INTRODUCTION Atmospheric aerosols,which are ubiquitous in the planetary boundary layer,have received much attention due to their effects on visibility,climate and human health.Water,a semi-volatile species,is ubiquitous in the atmosphere as well.The interaction between water vapor and aerosols plays a key role in determining the aerosol properties in the atmosphere and therefore has great effects on both air quality (Fig.la and b) and climate change (Fig.lc),which is depicted in Fig.1 as a function of atmospheric relative humidity (RH) and temperature.