Diurnal effects in limb scatter observations

Abstract

Instruments that measure UV/visible scattered light from the Earth's limb are emerging as an important class of sensors capable of providing high-quality profiles of aerosols and trace gases from the upper troposphere to the mesosphere. Critical to the inversion of limb scatter observations is the forward radiative transfer model. A fast and accurate radiative transfer model, VECTOR (Vector Orders-of-scattering Radiative transfer model), is presented that is able to account for the diurnal variation of species such as NO 2 and BrO along the observing line of sight and the incoming solar beam. VECTOR has been used to quantify for the first time diurnal effect errors in NO 2 and BrO with application to OSIRIS (Optical Spectrograph and Infra-Red Imager System) and SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography), two limb viewing satellite instruments. For a solar zenith angle near 90° at the tangent point, errors can exceed 50% for NO 2 and 100% for BrO in the lower stratosphere, with the largest errors generally occurring when viewing across, and at large angles to, the terminator. These results applied to OSIRIS NO 2 and SCIAMACHY BrO reveal that, diurnal effect errors are generally small (<10%). Yet 1 out of every 6 OSIRIS NO 2 profiles experiences large (10-35%) errors and 1 out of every 11 SCIAMACHY BrO profiles experiences large (10-100%, or larger) errors in the lower stratosphere. Copyright 2006 by the American Geophysical Union.