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ABSTRACT This paper is the second in a pair of papers on the topic of the generation of a two-colour artificial star [which we term a laser photometric ratio star (LPRS)] of de-excitation light from neutral sodium atoms in the mesosphere, for use in precision telescopic measurements in astronomy and atmospheric physics, and more specifically for the calibration of measurements of dark energy using type Ia supernovae. The two techniques, respectively, described in both this and the previous paper would each generate an LPRS with a precisely 1:1 ratio of yellow (589/590 nm) photons to near-infrared (819/820 nm) photons produced in the mesosphere. Both techniques would provide novel mechanisms for establishing a spectrophotometric calibration ratio of unprecedented precision, from above most of Earth’s atmosphere, for upcoming telescopic observations across astronomy and atmospheric physics; thus greatly improving the performance of upcoming measurements of dark energy parameters using type Ia supernovae. The technique described in this paper has the advantage of producing a much brighter (specifically, brighter by approximately a factor of 103) LPRS, using lower power (≤30 W average power) lasers, than the technique using a single 500 W average power laser described in the first paper of this pair. However, the technique described here would require polarization filters to be installed into the telescope camera in order to sufficiently remove laser atmospheric Rayleigh backscatter from telescope images, whereas the technique described in the first paper would only require more typical wavelength filters in order to sufficiently remove laser Rayleigh backscatter. 

ABSTRACT The largest uncertainty on measurements of dark energy using type Ia supernovae (SNeIa) is presently due to systematics from photometry; specifically to the relative uncertainty on photometry as a function of wavelength in the optical spectrum. We show that a precise constraint on relative photometry between the visible and near-infrared can be achieved at upcoming survey telescopes, such as at the Vera C. Rubin Observatory, via a laser source tuned to the 342.78 nm vacuum excitation wavelength of neutral sodium atoms. Using a high-power laser, this excitation will produce an artificial star, which we term a ‘laser photometric ratio star’ (LPRS) of de-excitation light in the mesosphere at wavelengths in vacuum of 589.16, 589.76, 818.55, and 819.70 nm, with the sum of the numbers of 589.16 and 589.76 nm photons produced by this process equal to the sum of the numbers of 818.55 and 819.70 nm photons, establishing a precise calibration ratio between, for example, the r and $z$ filters of the LSST camera at the Rubin Observatory. This technique can thus provide a novel mechanism for establishing a spectrophotometric calibration ratio of unprecedented precision for upcoming telescopic observations across astronomy and atmospheric physics; thus greatly improving the performance of upcoming measurements of dark energy parameters using type SNeIa. The second paper of this pair describes an alternative technique to achieve a similar, but brighter, LPRS than the technique described in this paper, by using two lasers near resonances at 589.16 and 819.71 nm, rather than the single 342.78 nm on-resonance laser technique described in this paper. 

Abstract. MethaneAIR is the airborne simulator of MethaneSAT, an area-mapping satellite currently under development with the goal of locating and quantifying large anthropogenic CH4 point sources as well as diffuse emissions at the spatial scale of an oil and gas basin. Built to closely replicate the forthcoming satellite, MethaneAIR consists of two imaging spectrometers. One detects CH4 and CO2 absorption around 1.65 and 1.61 µm, respectively, while the other constrains the optical path in the atmosphere by detecting O2 absorption near 1.27 µm. The high spectral resolution and stringent retrieval accuracy requirements of greenhouse gas remote sensing in this spectral range necessitate a reliable spectral calibration. To this end, on-ground laboratory measurements were used to derive the spectral calibration of MethaneAIR, serving as a pathfinder for the future calibration of MethaneSAT. Stray light was characterized and corrected for through fast-Fourier-transform-based Van Cittert deconvolution. Wavelength registration was examined and found to be best described by a linear relationship for both bands with a precision of ∼ 0.02 spectral pixel. The instrument spectral spread function (ISSF), measured with fine wavelength steps of 0.005 nm near a series of central wavelengths across each band, was oversampled to construct the instrument spectral response function (ISRF) at each central wavelength and spatial pixel. The ISRFs were smoothed with a Savitzky–Golay filter for use in a lookup table in the retrieval algorithm. The MethaneAIR spectral calibration was evaluated through application to radiance spectra from an instrument flight over the Colorado Front Range. 

Abstract. Formaldehyde (HCHO) has been measured from space for morethan 2 decades. Owing to its short atmospheric lifetime, satellite HCHOdata are used widely as a proxy of volatile organic compounds (VOCs; pleaserefer to Appendix A for abbreviations and acronyms), providing constraintson underlying emissions and chemistry. However, satellite HCHO products fromdifferent satellite sensors using different algorithms have received littlevalidation so far. The accuracy and consistency of HCHO retrievals remainlargely unclear. Here we develop a validation platform for satellite HCHOretrievals using in situ observations from 12 aircraft campaigns with a chemicaltransport model (GEOS-Chem) as the intercomparison method. Application tothe NASA operational OMI HCHO product indicates negative biases (−44.5 %to −21.7 %) under high-HCHO conditions, while it indicates high biases (+66.1 % to+112.1 %) under low-HCHO conditions. Under both conditions, HCHO a priorivertical profiles are likely not the main driver of the biases. By providingquick assessment of systematic biases in satellite products over largedomains, the platform facilitates, in an iterative process, optimization ofretrieval settings and the minimization of retrieval biases. It is alsocomplementary to localized validation efforts based on ground observationsand aircraft spirals.