Estimation of aerosol liquid water from optical scattering instruments using ambient and dried sample streams

Relative humidity (RH) is generally considered to be an interference (“bias”) factor for optical scattering instruments used for measuring aerosol mass concentrations. In this study, the “bias” of one kind of nephelometric monitors (Thermo PDR-1500) caused by high RH is shown to be quantitatively related to the aerosol liquid water (ALW). The method utilizes ambient and dried air samples measured by identical optical scattering instruments and a companion continuous mass concentration measurement, also sampling the dried air stream. This novel and simple method for estimating observed ALW (1) avoids the use of the complicated and expensive instruments for ALW measurement or estimation; (2) can be replicated elsewhere, (3) will improve the performance of models that predict ALW, and (4) indicates the correction of single channel nephelometers using RH alone and empirical equations is subject to error due to variation in the aerosol composition.