The chemistry of the atmosphere is undergoing significant changes, not only due to natural causes, but also due to human activities. The emissions of pollutants by human activities have caused significant increases in the concentration of several trace gases. This inadvertent experiment is predicted to have profound effects on air-chemistry and climate. The basic goal of the Center for Clouds, Chemistry and Climate (C4) is to develop the observational and the modeling base to understand and to predict the changing climate and air-chemistry. Climate-chemistry interactions, the role of clouds in such interactions, realistic treatment of clouds in global models and the role of human activities are the major research themes. The multi-institutional structure of the center involving universities, European institutions, federal labs and industries will facilitate unprecedented cross-fertilization between groups from diverse disciplines and accelerate the progress of research in these institutions. The proposed research should advance our knowledge of how micro-and macroscales interact and advance the treatment of clouds in global models. These fundamental advances should lead to improved predictions of the human impacts on the environment and to a better understanding of regional weather and climate phenomena, including global climate change by greenhouse gas warming. The utility and relevance of the research is very high. As indicated by the participation of industry, there is immediate technological relevance of experimental methods (data handling and image processing) developed for this project. The role of clouds is the fundamental unifying theme of the center which brings together scientists from several institutions in the U.S. and Europe who are involved in the study of chemistry, microphysics, radiation, nonlinear dynamics, weather and climate. Important regional scale climate changes are associated with clouds since they govern the rate of precipitation, sunlight, UV irradiance on the biosphere, soil moisture and removal of chemicals from the atmosphere. The radiative effects of clouds are so large that just a few percent change in cloudiness is sufficient to amplify or ameliorate by a factor of two the greenhouse effect of the trace gas increases during the last century. Even the sign of such changes are unknown. Clouds are the most poorly treated features in global models. The uncertain role of clouds in climate and chemistry is a Gordian knot of problems dealing with global changes. Clouds form because of microscale and macroscale interactions between chemistry, radiation, thermodynamics and dynamics and they in turn influence these processes significantly on the planetary scale. In the presence of sunlight, chemical reactions take place in cloud droplets converting sulfur and nitrogen compounds to sulfuric and nitric acids which are scavenged out by millimeter and larger sized rain drops. Deep clouds rapidly transport short-lived pollutants form the surface layer to the upper troposphere thus transferring a local problem into a global one. A fuller understanding of cloud phenomena requires a complex array of interrelated observations ranging from laboratory measurements, aircraft observations of a single cloud to satellite observations of the whole globe. It requires microphysical models to understand the interactions within a single cloud droplet to global models to unravel cloud feedback problems. It also requires a dedicated group to collaborate across disciplinary boundaries. The proposed research will address all of these requirements. Large volumes of satellite data will be analyzed to establish the link between sulfur chemistry and climate and to unravel the role of cloud radiative-dynamical interactions in major climate events such as the El-Nino of 1982/83 and the severe summer of 1988. Global models of chemistry and climate will be employed to understand the chemical coupling between nitrogen oxides, methane, other hydrocarbons and ozone in pristine and polluted environments. High resolution radar and satellite observations will be used in conjunction with microphysical and regional scale models to understand how the effects of individual clouds aggregate to govern regional scale climate and chemistry. C4 scientists will also participate in field measurements to develop and verify models of chemistry and climate. Useful results from the proposed research include: global distribution of UV irradiance on the biosphere; regional distributions of the anthropogenic radiative forcing during the last century; improved treatment of clouds in global models; and prediction of regional changes. Summer courses and workshops will be conducted to reach out to high school and college science teachers and other students. This outreach effort will take advantage of the effective educational programs currently in operation in the Chicago area.
