Section: New Results
Satellite data assimilation for air quality forecast
The latest generation of satellite sensors dedicated to atmospheric chemistry provide chemical measurements of the lowest layers of the troposphere: MOPITT (Measurement Of Pollution In The Troposphere) and OMI (Ozone Monitoring Instrument) are already operational on NASA platforms, whereas the IASI (Infrared Atmospheric Sounding Interferometer) and GOME-2 (Global Experiment Monitoring Experiment-2) have been launched by ESA in October 2006. All these instruments provide tropospheric columns of selected trace gases (ozone, nitrogen oxydes): the vertical integral in the troposphere of the gas concentration.
The objective of this work is to assess the feasibility of assimilating the 0-6km ozone column, to be provided by IASI, within the regional Chemistry-Transport Model Polyphemus, in view of improving air quality forecast at European scale. This study is led in the contexts of the ESA-EumetSat ``EPS-MetOp Research Annoucement of Opportunity'' (the Clime project being Principal Investigator), and of the TRAQ proposal (TRopospheric composition and Air Quality) for the future ESA tropospheric mission (around 2012), coordinated by KNMI, and comprises two main steps.
The first step is to assess the potential of the 0-6km ozone columns for air quality modelling, i.e. for improving the knowledge of ozone in the atmospheric boundary layer. A Polyphemus simulation for the month of July 2001, extensively validated, serves as reference and allows to conclude that the boundary layer represents a The sensitivity of boundary layer ozone to changes of concentration in the free troposphere is furthermore evaluated to a mean of 25%. This suggests that a better control of free troposphere ozone, to be provided by the satellite instrument, will in turn allow to better control boundary layer ozone.
The second step consists in leading data assimilation experiments. The IASI data not being available yet, a simulation chain has been developed, based on an atmosphere description (Polyphemus concentrations in the 0-5km domain, climatologies above), on the radiative transfer model LBLRTM (AER), on the IASI instrument model (kindly provided by IPSL/SA) and on the IASI operational inversion procedure SA-NN, developed by IPSL/SA. The simulated 0-6km columns have an average 27% error as compared to the reference, which is larger than the expected 20% accuracy of the IASI columns. The simulated data has been assimilated in a perturbated Polyphemus simulation, by means of an Optimal Interpolation procedure. A 5% improvement of boundary layer ozone concentrations has been observed. The next steps will concern the assimilation of the actual IASI data, expected during 2007.