The influence of functional groups on organic aerosol hygroscopicity

Description

Organic aerosols in the atmosphere are composed of a wide variety of species, reflecting the multitude of sources and growth processes of these particles. Especially challenging is predicting how these particles may act as cloud condensation nuclei (CCN). Köhler theory relates the particle’s dry diameter to its critical supersaturation. A hygroscopicity parameter, kappa, parameterizes this relationship in terms of the particle’s chemical composition. Previous studies have characterized kappa values for a range of organic model compounds. Here we extend these studies by designing new model systems that allow systematic investigation of the influence of the number and location of particular functional groups on the organic aerosols’ kappa value. Organic compounds were synthesized via gas-phase and liquid-phase reactions. Gas-phase products were collected on filters and extracted using ethyl acetate. Filter extracts and liquid-phase products were then fractionated by reversed-phase high-performance liquid chromatography using gradient elution with acetonitrile and water. The eluate was atomized, the solvent was removed by evaporation, and the residual aerosol particles were analyzed as a function of retention time using high-resolution scanning flow CCN analysis. Individual organic compounds eluting from the synthesized mixture were identified using thermal desorption particle beam mass spectrometry. These experiments yielded changes in kappa that can be attributed to the addition of one or more hydroxyl, nitrate, carboxyl, aldehyde, hydroperoxide, and methylene functional groups while otherwise maintaining the structure of the organic molecule. Our results show that the addition of hydroxyl and carboxyl groups can significantly increase a particle’s kappa value, while the addition of hydroperoxide, nitrate, and methylene groups does not. We anticipate that our results contribute to a mechanistic understanding of chemical aging and will help to guide input and parameterization choices in models that rely on simplified treatments such as the atomic oxygen-to-carbon ratio to predict the evolution of organic aerosol hygroscopicity.