Ocean flux of salt, sulfate, and organic components to atmospheric aerosol

Abstract

The oceans contribute to aerosol particles in the atmosphere through two different physical mechanisms: first by the production of sea spray aerosol (SSA), and second by emitting gases that condense to produce secondary marine aerosol (SMA). These aerosol emissions include three types of chemical compounds: salt particles account for >90% of the mass, most of which is >1 μm dry diameter; sulfate particles are mostly <0.5 μm, typically constituting most of the number and the largest impacts on clouds; organic components include the greatest variety of compounds and the most uncertain effects on clouds. Most SSA particles are expected to form from bubbles as film drops that are <1 μm dry diameter and form from flapping bursting bubbles, although >1 μm film drops can form by ligament fragmentation. SMA particles include contributions from marine biogenic gas emissions, including dimethylsulfide (DMS), isoprene, amines, and monoterpenes. The role of particles from the ocean in the atmosphere varies by region and by season, but since atmospheric concentrations of ocean-derived <1 μm particles are typically much smaller than the concentrations of their continental counterparts, they have the largest impacts on climate in regions where continental sources are limited. Most efforts to quantify global SSA and SMA emissions rely on global models, where representations of marine aerosol sources are constrained by a small number of field measurements. Satellite-based retrievals of coarse and marine aerosol optical depth provide near global coverage that has been linked to coincident wind speed, whitecaps, and biological productivity for >1 μm particles. The current best estimate of SSA flux of 5000 Tg/yr can be used to calculate SSA-related carbon flux as 35 TgC/yr, by approximating <1 μm SSA particles as 10% of SSA flux with 7% organic carbon and > 1 μm particles as 90% of SSA flux with no organic carbon. SMA is estimated to contribute 0.6 TgC/yr as DMS, 0.6 TgC/yr as amines, and an additional trace amount from isoprene and monoterpenes for a total of <2 TgC/yr. Because of the limited availability of observations to constrain SSA and SMA global estimates, oceanic fluxes to aerosol and aerosol precursors could vary by over two orders of magnitude. Key open questions that require additional observational constraints include the variability in >1 μm SSA mass size distributions, the relative contributions of SSA and SMA to number concentrations of particles <0.5 μm, and the regional and seasonal factors that may control these <0.5 μm particle concentrations.