atomospheric chemistry

Duke University Nicholas School of the Environment Honor Code Cover Sheet for Exams

Student Name: _____________________________________________________ (Please Print) STATEMENT FOR EXAMS Duke University is a community dedicated to scholarship, leadership, and service and to the principles of honesty, fairness, respect, and accountability. Citizens of this community commit to reflect upon and uphold these principles in all academic and nonacademic endeavors, and to protect and promote a culture of integrity. The Pledge Students affirm their commitment to uphold the values of the Duke University community by signing a pledge that states: To uphold the Duke Community Standard: I will not lie, cheat, or steal in my academic endeavors; I will conduct myself responsibly in all my endeavors; and I will act if the Standard is compromised. I have adhered to the Duke Community Standard in completing this exam. ________________________________________ _______________________ (Student Signature) Today’s Date

ENVIRON 239/EOS 239 FINAL EXAM 40 points

DUE BY: 5 pm, Thu May 2

IMPORTANT

1. Read the questions carefully.

2. Make sure that your answers precisely address the question - no credit will be given for

material that does not address the question.

3. Write clearly, and do not clutter up the page. The progression of your answer should be

self-evident. If I cannot figure out what you write, you will not get credit for your answer.

4. The exam is open-book – you may consult your class notes, the lecture slides, and the

readings. You are not allowed to consult any other sources or search the web for answers.

5. Late exams will be penalized 8 points (20% of total) for each day or part of day late.

SUBMISSION INSTRUCTIONS

Upload your completed final exam in electronic form to Sakai Drop Box – name the file

Final_LastName.pdf or Final_LastName.docx. Or slide a hard copy under my office

door. Also note that you should upload to Sakai Drop Box or email to me your PLAS

program for Question 8.

Question 1 (3 points) Briefly describe the two specific reasons that we discussed in class as to why HOx and NOx destroy O3 chemically in the stratosphere, but generally produce O3 chemically in the troposphere. Question 2 (2 points) Briefly explain why anthropogenic VOC emissions control measures in the 1990s did not significantly reduce regional surface ozone pollution in the U.S.

Question 3 (2 points) There is evidence that surface NOx emissions from the industrialized midlatitudes of the Northern Hemisphere occasionally contribute to elevated levels of NOx at the surface at high northern latitudes. Given the relatively short chemical lifetime of NOx (1-2 days), this seems surprising till one considers organic nitrates. Using PAN (peroxyacetyl nitrate) as an example of a typical organic nitrate, explain how surface NOx emissions from the industrialized midlatitudes of the Northern Hemisphere can occasionally lead to high NOx concentrations at the surface at high northern latitudes. Question 4 (3 points) Briefly summarize how humans have modified the global carbon cycle. What is the main gap in terms of our understanding of how humans have modified the global carbon cycle?

Question 5 (3 points) Briefly explain how anthropogenic aerosols have directly and indirectly counteracted some of the warming from anthropogenic greenhouse gases? Question 6 (4 points) We have covered a variety of topics over the course of the semester. From your perspective, what is one overarching thing that you learned about atmospheric chemistry? Give some details and explain why you consider it important.

Question 7 (2 points) Assuming a globally uniform atmospheric mixing ratio of 100 ppb CO, calculate the column density of CO in units of molecules cm-2. Assume that the total moles of air in the atmosphere is 1.8 x 1020 moles, and that the radius of the Earth is 6400 km.

Question 8 (3 points) Consider the elementary reaction NO3 + NO3 à 2NO2 + O2, with a rate constant k. (a) Assuming that the temperature dependence of k for this reaction is given by the Arrhenius expression, with the activation energy equal to 20360 J mole-1 and the pre- exponential factor equal to 8.5 x 10-13 cm3 molecule-1 s-1, what is the value of k (make sure you specify units) at a temperature of 298K for this reaction? (1 point) (b) Assuming atmospheric pressure = 1000 hPa, temperature =298K, and the following initial mixing ratios at time t=0: NO3 = 1 ppb, NO2 = 0, calculate and plot the mixing ratios of NO3 and NO2 (make sure you specify units) as function of time over 5 days using PLAS. (1 points) NOTE: Upload your PLAS program to Sakai Drop Box or email it to me. Include the plot as part of your written exam. (c) At the end of 5 days, what is the chemical lifetime of NO3 (make sure you specify units)? (1 point)

Question 9 (5 points) Using the simple 1-layer greenhouse model equations we considered in class (and assuming as we did in class that incoming shortwave radiation is 342 W m-2 and that the albedo is 0.3), calculate the temperature of the surface and the atmospheric layer if 100% of outgoing longwave radiation emitted from the surface of the earth is absorbed by the atmosphere? Draw a figure (like the one we discussed in class) showing quantitatively all relevant energy fluxes for the earth’s surface, for the atmospheric layer, and for the earth/atmosphere system as a whole.

Question 10 (5 points) It has been proposed that ship emissions can contribute significantly to NOx in the marine boundary layer. Consider the following idealized scenario: • Emissions from ship per unit surface area in the marine boundary layer = 10-10 moles

m-2 s-1 • A uniformly mixed (i.e. no vertical gradient in NOx mixing ratio) marine boundary

layer with a height of 1 km • Constant NOx chemical lifetime of 1.5 days • Constant number concentration of air throughout the marine boundary layer = 2.5 x

1019 molecules cm-3 For this idealized scenario, calculate the steady-state NOx mixing ratio (in ppb) in the marine boundary layer by assuming that NOx emissions from ships is balanced by NOx chemical loss.

Question 11 (8 points) NOTE: THIS PROBLEM DOES NOT REQUIRE PLAS – YOU SHOULD SHOW ALL THE STEPS IN YOUR CALCULATIONS Consider a parcel of air in the troposphere (number density of air = 2.5 x 1019 molecules cm-3) containing the following constant mixing ratios: 30 ppb of O3, 70 ppb CO, and 30 ppt NOx (note NOx = NO + NO2). Assume that the following simplified set of reactions occurs in this air parcel: Reaction Rate Constant 1. O3 + hν à 2OH j1 = 1 x 10

-6 s-1

2. CO + OH --> CO2 + HO2 k2 = 1 x 10 -13 cm3 molecule-1 s-1

3. HO2 + HO2 --> H2O2 + O2 k3 = 5 x 10 -12 cm3 molecule-1 s-1

4. HO2 + NO --> OH + NO2 k4 = 8 x 10 -12 cm3 molecule-1 s-1

5. NO + O3 --> NO2 + O2 k5 = 2 x 10 -14 cm3 molecule-1 s-1

6. NO2 + hν --> NO + O3 j6 = 1 x 10 -3 s-1

NOTE: Some reactions are not balanced stoichiometrically. Do not worry about this. The additional species required to balance these reactions do not affect the rates of these reactions and hence can be ignored in this calculation. (a) Assuming that HOx (note HOx = HO + HO2) is in steady-state, calculate the number

concentration of HO2 in units of molecules cm-3. (2 points) (b) Assuming that NO is in steady state, and noting that NOx = 30 ppt, calculate the

number concentration of NO and NO2 in units of molecules cm-3. (2 points) (c) Assuming that the partitioning between the HOx species (i.e. the OH/HO2 ratio) is

controlled only by the fast reactions 2 and 4, calculate the number concentration of OH in units of molecules cm-3. (2 points)

(d) Assuming that the parcel is representative of the whole troposphere, calculate the net O3 chemical tendency (i.e. O3 chemical production rate - O3 chemical destruction rate) in the troposphere in units of moles year-1. Assume that the total moles of air in the troposphere is 1.6 x 1020 moles. (2 points)