Relative Humidity Dependence of Hygroscopicity Parameter of Ambient Aerosols

The hygroscopicity parameter (kappa) has been widely used in studies of aerosol hygroscopic growth and cloud condensation nucleus activation, but the effect of relative humidity (RH) variations on ambient aerosol hygroscopicity is rarely discussed and is often ignored in the application of kappa-Kohler theory. In this study, an improved humidified nephelometer system was used to measure the scattering enhancement (f(RH)) of bulk aerosols and the RH-dependent kappa was derived. It was found that the kappa of both pure inorganic water-soluble electrolytes and ambient aerosols is significantly RH-dependent. The particle size had a great influence on f(RH), but kappa had no obvious size dependence. Compared with using the RH-dependent kappa, the scattering coefficient of ambient aerosols with the fixed kappa value would produce a relative error of -10%-25%. Parameterization schemes of RH-dependent kappa of ambient aerosols were established, which could serve as a reference for the future research.