Lower Tropospheric Composition by Combination of Operational Nadir Sounders with Limb-Emission Observations
van Weele, Michiel1; Siddans, Richard2; Peuch, Vincent-Henri3; Kerridge, Brian2; Kerridge, Brian2

The co-flight of MetOp-SG, including Sentinel-5, with the Earth Explorer 7 candidate mission PREMIER with two limb emission instruments in, respectively, IR and mm-wave, is expected to significantly improve the 3D-monitoring of the atmospheric composition. Especially noteworthy is the potentially improved information that could be derived from the operational nadir sounders on the lower tropospheric composition, i.e. on the atmospheric layers most important for the well-being of mankind in terms of air quality. Lower-tropospheric composition data will also improve on emission inversions based on total-column nadir observations which especially in the case of the CH4 column mean mixing ratio which critically depend on the assumed height of the tropopause and the UTLS CH4 profile.

In this presentation we will discuss the potential improvements for lower tropospheric O3, CO, CH4, and NO2, based on two ESA preparatory science and retrieval an performance studies performed in the framework of PREMIER. Although different approaches to limb-nadir combinations can be pursued which are equivalent in terms of their theoretical information content in an optimal estimation framework, substantial benefit may be expected from adopting mid-upper tropospheric retrieved distributions from limb-emission sounders as a priori information for nadir sounders. This approach will constrain the vertical distributions of trace gases in the higher layers such that independent, collocated information from the nadir sounders can be obtained which is concentrated on the lower troposphere. Reductions in retrieval uncertainty will be presented that show quantitatively the positive impact on the observation of (lower) tropospheric O3, CO, CH4, and NO2 of the combination of MetOp-SG observations with PREMIER limb-emission observations. The limb-information from PREMIER on mid-upper tropospheric NO2 will help to better characterize the NOX lightning source as well as the convective uplift of polluted air masses. Also constraints on stratospheric NO2 will provided by PREMIER. The largest benefits from limb IR NO2 observations on lower tropospheric NO2 are expected for low-to-intermediate tropospheric NO2 columns which critically depend on information of the vertical profile.

The synergistic assimilation of limb-profile data together with tropospheric information from nadir sounders is intensively being investigated within MACC-II. An independent study has recently been reported by Miyazaki et al.(ACP, 2012). Data assimilation helps to fill information gaps that result from the fact that observation systems can only provide discrete information in space and time and can also help to diagnose issues with the individual observational data sets. We will show some results from the joint assimilation of nadir and limb observations of O3 and CO.