Abstract. Water condensed on ambient aerosol particles plays significant
roles in atmospheric environment, atmospheric chemistry and climate. Before
now, no instruments were available for real-time monitoring of ambient
aerosol liquid water contents (ALWCs). In this paper, a novel method is
proposed to calculate ambient ALWC based on measurements of a
three-wavelength humidified nephelometer system, which measures aerosol light
scattering coefficients and backscattering coefficients at three wavelengths
under dry state and different relative humidity (RH) conditions, providing
measurements of light scattering enhancement factor f(RH). The proposed ALWC calculation method includes two steps: the first
step is the estimation of the dry state total volume concentration of ambient
aerosol particles, Va(dry), with a machine
learning method called random forest model based on measurements of the
“dry” nephelometer. The estimated Va(dry) agrees
well with the measured one. The second step is the estimation of the volume
growth factor Vg(RH) of ambient aerosol
particles due to water uptake, using f(RH) and the
Ångström exponent. The ALWC is calculated from the
estimated Va(dry) and Vg(RH). To validate the new method, the ambient ALWC calculated
from measurements of the humidified nephelometer system during the Gucheng
campaign was compared with ambient ALWC calculated from ISORROPIA
thermodynamic model using aerosol chemistry data. A good agreement was
achieved, with a slope and intercept of 1.14 and −8.6 µm3 cm−3 (r2 = 0.92), respectively. The
advantage of this new method is that the ambient ALWC can be obtained solely
based on measurements of a three-wavelength humidified nephelometer system,
facilitating the real-time monitoring of the ambient ALWC and promoting the
study of aerosol liquid water and its role in atmospheric chemistry,
secondary aerosol formation and climate change.
Compensation of Light Scattering Method for Real-Time Monitoring of Particulate Matters in Subway Stations
