DEVELOPMENT OF LIDAR TECHNIQUES TO ESTIMATE ATMOSPHERIC OPTICAL PROPERTIES by Mariana Adam A dissertation submitted to the Johns Hopkins University in conformity with the requirements for the degree of Doctor of Philosophy Baltimore, Maryland October, 2005 © Mariana Adam 2005 All rights reserved DEVELOPMENT OF LIDAR TECHNIQUES TO ESTIMATE ATMOSPHERIC OPTICAL PROPERTIES by Mariana Adam ABSTRACT The modified methodologies for one-directional and multiangle measurements, which were used to invert the data of the JHU elastic lidar obtained in clear and polluted atmospheres, are presented. The vertical profiles of the backscatter lidar signals at the wavelength 1064 nm were recorded in Baltimore during PM Supersite experiment. The profiles of the aerosol extinction coefficient over a broad range of atmospheric turbidity, which includes a strong haze event which occurred due to the smoke transport from Canadian forest fires in 2002, were obtained with the near-end solution, in which the boundary condition was determined at the beginning of the complete overlap zone. This was done using an extrapolation from the ground level of the aerosol extinction coefficient, calculated with the Mie theory. For such calculations the data of the ground-based in-situ instrumentation, the nephelometer and two particle size analyzers were used. An analysis of relative errors in the retrieved extinction profiles ii due to the uncertainties in the established boundary conditions was performed using two methods to determine the ground-level extinction coefficient, which in turn, imply two methods to determine aerosol index of refraction (using the nephelometer data and chemical species measurements). The comparison of the three analytical methods used to solve lidar equation (near-end, far-end and optical-depth solutions) is presented. An improved measurement methodology and modifications of a data processing technique are proposed to process the multiangle elastic-lidar data in clear atmospheres. The technique allows one to determine more accurate profiles of the optical depth and relative backscattering versus height. It is also shown that these profiles and the measured range-corrected signals can be used to determine the lidar overlap function versus range. The retrieved data allow one to analyze the influence of the local horizontal heterogeneity and measured lidar-data distortions, and thus, to estimate the retrieved data quality. The methodology and the data processing technique were tested with experimental data of two simultaneously scanning lidars operating in clear atmospheres. The experimental results obtained with the two lidars at different wavelengths are discussed. The results show that the multi-angle method is most suitable for the shortest wavelength (355 nm). Dissertation committee: Adviser: Dr. Marc B. Parlange Reader: Dr. Eugene D. Shchukin Reader: Dr. Vladimir A. Kovalev iii Acknowledgments I would like to thank my thesis adviser Prof. Marc B. Parlange for all his support and for giving me the freedom to do my own research. I appreciate his trust and his permanent and optimistic impulse: “Go for it, Mariana”. I want to thank my colleague Markus Pahlow, who introduced me to the lidar basics, for his patience in explaining different problems and for his willingness to share and discuss various experimental and theoretical lidar issues. I would like to express my all gratitude to Dr. Vladimir Kovalev (U.S.D.A., Fire Science Laboratory, Missoula, MT) along our collaboration for all his remarks and help. I really appreciate his patience and his willingness to help me become a lidar scientist. I am thankful for all his remarks, for all his support in overcoming my frustrations, and for trusting me. I thank Prof. Eugene D. Shchukin for accepting to be part of my thesis committee. I wish to thank my colleagues Jan Kleissl, Elie Bou-Zeid, Vijayant Kumar and Chad Higgins for their assistance in the field experiments and for coping together during “the best years of my life” in “the greatest city of America”. I am thankful for the “Chemistry team” from University of Maryland at College Park, for its nice and fruitful collaboration during Baltimore PM Supersite Experiment. Special thanks to Prof. John M. Ondov and Dr. David Harrison. I thank the lidar team from Swiss Federal Institute of Technology (EPFL), Lausanne, who helped us upgrade the lidar system. I really appreciate their efforts and I want to especially thank Dr. Valentin Simeonov, Pablo Ristori and Ioan Balin. iv I want to thank Jenny Newton and Cyle Wold for their assistance during Montana experiment as well as Wei Min Hao the project manager at Fire Science Laboratory who supported this experiment. I wish to thank all my friends along the journey, for being along my side during all good and bad times. Special thanks to my friend Anca – Monia Constantinescu. I want to thank my family for understanding and trusting me entirely to fulfill my goals. My gratification from everybody who reads this thesis. This gives me a sense and all my efforts were not in vain. v TABLE OF CONTENTS List of tables……………………………………………………………………........IX List of figures………………………………………………………………………..XI Chapter 1 1. Introduction………………………………………………………………………..1 1.1. Research context……………………………………………………..