100 Years of Progress in Cloud Physics, Aerosols, and Aerosol Chemistry

Pioneering researchers in the early through mid 20^thcentury laid the foundations of the field of cloud physics, through their discovery of cloud nuclei and their impacts on cloud microphysics and the climate system. Rapid progress has been made in the last 100 years in understanding the sources, evolution, and composition of the atmospheric aerosol, the interactions of particles with atmospheric water vapor, cloud microphysical processes, and the relationships between cloud physics and dynamics. Major breakthroughs in measurement capabilities and in theoretical understanding have elucidated the characteristics of cloud condensation nuclei and ice nucleating particles, and the role these play in shaping cloud microphysical properties and the formation of precipitation. The resulting indirect effects of aerosol on the climate system remain unacceptably large uncertainties in future climate projections, however, as not all of these interacting factors are yet fully understood. In particular, process-level understanding of aerosol-cloud interactions remains insufficient to support technological mitigation strategies such as intentional weather modification or geoengineering, offering great opportunity for continued major contributions to fundamental science that can be applied to addressing some of the world’s most pressing societal challenges.