO 2 +NO NO 2 +OH; NO 2 +hv NO+O; O+O 2 +M O 3 ; ; Net CO+2O 2 +hv CO 2 +O 3. c-iii) At very high NO x, HNO 3 is produced and removed by wet and dry deposition, and the rate of O 3 production begins to decrease.

7 7. Aerosols and Clouds (50 pts: 10, 10, 5, 5, 5, 5, 10) a) What is saturation? What is supersaturation? Include an equation in terms of the vapor pressure of water and the water saturation vapor pressure. b) How does the saturation of a warm, moist air parcel change as it rises? Explain what changes to cause this if the total amount of water in the air parcel is constant. c) What is nucleation? Give an example. d) What is a Köhler curve? e) Give the name and physical property which prevents water from nucleating in the Earth s atmosphere without a large enough preexisting particle. f) Give the name and physical property which helps water nucleate on a particle by providing energy from mixing. g) For particles composed of the soluble salt shown above, what supersaturation is required in order to activate droplets from particles which are 0.1 µm diameter at 99% RH? a) Saturation is the phase equilibrium between a component in a pure state with that same component in another phase which is a mixture. Supersaturation is 100*(e/e s -1), where e is the vapor pressure of water and e s is the saturation vapor pressure of water. b) The temperature of a moist air parcel decreases as it rises. This decrease in temperature causes a corresponding decrease in the saturation vapor pressure of water. The decrease in saturation vapor pressure means that the parcel may become supersaturated and its supersaturation will increase. c) Nucleation is the process by which a new phase is formed from a different phase. An example is aerosol particles known as CCN nucleate the formation of cloud droplets. d) Köhler curves are the curves shown in the figure above, which describe the competition between the surface tension and solubility effects in activating CCN. e) The Kelvin effect prevents water from nucleating in the atmosphere without a preexisting particle. f) The Raoult effect helps water nucleate on a particle by providing energy from mixing. g) The particle on the Köhler curve labeled 3 with a size of ~0.1 µm at 99% RH (-1% supersaturation) will activate at 0.2% supersaturation.

8 8. Measurement (40 pts: 10, 10, 10, 10) a) What type of energy transition is used in infrared spectroscopy? Illustrate with a diagram that shows energy states. b) What type of energy transition is used in X-ray fluorescence? Does this transition involve singlet or triplet states? c) Name three techniques for measuring the gas-phase chemical constituents of the atmosphere. For each technique, describe the principle of operation and whether it is an example of spectrometry or spectroscopy. d) What particle characteristics make it difficult to measure the chemical composition of particles? Include in your answer three characteristics of aerosol particles and describe how each impacts aerosol chemical measurements. a) IR measures the wavelength of energy associated with photon Absorption of photon absorption for the transition from the electronic ground state to and excited state. b) XRF measures the wavelength of energy associated with photon emission; the excited state for fluorescence is a singlet state (S 1 ). Fluorescence is faster (10-6 to 10-8 s). c-1) Chemiluminescence (spectroscopy) operates by measuring the light formed by the energy released when two molecules react. c-2) Fourier Transform InfraRed (FTIR) (spectroscopy) operates by measuring the infrared energy absorbed by each bond. c-3) Gas chromatography mass spectrometry (GCMS) (spectrometry) operates by ionizing molecules and then detecting their mass by time-of-flight, ion cyclotron, etc. d-1) The size of particles varies over 6 orders of magnitude. This characteristic makes it difficult to sample and deposit particles of different sizes. d-2) The internal mixing of many chemical constituents in individual aerosol particles make separation and detection difficult. d-3) The organic fraction of particles is complex and unresolved, with selective detection proving impossible for up to 80% of organic mass.

9 9. Climate Change (40 pts: 15, 15, 10) a) In the next three parts, contrast the 1 st (more than 4 Ga ago) and 3 rd (up to about 3 Ga ago) stages of the evolution of the Earth s atmosphere. i) What is the main difference in the chemical nature between the 1 st and 3 rd stages? ii) Name two major constituents of the 1 st and 3 rd stages that illustrate this chemical difference. iii) What major change led to the development of the composition of the 3 rd (most recent) stage? b) Refer to the diagram below for the following questions. i) Draw a circle on the diagram around the two major roles played by greenhouse gases. ii) What is the name for the specific range of the longwave radiation identified below? iii) What is the name given to the total reflected solar radiation identified on the diagram below? c) Consider how radiative forcing affects the atmospheric radiation balance? i) What type of chemical component of the atmosphere has a positive radiative forcing? Give a specific chemical example. ii) What type of chemical component of the atmosphere has a negative radiative forcing? Give a specific chemical example. a-i) The 1 st stage was reducing and the 3 rd stage was oxidizing. a-ii) The 1 st stage included H 2 O, H 2, CH 4, NH 3 ; the 3 rd stage included O 2, O 3, less CO 2. a-ii) The major change was the beginning of algae and other photosynthesizing plant life which released O 2 from CO 2. b-i) Greenhouse gases absorb and emit longwave radiation in the atmosphere. b-ii) The longwave radiation absorbed and re-emitted in the atmosphere is infrared. b-ii) The total reflected solar radiation is the albedo. c-i) Greenhouse gases have a positive radiative forcing; examples include CFCs, N 2 O, H 2 O, CO 2, CH 4. c-ii) Most aerosol particles have a negative radiative forcing; examples include sulfate, nitrate, and organic particles.

10 10. Extra Credit (10 pts) Name up to five Take-Home Messages that describe a role that aerosol particles play in atmosheric chemistry. 2. An ozone hole was unpredictable!!! 7. Particle SIZE matters!!! 8. Clouds are made from particles!!! 9. Most rain is acidic!!! 10. Particle pollution causes cooling!!!