OPAC, or the Optical Properties of Aerosols and Clouds, is a tool that allows users to study the spectral optical properties of atmospheric aerosols and clouds. Developed by the Atmospheric Radiation Measurement (ARM) Program, OPAC is a widely-used resource in atmospheric research. With the most recent version, OPAC 4.0b, users can access a database with over 40 unique aerosol types, each with their own set of optical properties. Visit the OPAC website at https://geisa.aeris-data.fr/opac/ to learn more about this powerful atmospheric research tool.