Biological Influence on δ<sup>13</sup>C and Organic Composition of Nascent Sea Spray Aerosol
Daniel R. Crocker, Ritchie E. Hernandez, Haonan D. Huang, Matthew A. Pendergraft, Ruochen Cao, Jiayin Dai, Clare K. Morris, Grant B. Deane, Kimberly A. Prather, M. H. Thiemens
Abstract
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 δ13C were measured for SSA (δ13CSSA) along with seawater particulate and dissolved organic carbon (δ13CPOC and δ13CDOC) to track changes in carbon transfer and composition between seawater and SSA. Contrary to common assumptions, δ13CSSA values were not equivalent to δ13CDOC. The consistently less negative δ13CSSA values indicate that nascent δ13CSSA 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 δ13CSSA and δ13CDOC 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 δ13CSSA 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.