Chamba, Guillaume, Rissanen, Matti, Barthelmess, Theresa, Saiz-Lopez, Alfonso, Rose, Clémence, Iyer, Siddharth, Saint-Macary, Alexia, Rocco, Manon, Safi, Karl, Deppeler, Stacy, Barr, Neill, Harvey, Mike, Engel, Anja , Dunne, Erin, Law, Cliff S. and Sellegri, Karine (2023) Evidence of nitrate-based nighttime atmospheric nucleation driven by marine microorganisms in the South Pacific. PNAS Proceedings of the National Academy of Sciences of the United States of America, 120 (48). Art.Nr. e2308696120. DOI 10.1073/pnas.2308696120.

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Abstract

Significance
Oceans represent 70% of our planet’s surface, housing a large spectrum of microorganisms that interact with the above atmosphere. Ocean microorganisms were proposed in the late 80’s to be at the center of a climate feedback loop involving dimethyl sulfide (DMS) that would form aerosols and modify cloud properties (CLAW hypothesis). In the present paper, we report observational evidence from semicontrolled experiments in the South Pacific that nitrate ions, yet hitherto not considered, is a key species involved in aerosol nucleation in the pristine marine atmosphere and which precursors are coemitted with DMS. Our results further indicate that nitrate ion formation would be related to short-term microbial processes, sensitive to environmental stressors, therefore potentially “closing the loop”.

Abstract
Our understanding of ocean–cloud interactions and their effect on climate lacks insight into a key pathway: do biogenic marine emissions form new particles in the open ocean atmosphere? Using measurements collected in ship-borne air–sea interface tanks deployed in the Southwestern Pacific Ocean, we identified new particle formation (NPF) during nighttime that was related to plankton community composition. We show that nitrate ions are the only species for which abundance could support NPF rates in our semicontrolled experiments. Nitrate ions also prevailed in the natural pristine marine atmosphere and were elevated under higher sub-10 nm particle concentrations. We hypothesize that these nucleation events were fueled by complex, short-term biogeochemical cycling involving the microbial loop. These findings suggest a new perspective with a previously unidentified role of nitrate of marine biogeochemical origin in aerosol nucleation.

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