A New Instrument for Determining the Coarse-Mode Sea Salt Aerosol Size Distribution
Abstract
Sea salt aerosol (SSA) plays a significant role in the atmosphere through aerosol direct and indirect effects, and in atmospheric chemistry as a source of tropospheric bromine. In situ measurements of coarse-mode SSA particles are limited because of their low concentration and relatively large sizes (dry radius rd > 0.5 μm). With this in mind, a new, low-cost, easily usable method for sampling coarse-mode SSA particles in the marine boundary layer was developed. An SSA particle sampler that uses an impaction method was designed and built using 3D printing and Arduino microcontrollers and sensors. It exposes polycarbonate slides to ambient airflow remotely on a kite-based platform to capture coarse-mode SSA particles. Because it is a smaller version of the Giant Nucleus Impactor (GNI), designed for use on aircraft, it is named the miniature Giant Nucleus Impactor (miniGNI). After sample collection, the same optical microscope methodology utilized by the GNI was used to analyze the wetted salt particles that impacted onto the slides. In this proof-of-concept study, multiple miniGNIs were attached serially to a kite string, allowing for sampling at multiple altitudes simultaneously. The robustness of the results from this new instrument and methodology for sampling at ambient RH (~75%) the SSA particle size distribution with rd > 3.3 μm are compared with a similar study. We find that the SSA particle number concentration decreases weakly with altitude and shows no correlation to instantaneous U10 wind speed along the windward coastline of Oʻahu in the Hawaiian Islands.
Significance Statement
The goal of our work is to determine the size distribution of large (dry salt radius > 3.3 μm) sea salt aerosol (SSA) particles in the atmosphere. Understanding the number concentration and size distribution of SSA particles is important because they play a role in many atmospheric processes, affecting visibility, atmospheric chemistry, cloud properties, precipitation formation, etc. We designed, built, and tested a new low-cost instrument for sampling large SSA particles. In this paper we compare our results to a similar prior study to give insight into the observations made with the new instrument. We flew the instrument on a kite for sampling, but future work could explore using the instrument on other platforms, such as a drone or unmanned aircraft system (UAS).
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