A novel method for calculating ambient aerosol liquid water contents based on measurements of a humidified nephelometer system

Abstract. Water condensed on ambient aerosol particles plays significant roles in atmospheric environment, atmospheric chemistry and climate. So far, no instruments are available for real-time monitoring of ambient aerosol liquid water contents (ALWC). In this paper, a novel method is proposed to calculate ambient ALWC based on measurements of a three-wavelength humidified nephelometer system. A humidified nephelometer system measures aerosol light scattering coefficients and backscattering coefficients at three wavelengths under dry and different relative humidity (RH) conditions, and therefore provides measurements of light scattering enhancement factor f(RH). The proposed method of calculating ALWC includes two steps. The first step is estimating total volume concentration of ambient aerosol particles in dry state (Va (dry)) with a machine learning method based on measurements of the “dry” nephelometer. The estimated Va (dry) agrees well with the measured Va (dry). The second step is estimating the volume growth factor Vg(RH) of ambient aerosol particles due to water uptake using f(RH) and Ångström exponent. The ALWC is calculated from the estimated Va (dry) and Vg(RH). Uncertainty analysis of the estimated Va (dry) and Vg(RH) is conducted. This research have bridged the gap between f(RH) and Vg(RH). The advantage of this new method is that the ambient ALWC can be obtained using only measurements from a three-wavelength humidified nephelometer system. This method will facilitate the real-time monitoring of the ambient ALWC and help for studying roles of aerosol liquid water in atmospheric chemistry, secondary aerosol formation and climate change.