Hygroscopicity of dimethylaminium-, sulfate-, and ammonium-containing nanoparticles

Abstract

Dimethylamine (DMA) and sulfuric acid (SA) are the important constituents of atmospheric aerosols. To accurately predict the behavior of DMA-containing aerosol systems, exact thermodynamic models are needed. The applicability of these models needs to be tested carefully in different experimental settings to continuously validate and improve their performance. In this work, the Extended Aerosol Inorganics Model (E-AIM) was used to simulate the hygroscopicity of aerosol particles generated from five different aqueous DMA-SA solutions. The applicability of the model was tested in the 10–200 nm size range and from DMA-SA molar ratios ranging from 1:3 to 2:1. The aerosol hygroscopic growth at 0–80% RH was determined with two tandem differential mobility analyzers, and the composition of the generated particles was measured with the Aerosol Mass Spectrometer (AMS), which revealed that the particles contained also ammonium. The model accurately captured the hygroscopicity for particles larger than 80 nm. With particles smaller than 80 nm, the model underestimated the hygroscopicity in all the studied experimental conditions. An increase in hygroscopicity parameter κ with decreasing particle size implied a plausible base evaporation in the experimental setup, which in turn may have affected the modeled hygroscopicity as the composition of the smallest particles may have differed from the AMS measurements. Coupling E-AIM to a dynamic evaporation model, however, could not produce compositions whose modeled hygroscopic behavior would match the measured hygroscopic growth at smaller sizes. Our results, therefore, suggest that DMA thermodynamics are not modeled correctly in E-AIM or there exists uncertainty in the physicochemical parameters.© 2018 American Association for Aerosol Research