ESSDD Earth System Science Data Discussions ESSDD Earth Syst. Sci. Data Discuss. 1866-3591 Göttingen, Germany 10.5194/essd-2026-541 The NASA Plankton, Aerosol, Cloud, ocean Ecosystem mission Postlaunch Airborne eXperiment (PACE-PAX) Knobelspiesse Kirk D. https://orcid.org/0000-0001-5986-1751 1 Cetinić Ivona https://orcid.org/0000-0002-1363-3136 1 2 Cairns Brian 3 Leblanc Samuel E. https://orcid.org/0000-0003-0173-3890 4 5 Ueyama Rei 4 NASA Goddard Space Flight Center, Greenbelt, MD, USA GESTAR II Morgan State University, Greenbelt, MD, USA NASA Goddard Institute for Space Studies, New York, NY, USA NASA Ames Research Center, Moffett Field, CA, USA Bay Area Environmental Research Institute, Moffett Field, CA, USA 07 09 2026 2026 1 55 Copyright: © 2026 Kirk D. Knobelspiesse et al. 2026 This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/ This article is available from https://essd.copernicus.org/preprints/essd-2026-541/ The full text article is available as a PDF file from https://essd.copernicus.org/preprints/essd-2026-541/essd-2026-541.pdf

The PACE Postlaunch Airborne eXperiment (PACE-PAX) was a field campaign conducted in September 2024 in California to collect validation data for the NASA Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) and JAXA/ESA Earth, Cloud, Aerosol and Radiation Explorer (EarthCARE) missions. PACE-PAX utilized coordinated observations from two research aircraft, several research vessels, and additional ocean and ground-based assets to collect comprehensive measurements of atmospheric aerosols, clouds, ocean color properties, and land surface characteristics. The NASA ER-2 high-altitude aircraft flew 13 research flights with 80.9 flight hours total, carrying remote sensing instruments serving as a proxy for those on PACE, plus a High Spectral Resolution Lidar. The Navy Postgraduate School CIRPAS Twin Otter flew 17 research flights totaling 60 flight hours at low altitude, equipped with in-situ aerosol and cloud microphysics instrumentation. The NOAA R/V Shearwater conducted 15 day trips collecting ocean color radiometry, bio-optical properties, and water samples in the Santa Barbara Channel. The R/V Blissfully, a 30-foot sailboat, conducted 9 sampling days in the San Pedro Channel collecting ocean color radiometry profiles with a HyperPro instrument and discrete water samples for phytoplankton pigment and absorption measurements. A Slocum glider occupied a 10 km transect in the Santa Barbara Channel for 24 days with in-situ CTD, ocean color radiometry, optical backscatter, and broadband acoustic measurements. Data were also collected as part of the PACE Validation Science Team (PVST) or other externally funded efforts. Operations were coordinated using a Validation Traceability Matrix (VTM) decision support algorithm to maximize validation opportunities while targeting diverse geophysical conditions. This included varying atmospheric, land and ocean properties throughout California and nearby regions, as well as observations of a major biomass burning event in the Los Angeles region and harmful algal blooms (HABs) in the Monterey Bay. PACE-PAX airborne data are archived at the NASA Atmospheric Science Data Center with standardized formatting at https://doi.org/10.5067/SUBORBITAL/PACE-PAX/DATA001 (Knobelspiesse et al., 2025a). Ocean data are archived at the SeaWiFS Bio-optical Archive and Storage System (SeaBASS), at https://doi.org/10.5067/SeaBASS/PACE-PAX/DATA001 (Knobelspiesse et al., 2025b). This comprehensive dataset provides critical early-mission validation data for novel PACE and EarthCARE remote sensing capabilities and is also a rich dataset for other purposes.