The content and composition of organic matter in sea spray aerosol (SSA) affects climatically relevant properties of these aerosols such as hygroscopicity, reflectivity, and cloud seeding ability. The formation of submicron and supermicron SSA primarily occurs via two separate mechanisms, where most submicron SSA is produced by bursting of the bubble film, and most supermicron SSA is generated by collapse of the bubble cavity. Because of the two distinct formation pathways, biologically-induced changes in seawater organic carbon composition are likely to exert different influences on submicron and supermicron SSA. Carbon isotopic composition (d13C) measurements have been previously employed to elucidate the contribution of different seawater carbon pools to nascent SSA, and in this study we extend these measurements to assess the effects of biological activity on processing of marine organic matter and transfer into size-segregated SSA. The d13C and OC amounts for submicron SSA, supermicron SSA, size-segregated particulate organic carbon (POC), and dissolved organic carbon (DOC) were measured throughout a mesocosm phytoplankton bloom experiment conducted in a wave channel filled with natural, coastal seawater. Similar to previous measurements on the total SSA population, the supermicron SSA was comprised of a mixture of bulk seawater POC and DOC with an increased contribution of “freshly produced” OC during and after the Chl-a peak. This contribution of “freshly produced” OC to supermicron SSA was modulated by bacterial activity emphasizing the importance of the microbial loop in determining the composition of supermicron SSA. In contrast, the submicron SSA did not appear to be influenced by biology activity and had much more negative d13C values indicative of a large anthropogenic carbon contribution. The d13C values for submicron SSA trend very closely with AFM surface tension measurements on the sea surface microlayer that probe top 0.1-1 µm of this layer. This suggests that surface-active anthropogenic compounds present in the coastal seawater were enriched in this topmost 0.1-1 µm layer, about the thickness of the bubble film, and preferentially transferred into submicron SSA. The extreme differences between supermicron and submicron SSA highlight the importance of SSA formation processes on organic composition, and illustrate that these two SSA populations should be separated when assessing their potential role in the climate system.

Carbon isotopic investigations of chemical composition differences between submicron and supermicron sea spray aerosol

ACS Spring Meeting 2021

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