Our ability to predict our future climate is limited in part by our lack of understanding of fundamental atmospheric chemical processes. This session will explore the current state of knowledge in our understanding of the chemical complexities of the atmosphere, and the impact this complexity has on climate. Much effort has been focused on greenhouse gases, which are long-lived species in our atmosphere, such as carbon dioxide, methane, and nitrous oxide. For the most part, scientists largely understand the sources, lifetime, and atmospheric fate of these key atmospheric species. In contrast, another key component in our atmosphere that plays an important role in climate is aerosols, short-lived forcing agents comprised of chemically complex particles. Aerosols, comprised of dust, sea spray, soot, and bioparticles, circle the globe on time-scales of days. They present a major challenge, since they vary spatially and temporally on extremely short time-scales. Most climate models ignore this complexity and treat aerosol chemistry in a simplistic manner. As a result, our understanding of the impacts of aerosols on climate represents the single largest uncertainty in our ability to make climate predictions. This session will focus on the most recent field and laboratory studies using state-of-the-art tools to explore the level of chemical detail needed to advance the field. Gaps in knowledge will be identified to address this very important 21st century environmental grand challenge.