Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 3 2005 10.5194/acpd-5-4115-2005 http://www.atmos-chem-phys-discuss.net/5/4115/2005/ http://www.atmos-chem-phys-discuss.net/5/4115/2005/acpd-5-4115-2005.html http://www.atmos-chem-phys-discuss.net/5/4115/2005/acpd-5-4115-2005.pdf 4115 4141 2005-06-27 Evolution of aerosol optical thickness over Europe during the August 2003 heat wave as seen from CHIMERE model simulations and POLDER data A. Hodzic R. Vautard H. Chepfer P. Goloub L. Menut P. Chazette J. L. Deuzé A. Apituley P. Couvert Laboratoire de Météorologie Dynamique, Institut Pierre-Simon Laplace, Palaiseau, France Laboratoire d’Optique Atmosphérique, Lille, France Laboratoire des Sciences du Climat et de l’Environnement, Institut Pierre-Simon Laplace, Gif sur Yvette, France National Institute of Public Health and the Environment, Bilthoven, The Netherlands This study describes the atmospheric aerosol load encountered during the large-scale pollution episode that occurred in August 2003, by means of the aerosol optical thicknesses (AOTs) measured at 865 nm by the Polarization and Directionality of the Earth's Reflectances (POLDER) sensor and the simulation by the CHIMERE chemistry-transport model. During this period many processes (stagnation, photochemistry, forest fires) led to unusually high particle concentrations and optical thicknesses. The observed/simulated AOT comparison helps understanding the ability of the model to reproduce most of the gross AOT features observed in satellite data, with a general agreement within a factor 2 and correlations in the 0.4–0.6 range. However some important aerosol features are missed by the simulation using regular sources. Additional simulations including emissions and high-altitude transport of smoke from wildfires that occurred in Portugal indicate that these processes could dominate the signal. This study also highlights the difficulties of comparing simulated and POLDER-derived AOTs due to large uncertainties in both cases. Observed AOT values are significantly lower than the simulated ones (30–50%). The simultaneous comparison with the ground-based Sun photometer Aerosol Robotic Network (AERONET) measurements suggests, for the European sites considered here, an underestimation of POLDER-derived aerosol levels with a factor between 1 and 2. AERONET AOTs compare better with simulations (no particular bias) than POLDER AOTs do.