GENERAL COMMENTS

The large portions of the GHG gases emit from global mega cities and point sources. Remote sensing form upper sky is a powerful tool to capture potential emission plumes, however, the amount of data with satellites and airplanes is limited. Local time of the existing satellites are around noon. The emission estimates from satellite data have large uncertainties due to local weather conditions such as wind speed and direction. Diurnal variation data from the fixed point will improve emission monitoring. The modification of the light source from a direct solar light (COCCON) to scattered light instrument and its characterization tests are well described in the manuscript. Technical portions are good. However, scientific discussions are needed. I have the following general comments. The discussions on additional characterization and applications will improve the scientific significance. Major revisions are needed.

(1) Retrieval

The present retrieval seems to be modification of the direct solar measurement such as COCCON to slant column densities. Aerosols over LA Basin causes large fluctuations with the large off nadir geometry. Are the authors planning to retrieve several parameters such as aerosol optical thickness, surface albedo, surface pressure from EM27 spectra?

Does the surface reflectance over the LA basin has strong dependency on solar zenith angles? Are there critical angles in viewing and solar zenith? Discussions on which parameters are retrieved and assumed will suggest the importance of the measurements.

(2) Instrument Resources

The EM25 spectrometer is still heavy and expensive, if we install several systems from different location. Is it possible to reduce size and weight by relaxing spectral resolution?

(3) Polarization sensitivity?

EM27/SUN for the direct sun does not care the input light polarization. However, surface reflected light and aerosol scattered light are polarized. Aerosol scattering is a major contamination source for slant viewing measurement over megacities. Do the authors characterize the instrument polarization? Do they try to measure the polarization of the input light by installing and rotating the polarizer in the front optics?

SPECIFIC COMMENTS

(1) P11, Line 189 “geometric assumptions, … not uniform”

It is not clear. More detailed description is needed.

(2) P14, Lines 283-284, “SNR …. is most compact”　

How the authors mean by “compact”?  Does it mean calculated SNR has low variation?

(3) Page 19 Figure 14 and Page 21, Figure 15,

Discussion on wind speed and direction, possible CO2 and CH4 emission sources and ground measured surface pressure in the LA basin will improve the readers understanding. Does the TCCON data at Caltech, Pasadena show the similar trend of the diurnal variation of that day with the West Pasadena data?

TECHNICAL CORRECTIONS

(1) Page 16, Figure 11 “RMS”

Dese it mean “RMS of SCD.

(2) Page 24, References. Journal title abbreviation　

Examples for reference types are available at https://www.atmospheric-measurement-techniques.net/submission.html.

The California Laboratory for Atmospheric Remote Sensing (CLARS) – Fourier Transform Spectrometer is an established facility located at Mt Wilson above the Los Angeles Basin. CLARS maps CO2 and CH4 in the urban atmosphere above LA by measurement of solar radiation scattered from targets around the basin below the observing station. Sequential measurements from a range of targets provides a mapping of CO2 and CH4 concentrations across the basin which can be used through meteorological modelling to elucidate local sources and sinks.

CLARS is a large, fixed spectrometer and thus not mobile or usable at other sites. This paper describes a valuable extension of the measurement principle to using a portable, low resolution FTS (modified Bruker EM27) called the EM27/SCA that can in principle be deployed at other suitable sites with little infrastructure requirement. The paper provides a good technical description of the EM27/SCA, an assessment of SNR and measurement precision, and a month of side-by-side measurements beside CLARS in the LA basin to assess overall performance.

This presentation is of high quality, clear and well presented, and certainly well suited to publication in AMT. However I feel it stops a bit short of its eventual purpose and usefulness and would be a much more valuable paper if the analysis could be extended as described in the general comments below. I have also made specific and minor technical comments directly in the attached pdf version of the manuscript.

General comments

1. The neglect of scattering in the RemoTeC retrieval of vertical column densities (VCDs) from spectra leads to significant artificial variability in retrieved VCDs as seen clearly in Fig 14. These are large enough to mask the true variability which is the ultimate aim of these measurements. This is most clearly seen in O2, which should be constant. Why has the analysis stopped at this point, with the neglect of scattering, when it is the main identified cause of the inaccuracy in the measurements? To my knowledge RemoTeC can handle scattering in its forward model, so why not include it? This may not be feasible, in this case the authors should explain why. If it is feasible, Figures such as Fig 14 would be immensely more informative because at present most of the diurnal variability we see is artefact; geophysical features of interest are masked by the artefact.
2. Although the principal aim of the paper is technical, it would benefit at the end with some simple interpretation of the observed variability (after allowing for the scattering arefacts in 1 above) in terms of local sources and sinks using wind and other meteorological data. This is the ultimate aim of the work, but underrepresented in the present version. A full analysis, say with tomography, is outside the scope of the paper, but some simplified interpretation would improve the paper considerably.
3. CO precision. The low CO precision is ascribed to low SNR due to the low throughput near the detector cutoff wavelength. What is the impact of solar CO on the retrievals and their accuracy and precision? Much of the solar spectrum is dominated by CO lines at very different conditions (temperature, pressure), and the solar CO lines from different parts of the solar disk are also shifted in wavelength relative to terrestrial CO lines. What is the impact of solar CO on the terrestrial CO retrieval?