Cation-Driven Lipopolysaccharide Morphological Changes Impact Heterogeneous Reactions of Nitric Acid with Sea Spray Aerosol Particles

Lipopolysaccharides (LPS) in sea spray aerosol (SSA) particles have recently been shown to undergo heterogeneous reactions with HNO3 in the atmosphere. Here, we integrate theory and experiment to further investigate how the most abundant sea salt cations, Na+, Mg2+, and Ca2+, impact HNO3 reactions with LPS-containing SSA particles. Aerosol reaction flow tube studies show that heterogeneous reactions of SSA particles with divalent cation (Mg2+ and Ca2+) and LPS signatures were less reactive with HNO3 than those dominated by monovalent cations (Na+). All-atom molecular dynamics simulations of model LPS aggregates suggest that divalent cations cross-link the oligosaccharide chains to increase molecular aggregation and rigidity, which changes the particle phase and morphology, decreases water diffusion, and consequently decreases the reactive uptake of HNO3. This study provides new insight into how complex chemical interactions between ocean-derived salts and biogenic organic species can impact the heterogeneous reactivity of SSA particles.