Effects of relative humidity and temperature on sulphate aerosol optical properties

van Ypersele de Strihou, Jean-Pascal [UCL] Bertrand, C.

During their interaction with water vapor, hygroscopic aerosols modify their size, shape and refractive index, which alters their radiative properties. Furthermore, when the temperature changes, the equilibrium between the water vapor pressure above the surface of the liquid aerosols and the ambient water vapor pressure is affected, leading to a modification on the aerosol size and refractive index. This paper presents a study of the influence of temperature on sulphate aerosol optical properties. Our results indicate that both sulphate specific extinction and asymmetry factor are affected by temperature changes via the temperature dependence of the growth factor and of the refractive index of the humidified particles. However these changes appears to be of minor importance compared to the modifications introduced in response to relative humidity changes. This finding seems to give credence to the often adopted assumption of using optical constants and thermodynamic properties of the aerosol chemical species at fixed temperature of 25°C at which the major measurement have been made.