Interconnections between aerosols, clouds and marine ecosystems

NOAA Pacific Marine Environmental Laboratory, USA

 

Title: Connections between marine ecosystems and atmospheric aerosols and clouds

 

Abstract: The impact of ocean ecosystems on marine boundary layer aerosols and clouds has been the subject of much research but remains uncertain. Five experiments were recently conducted in the western North Atlantic to assess if the seasonally recurring phytoplankton bloom affects aerosol properties. These experiments include the second Western Atlantic Climate Study and four North Atlantic Aerosols and Marine Ecosystem Study cruises. Measurements of unheated and heated number size distributions, cloud condensation nucleus (CCN) concentrations, and aerosol composition were used to identify primary and secondary aerosol components that could be related to the state of the bloom. Only periods of clean marine air, as defined by radon, particle number concentrations, aerosol light absorption coefficient, and back trajectories, were included in the analysis. Nonvolatile material was found to be prevalent in the Aitken mode size range after heating to 230°C, likely due to downward mixing from the free troposphere. CCN concentrations at 0.1% supersaturation were best correlated (r² = 0.73) with accumulation mode nss SO4⁼. Sea spray aerosol was only correlated with CCN during November when bloom accumulation had not yet occurred and dimethylsulfide concentrations were at a minimum. The fraction of CCN attributable to sea spray aerosol was less than 20% during March, May/June, and September, indicating the limited contribution of sea spray aerosol to the CCN population of the western North Atlantic atmosphere. The strongest link between the plankton bloom and aerosol and cloud properties appears to be due to biogenic non-sea salt SO4⁼.

 

Washington University in St. Louis, USA

 

Title: Aerosol properties and processes in the Eastern North Atlantic

 

Abstract: With their extensive coverage, marine low clouds greatly impact global climate. Presently, the response of marine low clouds to changes of atmospheric aerosols remains a major source of uncertainty in climate simulations. The Eastern North Atlantic (ENA) is a region of persistent but diverse subtropical marine boundary layer clouds, whose albedo and precipitation are highly susceptible to perturbations in aerosol properties. In addition, ENA is periodically impacted by continental aerosols from North America, making it an excellent location to study the cloud condensation nuclei (CCN) budget in a remote marine region periodically perturbed by anthropogenic emissions.

 

The aerosol properties and the processes that drive the CCN population in the ENA are examined by combining the airborne measurements during the Aerosol and Cloud Experiments in Eastern North Atlantic (ACE-ENA) campaign and long-term observations at a ground site on Graciosa Island in the Azores. The ACE-ENA campaign has two airborne deployments, which took place in the Azores from June 21 to July 20, 2017 and from January 15 to February 18, 2018, respectively. The vertical profiles of aerosol size distribution and chemical composition are presented. We focus on the key aerosol processes including new particle formation, the long-range transport of continental emissions, the entrainment of free troposphere air, and the growth of particles into CCN size range inside the marine boundary layer. The variations of the aerosol properties and processes with season and synoptic conditions are examined.

 

University of California, San Diego, USA

 

Title: Molecular drivers of nascent and aged sea spray aerosol phase state across a phytoplankton bloom

 

Abstract: The world’s oceans serve as a major reservoir of biogenic organic carbon and inorganic salts, which can become airborne in sea spray aerosol (SSA), constituting the greatest mass flux of aerosol globally. Physicochemical properties such as the phase state (viscosity) of SSA and its evolution due to atmospheric aging are not well known but such aerosol properties can affect atmospheric composition and aerosol interactions with clouds. During the Sea Spray Chemistry and Particle Evolution (SeaSCAPE) study, we sampled nascent SSA generated in a wave flume mesocosm during a controlled phytoplankton bloom. We employed a particle bounce apparatus and used extractive electrospray ionization mass spectrometry to measure modulations in the phase state and molecular composition of SSA during the bloom in real time. A potential aerosol mass oxidation flow reactor was employed to age the SSA in the presence of hydroxyl radicals. The observed particle bounce fractions indicate that nascent SSA is more viscous during peak biological activity. We discovered that the increase in particle bounce at the peak of the bloom was driven by production of higher molecular weight organic carbon with lower hygroscopicity, with estimated viscosities of up to 10³-10⁴ Pa·s at the peak of biological activity, compared to <10² Pa·s during periods of low biological activity. Studies in my lab further demonstrate that divalent cations, e.g., Ca²⁺, in SSA promote stronger intermolecular interactions with marine-relevant fatty acids, leading to more viscous SSA. Oxidative aging by hydroxyl radical generally promotes volatilization of high molecular weight organic species in SSA during the peak of the bloom, driving less viscous phase states of SSA. Estimates of SSA viscosity vertical profiles suggest that during the peak of a phytoplankton bloom, SSA is more viscous at lower altitudes. These results have important implications for SSA processing and ice nucleation in the marine boundary layer.