Impact of fatty acid coating on the CCN activity of sea salt particles

Abstract

This study investigates the impact of fatty acids on the cloud condensation nuclei (CCN) activity of sea salt aerosol of initial size 30, 50, 70 or 90 nm. Two of the major fatty acids in the marine environment, palmitic acid (C16) and stearic acid (C18), were investigated along with their unsaturated analogues palmitoleic acid and oleic acid, respectively. Sea salt seed particles were generated by aeration through a diffuser placed inside a sea spray tank. Fatty acids were added to the particles via condensation of fatty acid vapours in a heated flask at different temperatures. The diameters and CCN activity of particles before and after condensation of fatty acids were monitored. Based on the change in mobility diameter, a coating thickness and an organic volume fraction were inferred. Addition of the unsaturated acids to the core sea salt particles did not result in hindered water uptake for any organic volume fractions studied (25–96%) and critical supersaturations generally followed the kappa addition rule assuming a kappa value of zero for the fatty acids and assuming a constant surface tension equal to that of water. For the saturated fatty acids, a deviation from the Zdanovskii, Stokes and Robinson assumption (kappa mixing rule) in the direction of hindered water uptake was observed for organic volume fractions corresponding to thick (25–29 nm) coatings of palmitic acid and even thinner coatings of stearic acid.

Keywords:

• CCN
• marine aerosol
• sea salt particle
• coating
• fatty acids

Acknowledgements

We thank Robert Lange for his help with the initial experimental set-up and good discussions on the results. We also thank Dr. Eva Emanuelsson for her help with using the Igor software. We thank Dr. Mia Frosch for her Matlab script for calculations of theoretical supersaturations. Nadia Nasser Pedersen is also acknowledged for her help in calculating carbon chain length using ChemBio 3D Ultra 14.0. We also express our gratitude to Jacques Chevallier, Department of Physics, Aarhus University for his generous help with the CRYO-TEM experiment, results and discussions. We thank Dr. Patricia Quinn, Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, USA for providing organic volume fraction and kappa data from Quinn et al. (2014).