Physicochemical properties of nascent versus aged sea spray aerosol – A study from the eastern North Atlantic Ocean

Sea spray aerosol (SSA), produced by bubble bursting at the ocean's surface, plays a critical role in climate regulation and atmospheric chemistry. It also provides a unique microenvironment for gas-to-particle partitioning and aqueous-phase reactions. Understanding these processes requires a detailed examination of the physicochemical properties and the transformations of SSA during atmospheric aging. 
Hence, we designed a comprehensive experimental setup comprising a sea spray simulation tank for generating SSA, a chemical ionization mass spectrometer (CIMS) for analyzing molecular-level composition, an oxidation flow reactor (PAM) for simulating atmospheric oxidation, and a differential mobility particle counter (DMPS) for determining particle size distribution. We deployed this setup in May 2022 during the AGENA campaign on Graciosa Island in the Azores, Portugal, a remote marine site. We collected surface ocean water samples from the Atlantic, and generated SSA using a plunging jet. We used DMPS and CIMS to analyze physicochemical properties of SSA present in the tank headspace, and also collected filter samples for offline CIMS analysis. 
Our results revealed significant particle formation in the PAM chamber at an aging period equivalent to 3–3.5 days in the atmosphere. Notably, the increase was primarily restricted to particles below 100 nm, suggesting that new particle formation dominated over condensation in the PAM environment, likely due to high oxidant concentrations. This observation also indicates the presence of numerous volatile organic compounds (VOCs) in the nascent SSA, which may have condensed onto pre-existing particles in natural settings. Further analysis of the VOCs using CIMS showed that nascent SSA contained compounds with longer carbon chains (1–16 carbons) and higher oxidation states, indicating low volatility. In contrast, gases exiting the PAM chamber exhibited shorter carbon chains (1–10 carbons) and lower oxidation levels, suggesting condensation of oxidation products onto newly formed particles within the reactor. Additionally, we identified oxidation products of dimethyl sulfide (DMS), such as dimethyl sulfoxide (DMSO) and methanesulfonic acid (MSA), in both nascent and aged samples. Intriguingly, nascent SSA also exhibited strong signals for fluorinated compounds, including hydrofluoric acid, likely formed from protonation of fluoride ions (F⁻) and other fluoride-containing salts like MgF⁺, CaF⁺, and NaF⁺ found in sea salt. These findings provide valuable insights into the molecular composition and dynamic behaviour of SSA, with implications for understanding its role in atmospheric processes and climate.