Relative humidity impact on aerosol parameters in a Paris suburban area

Measurements of relative humidity (RH) and aerosol parameters (scattering cross section, size distribu- tions and chemical composition), performed in ambient at- mospheric conditions, have been used to study the influence of relative humidity on aerosol properties. The data were acquired in a suburban area south of Paris, between 18 and 24 July 2000, in the framework of the "Etude et Simulation de la Qualit´ e de l'air en Ile-de-France" (ESQUIF) program. According to the origin of the air masses arriving over the Paris area, the aerosol hygroscopicity is more or less pro- nounced. The aerosol chemical composition data were used as input of a thermodynamic model to simulate the varia- tion of the aerosol water mass content with ambient RH and to determine the main inorganic salt compounds. The cou- pling of observations and modelling reveals the presence of deliquescence processes with hysteresis phenomenon in the hygroscopic growth cycle. Based on the Hmodel, pa- rameterisations of the scattering cross section, the modal ra- dius of the accumulation mode of the size distribution and the aerosol water mass content, as a function of increasing RH, have been assessed. For the first time, a crosscheck of these parameterisations has been performed and shows that the hygroscopic behaviour of the accumulation mode can be coherently characterized by combined optical, size distribu- tion and chemical measurements.