| 07 Sep 2026
The NASA Plankton, Aerosol, Cloud, ocean Ecosystem mission Postlaunch Airborne eXperiment (PACE-PAX)
Abstract. 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.
This manuscript reviews the overarching design, realization, and outcomes of the PACE-PAX campaign, the intensive suborbital validation program for the PACE and EarthCARE satellite missions in September 2024 near California. A validation traceability matrix (VTM) approach is used to systematically coordinate the sampling by a diverse collection of platforms and instrument teams. Data from the campaign are already available on NASA repositories. The manuscript is well written with a few minor points to consider.
The scope is significantly larger than a typical data paper, covering instruments and measurements data across multiple disciplines. The VTM is critical to bring all pieces together. Beyond what are already provided in Table 1 and Fig. 8, it is recommended to include more details and justifications on the key decision-making scores. For example, how and why the importance numbers are chosen, and what are the formula for the fulfillment function and the remaining objective function. Those are the unique aspects of this paper, as many data sets are expected to be presented in greater details and higher accuracy in future publications.
Including preliminary, coordinated campaign vs. EarthCARE data will strengthen the results.
Minor points:
Spell out R/V at first appearance.
Line 133: Level 2 does not appear to be particularly "new" or "advanced". Some may argue Level 2 products are among the basic deliverables for a mission. Why not just "validate Level 2 retrieval parameters"?
Line 158: be specific in the objective, e.g., "test algorithms under specific geophysical processes or phenomena"
Figures 3 and 4: clarify what "ER-2 5-points" mean in the lower right panel.
Figure 5: the blue and red double arrows for PRISM and PICARD are narrower than their respective coverage. Are they merged like this? If so, it should be clarified in the text and caption. What is the meaning of shadings? Where were the data collected?
Line 367: check the citation.
Table 5: clarify "Facility" in the second column.
Caption of Fig. 10: check if the panel b is referred to at the right sentence.
Figure 11: plotting PACE data along the same UTC axis is confusing, especially with a vertical line indicating the PACE timing. Consider using distance along the flight leg, maybe a second axis for UTC. The red and magenta lines are difficult to separate; consider different line styles or more distinctive colors.