Dedication: We dedicate this chapter to the memory of our friend and colleague, Dr. Fitzgerald Booker. Fitz made many lasting contributions to air pollution science through his primary research and leadership in organizing informal and formal research collaborations, such as the Air Pollution Workshop and NE-1013. He also contributed to the career advancement of many by unselfishly sharing his lab, expertise and time. He was a meticulous editor and author, careful researcher and patient teacher. His life is a reminder to appreciate each day, be helpful to those around us, and do good work.