Oceanic sea spray is one of the largest contributors of atmospheric aerosol particles worldwide. The phase of aerosol particles is known to impact radiative forcing and cloud nucleation. However, as chemically complex aqueous systems that include mixtures of biological, organic, and salt constituents, it is a challenge to predict the phase of sea spray aerosol especially as they age in the atmosphere. In this study, phase behavior (liquid-liquid phase separation, LLPS, and crystallization) of sea surface microlayer (SSML) sample and chemical mimics are investigated using a microfluidic pervaporation approach. Internally mixed aqueous droplets with varying compositions and concentrations are trapped in microfluidic wells, and phase transitions of the droplets are optically determined in a slow dehydration process. A system containing SSML sample combined with an organic acid 3-methyl glutaric acid (3-MGA) undergoes multiple phase changes, including two crystallization and two LLPS events. The added organic acid increases the sample's organic-to-inorganic ratio and moves the system into the range typical of aged SSA. To better understand the contributing constituents to the observed phase changes, control experiments with inorganic salt components NaCl, MgCl2, and Na2SO4 are performed with and without 3-MGA, at varying organic to inorganic ratios. 3-MGA leads to LLPS, and the presence of Mg2+ more readily facilitates LLPS than Na+. With the systems studied, LLPS is more prevalent for the chemical mixtures in an intermediate OIR range. This study provides new insight into sea spray aerosol phase as a function of composition and relative humidity and demonstrates multistep phase transitions for these complex systems.
Bibliographical noteFunding Information:
This material is based upon work supported in part by the National Science Foundation under NSF CAREER Grant No. 1554936, including the measurements and analysis of the laboratory chemical mimic systems. This work was also supported by NSF through the NSF Center for Aerosol Impacts on Chemistry of the Environment (NSF-CAICE) under Grant No. CHE-1801971, including the sampling and measurements of the environmental SSML samples. Portions of this work were conducted in the Minnesota Nano Center, which is supported by the National Science Foundation through the National Nano Coordinated Infrastructure Network (NNCI) under Award No. ECCS-1542202.
Copyright © 2019 American Chemical Society.
- atmospheric aerosols
- droplet microfluidics
- liquid-liquid phase separation
- organic material
- sea spray aerosols
- sea surface microlayer