GeoCape Airborne Simulator (GCAS) archived data README August 2024 PI: Scott Janz - scott.janz@nasa.gov Alternate contact: Laura Judd - laura.m.judd@nasa.gov Description: This data file contains GCAS HCHO column densities in molecules/cm^2, derived using measured spectra obtained during the STAQS flights in summer 2023 ------Revision history----------- Release 0 (R0): Initial release of HCHO the vertical column retrieval. --------------------------------- VERY IMPORTANT NOTES: Some flights show some extreme striping across track during flight. Analysis should be careful to not draw conclusions from spatial variance due to this feature. The GCAS team will explore ways to minimize this striping and if a solution is found will result in a revision to this product. The retrieval assumes there are no clouds in the region. Vertical columns are not calculated for when the count_rate in the NO2 window is greater than 125000. However, this does not perfectly remove cloudy scenes, particularly under cumulus conditions. Therefore, we recommend investigating GOES imagery, land surface features, and the count_rate parameter in the file to future refine interpretation of the data in partially cloud scenes. Method: Absolute nadir radiances are measured and aggregated to 10 cross-track positions of approximately 750 m with an average along track length of 1250 m. Note: Sample areas are approximate given typical flight altitudes and ground speed. Pixel bounds Lat/Lon data provide actual sample area used. Each average sample spectrum is calibrated using a nadir reference spectrum which is an average under clear-sky conditions over a scene with clean scene absorption. For STAQS, this reference is unique to each day and the latitude, Longitude, time, and differential slant column are saved within the file. Conversion from a differential column to a vertical column includes a reference estimate from the GEOS-CF model over the reference scene. The result is analyzed in specific spectral windows to retrieve the molecular absorption using Differential Optical Absorption Spectroscopy techniques [1]. The software package developed at the Belgian Institute for Space Aeronomy (BIRA-IASB) QDOAS [2] is used to fit cross-section data to log-normalized spectra. Cross-sections are those used in the TROPOMI HCHO operational product. Median minimum sensitivity for the slant column product is ranges from ~3.7-4.5E15 molecules/cm2 depending on day (calculated from the differential slant column uncertainty). AMFs for slant-to-vertical column conversion are derived by running VLIDORT to calculate scattering weights using an a priori modeled atmosphere and BRDF parameters for surface reflectance from MODIS MCD43A1 averaged over the domain from June 2021 for land. Over water, an isotropic parameter 0.06 is used(median iso kernel over water from MCD43A1) plus the Cox-Monk kernel in VLIDORT to represent water surface reflectivity. MODIS MCD43A1 band is in the visible at ~470nm, which is converted to 342 nm (middle of the HCHO spectral window) using ratio between the two wavelengths within the OMI LER Climatology. HCHO modeled profiles are taken from GEOS-CF analysis. https://portal.nccs.nasa.gov/datashare/gmao/geos-cf/ Vertical columns are only calculated when roll angle of the aircraft is < 10 degrees. All geolocation and geometry parameters refer to the center of the pixel sample area and take into account aircraft yaw, pitch, and roll as well as view angle. -------Relevant References for this retrieval with GCAS------------- Judd, L. M., Al-Saadi, J. A., Szykman, J. J., Valin, L. C., Janz, S. J., Kowalewski, M. G., Eskes, H. J., Veefkind, J. P., Cede, A., Mueller, M., Gebetsberger, M., Swap, R., Pierce, R. B., Nowlan, C. R., Abad, G. G., Nehrir, A., and Williams, D.: Evaluating Sentinel-5P TROPOMI tropospheric NO2 column densities with airborne and Pandora spectrometers near New York City and Long Island Sound, 13, 6113–6140, https://doi.org/10.5194/amt-13-6113-2020, 2020. Kowalewski, M. G. and Janz, S. J.: Remote sensing capabilities of the GEO-CAPE airborne simulator, SPIE Optical Engineering + Applications, San Diego, California, United States, 92181I, https://doi.org/10.1117/12.2062058, 2014. Nowlan, C. R., Liu, X., Janz, S. J., Kowalewski, M. G., Chance, K., Follette-Cook, M. B., Fried, A., González Abad, G., Herman, J. R., Judd, L. M., Kwon, H.-A., Loughner, C. P., Pickering, K. E., Richter, D., Spinei, E., Walega, J., Weibring, P., and Weinheimer, A. J.: Nitrogen dioxide and formaldehyde measurements from the GEOstationary Coastal and Air Pollution Events (GEO-CAPE) Airborne Simulator over Houston, Texas, 1–36, https://doi.org/10.5194/amt-2018-156, 2018.