Section: New Results

Assimilation of NO ₂ columns for air quality forecast

Participants : Xiaoni Wang, Jean-Paul Berroir, Vivien Mallet, Isabelle Herlin, Marc Bocquet.

This study concerns the data assimilation of satellite observations for improving the air quality forecast, performed by the Polyphemus air quality system.

Nitrogen dioxide ( NO ₂ ) plays an important role in the tropospheric chemistry and has a direct impact on the public health. A better knowledge and forecast of NO ₂ concentration are important to all issues related to air quality. In this research work available satellite data are considered: the Ozone Monitoring Instrument (OMI), aboard NASA Aura satellite, provides NO ₂ column data with a good spatial resolution (13 by 24 km² ) and daily global coverage.

First, satellite data have been compared to Polyphemus simulations: the OMI column data and the Polyphemus simulations have both been averaged over November-December 2005 in Europe, demonstrating a good consistency in Spain, Italy and North Europe (see Figure 10 ), even if model simulations have higher values than satellite observations.

Figure 10. Top: the OMI column data averaged in Europe over November and December of 2005. Down: the Polyphemus simulation of model columns averaged in Europe in the same time period.

The satellite acquisitions are then assimilated in Polyphemus. The forecast obtained with and without assimilation are compared with ground observations for validation. It is found that assimilation of these satellite data improve the NO ₂ forecast, with the RMSE between model results and ground observations reduced after assimilation.