Humidity-dependent photoacoustic absorption biases and effective water mass accommodation coefficients for multicomponent aerosol particles

Water uptake in humid environments modifies the optical extinction, scattering, and absorption properties of aerosol particles. Cavity ring-down spectroscopy (CRDS) and photoacoustic spectroscopy (PAS) are commonly used for sensitive measurements of aerosol extinction and absorption, respectively. However, biases in PAS-measured absorption arise at elevated relative humidity (RH) when particles contain water. This work uses aerodynamically selected particles in combination with CRDS and PAS to quantify humidification effects on extinction and absorption and characterize RH-dependent biases in PAS measurements, from which effective mass accommodation coefficients are inferred for water on light-absorbing aqueous droplets. Aerodynamic particle size selection eliminates multiple-charge artifacts that complicate determinations of aerosol optical and hygroscopic properties from traditional mobility-selected measurements. Ammonium sulfate aerosol particles and dyed polystyrene spheres are used to validate humidified optical measurements. CRDS-measured extinction is then used to determine the hygroscopicity of light-absorbing droplets, containing either nigrosin or a 1:1 mass ratio mixture of nigrosin and ammonium sulfate, enabling predictions of RH-dependent absorption. Differences between predicted and PAS-measured absorption quantify the biases in PAS-measured absorption, which are compared to a coupled heat-mass transfer model to retrieve RH-dependent effective mass accommodation coefficients (αM) for water. The retrieved αM values are compared with a hierarchy of models that provide insights into aerosol particle composition, interfacial solute partitioning, and bulk transport limitations in the interrogated multicomponent aqueous droplets. This work opens new opportunities for interrogating effective water mass accommodation on light-absorbing aerosol particles and the effects of particle composition and bulk diffusivity.