California Laboratory for Atmospheric Remote Sensing
The FTS operates in two modes: the Spectralon Viewing Observations (SVO) and the Los Angeles Basin Surveys (LABS). In the SVO mode, the Fourier Transform Spectrometer (FTS) points at a Spectralon® plate placed immediately below the FTS telescope to quantify the total column trace gases above the Los Angeles megacity (above 1670m and the basin PBL height). The SVO measurements represent approximately free tropospheric background levels. In the LABS mode, the FTS points downward at 33 geographical points in the basin acquiring spectra from reflected sunlight in the near-infrared region. Our measurement technique from Mount Wilson mimics satellite observations that measure surface reflectance from space or atmospheric absorptions of GHGs along the optical path – (1) from the sun to the surface and (2) from the surface to the instrument. The locations of the reflection points are selected to provide the best coverage of the megacity.
In addition, reflection points are chosen with uniform surface albedo across the spectrometer field of view using a near-infrared camera. Reflection points sample from the San Bernardino Mountains in the east to the Pacific coast in the west and from the base of the San Gabriel Mountains in the north to Long Beach Harbor and Orange County in the south. The footprints in the Los Angeles megacity are ellipses with surface area ranging from 0.04 to 21.62 km2.
It is optimized for high spectral resolution reflected sunlight measurements with high spectral resolution in the near-infrared (NIR) region (4,000 –15,000 cm-1). The spectral resolution used in the CLARS-FTS measurement is 0.12 cm-1, with an angular radius of the field of view of 0.5 mrad.
To derive slant column densities (SCDs) of atmospheric trace gases from the measured absorption spectra, CLARS uses two standard operational retrieval algorithms: CLARS-GFIT developed by Fu et al. (2014) and GFIT3 developed by Zeng et al. (2021). CLARS-GFIT is a modified version of the GFIT program which is a state-of-the-art profile scaling algorithm to retrieve gas concentrations and related atmospheric and instrumental parameters from absorption spectra. It has been the primary retrieval algorithm for the Total Carbon Column Observing Network (TCCON) network, which has been the benchmark for validating satellite-based trace gas observations. GFIT3 is a full physics algorithm, which is also an extension of the GFIT model, to retrieve GHGs in polluted urban atmospheres from spectra of reflected solar radiation. GFIT3 accounts for aerosol scattering effects in polluted urban atmospheres. In particular, the algorithm includes coarse (including sea salt and dust) and fine (including organic carbon, black carbon, and sulfate) mode aerosols in the radiative transfer model.
Overall, the GHG retrievals from CLARS-FTS are potentially complementary to existing ground-based and space-borne observations to monitor anthropogenic GHG fluxes in the LA megacity.