Articles | Volume 8, issue 11
Research article
23 Nov 2015
Research article |  | 23 Nov 2015

Sensitivity of remotely sensed trace gas concentrations to polarisation

D. M. O'Brien, I. N. Polonsky, and J. B. Kumer

Abstract. Current and proposed space missions estimate column-averaged concentrations of trace gases (CO2, CH4 and CO) from high resolution spectra of reflected sunlight in absorption bands of the gases. The radiance leaving the top of the atmosphere is partially polarised by both reflection at the surface and scattering within the atmosphere. Generally, the polarisation state is unknown and could degrade the accuracy of the concentration measurements. The sensitivity to polarisation is modelled for the proposed geoCARB instrument, which will include neither polarisers nor polarisation scramblers to select particular polarisation states from the incident radiation. The radiometric and polarimetric calibrations proposed for geoCARB are outlined, and a model is developed for the polarisation properties of the geoCARB spectrographs. This model depends principally upon the efficiencies of the gratings to polarisations parallel and perpendicular to the rulings of the gratings. Next, an ensemble of polarised spectra is simulated for geoCARB observing targets in India, China and Australia from geostationary orbit at longitude 110° E. The spectra are analysed to recover the trace gas concentrations in two modes, the first denied access to the polarimetric calibration and the second with access. The retrieved concentrations using the calibration data are almost identical to those that would be obtained with polarisation scramblers, while the retrievals without calibration data contain outliers that do not meet the accuracies demanded by the mission.

Short summary
We investigate the sensitivity to polarisation of geoCARB, a proposed geostationary mission to measure column-averaged concentrations of CO2, CH4 and CO from spectra of reflected sunlight, through analysis of an ensemble of simulated polarised spectra for targets in India, China and Australia. Although geoCARB will not carry polarisers or polarisation scramblers, we show that the effects of polarisation can be minimised provided the polarisation signature of geoCARB is calibrated before flight.