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An Experimental Study of OH Uptake by Surfaces of Tropospheric Importance under Dry and Wet Conditions by Jong-Ho Park MASSACHUSETTS INSTITU OFTEOHNOLOGY M.S. Chemistry, Korea University, 2001 JUN 1 0 2008 Submitted to the Department of Chemistry LIBRARIES In partial fulfillment of the requirements for the degree of DOCTOR of PHILOSOPHY in CHEMISTRY p4-~fVU at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY January 2008 © Massachusetts Institute of Technology 2008. All rights reserved. Author... ....... ................ Department of Chemistry January 14, 2008 Certified by ..................... ............................. ............................ Mario J. Molina Institute Professor Thesis Supervisor Accepted by Robert W. Field Chairman, Department Committee on Graduate Students This doctoral thesis has been examined by a Committee of the Department of Chemistry as follows: Professor Jeffrey I. Steinfeld........................... h(• is Chairman Professor Mario J. Molina... ............ .......... ................ Research Advisor Professor William H. Green ................... Department ifhemical Engineering An Experimental Study of OH Uptake by Surfaces of Tropospheric Importance under Dry and Wet Conditions by Jong-Ho Park Submitted to the Department of Chemistry on January 14, 2008 in Partial Fulfillment of the Requirements for the Degree of DOCTOR of PHILOSOPHY in CHEMISTRY ABSTRACT The effect of relative humidity (RH) on OH uptake by surfaces of tropospheric importance was investigated. Due to diffusion limitation conditions, experiments were performed with parallel reactors packed with beads and using a surface dilution technique. A virtual cylindrical reactor approximation was developed to further quantify and confirm the reaction probability of OH for the surfaces of interest. While OH exposure of hydrophobic organic surfaces (paraffin wax, pyrene, and methane soot) did not result in measurable change in their relative hydrophilic properties, the presence of water vapor enhanced the OH reactivity on a hydrophilic organic surface (glutaric acid). The RH effect on OH uptake by sea salt and its components was dependent on the nature of the cations. Redistribution of surface ions under humid environments caused changes in pH on the inorganic surfaces, thereby influencing the rate- determining step in the reaction mechanism of heterogeneous OH uptake. This segregation effect explains why the RH effect on OH uptake by sea salt is determined by MgCl 2 rather than NaC1. Experimental evidence suggests that adsorbed water on the surfaces of SiO2 and A120 3 is responsible for enhancement in OH reactivity with mineral dust surfaces under high humidity conditions. Mass spectra of the gas-phase species produced from the heterogeneous reaction of OH with NaCl were obtained in order to characterize the reaction products and the kinetic mechanism. Evidence for gas-phase HC1, supplemented with kinetics modeling and experiments on heterogeneous 03 reactivity strongly suggest that C12, sole product of the heterogeneous reaction, transforms to HCI in the presence of H radicals. The C12 yields per OH collision were determined to be 0.020 and 0.022 at 0% and 6% of RH, respectively. Enhancement in C12 production was observed under wet conditions, consistent with a measured chlorine deficit on the NaCl surface. Two alternate reaction mechanisms are proposed to describe the heterogeneous OH uptake by NaC1. Thesis Supervisor : Mario J. Molina Title : Institute Professor To Eunsook, June, and Sean Acknowledgements I would not say that the last five years were easy for me. However, I could keep moving forward thanks to many people, and finally did a wonderful job. First, I want to thank my advisor, professor Mario Molina, for his invaluable advice, guidance, and encouragement. It was a great experience to work in his group. Even though he is one of the busiest scientists in the world, he has never been reluctant to share his time with me whenever I need him. I also admire his humble attitude. I remember that he delivered us mails in person going up and down the stairs in the Green Building. He is the only delivery man who won the Nobel prize. I appreciate the advices and helps from my former advisor, professor Jong-Ho Choi in Korea University. He is the person who initiated my enthusiasm for the physical chemistry. Working at the Molina group let me have many best friends. Thanks to my best Russian friend, Andrey Ivanov. He always welcomed and gave invaluable advices to solve the problems that I had. He also told me many Russian proverbs to encourage me when I was depressed. Kirsten Johnson, my best American friend, shared a lot of things including her experience about science and even her cookies. Thank