GeoCape Airborne Simulator (GCAS) archived data                               
README March 2023

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 TRACER-AQ flights in August-September 2021

------Revision history-----------
Release 0 (R0): Initial release of HCHO the vertical column retrieval.  
Release 1 (R1): Fixed geolocation errors, ozone correction, and surface reflectivity assumption errors from R0
---------------------------------

Method:

Absolute nadir radiances are measured and aggregated to 30 
cross-track positions of approximately 250 m with an average along track length of 560 m.
To reduce the noise, pixels are later aggregated to 750 x 1680 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 TRACER-AQ, this reference is from Sept. 25th
between 19.4325-19.4525 UTC with coincident TROPOMI measurements of 1.25E16 molecules/cm2 
in the column but this product has a noted 29% low bias which is accounted for in the reference.

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. 

Average minimum sensitivity for the slant column product is ~3.5E15 molecules/cm2.
 
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 0.03 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.  

The retrieval assumes there are no clouds in the region, therefore the cloud_glint_fraction should be applied when using this data. This flag is calculated using count_rates in the NO2 window where the likelihood of a cloud and sun glint impacts are expected for count rates > 125000.  The fraction compares from the fraction of pixels within the 3 x 3 aggregated grid that have count rates above this threshold.   

Pixels with slant column retrieval error > 2-sigma above the mean are removed. 

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.