……….1 1.2. Research presentation……………………………………………….………...4 Chapter 2 2. Light propagation in atmosphere and lidar technique …………………..….……..6 2.1. Atmosphere structure and properties……………………………….………...6 2.1.1. Overview………………………………………………………...…….6 2.1.2. Troposphere………………………………………….……..………...11 2.1.3. Atmospheric boundary layer ……………………………..………….11 2.1.3.1.Unstable boundary layer………..…………………….…………..14 2.1.4. Aerosols within troposphere…………………………….…….……...15 2.1.4.1.Continental aerosol………………………………………..……...16 2.2. Light propagation in atmosphere……………………………….…………...17 2.2.1. Overview…………………………………….……………………….17 2.2.2. Light scattering by molecules (Rayleigh theory)…………………….19 2.2.3. Light scattering by particles (Mie theory)…………………..………..20 2.3. Lidar system……………………………….………………………………...23 2.3.1. Overview of a backscatter elastic lidar..……………………………...23 2.3.2. JHU lidar system biaxial configuration………………..……………..25 2.3.3. JHU lidar system coaxial configuration………………….…………..27 2.3.4. FSL lidar system biaxial configuration………………………..……..28 vi 2.4. Lidar equation and inversion methods……………………………..………..29 2.4.1. Lidar equation…………………………………………………….......29 2.4.2. Analytical solutions for lidar equation……………………………….30 2.4.2.1.Boundary point solution (far-end and near-end solutions)……….32 2.4.2.2.Optical depth solution……………………………………………32 2.4.2.3.Multiangle methods………………………………………..……..34 Chapter 3 3. Aerosol optical characterization during Baltimore PM Supersite Experiment…..39 3.1. Baltimore PM Supersite field experiment…………………………………...39 3.2. Aerosols optical properties at ground level………………………………….42 3.3. Application of the near-end solution to determine vertical profile of aerosol extinction coefficient……………………………………………………...…53 3.4. Comparison of near, far and optical depth solution………………………....64 3.5. Uncertainties arising from estimation of the boundary condition in near-end solution case………………………………………………………………....68 3.6. Conclusions………………………………………………………………….76 Chapter 4 4. Applications of Kano-Hamilton multi-angle method to determine vertical profile of aerosol optical properties and lidar overlap………………………………...…80 4.1. Real lidar in an ideal atmosphere: simulations……………………………...81 4.2. Methodology……………………………………………………………...…88 4.3. Determination of the lidar effective overlap………………………………...98 4.4. Instrumentation and measurement procedure………………………….......104 4.4.1. Instrumentation……………………………………………………...104 4.4.2. Measurement procedure…………………………………………….104 vii 4.5. Results and discussion……………………………………………………...110 4.6. Procedures to determine aerosol extinction and backscatter coefficients….131 4.6.1. Determination of the aerosol extinction coefficient………………...131 4.6.2. Determination of the aerosol backscatter coefficient…………….....138 4.6.3. Application on synthetic lidar signals………………………………140 4.7. Summary…………………………………………………………………...151 Chapter 5 5. Conclusions……………………………………………………………………..158 References Appendix I: Rayleigh scattering………………………………….…………......162 Appendix II: Mie theory………………………………………………………...169 Appendix III: Derivation of the lidar equation……………………………….....175 Appendix IV: Analytical solutions for lidar equation…………………….….....178 Appendix V: Multianlge methods…………………………………….….…......184 Appendix VI: Background subtraction and SNR estimation………….…..……186 Appendix VII: Multiangle methods improved measurement procedure.……..............190 Appendix VIII: Notations of the main variables and parameters……………….198 Bibliography………………………………………………………….…………202 Curriculum vitae………………………………………………………….…….....214 viii LIST OF TABLES Table 2.1. Atmospheric composition. Components are listed by mixing ratios representative in troposphere (TP) or stratosphere (ST), their vertical distribution and controlling processes Table 2.2. JHU lidar system, biaxial configuration. Principal characteristics Table 3.1. Correlation coefficients between measured and computed aerosol scattering and between measured aerosol scattering and mass concentration Table 3.2. Mean, standard deviation (STD), maximum, minimum and median for measured (nephelometer) and computed (Mie theory) aerosol scattering, computed (Mie theory) aerosol extinction coefficient, derived mass and mass scattering coefficient (using measured and computed scattering