A universal aerosol composition analysis method for optical tweezers measurement and its application to determine hygroscopic growth factor of single-particle aerosol

Abstract. Traditional hygroscopicity bulk measurements of aerosols using humidified tandem differential mobility analyzer (HTDMA) are limited to population-averaged properties, potentially overlooking individual particle growth processes. Although aerosol optical tweezers enable single-particle measurements, no universal and accurate method exists to determine particle dry radius and hygroscopic growth factor (GF). Here, we develop a robust method using optical tweezers to quantify GF of individual particles accurately. Solution densities were accurately predicted via apparent molar volume, and refractive indices were predicted using the molar refraction method. By fitting particle radius and refractive index across multiple relative humidities under the conservation of solute mass, we retrieve dry particle size and hygroscopic growth curves. Application to typical aerosols, including ammonium sulfate, sodium chloride, and sucrose, yields GF in excellent agreement with reported values and thermodynamic models, while extension to mixed-component particles also demonstrates broad applicability. This study provides the first accurate characterization of single-particle hygroscopic growth with optical tweezers, yielding a self-consistent set of physical parameters and a framework to test and refine thermodynamic models, while improving the representation of aerosol–radiation–cloud interactions in climate models.