Surfactant Properties Influence Hygroscopic Growth of Submicron and Supermicron Model Sea Salt Particles

AMANDA FROSSARD, Rachel Bramblett, Ben Swanson, University of Georgia

     Abstract Number: 338
     Working Group: Aerosol Physical Chemistry and Microphysics

Abstract
The size and composition of atmospheric aerosol particles modulate their interactions with solar radiation, and thus their influence on climate and visibility. The radiative effects of aerosol particles remain a large uncertainty in accurately modeling and predicting current and future climates. Sea spray aerosol particles are a significant fraction of particle mass and number in the atmosphere, and their size and composition influence their radiative properties and cloud condensation nuclei activity. Hygroscopic growth of these particles is dependent on their size and the hygroscopicity of the sea salt and organic fractions. Recent work has demonstrated the presence of organic surfactant molecules in sea spray aerosol particles, in both submicron and supermicron size regimes. The role of the chemical and interfacial properties of surfactants and their overall influence on the hygroscopic growth of aerosol particles is still unknown. Here, we demonstrate the influence of surfactants with different properties on the hygroscopic growth and deliquescence of model inorganic salt aerosol particles. We compare the hygroscopic growth of individual, supermicron particles measured with an aerosol optical trap to that of a distribution of submicron aerosol particles measured with a humidified tandem differential mobility analyzer. The properties of surfactants within individual, supermicron aqueous NaCl particles, including molecular size, structure, and ionicity, determine their effect on NaCl particle hygroscopic growth. Supermicron particles containing both NaCl and large, nonionic surfactants showed an increase in hygroscopic growth from that of pure aqueous NaCl particles by up to 3%. However, this deviation was not observed in aqueous NaCl particles containing anionic or cationic surfactants, suggesting different electrostatic interactions between the different surfactants and NaCl ions in solution. This work demonstrates that the charge of the surfactant polar head group controls the influence on particle hygroscopic growth for supermicron particles, due in part to interactions with the salt ions. For the submicron particle sizes, the presence of surfactants alters the growth factors of sea salt aerosol particles differently at humidities above and below the sea salt deliquescence relative humidity. In this study, the addition of ionic surfactants increased sea salt hygroscopic growth factors by up to 4% at 60% relative humidity but decreased growth factors by up to 6% at 85% relative humidity. Additionally, the presence of ionic surfactants in submicron sea salt particles decreased their characteristic deliquescence relative humidity. The changes in sea salt particle hygroscopic growth varied for the different surfactants studied demonstrating the influence of individual surfactant properties, most notably ionicity, on particle hygroscopic growth.