Physicochemical Properties of Nascent vs. Aged Sea Spray Aerosols in the Eastern North Atlantic

Sea spray aerosols (SSA) represent one of the largest sources of atmospheric particles since over two-thirds of the Earth’s surface is covered by oceans. They play an important role in climate and atmospheric chemistry, however, despite this a series of knowledge gaps hinder us from constraining their relevance. One critical question is why the physicochemical properties of nascent particles generated in the laboratory are so different from those measured in the ambient marine atmosphere. For example, a series of studies have highlighted that SSA generated in the laboratory exhibit essentially the same ability to act as cloud condensation nuclei as inorganic sea salt, regardless of the amounts of organic substances present in the seawater from which they were generated (e.g., Collins et al., 2016). This is in stark contrast to observations of ambient marine aerosols - their ability to act as cloud condensation nuclei is often significantly reduced in comparison (Swietlicki et al., 2000).

To address this discrepancy, we prepared a novel experimental setup in which we deployed a chemical ionisation mass spectrometer (CIMS) with an Aim inlet in a setup together with a sea spray simulation chamber, an oxidative flow reactor (OFR), and a differential mobility particle sizer (DMPS) at Graciosa Island, Azores, in the eastern north Atlantic Ocean during summer 2022 as a part of the AGENA campaign.

We used freshly-sampled ocean water to generate SSA that were aged in an OFR for an equivalent period of 3 to 3.5 days in the atmosphere. We recorded the gas-phase chemical composition of nascent and aged aerosols using the AIM-CIMS with multiple reagent ions, collected filter samples for offline analysis of the particle-phase chemical composition, and used a DMPS to compare the particle size distribution and concentration.

The first results of our study show that the volatile organic compounds released from the sampled ocean water considerably nucleate when they are oxidized in the OFR. Furthermore, the chemical analysis of these gases reveals an increase in the concentration of DMS oxidation products, such as methane sulfonic acid, when the nascent SSAs along with the gases in the tank headspace are exposed to oxidants in the OFR. However, we did not observe any substantial differences in the concentration and size distribution of the accumulation and larger-mode particles for primary and aged SSA. This could be attributed to extensive nucleation taking place in the OFR. It is possible that in the real world, these VOCs would rather condense on the primary SSA than form new particles.

In this presentation we will compare the properties of ambient SSA particles in the Eastern North Atlantic and those generated and aged with our experimental setup using real seawater in an attempt to address the discrepancy.

Collins, D. B., et al., Geophys. Res. Lett. 2016, 43 (18), 9975-9983.

Swietlicki, E., et al., Tellus B: Chemical and Physical Meteorology 2000, 52 (2), 201-227