In this study, we evaluated photosensitized chemistry at the air−sea interface as a source of secondary organic aerosols (SOA). Our results show that, in addition to biogenic emissions, abiotic processes could also be important in the marine boundary layer. Photosensitized production of marine secondary organic aerosol was studied in a custom-built multiphase atmospheric simulation chamber. The experimental chamber contained water, humic acid (1−10 mg L −1) as a proxy for dissolved organic matter, and nonanoic acid (0.1−10 mM), a fatty acid proxy which formed an organic film at the air−water interface. Dark secondary reaction with ozone after illumination resulted in SOA particle concentrations in excess of 1000 cm −3 , illustrating the production of unsaturated compounds by chemical reactions at the air−water interface. SOA numbers via photosensitization alone and in the absence of ozone did not exceed background levels. From these results, we derived a dependence of SOA numbers on nonanoic acid surface coverage and dissolved organic matter concentration. We present a discussion on the potential role of the air−sea interface in the production of atmospheric organic aerosol from photosensitized origins. ■ INTRODUCTION Although the dominant mass fraction of sea-spray aerosol is inorganic sea salt, organic matter can also contribute to the overall mass of aerosols in the marine boundary layer (MBL). 1,2 Recent field measurements clearly documented the presence of organic matter in oceanic particles. 3,4 Cavalli et al. 5 and O'Dowd et al. 6 found a significant and dominating fraction of organic matter in the submicrometer size range, while the supermicrometer size range predominately consisted of inorganic sea salt. During high biological activity, the organic fraction ranges from 40 to 60% of the submicrometer aerosol mass, while during low biological activity periods, this fraction decreases to 10−15%. Concentrations of organic aerosol mass in air advected over regions of high biological activity were up to 4 μg m −3 and comparable to polluted air masses. 7 The concentration of organic aerosol formed by secondary processes has also been correlated with biological activity. 8 Volatile sulfur species greatly impact the formation of secondary marine aerosols 9−13 and are included in general circulation models predicting climate evolution. 13−15 Together, these findings potentially link ocean biota with marine derived organic aerosols. 16 As a result, the organic fraction of the marine aerosols as well as the trace gas composition over the ocean are controlled by the chemical and physical properties of the sea-surface microlayer (SML). 17−19 Indeed, recent studies reported the use of natural seawater to generate sea spray aerosol (SSA) in order to evaluate how SML composition drives the composition and associated properties of freshly emitted SSA. 20,21 Organic material present at the sea surface includes amphiphiles derived from oceanic biota (fatty acids, fatty alcohols, sterols, amines, etc.) and more complex colloids and aggregates exuded by phytoplankton, which mainly consist of lipopolysaccharides. 22−31 All of these compounds can be highly enriched in this microlayer. 32,33 The presence of complex and potentially photoactive compounds, such as a fatty acid film at the air−sea interface and therefore in the primary marine aerosol, was reported on the surface of continental and marine aerosols. 34−36 This could give rise to the assumption that new processes affect the chemistry in the MBL. Indeed, Reeser et al. 37,38 showed that photoexcited chlorophyll can oxidize halide anions at the salt water surface, producing atomic halogens. A similar chemistry is expected for nitrate and nitrite anions, suggesting a rich new source of oxidants in the MBL. These studies stress the need for a better understanding of the chemistry and potential photochemistry of the surface micro-layer. Indeed, the photochemistry at the air−sea interface has not been adequately considered over the past years. 39 Previous works from our group have shown that such photochemical processing of a surfactant in the presence of a photosensitizer led to the formation of unsaturated and highly functionalized volatile organic compounds (VOCs). 40,41 The use of humic acid as a photosensitizer initiates chemical transformation of surfactants, such as nonanoic acid 40 and octanol, 41 through multiple pathways. The initial step is H-abstraction on the alkyl