Aerosol hygroscopicity is crucial for understanding roles
of aerosol particles in atmospheric chemistry and aerosol climate effects.
Light-scattering enhancement factor f(RH, λ) is one of the
parameters describing aerosol hygroscopicity, which is defined as
f(RH, λ) = σsp(RH, λ)∕σsp(dry, λ), where
σsp(RH, λ) or σsp(dry, λ) represents σsp at
wavelength λ under certain relative humidity (RH) or dry conditions.
Traditionally, an overall hygroscopicity parameter κ can be
retrieved from measured f(RH, λ), hereinafter referred to as
κf(RH), by combining concurrently measured particle number
size distribution (PNSD) and mass concentration of black carbon. In this
paper, a new method is proposed to directly derive κf(RH)
based only on measurements from a three-wavelength humidified nephelometer
system. The advantage of this newly proposed approach is that
κf(RH) can be estimated without any additional information
about PNSD and black carbon. This method is verified with measurements from
two different field campaigns. Values of κf(RH) estimated
from this new method agree very well with those retrieved by using the
traditional method: all points lie near the 1 : 1 line and the square of
correlation coefficient between them is 0.99. The verification results
demonstrate that this newly proposed method of deriving
κf(RH) is applicable at different sites and in seasons of
the North China Plain and might also be applicable in other regions around
the world