Molecular Drivers of Nascent and Aged Marine Aerosol Phase States
JONATHAN SLADE, Paul Tumminello, Samantha Kruse, Karen Lopo Zepeda,
University of California San Diego Abstract Number: 181
Working Group: Aerosol Physical Chemistry and Microphysics
AbstractThe world’s oceans serve as a major reservoir of organic carbon and inorganic salts, which can become airborne in sea spray aerosol (SSA), constituting the greatest mass flux of aerosol globally. The phase state (viscosity) of SSA and the evolution of SSA phase state due to atmospheric aging are not well known but aerosol viscosity can affect atmospheric composition and the aerosol’s ability to serve as cloud condensation and ice nuclei. Here, nascent SSA was generated in a wave flume mesocosm during a controlled phytoplankton bloom. We employed a particle bounce apparatus and used extractive electrospray ionization mass spectrometry to measure modulations in the phase state and molecular composition of the aerosol in real time. The observed particle bounce fractions indicate that nascent SSA is more viscous during peak biological activity with estimated viscosities of the organic material of up to 10
3-10
4 Pa·s at the peak of biological activity, compared to <10
2 Pa·s during periods of low biological activity. These differences are attributed to differences in the molecular weights and hygroscopicity of the organic material in SSA dependent upon the stage of the bloom. The nascent SSA was further aged in an oxidative flow reactor by hydroxyl radicals (OH) between the equivalent of 5 and >21 days of atmospheric exposure at [OH]=1.5×10
6 molecules cm
-3 in the presence of marine volatile compounds. At low levels of OH exposure, the particle bounce fraction increased with aging, whereas extensive aging led to less viscous phase states. To better constrain the role of heterogeneous OH oxidation on the phase state of SSA, laboratory mimics of SSA composed of mixtures of fatty acids and inorganic salts (NaCl and CaCl
2) were aged in an oxidation flow reactor under varying OH exposures between 0 and 5 days equivalent aging in the atmosphere. In the absence of OH, divalent Ca
2+ in the SSA mimic promoted stronger intermolecular interactions with marine-relevant fatty acids and more viscous particles compared to mixtures composed of monovalent Na+. Oxidative aging of pure fatty acid organic aerosol by OH decomposed and volatilized the organic material leading to a reduction in particle volume of 80%, whereas the presence of NaCl under the same OH exposures decreased particle volume by up to 20%. Particle bounce fractions increased with increasing OH exposure for the pure fatty acid aerosol, whereas mixtures with NaCl led to no change, suggesting that major decomposition products such as aldehydes in the aerosol phase may condense in the more acidic particle environment with lower levels of inorganic salts, promoting more viscous, bouncier particles with aging. These results have important implications for SSA lifetimes and potentially processes such as ice nucleation in the marine atmospheric environment.