Marine Organic Aerosols at Mace Head: Effects from Phytoplankton and Source Region Variability

Abstract. Organic aerosols play a significant role in atmospheric chemistry and climate, yet their sources and transformations remain poorly characterised over marine regions. This study aims at representing the clean Northeast Atlantic background by presenting the first source apportionment of organic aerosols (OA) at Mace Head, Ireland. Resolved organic aerosol sources were characterised by prevailing pristine marine boundary layer air masses, with balanced mass contributions from primary and secondary sources. Positive matrix factorisation resolved four OA types: primary marine organic aerosols (42%), methanesulphonic acid aerosols (17%), more oxidised oxygenated organic aerosols (32%), and peat-derived aerosols (9%). Transfer entropy is introduced as a novel method to trace secondary organic aerosol origins, revealing that aged organics form from both open Ocean air masses undergoing ozonolysis as well as oxidation from local peat burning emissions. Conversely this approach further demonstrates that primary marine organic aerosols and methanesulphonic acid aerosols are fully exempt of anthropogenic influences. This work also presents a detailed characterisation of marine sources, building on previously identified tracers. PMOA reflects phytoplankton extracellular metabolic processes largely shaped by abacterial processes whereas MSA-OA is marked by stress enzymes. By integrating diatoms, coccolithophores, cyanobacteria, and green algae from the NASA Ocean Biogeochemical Model (NOBM) this manuscript finally concludes that MSA-OA are closely tied to coccolithophores blooms while PMOA  are rather linked to diatoms, chlorophytes, and cyanobacteria instead. These findings highlight the need for  extended investigations into marine aerosols chemical pathways and associations with phytoplanktons as drivers of marine OA.