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This technology demonstration paper reports on thedevelopment, demonstration, performance assessment, and initial data analysisof a benchtop prototype quantum cascade laser heterodyne spectroradiometer,operating within a narrow spectral window of  ∼  1 cm−1around 953.1 cm−1 in transmission mode and coupled to a passive Suntracker. The instrument has been specifically designed for accurate dry airtotal column, and potentially vertical profile, measurements of CO2.Data from over 8 months of operation in 2015 near Didcot, UK, confirm thatatmospheric measurements with noise levels down to 4 times the shot noiselimit can be achieved with the current instrument. Over the 8-month period,spectra with spectral resolutions of 60 MHz (0.002 cm−1) and 600 MHz(0.02 cm−1) have been acquired with median signal-to-noise ratios of113 and 257, respectively, and a wavenumber calibration uncertainty of0.0024 cm−1. Using the optimal estimation method and RFM as the radiative transferforward model, prior analysis and theoretical benchmark modelling had beenperformed with an observation system simulator (OSS) to target an optimizedspectral region of interest. The selected narrow spectral window includesboth CO2 and H2O ro-vibrational transition lines to enable themeasurement of dry air CO2 column from a single spectrum. The OSS andpreliminary retrieval results yield roughly 8 degrees of freedom for signal(over the entire state vector) for an arbitrarily chosen a priori state withrelatively high uncertainty ( ∼  4 for CO2). Preliminarytotal column mixing ratios obtained are consistent with GOSAT monthly data.At a spectral resolution of 60 MHz with an acquisition time of 90 s,instrumental noise propagation yields an error of around 1.5 ppm on the dryair total column of CO2, exclusive of biases and geophysical parameterserrors at this stage

Thermal infrared laser heterodyne spectroradiometry for solar occultation atmospheric CO&lt;sub&gt;2&lt;/sub&gt; measurements

Thermal infrared laser heterodyne spectroradiometry for solar occultation atmospheric CO<sub>2</sub> measurements