GLOBAL SIMULATION OF NITRATE AND AMMONIUM AEROSOLS AND THEIR RADIATIVE EFFECTS AND COMPARISON OF SATELLITE-BASED AND MODELED AEROSOL INDIRECT FORCING by Li Xu A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Atmospheric and Space Sciences) in The University of Michigan 2012 Doctoral Committee: Professor Joyce E. Penner, Co-Chair Assistant Professor Xianglei Huang, Co-Chair Associate Professor Christopher J. Poulsen Assistant Professor Derek J. Posselt © Li Xu 2012 ACKNOWLEDGEMENTS I wish to express my sincere gratitude to my advisor, Professor Joyce Penner, for her insightful guidance and continuous encouragement, which motivate and inspire me to conduct and finish this research work. I felt very fortunate just to be around her all these years, observing how she thinks, how she approaches the problem and solves it, and how she dedicates to science. It really helps me grow intellectually. I am sure I am going to benefit from this in my future career. I would like to thank my co-advisor, Assistant Professor Xianglei Huang, for sharing his own research experience and for his numerous help and enthusiastic encouragements towards the completion of this dissertation. I also wish to thank all committee members, collaborators, group members, and friends who helped me and encouraged me during the accomplishment of this dissertation. Finally, I am grateful of countless support from my family. ii TABLE OF CONTENTS ACKNOWLEDGEMENTS ……………………………………………………………..ii LIST OF FIGURES …………………………………………………………………....vi LIST OF TABLES…………………………………………………………………….xiv CHAPTER I. INTRODUCTION .................................................................................... 1 1.1 Global climate system and climate change ............................................1 1.1.1 Energy budget in the Earth system .........................................1 1.1.2 Climate change and radiative forcing .....................................4 1.2 Atmospheric aerosols and their effects on climate ................................8 1.2.1 An overview of atmospheric aerosols .....................................8 1.2.2 Aerosol direct effect ..............................................................11 1.2.3 Aerosol indirect effect...........................................................11 1.2.4 Global observed and modeled aerosol effects ......................13 1.2.5 Modeling semi-volatile inorganic aerosols ...........................18 1.3 Overview of the dissertation ................................................................19 II. A COMPARISON OF INORGANIC AEROSOL THERMODYNAMIC PROPERTIES PREDICTED BY EQSAM4 AND EQUISOLV II …………22 2.1 Introduction ..........................................................................................22 2.2 Description of two thermodynamic equilibrium models ....................25 2.3 Comparison of simulation results ........................................................33 2.3.1 Aerosol water .......................................................................39 2.3.2 Total particulate matter ........................................................40 2.3.3 Particulate nitrate ..................................................................45 2.3.4 Particulate ammonium ..........................................................50 2.3.5 Particulate chloride ...............................................................51 2.3.6 Potential of hydrogen ............................................................51 2.3.7 Dominant solid PM compounds............................................52 2.3.8 Hygroscopic growth factor (HGF) ........................................56 iii 2.4 Comparison with MINOS observations ...............................................58 2.5 Discussion and conclusions .................................................................68 III. BOX MODEL SIMULATIONS OF NITRATE AND AMMONIUM AEROSOLS ……………...……………………………………………………73 3.1 Introduction ..........................................................................................73 3.2 Methods................................................................................................78 3.2.1 Aerosol thermodynamic module ...........................................78 3.2.2 Diffusion-limited mass transfer ............................................80 3.2.3 Hybrid dynamic method .......................................................84 3.2.4 Simple kinetic-limited equilibrium method ..........................92 3.2.5 Aerosol mixing state ............................................................98 3.3 Results ................................................................................................105 3.3.1 Nitrate treatment using EM and IM ....................................105 3.3.2 Nitrate treatment using KEQ and HDYN ...........................114 3.4 Discussion and conclusions ..............................................................118 IV. GLOBAL SIMULATIONS OF NITRATE AND AMMONIUM AEROSOLS AND THIR RADIATIVE EFFECTS……….……….………..122 4.1 Introduction ........................................................................................122 4.2 Model descriptions .............................................................................129 4.2.1 Global aerosol and chemistry transport model ...................129 4.2.2 Nitrogen chemistry ..............................................................134 4.2.3 Treatment of nitrate and ammonium ...................................135 4.2.4 Emission Scenarios .............................................................138 4.3 Present-day global aerosol concentration fields ................................143 4.3.1 Sulfate .................................................................................143 4.3.2 Nitric acid and nitrate ..........................................................144 4.3.3 Ammonia and ammonium...................................................149 4.3.4 Aerosol water ......................................................................152 4.3.5 Zonal average mixing ratios ................................................153 4.4 Global budgets ...................................................................................156 4.4 Aerosol optical properties ..................................................................168 4.6 Radiative forcing of nitrate and ammonium ......................................172 4.6.1 Direct forcing of nitrate and ammonium ............................173 4.6.2 Indirect forcing of nitrate and ammonium ..........................179 4.7 Summary and discussion....................................................................195 V. COMPARISON O SATELLITE-BASED AND MODELED AEROSOL INDIRECT FORCING……………….……………………………………….201 5.1 Introduction ........................................................................................201 5.2 Methods..............................................................................................209 5.2.1 The coupled IMPACT/CAM model ...................................209 5.2.2 Calculation of cloud droplet number concentration and the first aerosol indirect forcing .........................................................211 5.2.3 Calculation of aerosol optical properties ............................214 iv 5.3 Empirical measures of aerosol-cloud interactions .............................215 5.3.1 Aerosol-cloud interaction relationships based on theory ....215 5.3.2 Statistical relationship following satellite-based method ..220 5.4 Global aerosol mass budgets in PD and PI simulations .....................223 5.5 Results of PD and PI simulations.......................................................224 5.5.1 Statistical relationship between CDNC and AOD/AI .........224 5.5.2 Sensitivity study of CCN activation with the change of aerosol concentration ..................................................................230 5.5.3 Cloud droplet number concentration and cloud effective radius ............................................................................................237 5.5.4 Aerosol optical depth and aerosol index .............................241 5.5.5 Aerosol indirect forcing ......................................................243 5.6 Discussion and conclusions ...............................................................249 VI. SUMMARY AND FUTURE WORK .................................................. 253 6.1 Summary ............................................................................................253 6.2 Future work ........................................................................................264 BIBLIOGRAPHY ……………………………………………………………………..267 v LIST OF FIGURES Figure 1.1 The global annual mean Earth’s energy budget for the March 2000 to May 2004 period (W m-2). The broad arrows indicated schematic flow of energy in proportion to their importance. Source: Trenberth et al. (2009).........................3 1.2 Global mean temperature and CO2 concentration from 1880 to 2009. Source: National Climatic Data Center…………………….…………………………...5 1.3 Comparison of outgoing shortwave radiation flux anomalies (in W m-2, calculated relative to the entire time period from 1984 to 1999) from several models in the MMD archive at PCMDI (colored lines) with ERBE satellite data (black with starts) and ISCCP flux data set (black with squares). Source: Intergovernmental Panels on Climate Change (IPCC), 2007…………………6 1.4 Global mean radiative forcing from human activities and natural processes for the year 2005 relative