Abstract
Abstract. 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.
Highlights
Atmospheric aerosols contribute significantly to the uncertainties in the prediction of climate radiative forcing (IPCC, 2007)
The relative humidity (RH) dependence of aerosol optical properties, e.g. light scattering, is a crucial input parameter for accurate estimation of the direct radiative forcing by aerosols
The information of aerosol hygroscopicity is always insufficiently implemented in climate models
Summary
Atmospheric aerosols contribute significantly to the uncertainties in the prediction of climate radiative forcing (IPCC, 2007). By varying the insoluble mass fraction, with modulating the comparison results between the measured and calculated f (RH) at a specific RH to the minimum deviation, the aerosol hygroscopicity parameter can be obtained It provides a possible way for the prediction of aerosol activation property, building a bridge for the estimation of CCN number concentration with the measured PNSDs. We show that the retrieval algorithm for κ based on f (RH) measurements is of significant utility and applicability. An application of the retrieved κ in the prediction of the CCN number concentration (NCCN), along with the comparison study of NCCN between the calculated and in situ measured values, is performed These results are of great reference value to the relevant model simulations of aerosol particles in the northern part of the NCP region
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