Organosulfate Formation in Proxies for Aged Sea Spray Aerosol: Reactive Uptake of Isoprene Epoxydiols to Acidic Sodium Sulfate

Oxidation of isoprene, the biogenic volatile organic compound (BVOC) with the highest emissions globally, is a large source of secondary organic aerosol (SOA) in the atmosphere. Particulate organosulfates formed from acid-driven reactions of the oxidation products isoprene epoxydiol (IEPDX) isomers are important contributors to SOA mass. Most studies have focused on organosulfate formation on ammonium sulfate particles, often at low pH. However, recent work has shown that sea spray aerosol (SSA) in the accumulation mode (similar to 100 nm) is quite acidic (pH similar to 2) and undergoes further heterogeneous reactions with H2SO4 to form Na2SO4. Herein, we demonstrate that substantial SOA, including organosulfates, are formed on acidic sodium sulfate particles (pH = 1.4 +/- 0.1) via controlled laboratory experiments. Comparable organosulfate formation was observed for acidic sodium and ammonium sulfate particles even though acidic particles with sodium versus ammonium as the primary cation formed less SOA volume. Both exhibited core-shell morphology after the reactive uptake of IEPDX; however, organosulfates were identified with Raman microspectroscopy in the core and shell of ammonium sulfate SOA particles, but only in the core for sodium sulfate SOA. Key organosulfates were also identified in ambient samples from the Galkpagos Island. Our results suggest that isoprene-derived SOA formed on aged SSA is potentially an important, but underappreciated, source of SOA and organosulfates in marine and coastal regions that could modify SOA budgets.

Automated summary

Oxidation of isoprene, the highest globally emitted biogenic volatile organic compound (BVOC), is a significant source of secondary organic aerosol (SOA) in the atmosphere. The formation of particulate organosulfates from acid-driven reactions of isoprene epoxydiol (IEPDX) isomers contributes to SOA mass. While most studies have focused on organosulfate formation on ammonium sulfate particles, recent research has shown that sea spray aerosol (SSA) in the accumulation mode undergoes further reactions with sulfuric acid to form sodium sulfate. This study demonstrates that significant SOA, including organosulfates, can also form on acidic sodium sulfate particles via laboratory experiments, suggesting that aged SSA may be an underappreciated source of SOA and organosulfates in marine environments.