Aerosol Retrieval and Assimilation, phase 2 (ARIA-2)

The objective of the ARIA-1 project was to create a working prototype of a system to estimate the global distribution of aerosol by the assimilation of satellite retrieved aerosol optical depths (AOD) in the global atmospheric transport/chemistry model TM3. In the second phase (ARIA-2) described in this report, the global assimilation system is updated, and tested both for Europe and the INDOEX region. In addition a regional assimilation system using the LOTOS-EUROS model with higher spatial resolution is applied over Europe, yielding estimates of aerosol emission and PM 2.5 concentration. It is shown that as a result of the assimilation both models are adjusted towards the retrieved AOD values. Assimilation of observational data into the global TM4 model effectively acts as a source of aerosols, improving the agreement with independent surface observations of AOD. However, the free running models both systematically underestimate the AOD values significantly. As a consequence the assimilation schemes induce in LOTOS-EUROS a large and unrealistic change in the emissions and in TM4 strong discontinuities in the distribution of aerosols in space and time. These results indicate the need to improve the modelling of the total aerosol mass and the subsequent calculation of aerosol optical depths. Moreover it shown that more accurate retrieved AOD data are needed to improve emission estimates by the LOTOS-EUROS assimilation system. It is also suggested to constrain the models in the future by ground based measurements, and to further increase the spatial resolution.