you so much for your kindness, Kirsten. Ingrid Kohl is my best Austrian friend that I have to emphasize her helps. She was willing to do the experiment with me when I just joined the group. Even though it was not so long before she left for Austria, she made me familiar with the experiment by sharing her precious experience. Thanks to the other best friends: Rainer Volkamer, Jun Noda, Miguel Zavala, Elizabeth Fitzgerald, Edward Dunlea, Gregory Poskrebyshev, Philip Sheehy, and Van Tran. I also express my thanks to Luisa Molina. Thanks to my Korean friends: Hohjai Lee and his wife, Li Zhang, Keehyun Choi, Changsik Song, Jeongkwon Kim, Jae-Gook Lee, Byungkwon Min, Chankyu Joo and Mi Hee Lim. I could not have survived from the tough five years without you all. My best thanks to my parents and my brother. I can not imagine how much I owe from them in my life. My dad and mom always trust me to give the best supports. I have been learning from them almost everything that I must know to live as a good citizen, a good son, and a good parent. They also provided me Jong-Sung. He is not only my brother, but also my friend and supporter. It is wonderful to be your son and brother. I also give my appreciation to Jeong-Eun Hong, my sister-in-law. Finally, no words can be enough for my wife, Eunsook, and my two sons, June and Sean. This is for you. Contents 1. Introduction 14 1.1. OH Radicals in the Troposphere ................................. .................... 15 1.2. Discrepancy between Models and Field Measurements of OH Concentrations in the Troposphere ......................... ...................................... 17 1.3. Heterogeneous Losses of OH as Additional Missing Sinks .................... 18 1.4. Reaction Probabilities of OH on Aerosol Surfaces ....................... ... 19 1.5. Tropospheric Aerosols .................... .... ........... .................... 21 1.5.1. Organic Aerosols......................... .................... 21 1.5.2. Sea Salt ...................................... .. .. ...................... 23 1.5.3. M ineral Dust Particles ......................................................... 25 1.6. Heterogeneous Reactions under High Humidity Conditions ................... 26 1.6.1. Molecular Water Bonding to Surfaces ...................................... 27 1.6.2. Technical problems in experiments with water vapor ................... 27 1.7. Chemical Ionization Mass Spectrometry (CIMS)................................28 1.8. Thesis O utline ........................ ............. ... ................................. 29 References for Chapter 1 ................. ........................ ...................... 31 2. Experimental 38 2.1. The Flow Tube System ................................................. .. 39 2.2. OH Production and Calibration............... ............................... .........42 2.3. Water Bubbler ................ ..... ... .... .......................... 45 2.4. Parallel Reactor Tubes . ....... ......... ................ .. ................... 47 2.5. Flow Rates and Velocities ................................................ .......... 49 2.6. Pressure Reduction ................... ............................ 50 2.7. Detection Method ............................. .......................... 50 2.7.1. CI region .................................... ........ 51 2.7.2. Mass Spectrometer ........................................................... 54 References for Chapter 2 ........................ ....................... 56 3. The Effects of Relative Humidity on OH Uptake by Surfaces of Tropospheric Importance 58 3. 1. Introduction........ ....................................... 59 3. 2. Experimental ................................................................... .. 60 3.2.1. Radical Production ................ ... ........................... 60 3.2.2. Experimental Procedure ................................. ............. 61 3.2.3. Surface Preparation ................... ..................... 61 3.2.4. Theoretical Method ..................... ................ 66 3. 3. Results and Discussion .................................................................. 67 3.3.1. Reaction Probability, y ........................ ..........................67 3.3.2. The Relative Intensity of OH, Rsurface .................. ................... 68 3.3.3. The Virtual Cylindrical Reactor (VCR) Approximation ........... 70 3.3.3.1. The Validity of the VCR Approximation ..................... 72 3.3.3.2. Beads-Packing Technique (Revisited) ....................... 74 3.3.3.3. Technique of Dilution for Better Sensitivity................ 74 3.3.3.4. Determination of YOH from Rsurfce ........................... 75 3.3.4. OH-H 20 Complexes .............. .. ............ ............ 75 3.3.4.1. Energy State of the Complexes