Abstract
Abstract. The Sub-Millimetre Radiometer (SMR) on board the Odin satellite performs limb sounding measurements of the middle atmosphere to detect molecular emission from different species. Carbon monoxide (CO) is an important tracer of atmospheric dynamics at these altitudes, due to its long photochemical lifetime and high vertical concentration gradient. In this study, we have successfully recovered over 18 years of SMR observations, providing the only dataset to date being so extended in time and stretching out to the polar regions, with regards to satellite-measured mesospheric CO. This new dataset is part of the Odin/SMR version 3.0 level 2 data. Much of the level 1 dataset – except the October 2003 to October 2004 period – was affected by a malfunctioning of the phase-lock loop (PLL) in the front end used for CO observations. Because of this technical issue, the CO line could be shifted away from its normal frequency location, causing the retrieval to fail or leading to an incorrect estimation of the CO concentration. An algorithm was developed to locate the CO line and shift it to its correct location. Nevertheless, another artefact causing an underestimation of the concentration, i.e. a line broadening, stemmed from the PLL malfunctioning. This was accounted for by using a broader response function. The application of these corrections resulted in the recovery of a large amount of data that was previously being flagged as problematic and therefore not processed. A validation study has been carried out, showing how SMR CO volume mixing ratios are in general in good accordance with the other instruments considered in the study. Overall, the agreement is very good between 60 and 80 km altitude, with relative differences close to zero. A positive bias at low altitudes (50–60 km) up to +20 % and a negative bias up to −20 % at high altitudes (80–100 km) were found with respect to the comparison instruments.
Highlights
Of the carbon monoxide (CO) produced at surface level by anthropogenic sources as well as by oceans and biogenic sources, very little is transported upwards to the stratosphere, as it is chemically destroyed by reacting with the hydroxyl radical (OH)
High altitudes are characterised by strong fluxes of radiation in the Schumann–Runge bands and continuum and Lyman-α wavelengths which are strongly absorbed by CO2; CO2 photolysis becomes more significant with height in the upper mesosphere and lower thermosphere, providing a major source of CO and resulting in a large vertical gradient in its concentration
Before the application of the corrections described in this study, almost the whole Odin/Sub-Millimetre Radiometer (SMR) CO dataset was unusable due to line shifts and broadening of instrumental origin, due to the phase-lock loop malfunctioning
Summary
Of the carbon monoxide (CO) produced at surface level by anthropogenic sources (e.g. industrial activities, biomass burning, transport and heating) as well as by oceans and biogenic sources, very little is transported upwards to the stratosphere, as it is chemically destroyed by reacting with the hydroxyl radical (OH). The first retrieval results from CO measurements obtained with the Sub-Millimetre Radiometer (SMR) on board Odin were presented by Dupuy et al (2004) They observed seasonal variations of CO concentration related to global circulation and chemical processes as predicted from the Whole Atmosphere Community Climate Model (WACCM), to which they compared their results obtaining an overall agreement within 2 orders of magnitude. There are not many studies about Odin/SMR measurements of CO, such measurements have been performed from August 2001 until today This is due to a malfunctioning of the phase-lock loop (PLL) in the front end used for CO observations, which caused the majority of CO data to present artefacts which made them unusable for retrievals. We present comparisons with ground-based measurements from the Onsala Space Observatory
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