Mechanisms controlling giant sea salt aerosol size distributions along a tropical orographic coastline

Abstract

Abstract. Sea salt aerosol (SSA) is a naturally occurring phenomenon that arises from the breaking of waves and consequent bubble bursting on the ocean's surface. The resulting particles exhibit a bimodal distribution spanning orders of magnitude in size that introduces significant uncertainties when estimating the total annual mass of SSA on a global scale. Although estimates of mass and volume are significantly influenced by the presence of giant particles (dry radius >1 µm), effectively observing and quantifying these particles proves to be challenging. Additionally, uncertainties persist regarding the contribution of SSA production along coastlines, but preliminary studies suggest that coastal interactions may increase SSA particle concentrations by orders of magnitude. Moreover, our knowledge regarding the vertical distribution of SSA particles in the marine boundary layer remains limited, resulting in significant gaps in understanding the vertical mixing of giant aerosol particles and specific environmental conditions facilitating their dispersion. By addressing these uncertainties, particularly in regions where SSA particles constitute a substantial percentage of total aerosol loading, we can enhance our comprehension of the complex relationships between the air, sea, aerosols, and clouds. A case study conducted on the Hawaiian island of O`ahu offers insight into the influence of coastlines and orography on the production and vertical distribution of giant SSA size distributions. Along the coastline, the frequency of breaking waves is accelerated, serving as an additional source of SSA production. Furthermore, the steep island orography generates strong and consistent uplift under onshore trade wind conditions, facilitating vertical mixing of SSA particles along windward coastlines. To investigate this phenomenon, in situ measurements of SSA size distributions for particles with dry radii (rd) ≥ 2.8 µm were conducted for various altitudes, ranging from approximately 80 to 650 m altitude along the windward coastline and from 80 to 250 m altitude aboard a ship offshore. Comparing size distributions onshore and offshore confirmed significantly higher concentrations along the coastline, with 2.7–5.4 times greater concentrations than background open-ocean concentrations for supermicron particles. These size distributions were then analyzed in relation to environmental variables influencing SSA production and atmospheric dynamics. It was found that significant wave height exhibited the strongest correlation with changes in SSA size distributions. Additionally, simulated sea salt particle trajectories provided valuable insight into how production distance from the coastline impacts the horizontal and vertical advection of SSA particles of different sizes under varying trade wind speeds. Notably, smaller particles demonstrated reduced dependence on local wind speeds and production distance from the coastline, experiencing minimal dry deposition and high average maximum altitudes relative to larger particles. This research not only highlights the role of coastlines in enhancing the presence and vertical mixing potential of giant SSA particles, but also emphasizes how important it is to consider the influence of local factors on aerosol observations at different altitudes.