Evaluation of an aerosol optical scheme in the chemistry-transport model CHIMERE

This paper presents an aerosol optical scheme developed in the chemistry-transport model CHIMERE dedicated to calculate optical properties of particles. Such developments are very helpful as they complement the usual validation with PM (Particulate Matter) ground-based measurements by using surface (AERONET/PHOTONS network) and satellite (MODIS) remote sensing observations. To reach this goal, Aerosol Optical Thickness (AOT), column-averaged Single Scattering Albedo (SSA) and asymmetry parameter (g) are calculated at 440 nm, 675 nm, 870 nm and 1020 nm (AERONET wavelengths) under three hypotheses on the particle mixing state (external, internally homogeneous and core-shell). Furthermore and in addition to optical calculations, an original development has been made to estimate column volume size distributions in CHIMERE, directly comparable with AERONET retrievals. Comparisons between simulations and observations are made over Western Europe for the year 2003 but also for one specific case focused on ammonium nitrate aerosols. Observed AOT display a seasonal cycle (with highest values during summer) rather well reproduced by the model but biases with observational data have been found depending on seasons. In fall, winter and early spring, modeled AOT values agree well with AERONET retrievals with small negative biases. Focus on a pollution episode of ammonium nitrate origin during March 2003 reveals that CHIMERE is able to well reproduce the fine mode volume size distribution retrieved by AERONET, leading to good agreements between modeled and observed AOT. In late spring and summer, AERONET AOT values are underpredicted by the model, which could be due to uncertainties in modeling secondary species.