Elucidating the influence of oceanic biological activity on the organic composition of sea spray aerosol (SSA) is crucial to understanding marine cloud properties relevant to climate. Numerous marine field studies designed to address this topic have yielded conflicting results mainly as a result of the inability to distinguish primary SSA composition from terrestrial and marine secondary organic aerosols. In this study, two laboratory-induced phytoplankton blooms were conducted in an isolated system without background aerosol contributions. Values for δ 13 C were measured for SSA (δ 13 C SSA ) along with seawater particulate and dissolved organic carbon (δ 13 C POC and δ 13 C DOC ) to track changes in carbon transfer and composition between seawater and SSA. Contrary to common assumptions, δ 13 C SSA values were not equivalent to δ 13 C DOC . The consistently less negative δ 13 C SSA values indicate that nascent δ 13 C SSA reflects specific changes in relative contributions to SSA from the available seawater carbon pools, as a function of biological activity. A dual-source isotopic mixing model revealed that the difference between δ 13 C SSA and δ 13 C DOC was explained by increased relative contributions of "freshly produced" organic carbon (OC) to SSA, with the largest contribution of "freshly produced" OC occurring 2−3 days after the maximum chlorophyll-a concentrations. This finding is consistent with previous mesocosm studies, showing that organic enrichment in SSA requires processing by heterotrophic bacteria after periods of high primary productivity. This work examining the biological influences on SSA organic composition and nascent δ 13 C SSA values provides new insights into ocean-to-SSA carbon transfer dynamics, which can be used in future field studies to improve estimates of anthropogenic influences on the carbon composition of the marine environment.

To elucidate the seawater biological and physicochemical factors driving differences in organic composition between supermicron and submicron sea spray aerosol (SSAsuper and SSAsub), carbon isotopic composition (δ13C) measurements were performed on size-segregated, nascent SSA collected during a phytoplankton bloom mesocosm experiment. The δ13C measurements indicate that SSAsuper contains a mixture of particulate and dissolved organic material in the bulk seawater. After phytoplankton growth, a greater amount of freshly produced carbon was observed in SSAsuper with the proportional contribution being modulated by bacterial activity, emphasizing the importance of the microbial loop in controlling the organic composition of SSAsuper. Conversely, SSAsub exhibited no apparent relationship with biological activity but tracked closely with surface tension measurements probing the topmost ∼0.2–1.5 μm of the sea surface microlayer. This probing depth is similar to a bubble’s film thickness at the ocean surface, suggesting that SSAsub organic composition may be influenced by the presence of surfactants at the air–sea interface that are transferred into SSAsub by bubble bursting. Our findings illustrate the substantial impact of seawater dynamics on the pronounced organic compositional differences between SSAsuper and SSAsub and demonstrate that these two SSA populations should be considered separately when assessing their contribution to marine aerosols and climate.

Studies over the last two decades have shown that submicron particulates (SMPs) can be transferred from the seawater into sea spray aerosol (SSA), potentially impacting SSA cloud seeding ability. This work reports the first concurrent bulk and sea surface microlayer (SSML) SMP (0.4–1.0 μm) measurements, made during two mesocosm phytoplankton blooms in a region devoid of active wave breaking and bubble formation, providing insight into how biological and physicochemical processes influence seawater SMP distributions. Modal analyses of the SMP size distributions revealed contributions from multiple, biologically related particulate populations that were controlled by the microbial loop. With negligible bubble scavenging occurring, SSML enrichment of SMPs remained low throughout both experiments, suggesting scavenging is vital for SMP enrichment in the SSML. Our findings are discussed in the context of SMP transfer into SSA and its potential importance for SSA cloud seeding ability.

Increases in seawater submicron particulates during a phytoplankton bloom are size-dependent and influenced by the bloom phase.• Particulate enrichment in the sea surface microlayer is low in the absence of wave breaking and bubble scavenging• Augmented biological production of submicron particulates may enhance their entrainment in sea spray aerosol particlesESSOAr |