Quantification of the presence and production of giant sea salt aerosol in the global marine atmosphere

Abstract

Giant sea salt aerosol (GSSA, dry radius > 1 μm) plays a crucial role in atmospheric processes, yet significant uncertainties remain regarding its production, vertical transport, and global distribution. These large particles influence global radiative balance and cloud interactions, but observational limitations have hindered accurate quantification of their production and consequently global presence in the marine atmosphere. This study integrates in-situ measurements and models to build upon our understanding of GSSA production, presence, and dynamics that control their existence in the atmosphere. Recognizing the aerosol concentrations are built up over time, HYSPLIT back-trajectories and ERA5 reanalysis data are used to augment information from instantaneous observations, and ultimately account for the cumulative effects of historical environmental conditions. I assessed the influence of past wind speeds and ocean conditions on GSSA concentrations and determined that traditional correlations with instantaneous environmental variables may overestimate the significance of local conditions, underscoring the need to consider air mass history when interpreting GSSA measurements. Expanding on this idea of utilizing historical information to inform observations, I developed a new GSSA source function based on over 700 size distributions collected across four field campaigns from regions around the world. GSSA particle sizes are often neglected in traditional sea salt aerosol source functions, or their size ranges have gone untested. This new function, derived using a novel, 1-D quasi-concentration build-up model, incorporates information from historical environmental influences to better capture GSSA production. Comparison of this new parameterization alongside existing source functions from earth system models demonstrates a reduction in root mean squared error, ultimately enhancing our ability to estimate SSA contributions and their atmospheric impacts more accurately. Lastly, this research investigates how local effects, like coastal wave breaking, may alter GSSA concentrations in places like Hawai‘i. Historical studies determined that coastal processes and orographic effects can enhance atmospheric concentrations of GSSA particles by orders of magnitude, but the extent of these contributions remains poorly constrained. Field observations from the Hawaiian Island of O‘ahu confirm that coastal wave action and orographic uplift significantly increase GSSA concentrations and facilitate vertical mixing. Measurements collected reveal that coastal concentrations of GSSA particles are 2.7–5.4 times greater than open-ocean levels. Furthermore, significant wave height exhibits the strongest correlation with SSA variability, emphasizing the role of coastal processes in modifying aerosol distributions. This research highlights the importance of accounting for air mass history in aerosol observations, the need for refined source functions to improve global GSSA modeling, and the critical role of coastal environments in GSSA production, ultimately providing important context to highly nuanced observations that have been historically understudied.