CAL_LID_L3_Tropospheric_APro_CloudFree-Standard-V4-21 is the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) Lidar Level 3 Tropospheric Aerosol Profiles, Cloud Free Data, Standard Version 4-21 data product. This data product was collected using the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument. Data collection for this product is ongoing.The CALIPSO lidar level 3 aerosol data product reports monthly mean profiles of aerosol optical properties on a uniform spatial grid. It is intended to be a tropospheric product, so data are only reported below altitudes of 12 km. All level 3 parameters are derived from the version 4.21 CALIOP level 2 aerosol profile product and have been quality screened before averaging. The primary quantities reported are vertical profiles of the aerosol extinction coefficient at 532nm and its vertical integral, the aerosol optical depth (AOD). Aerosol type and spatial distribution information are also included. Averaged profile data is reported for all aerosols, regardless of type, and for mineral dust aerosols only. Classification of dust is based on the aerosol-type flags in the level 2 profile product. To keep level 3 file sizes manageable, four different types of level 3 files are produced, depending on the sky condition and the temporal coverage of the data prior to averaging.Description of the Four Sky Conditions (Day, Night)1) All Sky: All level 2 columns are averaged, regardless of cloud occurrence2) Cloud-Free: Only cloud-free level 2 columns are averaged3) Cloudy-Sky, Transparent: Only level 2 columns containing transparent clouds are averaged4) Cloud-Sky, Opaque: Only level 2 columns containing opaque clouds are averagedCALIPSO was launched on April 28, 2006, and continues to collect data necessary to study the impact of clouds and aerosols on the Earth's radiation budget and climate. It flies in the international A-Train constellation for coincident Earth observations. The CALIPSO satellite comprises three instruments: CALIOP, Imaging Infrared Radiometer (IIR), and Wide Field Camera (WFC). CALIPSO is a joint satellite mission between NASA and the French Agency CNES (Centre national d'études spatiales).