Organic Nitrogen, Bimodal Size Distributions, and Polluted Marine Air in an Ocean-in-Lab Experiment

Abstract:

We report size-resolved chemical composition of fresh and aged marine aerosol formed via wave breaking of real ocean water in an innovative ocean-atmosphere facility during the Sea Spray Chemistry and Particle Evolution (SeaSCAPE) experiment. We observe that the size distribution and chemical composition of primary marine aerosol, measured by high-resolution time-of-flight aerosol-mass-spectrometry (HR-TOF-AMS), does not significantly change over the course of phytoplankton and bacterial blooms. However, we reproducibly observe a bimodal aerosol size distribution when primary marine aerosol and air from the isolated headspace of the tank containing a few ppb of NO_x are rapidly co-oxidized by OH radicals in an oxidation flow reactor. We attribute the emergence of an additional size mode to nucleation and growth of new particles. This new smaller size mode consists mostly of reduced organic nitrogen species (C_xH_yN), while the aged sea spray aerosol mode is mostly particulate nitrate (pNO₃). We hypothesize that the particulate reduced organic nitrogen is the product of gas-phase reactions of reduced organic nitrogen species with acids formed in the oxidation flow reactor, initiated by the formation of HNO₃ from NO_x and subsequent anion substitution of chloride in NaCl to form NaNO₃. We further investigate the relevance of this mechanism of aerosol formation and potential impacts on the cloud condensation nuclei activity in polluted marine environments.