Marine Organic Aerosols Reflect Ecosystem Variability from Phytoplankton Functional Types to Micronekton

Abstract. Marine organic aerosols remain a major source of uncertainty in aerosol cloud–climate interactions, in part because marine ecosystem structure and biological drivers are often represented in overly simplified terms, typically reduced to bulk chlorophyll‑a. Here, a full year of high-resolution aerosol mass spectrometry measurements at Mace Head (west coast of Ireland) is combined with HYSPLIT air-masses exposure metrics and gap-free phytoplankton functional type (PFT) fields to explore influences on primary marine organic aerosol (PMOA) and methane sulphonic acid (MSA). During the spring-summer diatom climax, PMOA correlates with dominant bloom taxa (R=0.65-0.70) and micronekton (R=0.55), with rapid 1-3 day responses and secondary maxima at ~25 days, consistent with early labile release and later lysis/grazing. During that same phase, MSA also showed a lagged responses to both PFT and micronekton reflecting delayed DMS production and oxidation. However, comparable phytoplankton air-mass exposure in the late summer of that same year (i.e. early depletion phase) did not reproduce such high correlations, with time-scale analyses indicating weakened coupling at warmer sea-surface temperatures despite moderately stronger winds. These results imply that structured ecosystem composition and physical forcing both contribute to cross-basin seasonal differences in marine organic aerosols formation. This motivates future research vessel campaigns and mesocosm experiments to explicitly manipulate PFT interactions and air-sea physics.