Aerosol hygroscopicity parameter derived from the light scattering enhancement factor measurements in the North China Plain

The relative humidity (RH) dependence of aerosol light scattering is an essential parameter for accurate estimation of the direct radiative forcing induced by aerosol particles. Because of insufficient information on aerosol hygroscopicity in climate models, a more detailed parameterization of hygroscopic growth factors and resulting optical properties with respect to location, time, sources, aerosol chemistry and meteorology are urgently required. In this paper, a retrieval method to calculate the aerosol hygroscopicity parameter, κ , is proposed based on the in situ measured aerosol light scattering enhancement factor, namely f (RH), and particle number size distribution (PNSD) obtained from the HaChi (Haze in China) campaign. Measurements show that f (RH) increases sharply with increasing RH, and that the time variance of f (RH) is much greater at higher RH. A sensitivity analysis reveals that the f (RH) is more sensitive to the aerosol hygroscopicity than PNSD. f (RH) for polluted cases is distinctly higher than that for clean periods at a specific RH. The derived equivalent κ , combined with the PNSD measurements, is applied in the prediction of the cloud condensation nuclei (CCN) number concentration. The predicted CCN number concentration with the derived equivalent κ agrees well with the measured ones, especially at high supersaturations. The proposed calculation algorithm of κ with the f (RH) measurements is demonstrated to be reasonable and can be widely applied.