Effects of Wind Speed on Size-Dependent Morphology and Composition of Sea Spray Aerosols
Chamika K. Madawala, Carolina Molina, Deborah Kim, Dilini Kirindigoda Gamage, Mengnan Sun, Raymond J. Leibensperger, Lincoln Mehndiratta, Jennie Lee, Chathuri P. Kaluarachchi, Ke’La A. Kimble, Greg Sandstrom, Charbel Harb, Julie Dinasquet, Francesca Malfatti, Kimberly A. Prather, Grant B. Deane, M. Dale Stokes, Christopher T. Lee, Jonathan H. Slade, Elizabeth A. Stone, Vicki H. Grassian, Alexei V. Tivanski
Abstract
Variable wind speeds over the ocean can have a significant impact on the formation mechanism and physical-chemical properties of sea spray aerosols (SSA), which in turn influence their climate-relevant impacts. Herein, for the first time, we investigate the effects of wind speed on size-dependent morphology and composition of individual nascent SSA generated from wind-wave interactions of natural seawater within a wind-wave channel as a function of size and their particle-to-particle variability. Filter-based thermal optical analysis, atomic force microscopy (AFM), AFM infrared spectroscopy (AFM-IR), and scanning electron microscopy (SEM) were employed in this regard. This study focuses on SSA with sizes within 0.04-1.8 μm generated at two wind speeds: 10 m/s, representing a wind lull scenario over the ocean, and 19 m/s, indicative of the wind speeds encountered in stormy conditions. Filter-based measurements revealed a reduction of the organic mass fraction as the wind speed increases. AFM imaging at 20% relative humidity of individual SSA identified six main morphologies: prism-like, rounded, core-shell, rod, rod inclusion core-shell, and aggregates. At 10 m/s, most SSA were rounded, while at 19 m/s, core-shells became predominant. Based on AFM-IR, rounded SSA at both wind speeds had similar composition, mainly composed of aliphatic and oxygenated species, whereas the shells of core-shells displayed more oxygenated organics at 19 m/s and more aliphatic organics at 10 m/s. Collectively, our observations can be attributed to the disruption of the sea surface microlayer film structure at higher wind speeds. The findings reveal a significant impact of wind speed on morphology and composition of SSA, which should be accounted for accurate assessment of their climate effects.